srfi-13.txt

The SRFI 13 string libraries					-*- outline -*-
Olin Shivers
1999/10/2
Last Update: 2001/12/23

Emacs should display this document in outline mode. Say c-h m for
instructions on how to move through it by sections (e.g., c-c c-n, c-c c-p).

* Table of contents
-------------------
Abstract
Procedure index
Rationale
  Strings are code-point sequences
  String operations are locale- and context-independent
  Internationalisation & super-ASCII character types
    Case mapping and case folding
    String equality & string normalisation
    String inequality
  Naming conventions
  Shared storage
  R4RS/R5RS procedures
  Extra-SRFI recommendations
Procedure Specification
  Main procedures
    Predicates
    Constructors
    List & string conversion
    Selection
    Modification
    Comparison
    Prefixes & suffixes
    Searching
    Alphabetic case mapping
    Reverse & append
    Fold, unfold & map
    Replicate & rotate
    Miscellaneous: insertion, parsing
    Filtering & deleting
  Low-level procedures
    START/END optional argument parsing & checking utilities
    Knuth-Morris-Pratt searching
Reference implementation
Acknowledgements
References & links
Copyright

-------------------------------------------------------------------------------
* Abstract
----------

R5RS Scheme has an impoverished set of string-processing utilities, which is 
a problem for authors of portable code.  This SRFI proposes a coherent and
comprehensive set of string-processing procedures; it is accompanied by a
reference implementation of the spec. The reference implementation is
    - portable
    - efficient
    - open source

The routines in this SRFI are backwards-compatible with the string-processing
routines of R5RS.

-------------------------------------------------------------------------------
* Procedure index
-----------------
Here is a list of the procedures provided by the string-lib 
and string-lib-internals packages. "#" marks R5RS procedures; "+" 
marks extended R5RS procedures.

Predicates
#   string?
    string-null? 
    string-every string-any

Constructors
#   make-string
#   string
    string-tabulate

List & string conversion
+   string->list 
#   list->string
    reverse-list->string
    string-join

Selection
#   string-length
#   string-ref
+   string-copy
    substring/shared
    string-copy! 
    string-take string-take-right
    string-drop string-drop-right
    string-pad  string-pad-right 
    string-trim string-trim-right string-trim-both 

Modification
#   string-set!
+   string-fill! 

Comparison
    string-compare string-compare-ci
    string<>     string=    string<    string>    string<=    string>=
    string-ci<>  string-ci= string-ci< string-ci> string-ci<= string-ci>=
    string-hash  string-hash-ci

Prefixes & suffixes
    string-prefix-length    string-suffix-length
    string-prefix-length-ci string-suffix-length-ci

    string-prefix?    string-suffix?    
    string-prefix-ci? string-suffix-ci? 

Searching
    string-index string-index-right
    string-skip  string-skip-right 
    string-count 
    string-contains string-contains-ci

Alphabetic case mapping
    string-titlecase  string-upcase  string-downcase
    string-titlecase! string-upcase! string-downcase!

Reverse & append
    string-reverse string-reverse!
#   string-append
    string-concatenate
    string-concatenate/shared string-append/shared
    string-concatenate-reverse string-concatenate-reverse/shared

Fold, unfold & map
    string-map      string-map!
    string-fold     string-fold-right
    string-unfold   string-unfold-right
    string-for-each string-for-each-index

Replicate & rotate
    xsubstring string-xcopy!

Miscellaneous: insertion, parsing
    string-replace string-tokenize

Filtering & deleting
    string-filter string-delete 

Low-level procedures
    string-parse-start+end
    string-parse-final-start+end
    let-string-start+end

    check-substring-spec
    substring-spec-ok?

    make-kmp-restart-vector kmp-step string-kmp-partial-search


-------------------------------------------------------------------------------
* Rationale
-----------

This SRFI defines two libraries that provide a rich set of operations for
manipulating strings. These are frequently useful for scripting and other
text-manipulation applications. The library's design was influenced by the
string libraries found in MIT Scheme, Gambit, RScheme, MzScheme, slib, Common
Lisp, Bigloo, guile, Chez, APL, Java, and the SML standard basis.

All procedures involving character comparison are available in
both case-sensitive and case-insensitive forms.

All functionality is available in substring and full-string forms.

** Strings are code-point sequences
===================================
This SRFI considers strings simply to be a sequence of "code points" or
character encodings. Operations such as comparison or reversal are always done
code point by code point. See the comments below on super-ASCII character
types for implications that follow.

It's entirely possible that a legal string might not be a sensible "text"
sequence. For example, consider a string comprised entirely of zero-width
Unicode accent characters with no preceding base character to modify --
this is a legal string, albeit one that does not make a great deal of sense
when interpreted as a sequence of natural-language text. The routines in
this SRFI do not handle these "text" concerns; they restrict themselves
to the underlying view of strings as merely a sequence of "code points."

** String operations are locale- and context-independent
========================================================
This SRFI defines string operations that are locale- and context-independent.
While it is certainly important to have a locale-sensitive comparison or
collation procedure when processing text, it is also important to have a suite
of operations that are reliably invariant for basic string processing ---
otherwise, a change of locale could cause data structures such as hash tables,
b-trees, symbol tables, directories of filenames, etc. to become corrupted.

Locale- and context-sensitive text operations, such as collation, are
explicitly deferred to a subsequent, companion "text" SRFI.

** Internationalisation & super-ASCII character types
=====================================================
The major issue confronting this SRFI is the existence of super-ASCII
character encodings, such as eight-bit Latin-1 or 16- and 32-bit Unicode.  It
is a design goal of this SRFI for the API to be portable across string
implementations based on at least these three standard encodings.
Unfortunately, this places strong limitations on the API design. Here are
some relevant issues. Be warned that life in a super-ASCII world is
significantly more complex; there are no easy answers for many of these issues.

*** Case mapping and case-folding
=================================
Upper- and lower-casing characters is complex in super-ASCII encodings.

- Some characters case-map to more than one character. For example,
  the Latin-1 German eszet character upper-cases to "SS." 

  + This means that the R5RS function CHAR-UPCASE is not well-defined, 
    since it is defined to produce a (single) character result. 

  + It means that an in-place STRING-UPCASE! procedure cannot be reliably
    defined, since the original string may not be long enough to contain
    the result -- an N-character string might upcase to a 2N-character result.

  + It means that case-insensitive string-matching or searching is quite
    tricky. For example, an n-character string S might match a 2N-character
    string S'.

- Some characters case-map in different ways depending upon their surrounding
  context. For example, the Unicode Greek capital sigma character downcases
  differently depending upon whether or not it is the final character in a
  word. Again, this spells trouble for the simple R5RS CHAR-DOWNCASE function.

- Unicode defines three cases: lowercase, uppercase and titlecase. The
  distinction between uppercase and titlecase arises in the presence of
  Unicode's compound characters. For example, Unicode has a single character
  representing the compound pair "dz." Uppercasing the "dz" character produces
  the compound character "DZ", while titlecasing (or, as Americans say,
  capitalizing) it produces compound character "Dz".

- Turkish actually has different case-mappings from other languages.

The Unicode Consortium's web site
    http://www.unicode.org/
has detailed discussions of the issues. See in particular technical report
21 on case mappings
    http://www.unicode.org/unicode/reports/tr21/

SRFI 13 makes no attempt to deal with these issues; it uses a simple 1-1
locale- and context-independent case-mapping, specifically Unicode's 1-1
case-mappings given in

    ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt

The format of this file is explained in

    ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.html

Note that this means that German eszet upper-cases to itself, not "SS".

Case-mapping and case-folding operations in SRFI 13 are locale-independent so
that shifting locales won't wreck hash tables, b-trees, symbol tables, etc.


*** String equality & string normalisation
==========================================
Comparing strings for equality is complicated because in some cases Unicode
actually provides multiple encodings for the "same" character, and because
what we usually think of as a "character" can be represented in Unicode as a
*sequence* of several code-points. For example, consider the letter "e" with
an acute accent. There is a single Unicode character for this. However,
Unicode also allows one to represent this with a two-character sequence: the
"e" character followed by a zero-width acute-accent character. As another
example, Unicode provides some Asian characters in "narrow" and "full" widths.

There are multiple ways we might want to compare strings for equality. In
(roughly) decreasing order of precision,

- we might want a precise comparison of the actual encoding, so that
  <e-acute> would *not* compare equal to <e, acute>.

- We might want a "normalised" comparison, where these two sequences would 
  compare equal.

- We might want an even more-permissive normalisation, where visually-distinct
  properties of "the same" character would be ignored. For example, we might
  want narrow/full-width versions of the same Asian character to compare equal.

- We might want comparisons that are insensitive to accents and diacritical
  marks.

- We might want comparisons that are case-insensitive.

- We might want comparisons that are insensitive to several of the above
  properties.

- We might want ways to "normalise" strings into various canonical forms.

This library does not address these complexities. SRFI 13 string equality is
simply based upon comparing the encoding values used for the characters.
Accent-insensitive and other types of comparison are not provided; only
a simple form of case-insensitive comparison is provided, which uses the
1-1 case mappings specified by Unicode in
    ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt
These are adequate for "program" or "systems" use of strings (e.g., to 
manipulate program identifiers and operating-system filenames).

*** String inequality
=====================
Above and beyond the issues arising in string-equality, when we attempt
to order strings there are even further considerations.

- French orders accents with right-to-left significance -- the reverse of
  the significance of the characters.

- Case-insensitive ordering is not well defined by simple "code-point"
  considerations, even for simple ASCII: there are punctuation characters
  between the ASCII's upper-case range of letters and its lower-case range
  (left-bracket, backslash, right-bracket, caret, underbar and backquote).
  Does left-bracket compare less-than or greater-than "a" in a
  case-insensitive comparison? 

- The German eszet character should sort as if it were the *pair* of
  letters "ss".

Unicode defines a complex set of machinery for ordering or "collating"
strings, which involves mapping each string to a multi-byte sort key,
and then doing simple lexicographic sorting with these keys. These rules
can be overlaid by additional domain- or language-specific rules. Again,
this SRFI does not address these issues. SRFI 13 string ordering is strictly
based upon a character-by-character comparison of the values used for
representing the string.

** Naming conventions
=====================

This library contains a large number of procedures, but they follow
a consistent naming scheme, and are consistent with the conventions
developed in SRFI 1. The names are composed of smaller lexemes
in a regular way that exposes the structure and relationships between the
procedures. This should help the programmer to recall or reconstitute the name
of the particular procedure that he needs when writing his own code. In
particular

    - Procedures whose names end in "-ci" are case-insensitive variants.

    - Procedures whose names end in "!" are side-effecting variants.
      What values these procedures return is usually not specified.

    - The order of common parameters is consistent across the
      different procedures.

    - Left/right/both directionality
      Procedures that have left/right directional variants
      use the following convention:
      
	  Direction	    Naming convention
	  -------------	    -----------------
	  left-to-right	    no suffix
	  right-to-left	    -RIGHT suffix
	  both		    -BOTH suffix

      This is a general convention that was established in SRFI 1.
      The value of a convention is proportional to the extent of its 
      use.
      
** Shared storage
=================
Some Scheme implementations, e.g. guile and T, provide ways to construct
substrings that share storage with other strings. This facility is called
"shared-text substrings." Shared-text substrings can be used to eliminate the
allocation and copying time and space required to produce substrings, which
can be a tremendous savings for some applications, reducing a linear-time
operation to constant time.  Additionally, some algorithms rely on the sharing
property of these substrings -- the application assumes that if the underlying
storage is mutated, then all strings sharing that storage will show the
change.  However, shared-text substrings are not a common feature; most Scheme
implementations do not provide them.

SRFI 13 takes a middle ground with respect to shared-text substrings.  In
particular, a Scheme implementation does not need to have shared-text
substrings in order to implement this SRFI.

There is an additional form of storage sharing enabled by some SRFI 13
procedures, even without the benefit of shared-text substrings. In 
some cases, some SRFI 13 routines are allowed to return as a result one
of the strings that was passed in as a parameter. For example, when
constructing a substring with the SUBSTRING/SHARED procedure, if the
requested substring is the entire string, the procedure is permitted
simply to return the original value. That is,

   (eq? s (substring/shared s 0 (string-length s))) => true or false

whereas the R5RS SUBSTRING function is required to allocate a fresh copy

   (eq? s (substring s 0 (string-length s))) => false.

In keeping with SRFI 13's general approach to sharing, compliant
implementations are allowed, but not required, to provide this kind of
sharing. Hence, procedures may not *rely* upon sharing in these cases.

Most procedures that permit results to share storage with inputs have
equivalent procedures that require allocating fresh storage for results. 
If an application wishes to be sure a new, fresh string is allocated, then 
these "pure" procedures should be used.
	Fresh copy guaranteed		Sharing permitted
	---------------------		-----------------
	string-copy			substring/shared
	string-copy			string-take string-take-right
	string-copy			string-drop string-drop-right
	string-concatenate		string-concatenate/shared
	string-append			string-append/shared
	string-concatenate-reverse	string-concatenate-reverse/shared
					string-pad string-pad-right
					string-trim string-trim-right string-trim-both
					string-filter string-delete

On the other hand, the functionality is present to allow one to write
efficient code *without* shared-text substrings. You can write efficient code
that works by passing around start/end ranges indexing into a string instead
of simply building a shared-text substring. The API would be much simpler
without this consideration -- if we had cheap shared-text substrings, all the
start/end index parameters would vanish. However, since SRFI 13 does not
require implementations to provide shared-text substrings, the extended
API is provided.

** R4RS/R5RS procedures
=======================
The R4RS and R5RS reports define 22 string procedures. The string-lib
package includes 8 of these exactly as defined, 3 in an extended,
backwards-compatible way, and drops the remaining 11 (whose functionality
is available via other bindings).

The 8 procedures provided exactly as documented in the reports are
    string?
    make-string
    string
    string-length
    string-ref
    string-set!
    string-append
    list->string

The eleven functions not included are
    string=?  string-ci=?
    string<?  string-ci<?
    string>?  string-ci>?
    string<=? string-ci<=?
    string>=? string-ci>=?
    substring
The string-lib package provides alternate bindings and extended functionality.

Additionally, the three extended procedures are
    string-fill! s char [start end] -> unspecified
    string->list s [start end] -> char-list
    string-copy  s [start end] -> string

They are uniformly extended to take optional start/end parameters specifying
substring ranges.

** Extra-SRFI recommendations
=============================
This SRFI recommends the following

- A SRFI be defined for shared-text substrings, allowing programs to
  be written that actually rely on the shared-storage properties of these data
  structures.

- A SRFI be defined for manipulating Unicode text -- various normalisation
  operations, collation, searching, etc. Collation operations might be
  parameterised by a "collation" structure representing collation rules
  for a particular locale or language. Alternatively, a data structure
  specifying collation rules could be activated with dynamic scope by
  special procedures, possibly overridden by allowing collation rules
  to be optional arguments to procedures that need to order strings, e.g.

      (with-locale* denmark-locale
        (lambda () 
          (f x)
          (g 42)))

      (with-locale taiwan-locale
        (f x)
	(h denmark-locale)
        (g 42))

      (set-locale! denmark-locale)

- A SRFI be defined for manipulating characters that is portable across
  at least ASCII, Latin-1 and Unicode. 

  - For backwards-compatibility, CHAR-UPCASE and CHAR-DOWNCASE should 
    be defined to use the 1-1 locale- and context-insensitive case
    mappings given by Unicode's UnicodeData.txt table.
  
  - numeric codes for standard functions that map between characters and
    integers should be required to use the Unicode/Latin-1/ASCII mapping. This
    allows programmers to write portable code.

  - CHAR-TITLECASE be added to CHAR-UPCASE and CHAR-DOWNCASE

  - CHAR-TITLECASE? be added to CHAR-UPCASE? and CHAR-DOWNCASE?

  - Title/up/down-case functions be added to the character-processing suite
    which allow 1->n case maps by returning immutable,
    possibly-multi-character strings instead of single characters. These case
    mappings need not be locale- or context-sensitive.

  These recommendations are not a part of the SRFI 13 spec. Note also that
  requiring a Unicode/Latin-1/ASCII interface to integer/char mapping
  functions does not imply anything about the actual underlying encodings of
  characters.


-------------------------------------------------------------------------------
* Procedure Specification
-------------------------

In the following procedure specifications:
    - An S parameter is a string.

    - A CHAR parameter is a character.

    - START and END parameters are half-open string indices specifying 
      a substring within a string parameter; when optional, they default
      to 0 and the length of the string, respectively. When specified, it
      must be the case that 0 <= START <= END <= (string-length S), for
      the corresponding parameter S. They typically restrict a procedure's
      action to the indicated substring.

    - A PRED parameter is a unary character predicate procedure, returning 
      a true/false value when applied to a character.

    - A CHAR/CHAR-SET/PRED parameter is a value used to select/search
      for a character in a string. If it is a character, it is used in
      an equality test; if it is a character set, it is used as a
      membership test; if it is a procedure, it is applied to the 
      characters as a test predicate.

    - An I parameter is an exact non-negative integer specifying an index
      into a string.
    
    - LEN and NCHARS parameters are exact non-negative integers specifying a
      length of a string or some number of characters.

    - An OBJ parameter may be any value at all.

Passing values to procedures with these parameters that do not satisfy these
types is an error.

If a procedure is said to return "unspecified," this means that nothing at all
is said about what the procedure returns, not even the number of return
values. Such a procedure is not even required to be consistent from call to
call in the nature or number of its return values. It is specifically permitted
for such a procedure to return zero values, e.g., by calling (VALUES).

Parameters given in square brackets are optional. Unless otherwise noted in
the text describing the procedure, any prefix of these optional parameters may
be supplied, from zero arguments to the full list. When a procedure returns
multiple values, this is shown by listing the return values in square
brackets, as well. So, for example, the procedure with signature

    halts? f [x init-store] -> [boolean integer]

would take one (F), two (F, X) or three (F, X, INPUT-STORE) input parameters,
and return two values, a boolean and an integer.

A parameter followed by "..." means zero-or-more elements. So the procedure
with the signature
    sum-squares x ... -> number
takes zero or more arguments (X ...), while the procedure with signature
    spell-check doc dict1 dict2 ... -> string-list
takes two required parameters (DOC and DICT1) and zero or more
optional parameters (DICT2 ...).

If a procedure is said to return "unspecified," this means that nothing at all
is said about what the procedure returns. Such a procedure is not even
required to be consistent from call to call. It is simply required to return a
value (or values) that may be passed to a command continuation, e.g. as the
value of an expression appearing as a non-terminal subform of a BEGIN
expression. Note that in R5RS, this restricts such a procedure to returning a
single value; non-R5RS systems may not even provide this restriction.

** Main procedures
==================

In a Scheme system that has a module or package system, these procedures
should be contained in a module named "string-lib".

*** Predicates
==============

string? obj -> boolean						    	R5RS
  Returns #t if OBJ is a string, otherwise returns #f.

string-null? s -> boolean
    Is S the empty string?

string-every char/char-set/pred s [start end] -> value
string-any   char/char-set/pred s [start end] -> value
    Checks to see if the given criteria is true of every / any character in S,
    proceeding from left (index START) to right (index END).

    If CHAR/CHAR-SET/PRED is a character, it is tested for equality with
    the elements of S.

    If CHAR/CHAR-SET/PRED is a character set, the elements of S are tested
    for membership in the set.

    If CHAR/CHAR-SET/PRED is a predicate procedure, it is applied to the 
    elements of S. The predicate is "witness-generating:"
    
      - If STRING-ANY returns true, the returned true value is the one produced
        by the application of the predicate.
    
      - If STRING-EVERY returns true, the returned true value is the one
        produced by the final application of the predicate to S[END]. 
        If STRING-EVERY is applied to an empty sequence of characters, 
        it simply returns #T.

      If STRING-EVERY or STRING-ANY apply the predicate to the final element
      of the selected sequence (i.e., S[END-1]), that final application is a
      tail call.

    The names of these procedures do not end with a question mark -- this is to
    indicate that, in the predicate case, they do not return a simple boolean
    (#T or #F), but a general value.


*** Constructors
================

make-string len [char] -> string					R5RS
  MAKE-STRING returns a newly allocated string of length LEN.  If
  CHAR is given, then all elements of the string are initialized
  to CHAR, otherwise the contents of the string are unspecified.

string char1 ... -> string						R5RS
  Returns a newly allocated string composed of the argument characters.
    
string-tabulate proc len -> string
    PROC is an integer->char procedure. Construct a string of size LEN
    by applying PROC to each index to produce the corresponding string
    element. The order in which PROC is applied to the indices is not
    specified.


*** List & string conversion
============================

string->list s [start end] -> char-list					R5RS+
list->string char-list -> string					R5RS
    STRING->LIST returns a newly allocated list of the characters
    that make up the given string.  LIST->STRING returns a newly
    allocated string formed from the characters in the list CHAR-LIST,
    which must be a list of characters. STRING->LIST and LIST->STRING 
    are inverses so far as EQUAL? is concerned.

    STRING->LIST is extended from the R5RS definition to take optional
    START/END arguments.

reverse-list->string char-list -> string
    An efficient implementation of (compose list->string reverse):
	(reverse-list->string '(#\a #\B #\c)) -> "cBa"
    This is a common idiom in the epilog of string-processing loops
    that accumulate an answer in a reverse-order list. (See also
    STRING-CONCATENATE-REVERSE for the "chunked" variant.)

string-join string-list [delimiter grammar] -> string
    This procedure is a simple unparser --- it pastes strings together using
    the delimiter string. 

    The GRAMMAR argument is a symbol that determines how the delimiter is
    used, and defaults to 'infix.
    
      - 'infix means an infix or separator grammar: insert the delimiter
        between list elements.  An empty list will produce an empty string --
	note, however, that parsing an empty string with an infix or separator
	grammar is ambiguous. Is it an empty list, or a list of one element,
        the empty string?
    
      - 'strict-infix means the same as 'infix, but will raise an error
        if given an empty list.
    
      - 'suffix means a suffix or terminator grammar: insert the delimiter
        after every list element. This grammar has no ambiguities.

      - 'prefix means a prefix grammar: insert the delimiter
        before every list element. This grammar has no ambiguities.

    The delimiter is the string used to delimit elements; it defaults to
    a single space " ".

        (string-join '("foo" "bar" "baz") ":")         => "foo:bar:baz"
        (string-join '("foo" "bar" "baz") ":" 'suffix) => "foo:bar:baz:"

	;; Infix grammar is ambiguous wrt empty list vs. empty string,
	(string-join '()   ":") => ""
	(string-join '("") ":") => ""

	;; but suffix & prefix grammars are not.
	(string-join '()   ":" 'suffix) => ""
	(string-join '("") ":" 'suffix) => ":"


*** Selection
=============

string-length s -> integer						R5RS
  Returns the number of characters in the string S.

string-ref s i -> char							R5RS
  Return character S[I] using zero-origin indexing.
  I must be a valid index of S.

string-copy      s [start end] -> string				R5RS+
substring/shared s start [end] -> string
    SUBSTRING/SHARED returns a string whose contents are the characters of S
    beginning with index START (inclusive) and ending with index END
    (exclusive). It differs from the R5RS SUBSTRING in two ways:
      - The END parameter is optional, not required.
      - SUBSTRING/SHARED may return a value that shares memory with S or
        is EQ? to S.
    
    STRING-COPY is extended from its R5RS definition by the addition of
    its optional START/END parameters. In contrast to SUBSTRING/SHARED,
    it is guaranteed to produce a freshly-allocated string.

    Use STRING-COPY when you want to indicate explicitly in your code that you
    wish to allocate new storage; use SUBSTRING/SHARED when you don't care if 
    you get a fresh copy or share storage with the original string.

	(string-copy "Beta substitution") => "Beta substitution"
	(string-copy "Beta substitution" 1 10) 
	    => "eta subst"
	(string-copy "Beta substitution" 5) => "substitution"

string-copy! target tstart s [start end] -> unspecified
    Copy the sequence of characters from index range [START,END) in
    string S to string TARGET, beginning at index TSTART. The characters 
    are copied left-to-right or right-to-left as needed -- the copy is
    guaranteed to work, even if TARGET and S are the same string.

    It is an error if the copy operation runs off the end of the target
    string, e.g.
	(string-copy! (string-copy "Microsoft") 0
                      "Regional Microsoft Operating Companies") => error


string-take s nchars -> string
string-drop s nchars -> string
string-take-right s nchars -> string
string-drop-right s nchars -> string
    STRING-TAKE returns the first NCHARS of STRING; 
    STRING-DROP returns all but the first NCHARS of STRING.
    STRING-TAKE-RIGHT returns the last NCHARS of STRING;
    STRING-DROP-RIGHT returns all but the last NCHARS of STRING.
    If these procedures produce the entire string, they may return either
    S or a copy of S; in some implementations, proper substrings may share
    memory with S.

	(string-take "Pete Szilagyi" 6) => "Pete S"
	(string-drop "Pete Szilagyi" 6) => "zilagyi"

	(string-take-right "Beta rules" 5) => "rules"
	(string-drop-right "Beta rules" 5) => "Beta "

    It is an error to take or drop more characters than are in the string:
        (string-take "foo" 37) => *error*

string-pad       s len [char start end] -> string
string-pad-right s len [char start end] -> string
    Build a string of length LEN comprised of S padded on the left (right)
    by as many occurrences of the character CHAR as needed. If S has more
    than LEN chars, it is truncated on the left (right) to length LEN. CHAR
    defaults to #\space.

    If LEN <= END-START, the returned value is allowed to share storage
    with S, or be exactly S (if LEN = END-START).

	(string-pad     "325" 5) => "  325"
	(string-pad   "71325" 5) => "71325"
	(string-pad "8871325" 5) => "71325"


string-trim       s [char/char-set/pred start end] -> string
string-trim-right s [char/char-set/pred start end] -> string
string-trim-both  s [char/char-set/pred start end] -> string
    Trim S by skipping over all characters on the left / on the right /
    on both sides that satisfy the second parameter CHAR/CHAR-SET/PRED:
	- if it is a character CHAR, characters equal to CHAR are trimmed;
        - if it is a char set CS, characters contained in CS are trimmed;
	- if it is a predicate PRED, it is a test predicate that is applied
	  to the characters in S; a character causing it to return true
	  is skipped.
    CHAR/CHAR/SET-PRED defaults to the character set CHAR-SET:WHITESPACE
    defined in SRFI 14.

    If no trimming occurs, these functions may return either S or a copy of S;
    in some implementations, proper substrings may share memory with S.

    (string-trim-both "  The outlook wasn't brilliant,  \n\r")
	=> "The outlook wasn't brilliant,"


*** Modification
================

string-set! s i char -> unspecified					R5RS
  I must be a valid index of S.  STRING-SET! stores CHAR in
  element I of S. Constant string literals appearing in code are 
  immutable; it is an error to use them in a STRING-SET!.

  (define (f) (make-string 3 #\*))
  (define (g) "***")
  (string-set! (f) 0 #\?)                ==>  *unspecified*
  (string-set! (g) 0 #\?)                ==>  *error*
  (string-set! (symbol->string 'immutable)
               3
               #\?)                      ==>  *error*

string-fill! s char [start end] -> unspecified				R5RS+
    Stores CHAR in every element of S.

    STRING-FILL is extended from the R5RS definition to take optional
    START/END arguments.


*** Comparison
==============

string-compare    s1 s2 proc< proc= proc> [start1 end1 start2 end2] -> values
string-compare-ci s1 s2 proc< proc= proc> [start1 end1 start2 end2] -> values
    Apply PROC<, PROC=, or PROC> to the mismatch index, depending
    upon whether S1 is less than, equal to, or greater than S2.
    The "mismatch index" is the largest index i such that for
    every 0 <= j < i, s1[j] = s2[j] -- that is, i is the first 
    position that doesn't match.

    STRING-COMPARE-CI is the case-insensitive variant. Case-insensitive
    comparison is done by case-folding characters with the operation
	(char-downcase (char-upcase c))
    where the two case-mapping operations are assumed to be 1-1, locale- and
    context-insensitive, and compatible with the 1-1 case mappings specified
    by Unicode's UnicodeData.txt table:
        ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt

    The optional start/end indices restrict the comparison to the indicated
    substrings of S1 and S2. The mismatch index is always an index into S1;
    in the case of PROC=, it is always END1; we observe the protocol
    in this redundant case for uniformity.

    (string-compare "The cat in the hat" "abcdefgh" 
		    values values values
		    4 6		; Select "ca" 
		    2 4)	; & "cd"
	=> 5	; Index of S1's "a"

    Comparison is simply done on individual code-points of the string. 
    True text collation is not handled by this SRFI.

string=  s1 s2 [start1 end1 start2 end2] -> boolean
string<> s1 s2 [start1 end1 start2 end2] -> boolean
string<  s1 s2 [start1 end1 start2 end2] -> boolean
string>  s1 s2 [start1 end1 start2 end2] -> boolean
string<= s1 s2 [start1 end1 start2 end2] -> boolean
string>= s1 s2 [start1 end1 start2 end2] -> boolean
    These procedures are the lexicographic extensions to strings of the
    corresponding orderings on characters.  For example, STRING< is the
    lexicographic ordering on strings induced by the ordering CHAR<? on
    characters.  If two strings differ in length but are the same up to 
    the length of the shorter string, the shorter string is considered to 
    be lexicographically less than the longer string.

    The optional start/end indices restrict the comparison to the indicated
    substrings of S1 and S2. 

    Comparison is simply done on individual code-points of the string. 
    True text collation is not handled by this SRFI.

string-ci=  s1 s2 [start1 end1 start2 end2] -> boolean
string-ci<> s1 s2 [start1 end1 start2 end2] -> boolean
string-ci<  s1 s2 [start1 end1 start2 end2] -> boolean
string-ci>  s1 s2 [start1 end1 start2 end2] -> boolean
string-ci<= s1 s2 [start1 end1 start2 end2] -> boolean
string-ci>= s1 s2 [start1 end1 start2 end2] -> boolean
    Case-insensitive variants.

    Case-insensitive comparison is done by case-folding characters with 
    the operation
       (char-downcase (char-upcase c))
    where the two case-mapping operations are assumed to be 1-1, locale- and
    context-insensitive, and compatible with the 1-1 case mappings specified
    by Unicode's UnicodeData.txt table:
        ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt

string-hash    s [bound start end] -> integer
string-hash-ci s [bound start end] -> integer
    Compute a hash value for the string S. BOUND is a non-negative
    exact integer specifying the range of the hash function. A positive
    value restricts the return value to the range [0,BOUND).

    If BOUND is either zero or not given, the implementation may use an
    implementation-specific default value, chosen to be as large as
    is efficiently practical. For instance, the default range might be chosen
    for a given implementation to map all strings into the range of
    integers that can be represented with a single machine word.

    The optional start/end indices restrict the hash operation to the 
    indicated substring of S.

    STRING-HASH-CI is the case-insensitive variant. Case-insensitive
    comparison is done by case-folding characters with the operation
	(char-downcase (char-upcase c))
    where the two case-mapping operations are assumed to be 1-1, locale- and
    context-insensitive, and compatible with the 1-1 case mappings specified
    by Unicode's UnicodeData.txt table:
         ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt

    Invariants:
       (<= 0 (string-hash s b) (- b 1))	; When B > 0.
       (string=    s1 s2)  =>  (= (string-hash s1 b)    (string-hash s2 b))
       (string-ci= s1 s2)  =>  (= (string-hash-ci s1 b) (string-hash-ci s2 b))

    A legal but nonetheless discouraged implementation:
        (define (string-hash    s . other-args) 1)
        (define (string-hash-ci s . other-args) 1)

    Rationale: allowing the user to specify an explicit bound simplifies user
    code by removing the mod operation that typically accompanies every hash
    computation, and also may allow the implementation of the hash function to
    exploit a reduced range to efficiently compute the hash value. E.g., for
    small bounds, the hash function may be computed in a fashion such that
    intermediate values never overflow into bignum integers, allowing the
    implementor to provide a fixnum-specific "fast path" for computing the
    common cases very rapidly.

*** Prefixes & suffixes
=======================

string-prefix-length    s1 s2 [start1 end1 start2 end2] -> integer
string-suffix-length    s1 s2 [start1 end1 start2 end2] -> integer
string-prefix-length-ci s1 s2 [start1 end1 start2 end2] -> integer
string-suffix-length-ci s1 s2 [start1 end1 start2 end2] -> integer
    Return the length of the longest common prefix/suffix of the two strings.
    For prefixes, this is equivalent to the "mismatch index" for the strings
    (modulo the STARTi index offsets).

    The optional start/end indices restrict the comparison to the indicated
    substrings of S1 and S2.

    STRING-PREFIX-LENGTH-CI and STRING-SUFFIX-LENGTH-CI are the
    case-insensitive variants. Case-insensitive comparison is done by
    case-folding characters with the operation
       (char-downcase (char-upcase c))
    where the two case-mapping operations are assumed to be 1-1, locale- and
    context-insensitive, and compatible with the 1-1 case mappings specified
    by Unicode's UnicodeData.txt table:
        ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt

    Comparison is simply done on individual code-points of the string. 

string-prefix?    s1 s2 [start1 end1 start2 end2] -> boolean
string-suffix?    s1 s2 [start1 end1 start2 end2] -> boolean
string-prefix-ci? s1 s2 [start1 end1 start2 end2] -> boolean
string-suffix-ci? s1 s2 [start1 end1 start2 end2] -> boolean
    Is S1 a prefix/suffix of S2?

    The optional start/end indices restrict the comparison to the indicated
    substrings of S1 and S2.

    STRING-PREFIX-CI? and STRING-SUFFIX-CI? are the case-insensitive variants.
    Case-insensitive comparison is done by case-folding characters with the
    operation
       (char-downcase (char-upcase c))
    where the two case-mapping operations are assumed to be 1-1, locale- and
    context-insensitive, and compatible with the 1-1 case mappings specified
    by Unicode's UnicodeData.txt table:
        ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt

    Comparison is simply done on individual code-points of the string. 


*** Searching
=============

string-index       s char/char-set/pred [start end] -> integer or #f
string-index-right s char/char-set/pred [start end] -> integer or #f
string-skip        s char/char-set/pred [start end] -> integer or #f
string-skip-right  s char/char-set/pred [start end] -> integer or #f
    STRING-INDEX (STRING-INDEX-RIGHT) searches through the string from the 
    left (right), returning the index of the first occurrence of a character 
    which
	- equals CHAR/CHAR-SET/PRED (if it is a character);
	- is in CHAR/CHAR-SET/PRED (if it is a character set);
	- satisfies the predicate CHAR/CHAR-SET/PRED (if it is a procedure).
    If no match is found, the functions return false.

    The START and END parameters specify the beginning and end indices of
    the search; the search includes the start index, but not the end index.
    Be careful of "fencepost" considerations: when searching right-to-left, 
    the first index considered is
	  END-1
    whereas when searching left-to-right, the first index considered is
	  START
    That is, the start/end indices describe a same half-open interval
    [start,end) in these procedures that they do in all the other SRFI 13
    procedures.

    The skip functions are similar, but use the complement of the criteria:
    they search for the first char that *doesn't* satisfy the test. E.g., 
    to skip over initial whitespace, say
        (cond ((string-skip s char-set:whitespace) =>
               (lambda (i) ...)) ; s[i] is not whitespace.
              ...)

string-count s char/char-set/pred [start end] -> integer
    Return a count of the number of characters in S that satisfy the
    CHAR/CHAR-SET/PRED argument. If this argument is a procedure, 
    it is applied to the character as a predicate; if it is a character set, 
    the character is tested for membership; if it is a character, it is 
    used in an equality test.

string-contains    s1 s2 [start1 end1 start2 end2] -> integer or false
string-contains-ci s1 s2 [start1 end1 start2 end2] -> integer or false
    Does string S1 contain string S2?

    Return the index in S1 where S2 occurs as a substring, or false.
    The optional start/end indices restrict the operation to the
    indicated substrings.

    The returned index is in the range [start1,end1). A successful match
    must lie entirely in the [start1,end1) range of S1.

    (string-contains "eek -- what a geek." "ee"
                     12 18) ; Searches "a geek"
        => 15

    STRING-CONTAINS-CI is the case-insensitive variant. Case-insensitive
    comparison is done by case-folding characters with the operation
       (char-downcase (char-upcase c))
    where the two case-mapping operations are assumed to be 1-1, locale- and
    context-insensitive, and compatible with the 1-1 case mappings specified
    by Unicode's UnicodeData.txt table:
        ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt

    Comparison is simply done on individual code-points of the string. 

    The names of these procedures do not end with a question mark -- this is to
    indicate that they do not return a simple boolean (#T or #F). Rather,
    they return either false (#F) or an exact non-negative integer.

*** Alphabetic case mapping
===========================

string-titlecase  s [start end] -> string
string-titlecase! s [start end] -> unspecified
    For every character C in the selected range of S, if C is preceded
    by a cased character, it is downcased; otherwise it is titlecased.
    
    STRING-TITLECASE returns the result string and does not alter its S
    parameter. STRING-TITLECASE! is the in-place side-effecting variant.

    (string-titlecase "--capitalize tHIS sentence.") =>
      "--Capitalize This Sentence."
    
    (string-titlecase "see Spot run. see Nix run.") =>
      "See Spot Run. See Nix Run."
    
    (string-titlecase "3com makes routers.") =>
      "3Com Makes Routers."

    Note that if a START index is specified, then the character
    preceding S[START] has no effect on the titlecase decision for
    character S[START]:

    (string-titlecase "greasy fried chicken" 2) => "Easy Fried Chicken"

    Titlecase and cased information must be compatible with the Unicode
    specification.

string-upcase    s [start end] -> string
string-upcase!   s [start end] -> unspecified
string-downcase  s [start end] -> string
string-downcase! s [start end] -> unspecified
    Raise or lower the case of the alphabetic characters in the string.

    STRING-UPCASE and STRING-DOWNCASE return the result string and do not
    alter their S parameter. STRING-UPCASE! and STRING-DOWNCASE! are the
    in-place side-effecting variants.    

    These procedures use the locale- and context-insensitive 1-1 case mappings
    defined by Unicode's UnicodeData.txt table:
         ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt


*** Reverse & append
====================

string-reverse  s [start end] -> string
string-reverse! s [start end] -> unspecified
    Reverse the string.

    STRING-REVERSE returns the result string and does not alter its S
    parameter. STRING-REVERSE! is the in-place side-effecting variant.

    (string-reverse "Able was I ere I saw elba.") 
        => ".able was I ere I saw elbA"

    ;;; In-place rotate-left, the Bell Labs way:
    (lambda (s i)
      (let ((i (modulo i (string-length s))))
        (string-reverse! s 0 i)
        (string-reverse! s i)
        (string-reverse! s)))

    Unicode note: Reversing a string simply reverses the sequence of
    code-points it contains. So a zero-width accent character A coming *after*
    a base character B in string S would come out *before* B in the reversed
    result.

string-append s1 ... -> string						R5RS
  Returns a newly allocated string whose characters form the
  concatenation of the given strings.

string-concatenate string-list -> string
    Append the elements of STRING-LIST together into a single string.
    Guaranteed to return a freshly allocated string.

    Note that the (APPLY STRING-APPEND STRING-LIST) idiom is
    not robust for long lists of strings, as some Scheme implementations
    limit the number of arguments that may be passed to an n-ary procedure.

string-concatenate/shared string-list -> string
string-append/shared s1 ... -> string
    These two procedures are variants of STRING-CONCATENATE and STRING-APPEND
    that are permitted to return results that share storage with their
    parameters. In particular, if STRING-APPEND/SHARED is applied to just 
    one argument, it may return exactly that argument, whereas STRING-APPEND
    is required to allocate a fresh string.

string-concatenate-reverse        string-list [final-string end] -> string
string-concatenate-reverse/shared string-list [final-string end] -> string
    With no optional arguments, these functions are equivalent to
        (string-concatenate (reverse string-list))
    and
        (string-concatenate/shared (reverse string-list))
    respectively.

    If the optional argument FINAL-STRING is specified, it is consed
    onto the beginning of STRING-LIST before performing the list-reverse 
    and string-concatenate operations.

    If the optional argument END is given, only the first END characters
    of FINAL-STRING are added to the string list, thus producing
        (string-concatenate (reverse (cons (substring/shared final-string 0 end)
                                           string-list)))
    E.g.
	(string-concatenate-reverse '(" must be" "Hello, I") " going.XXXX" 7)
          => "Hello, I must be going."

    This procedure is useful in the construction of procedures that 
    accumulate character data into lists of string buffers, and wish to
    convert the accumulated data into a single string when done.

    Unicode note: Reversing a string simply reverses the sequence of
    code-points it contains. So a zero-width accent character AC coming *after*
    a base character BC in string S would come out *before* BC in the reversed
    result.


*** Fold, unfold & map
======================

string-map  proc s [start end] -> string
string-map! proc s [start end] -> unspecified
    PROC is a char->char procedure; it is mapped over S.
    
    STRING-MAP returns the result string and does not alter its S parameter.
    STRING-MAP! is the in-place side-effecting variant.

    Note: The order in which PROC is applied to the elements of S
    is not specified.

string-fold       kons knil s [start end] -> value
string-fold-right kons knil s [start end] -> value
    These are the fundamental iterators for strings.

    The left-fold operator maps the KONS procedure across the
    string from left to right
	(... (kons s[2] (kons s[1] (kons s[0] knil))))
    In other words, STRING-FOLD obeys the (tail) recursion
	(string-fold kons knil s start end) =
	    (string-fold kons (kons s[start] knil) start+1 end)

    The right-fold operator maps the KONS procedure across the
    string from right to left
	(kons s[0] (... (kons s[end-3] (kons s[end-2] (kons s[end-1] knil)))))
    obeying the (tail) recursion
	(string-fold-right kons knil s start end) =
	    (string-fold-right kons (kons s[end-1] knil) start end-1)
	
    Examples: 
	;;; Convert a string to a list of chars.
	(string-fold-right cons '() s)

	;;; Count the number of lower-case characters in a string.
	(string-fold (lambda (c count)
                       (if (char-lower-case? c)
                           (+ count 1)
                           count))
                     0
                     s)

	;;; Double every backslash character in S.
	(let* ((ans-len (string-fold (lambda (c sum)
                                       (+ sum (if (char=? c #\\) 2 1)))
				     0 s))
	       (ans (make-string ans-len)))
	  (string-fold (lambda (c i)
		         (let ((i (if (char=? c #\\)
                                      (begin (string-set! ans i #\\) (+ i 1))
				      i)))
			   (string-set! ans i c)
			   (+ i 1)))
		       0 s)
	  ans)

    The right-fold combinator is sometimes called a "catamorphism."

string-unfold p f g seed [base make-final] -> string
    This is a fundamental constructor for strings. 
    - G is used to generate a series of "seed" values from the initial seed:
	SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...
    - P tells us when to stop -- when it returns true when applied to one 
      of these seed values.
    - F maps each seed value to the corresponding character 
      in the result string. These chars are assembled into the
      string in a left-to-right order.
    - BASE is the optional initial/leftmost portion of the constructed string;
      it defaults to the empty string "".
    - MAKE-FINAL is applied to the terminal seed value (on which P returns
      true) to produce the final/rightmost portion of the constructed string.
      It defaults to (LAMBDA (X) "").

    More precisely, the following (simple, inefficient) definitions hold:

    ;;; Iterative
    (define (string-unfold p f g seed base make-final)
      (let lp ((seed seed) (ans base))
        (if (p seed) 
            (string-append ans (make-final seed))
	    (lp (g seed) (string-append ans (string (f seed)))))))
			                
    ;;; Recursive
    (define (string-unfold p f g seed base make-final)
      (string-append base
                     (let recur ((seed seed))
                       (if (p seed) (make-final seed)
		           (string-append (string (f seed))
                                          (recur (g seed)))))))

    STRING-UNFOLD is a fairly powerful string constructor -- you can use it to
    convert a list to a string, read a port into a string, reverse a string,
    copy a string, and so forth. Examples:

    (port->string p) = (string-unfold eof-object? values
                                      (lambda (x) (read-char p))
                                      (read-char p))

    (list->string lis) = (string-unfold null? car cdr lis)

    (tabulate-string f size) = (string-unfold (lambda (i) (= i size)) f add1 0)

    To map F over a list LIS, producing a string:
	(string-unfold null? (compose f car) cdr lis)

    Interested functional programmers may enjoy noting that STRING-FOLD-RIGHT 
    and STRING-UNFOLD are in some sense inverses. That is, given operations 
    KNULL?, KAR, KDR, KONS, and KNIL satisfying
	(kons (kar x) (kdr x)) = x  and (knull? knil) = #t
    then
	(STRING-FOLD-RIGHT kons knil (STRING-UNFOLD knull? kar kdr x)) = x
    and
	(STRING-UNFOLD knull? kar kdr (STRING-FOLD-RIGHT kons knil s)) = s.

    The final string constructed does not share storage with either BASE
    or the value produced by MAKE-FINAL.

    This combinator sometimes is called an "anamorphism."

    Note: implementations should take care that runtime stack limits do not
    cause overflow when constructing large (e.g., megabyte) strings with
    STRING-UNFOLD.


string-unfold-right p f g seed [base make-final] -> string
    This is a fundamental constructor for strings. 
    - G is used to generate a series of "seed" values from the initial seed:
	SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...
    - P tells us when to stop -- when it returns true when applied to one 
      of these seed values.
    - F maps each seed value to the corresponding character 
      in the result string. These chars are assembled into the
      string in a right-to-left order.
    - BASE is the optional initial/rightmost portion of the constructed string;
      it defaults to the empty string "".
    - MAKE-FINAL is applied to the terminal seed value (on which P returns
      true) to produce the final/leftmost portion of the constructed string.
      It defaults to (LAMBDA (X) "").

    More precisely, the following (simple, inefficient) definitions hold:

    ;;; Iterative
    (define (string-unfold-right p f g seed base make-final)
      (let lp ((seed seed) (ans base))
        (if (p seed) 
            (string-append (make-final seed) ans)
	    (lp (g seed) (string-append (string (f seed)) ans)))))

    ;;; Recursive
    (define (string-unfold-right p f g seed base make-final)
      (string-append (let recur ((seed seed))
                       (if (p seed) (make-final seed)
                           (string-append (recur (g seed))
                                          (string (f seed)))))
                     base))

    Interested functional programmers may enjoy noting that STRING-FOLD
    and STRING-UNFOLD-RIGHT are in some sense inverses. That is, given 
    operations KNULL?, KAR, KDR, KONS, and KNIL satisfying
	(kons (kar x) (kdr x)) = x  and (knull? knil) = #t
    then
	(STRING-FOLD kons knil (STRING-UNFOLD-RIGHT knull? kar kdr x)) = x
    and
	(STRING-UNFOLD-RIGHT knull? kar kdr (STRING-FOLD kons knil s)) = s.

    The final string constructed does not share storage with either BASE
    or the value produced by MAKE-FINAL.

    Note: implementations should take care that runtime stack limits do not
    cause overflow when constructing large (e.g., megabyte) strings with
    STRING-UNFOLD-RIGHT.


string-for-each  proc s [start end] -> unspecified
    Apply PROC to each character in S.
    STRING-FOR-EACH is required to iterate from START to END
    in increasing order.

string-for-each-index proc s [start end] -> unspecified
    Apply PROC to each index of S, in order. The optional START/END
    pairs restrict the endpoints of the loop. This is simply a
    method of looping over a string that is guaranteed to be safe
    and correct.

    Example:
        (let* ((len (string-length s))
               (ans (make-string len)))
          (string-for-each-index
              (lambda (i) (string-set! ans (- len i) (string-ref s i)))
              s)
          ans)


*** Replicate & rotate
======================

xsubstring s from [to start end] -> string
    This is the "extended substring" procedure that implements replicated
    copying of a substring of some string.

    S is a string; START and END are optional arguments that demarcate
    a substring of S, defaulting to 0 and the length of S (i.e., the whole
    string). Replicate this substring up and down index space, in both the
    positive and negative directions. For example, if S = "abcdefg", START=3, 
    and END=6, then we have the conceptual bidirectionally-infinite string
	...  d  e  f  d  e  f  d  e  f  d  e  f  d  e  f  d  e  f  d ...
	... -9 -8 -7 -6 -5 -4 -3 -2 -1  0  1  2  3  4  5  6  7  8  9 ...
    XSUBSTRING returns the substring of this string beginning at index FROM,
    and ending at TO (which defaults to FROM+(END-START)).

    You can use XSUBSTRING to perform a variety of tasks:
    - To rotate a string left:  (xsubstring "abcdef" 2)  => "cdefab"
    - To rotate a string right: (xsubstring "abcdef" -2) => "efabcd"
    - To replicate a string:    (xsubstring "abc" 0 7) => "abcabca"

    Note that 
      - The FROM/TO indices give a half-open range -- the characters from
	index FROM up to, but not including, index TO.
      - The FROM/TO indices are not in terms of the index space for string S.
	They are in terms of the replicated index space of the substring
	defined by S, START, and END.

    It is an error if START=END -- although this is allowed by special
    dispensation when FROM=TO.

string-xcopy! target tstart s sfrom [sto start end] -> unspecified
    Exactly the same as XSUBSTRING, but the extracted text is written
    into the string TARGET starting at index TSTART.
    This operation is not defined if (EQ? TARGET S) or these two arguments
    share storage -- you cannot copy a string on top of itself.



*** Miscellaneous: insertion, parsing
=====================================

string-replace s1 s2 start1 end1 [start2 end2] -> string
    Returns
	(string-append (substring/shared s1 0 start1)
                       (substring/shared s2 start2 end2)
                       (substring/shared s1 end1 (string-length s1)))

    That is, the segment of characters in S1 from START1 to END1
    is replaced by the segment of characters in S2 from START2 to END2.
    If START1=END1, this simply splices the S2 characters into S1 at the
    specified index.

    Examples:
	(string-replace "The TCL programmer endured daily ridicule."
                        "another miserable perl drone" 4 7 8 22 ) =>
            "The miserable perl programmer endured daily ridicule."

	(string-replace "It's easy to code it up in Scheme." "lots of fun" 5 9) =>
            "It's lots of fun to code it up in Scheme."

	(define (string-insert s i t) (string-replace s t i i))

	(string-insert "It's easy to code it up in Scheme." 5 "really ") =>
            "It's really easy to code it up in Scheme."

string-tokenize s [token-set start end] -> list
   Split the string S into a list of substrings, where each substring is
   a maximal non-empty contiguous sequence of characters from the character set
   TOKEN-SET.

       - TOKEN-SET defaults to CHAR-SET:GRAPHIC 
         (see SRFI 14 for more on character sets and CHAR-SET:GRAPHIC).
       - If START or END indices are provided, they restrict STRING-TOKENIZE
         to operating on the indicated substring of S.

    This function provides a minimal parsing facility for simple applications.
    More sophisticated parsers that handle quoting and backslash effects can
    easily be constructed using regular-expression systems; be careful not
    to use STRING-TOKENIZE in contexts where more serious parsing is needed.

        (string-tokenize "Help make programs run, run, RUN!") =>
	  ("Help" "make" "programs" "run," "run," "RUN!")


*** Filtering & deleting
========================

string-filter s char/char-set/pred [start end] -> string
string-delete s char/char-set/pred [start end] -> string
    Filter the string S, retaining only those characters that
    satisfy / do not satisfy the CHAR/CHAR-SET/PRED argument. If
    this argument is a procedure, it is applied to the character
    as a predicate; if it is a char-set, the character is tested
    for membership; if it is a character, it is used in an equality test.

    If the string is unaltered by the filtering operation, these
    functions may return either S or a copy of S.


** Low-level procedures
=========================
The following procedures are useful for writing other string-processing
functions. In a Scheme system that has a module or package system, these
procedures should be contained in a module named "string-lib-internals".

*** START/END optional-argument parsing & checking utilities
============================================================

string-parse-start+end proc s args -> [rest start end]
string-parse-final-start+end proc s args -> [start end]
    STRING-PARSE-START+END may be used to parse a pair of optional START/END 
    arguments from an argument list, defaulting them to 0 and the length of 
    some string S, respectively. Let the length of string S be SLEN.
    - If ARGS = (), the function returns (values '() 0 slen)
    - If ARGS = (i), I is checked to ensure it is an exact integer, and
      that 0 <= i <= slen. Returns (values (cdr args) i slen).
    - If ARGS = (i j ...), I and J are checked to ensure they are exact
      integers, and that 0 <= i <= j <= slen. Returns (values (cddr args) i j).
    If any of the checks fail, an error condition is raised, and PROC is used
    as part of the error condition -- it should be the client procedure whose
    argument list STRING-PARSE-START+END is parsing.
    
    STRING-PARSE-FINAL-START+END is exactly the same, except that the args list
    passed to it is required to be of length two or less; if it is longer,
    an error condition is raised. It may be used when the optional START/END 
    parameters are final arguments to the procedure.

    Note that in all cases, these functions ensure that S is a string
    (by necessity, since all cases apply STRING-LENGTH to S either to
    default END or to bounds-check it).

let-string-start+end (start end [rest]) proc-exp s-exp args-exp body ... Syntax
    Syntactic sugar for an application of STRING-PARSE-START+END or
    STRING-PARSE-FINAL-START+END.
      
    If a REST variable is given, the form is equivalent to
      (CALL-WITH-VALUES
          (LAMBDA () (STRING-PARSE-START+END proc-exp s-exp args-exp))
        (LAMBDA (rest start end) body ...))
        
    If no REST variable is given, the form is equivalent to
      (CALL-WITH-VALUES
          (LAMBDA () (STRING-PARSE-FINAL-START+END proc-exp s-exp args-exp))
        (LAMBDA (start end) body ...))

check-substring-spec proc s start end -> unspecified
substring-spec-ok? s start end -> boolean
    Check values S, START and END to ensure they specify a valid substring.
    This means that S is a string, START and END are exact integers, and 
	0 <= START <= END <= (STRING-LENGTH S)

    If the values are not proper
        - CHECK-SUBSTRING-SPEC raises an error condition. PROC is used
          as part of the error condition, and should be the procedure whose 
          parameters we are checking.
	- SUBSTRING-SPEC-OK? returns false.
    Otherwise, SUBSTRING-SPEC-OK? returns true, and CHECK-SUBSTRING-SPEC
    simply returns (what it returns is not specified).



*** Knuth-Morris-Pratt searching
================================

The Knuth-Morris-Pratt string-search algorithm is a method of rapidly scanning
a sequence of text for the occurrence of some fixed string.  It has the
advantage of never requiring backtracking -- hence, it is useful for searching
not just strings, but also other sequences of text that do not support
backtracking or random-access, such as input ports.  These routines package up
the initialisation and searching phases of the algorithm for general use. They
also support searching through sequences of text that arrive in buffered
chunks, in that intermediate search state can be carried across applications
of the search loop from the end of one buffer application to the next.

A second critical property of KMP search is that it requires the allocation of
auxiliary memory proportional to the length of the pattern, but *constant*
in the size of the character type. Alternate searching algorithms frequently
require the construction of a table with an entry for every possible
character -- which can be prohibitively expensive in a 16- or 32-bit character
representation.

make-kmp-restart-vector s [c= start end] -> integer-vector
    Build a Knuth-Morris-Pratt "restart vector," which is useful for quickly
    searching character sequences for the occurrence of string S (or the
    substring of S demarcated by the optional START/END parameters, if
    provided). C= is a character-equality function used to construct the
    restart vector. It defaults to CHAR=?; use CHAR-CI=? instead for
    case-folded string search.
    
    The definition of the restart vector RV for string S is:
    If we have matched chars 0..i-1 of S against some search string SS, and
    S[i] doesn't match SS[k], then reset i := RV[i], and try again to
    match SS[k].  If RV[i] = -1, then punt SS[k] completely, and move on to
    SS[k+1] and S[0].

    In other words, if you have matched the first i chars of S, but
    the i+1'th char doesn't match, RV[i] tells you what the next-longest
    prefix of S is that you have matched.

    The following string-search function shows how a restart vector is used to
    search. Note the attractive feature of the search process: it is "on
    line," that is, it never needs to back up and reconsider previously seen
    data. It simply consumes characters one-at-a-time until declaring a
    complete match or reaching the end of the sequence. Thus, it can be easily
    adapted to search other character sequences (such as ports) that do not
    provide random access to their contents.

    (define (find-substring pattern source start end)
      (let ((plen (string-length pattern))
	    (rv (make-kmp-restart-vector pattern)))

	;; The search loop. SJ & PJ are redundant state.
	(let lp ((si start) (pi 0)
		 (sj (- end start))	; (- end si)  -- how many chars left.
		 (pj plen))		; (- plen pi) -- how many chars left.

	  (if (= pi plen) (- si plen)			; Win.

	      (and (<= pj sj)				; Lose.

		   (if (char=? (string-ref source si)		; Test.
			       (string-ref pattern pi))
		       (lp (+ 1 si) (+ 1 pi) (- sj 1) (- pj 1))	; Advance.

		       (let ((pi (vector-ref rv pi)))		; Retreat.
			 (if (= pi -1)
			     (lp (+ si 1)  0   (- sj 1)  plen)	; Punt.
			     (lp si        pi  sj        (- plen pi))))))))))

    The optional START/END parameters restrict the restart vector to the
    indicated substring of PAT; RV is END - START elements long. If START > 0,
    then RV is offset by START elements from PAT. That is, RV[I] describes
    pattern element PAT[I + START]. Elements of RV are themselves indices
    that range just over [0, END-START), *not* [START, END).

    Rationale: the actual value of RV is "position independent" -- it
    does not depend on where in the PAT string the pattern occurs, but
    only on the actual characters comprising the pattern.

kmp-step pat rv c i c= p-start -> integer
    This function encapsulates the work performed by one step of the
    KMP string search; it can be used to scan strings, input ports,
    or other on-line character sources for fixed strings. 

    PAT is the non-empty string specifying the text for which we are searching.
    RV is the Knuth-Morris-Pratt restart vector for the pattern, as constructed
    by MAKE-KMP-RESTART-VECTOR. The pattern begins at PAT[P-START], and is
    (STRING-LENGTH RV) characters long. C= is the character-equality function
    used to construct the restart vector, typically CHAR=? or CHAR-CI=?.

    Suppose the pattern is N characters in length: PAT[P-START, P-START + N).
    We have already matched I characters: PAT[P-START, P-START + I). (P-START
    is typically zero.) C is the next character in the input stream. KMP-STEP
    returns the new I value -- that is, how much of the pattern we have
    matched, *including* character C. When I reaches N, the entire pattern has
    been matched.

    Thus a typical search loop looks like this:
        (let lp ((i 0))
	  (or (= i n)				; Win -- #t
	      (and (not (end-of-stream))	; Lose -- #f
	           (lp (kmp-step pat rv (get-next-character) i char=? 0)))))

    Example:
       ;; Read chars from IPORT until we find string PAT or hit EOF.
       (define (port-skip pat iport)
         (let* ((rv (make-kmp-restart-vector pat))
                (patlen (string-length pat)))
           (let lp ((i 0) (nchars 0))
             (if (= i patlen) nchars			; Win -- nchars skipped
                 (let ((c (read-char iport)))
                   (if (eof-object? c) c		; Fail -- EOF
                       (lp (kmp-step pat rv c i char=? 0) ; Continue
                           (+ nchars 1))))))))

    This procedure could be defined as follows:

    (define (kmp-step pat rv c i c= p-start)
      (let lp ((i i))
        (if (c= c (string-ref pat (+ i p-start)))     ; Match =>
            (+ i 1)                                   ;   Done.
            (let ((i (vector-ref rv i)))              ; Back up in PAT.
              (if (= i -1) 0                          ; Can't back up more.
                  (lp i)))))))                        ; Keep going.

    Rationale: this procedure takes no optional arguments because it
    is intended as an inner-loop primitive and we do not want any
    run-time penalty for optional-argument parsing and defaulting,
    nor do we wish barriers to procedure integration/inlining.

string-kmp-partial-search pat rv s i [c= p-start s-start s-end] -> integer
    Applies KMP-STEP across S; optional S-START/S-END bounds parameters 
    restrict search to a substring of S. The pattern is (VECTOR-LENGTH RV)
    characters long; optional P-START index indicates non-zero start of 
    pattern in PAT.

    Suppose PLEN = (VECTOR-LENGTH RV) is the length of the pattern.
    I is an integer index into the pattern (that is, 0 <= I < PLEN)
    indicating how much of the pattern has already been matched. 
    (This means the pattern must be non-empty -- PLEN > 0.)

    - On success, returns -J, where J is the index in S bounding
      the *end* of the pattern -- e.g., a value that could be used as 
      the END parameter in a call to SUBSTRING/SHARED.

    - On continue, returns the current search state I' (an index into RV)
      when the search reached the end of the string. This is a non-negative
      integer.

    Hence:
    - A negative return value indicates success, and says
      where in the string the match occured.

    - A non-negative return value provides the I to use for
      continued search in a following string.

    This utility is designed to allow searching for occurrences of a fixed
    string that might extend across multiple buffers of text. This is
    why, for example, we do not provide the index of the *start* of the
    match on success -- it may have occurred in a previous buffer.

    To search a character sequence that arrives in "chunks," write a
    loop of this form:
        (let lp ((i 0))
	  (and (not (end-of-data?))		; Lose -- return #f.
               (let* ((buf (get-next-chunk))	; Get or fill up the buffer.
                      (i (string-kmp-partial-search pat rv buf i)))
                 (if (< i 0) (- i) 		; Win -- return end index.
                     (lp i)))))			; Keep looking.

    Modulo start/end optional-argument parsing, this procedure could
    be defined as follows:
        (define (string-kmp-partial-search pat rv s i c= p-start s-start s-end)
	  (let ((patlen (vector-length rv)))
            (let lp ((si s-start)	; An index into S.
	             (vi i))		; An index into RV.
	      (cond ((= vi patlen) (- si))	; Win.
                    ((= si end) vi)		; Ran off the end.
                    (else (lp (+ si 1)		; Match s[si] & loop.
                              (kmp-step pat rv (string-ref s si)
                                        vi c= p-start)))))))


-------------------------------------------------------------------------------
* Reference implementation
--------------------------

This SRFI comes with a reference implementation. It can be found at:
    http://srfi.schemers.org/srfi-13/srfi-13.scm
I have placed this source on the Net with an unencumbered, "open" copyright.
The prefix/suffix and comparison routines in this code had (extremely distant)
origins in MIT Scheme's string lib, and were substantially reworked by myself.
Being derived from that code, they are covered by the MIT Scheme copyright,
which is a generic BSD-style open-source copyright. See the source file for
details.

The KMP string-search code was influenced by implementations written by
Stephen Bevan, Brian Denheyer and Will Fitzgerald. However, this version was
written from scratch by myself.

The remainder of the code was written by myself for scsh or for this SRFI; I
have placed this code under the scsh copyright, which is also a generic
BSD-style open-source copyright.

The code is written for portability and should be straightforward to port to
any Scheme. The source comments contains detailed notes describing the non-R5RS
dependencies.

The library is written for clarity and well-commented; the current source is
approximately 1000 lines of source code and 1000 lines of comments and white
space. It is also written for efficiency. Fast paths are provided for common
cases. This is not to say that the implementation can't be tuned up for a
specific Scheme implementation. There are notes in the comments addressing
ways implementors can tune the reference implementation for performance.

In short, I've written the reference implementation to make it as painless
as possible for an implementor -- or a regular programmer -- to adopt this
library and get good results with it.


-------------------------------------------------------------------------------
* Acknowledgements
------------------

The design of this library benefited greatly from the feedback provided during
the SRFI discussion phase. Among those contributing thoughtful commentary and
suggestions, both on the mailing list and by private discussion, were Paolo
Amoroso, Lars Arvestad, Alan Bawden, Jim Bender, Dan Bornstein, Per Bothner,
Will Clinger, Brian Denheyer, Mikael Djurfeldt, Kent Dybvig, Sergei Egorov,
Marc Feeley, Matthias Felleisen, Will Fitzgerald, Matthew Flatt, Arthur A.
Gleckler, Ben Goetter, Sven Hartrumpf, Erik Hilsdale, Richard Kelsey, Oleg
Kiselyov, Bengt Kleberg, Donovan Kolbly, Bruce Korb, Shriram Krishnamurthi,
Bruce Lewis, Tom Lord, Brad Lucier, Dave Mason, David Rush, Klaus Schilling,
Jonathan Sobel, Mike Sperber, Mikael Staldal, Vladimir Tsyshevsky, Donald
Welsh, and Mike Wilson. I am grateful to them for their assistance.

I am also grateful the authors, implementors and documentors of all the systems
mentioned in the introduction. Aubrey Jaffer and Kent Pitman should be noted
for their work in producing Web-accessible versions of the R5RS and Common
Lisp spec, which was a tremendous aid.

This is not to imply that these individuals necessarily endorse the final
results, of course. 

During this document's long development period, great patience was exhibited
by Mike Sperber, who is the editor for the SRFI, and by Hillary Sullivan,
who is not.

-------------------------------------------------------------------------------
* References & links
--------------------

[Case-map]
    Case mappings.
    Unicode Technical Report 21.
    http://www.unicode.org/unicode/reports/tr21/

[CommonLisp]
    Common Lisp: the Language
    Guy L. Steele Jr. (editor).
    Digital Press, Maynard, Mass., second edition 1990.
    Available at http://www.elwood.com/alu/table/references.htm#cltl2

    The Common Lisp "HyperSpec," produced by Kent Pitman, is essentially
    the ANSI spec for Common Lisp:
    http://www.harlequin.com/education/books/HyperSpec/

[Java]
    The following URLs provide documentation on relevant Java classes.

    http://java.sun.com/products/jdk/1.2/docs/api/java/lang/Character.html
    http://java.sun.com/products/jdk/1.2/docs/api/java/lang/String.html
    http://java.sun.com/products/jdk/1.2/docs/api/java/lang/StringBuffer.html
    http://java.sun.com/products/jdk/1.2/docs/api/java/text/Collator.html
    http://java.sun.com/products/jdk/1.2/docs/api/java/text/package-summary.html

[MIT-Scheme]
    http://www.swiss.ai.mit.edu/projects/scheme/

[R5RS]
    Revised^5 report on the algorithmic language Scheme.
    R. Kelsey, W. Clinger, J. Rees (editors).
    Higher-Order and Symbolic Computation, Vol. 11, No. 1, September, 1998.
    and ACM SIGPLAN Notices, Vol. 33, No. 9, October, 1998.

    Available at http://www.schemers.org/Documents/Standards/

[SRFI]
    The SRFI web site.
    http://srfi.schemers.org/

[SRFI-13]
    SRFI-13: String libraries.
    http://srfi.schemers.org/srfi-13/

    This document, in HTML:
        http://srfi.schemers.org/srfi-13/srfi-13.html
    This document, in plain text format:
        http://srfi.schemers.org/srfi-13/srfi-13.txt
    Source code for the reference implementation:
        http://srfi.schemers.org/srfi-13/srfi-13.scm
    Scheme 48 module specification, with typings:
        http://srfi.schemers.org/srfi-13/srfi-13-s48-module.scm

[SRFI-14]
    SRFI-14: Character-set library.
    http://srfi.schemers.org/srfi-14/

    The SRFI 14 char-set library defines a character-set data type,
    which is used by some procedures in this library.

[Unicode]
    http://www.unicode.org/

[UnicodeData]
    The Unicode character database.
    ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt
    ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.html


-------------------------------------------------------------------------------
* Copyright
-----------

Certain portions of this document -- the specific, marked segments of text
describing the R5RS procedures -- were adapted with permission from the R5RS
report.
    
All other text is copyright (C) Olin Shivers (1998, 1999, 2000). 
All Rights Reserved. 

This document and translations of it may be copied and furnished to others,
and derivative works that comment on or otherwise explain it or assist in its
implementation may be prepared, copied, published and distributed, in whole or
in part, without restriction of any kind, provided that the above copyright
notice and this paragraph are included on all such copies and derivative
works. However, this document itself may not be modified in any way, such as
by removing the copyright notice or references to the Scheme Request For
Implementation process or editors, except as needed for the purpose of
developing SRFIs in which case the procedures for copyrights defined in the
SRFI process must be followed, or as required to translate it into languages
other than English.

The limited permissions granted above are perpetual and will not be revoked by
the authors or their successors or assigns.

This document and the information contained herein is provided on an "AS IS"
basis and THE AUTHORS AND THE SRFI EDITORS DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


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