serialization.md

Serialization

Prism ships with the ability to serialize a syntax tree to a single string. The string can then be deserialized back into a syntax tree using a language other than C. This is useful for using the parsing logic in other tools without having to write a parser in that language. The syntax tree still requires a copy of the original source, as for the most part it just contains byte offsets into the source string.

Types

Let us define some simple types for readability.

varint

A variable-length integer with the value fitting in uint32_t using between 1 and 5 bytes, using the LEB128 encoding. This drastically cuts down on the size of the serialized string, especially when the source file is large.

string

# bytes	field
varint	the length of the string in bytes
...	the string bytes

location

# bytes	field
varint	byte offset into the source string where this location begins
varint	length of the location in bytes in the source string

comment

The comment type is one of:

• 0=INLINE (# comment)
• 1=EMBEDDED_DOCUMENT (=begin/=end)
• 2=__END__ (after __END__)

# bytes	field
1	comment type
location	the location in the source of this comment

magic comment

# bytes	field
location	the location of the key of the magic comment
location	the location of the value of the magic comment

diagnostic

# bytes	field
string	diagnostic message (ASCII-only characters)
location	the location in the source this diagnostic applies to

Structure

The serialized string representing the syntax tree is composed of three parts: the header, the body, and the constant pool. The header contains information like the version of prism that serialized the tree. The body contains the actual nodes in the tree. The constant pool contains constants that were interned while parsing.

The header is structured like the following table:

# bytes	field
5	"PRISM"
1	major version number
1	minor version number
1	patch version number
1	1 indicates only semantics fields were serialized, 0 indicates all fields were serialized (including location fields)
string	the encoding name
varint	the start line
varint	number of comments
comment*	comments
varint	number of magic comments
magic comment*	magic comments
varint	number of errors
diagnostic*	errors
varint	number of warnings
diagnostic*	warnings
4	content pool offset
varint	content pool size

After the header comes the body of the serialized string. The body consists of a sequence of nodes that is built using a prefix traversal order of the syntax tree. Each node is structured like the following table:

# bytes	field
1	node type
location	node location

Every field on the node is then appended to the serialized string. The fields can be determined by referencing config.yml. Depending on the type of field, it could take a couple of different forms, described below:

• node - A field that is a node. This is structured just as like parent node.
• node? - A field that is a node that is optionally present. If the node is not present, then a single 0 byte will be written in its place. If it is present, then it will be structured just as like parent node.
• node[] - A field that is an array of nodes. This is structured as a variable-length integer length, followed by the child nodes themselves.
• string - A field that is a string. For example, this is used as the name of the method in a call node, since it cannot directly reference the source string (as in @- or foo=). This is structured as a variable-length integer byte length, followed by the string itself (without a trailing null byte).
• constant - A variable-length integer that represents an index in the constant pool.
• constant? - An optional variable-length integer that represents an index in the constant pool. If it's not present, then a single 0 byte will be written in its place.
• location - A field that is a location. This is structured as a variable-length integer start followed by a variable-length integer length.
• location? - A field that is a location that is optionally present. If the location is not present, then a single 0 byte will be written in its place. If it is present, then it will be structured just like the location child node.
• uint32 - A field that is a 32-bit unsigned integer. This is structured as a variable-length integer.

After the syntax tree, the content pool is serialized. This is a list of constants that were referenced from within the tree. The content pool begins at the offset specified in the header. Constants can be either "owned" (in which case their contents are embedded in the serialization) or "shared" (in which case their contents represent a slice of the source string). The most significant bit of the constant indicates whether it is owned or shared.

In the case that it is owned, the constant is structured as follows:

# bytes	field
4	the byte offset in the serialization for the contents of the constant
4	the byte length in the serialization

Note that you will need to mask off the most significant bit for the byte offset in the serialization. In the case that it is shared, the constant is structured as follows:

# bytes	field
4	the byte offset in the source string for the contents of the constant
4	the byte length in the source string

After the constant pool, the contents of the owned constants are serialized. This is just a sequence of bytes that represent the contents of the constants. At the end of the serialization, the buffer is null terminated.

APIs

The relevant APIs and struct definitions are listed below:

// A pm_buffer_t is a simple memory buffer that stores data in a contiguous
// block of memory. It is used to store the serialized representation of a
// prism tree.
typedef struct {
  char *value;
  size_t length;
  size_t capacity;
} pm_buffer_t;

// Free the memory associated with the buffer.
void pm_buffer_free(pm_buffer_t *);

// Parse and serialize the AST represented by the given source to the given
// buffer.
void pm_serialize_parse(pm_buffer_t *buffer, const uint8_t *source, size_t length, const char *data);

Typically you would use a stack-allocated pm_buffer_t and call pm_serialize_parse, as in:

void
serialize(const uint8_t *source, size_t length) {
  pm_buffer_t buffer = { 0 };
  pm_serialize_parse(&buffer, source, length, NULL);

  // Do something with the serialized string.

  pm_buffer_free(&buffer);
}

The final argument to pm_serialize_parse is an optional string that controls the options to the parse function. This includes all of the normal options that could be passed to pm_parser_init through a pm_options_t struct, but serialized as a string to make it easier for callers through FFI. Note that no varint are used here to make it easier to produce the data for the caller, and also serialized size is less important here. The format of the data is structured as follows:

# bytes	field
4	the length of the filepath
...	the filepath bytes
4	the line number
4	the length the encoding
...	the encoding bytes
1	frozen string literal
1	suppress warnings
4	the number of scopes
...	the scopes

Each scope is layed out as follows:

# bytes	field
4	the number of locals
...	the locals

Each local is layed out as follows:

# bytes	field
4	the length of the local
...	the local bytes

The data can be NULL (as seen in the example above).