[8_12] improve strategy of expanding string for performance

* Revert "[8_12] Revert: improve strategy of expanding string for performance"

This reverts commit d4cfb60.

* adjust string layout

* improve performance of string initialization

Kernel/Types/string.cpp

@@ -31,9 +31,11 @@ round_length (int n) {
   return i;
 }
 
-string_rep::string_rep (int n2)
-    : n (n2),
-      a ((n == 0) ? ((char*) NULL) : tm_new_array<char> (round_length (n))) {}
+string_rep::string_rep (int n2) : n (n2), a_N (round_length (n2)), a (nullptr) {
+  if (a_N != 0) {
+    a= tm_new_array<char> (a_N);
+  }
+}
 
 void
 string_rep::resize (int m) {
@@ -42,17 +44,53 @@ string_rep::resize (int m) {
   if (mm != nn) {
     if (mm != 0) {
       if (nn != 0) {
-        a= tm_resize_array<char> (mm, a);
+        a_N= mm;
+        a  = tm_resize_array<char> (mm, a);
       }
       else {
-        a= tm_new_array<char> (mm);
+        a_N= mm;
+        a  = tm_new_array<char> (mm);
       }
     }
-    else if (nn != 0) tm_delete_array (a);
+    else if (nn != 0) {
+      tm_delete_array (a);
+      a_N= 0;
+      a  = NULL;
+    };
   }
   n= m;
 }
 
+int
+string_rep::expand_or_shrink_by (int delta) {
+  int old_n= n;
+  n+= delta;
+  reserve (n);
+  return old_n;
+}
+
+void
+string_rep::reserve (int new_n) {
+  int old_size= a_N;
+  if (new_n != 0) {
+    if (old_size == 0) {
+      a_N= round_length (new_n);
+      a  = tm_new_array<char> (a_N);
+    }
+    else if (old_size < new_n) {
+      a_N= round_length (new_n);
+      a  = tm_resize_array<char> (a_N, a);
+    }
+  }
+  else {
+    if (old_size != 0) {
+      tm_delete_array (a);
+      a  = NULL;
+      a_N= 0;
+    };
+  }
+}
+
 string::string (char c) {
   rep      = tm_new<string_rep> (1);
   rep->a[0]= c;
@@ -138,17 +176,17 @@ copy (string s) {
 
 string&
 operator<< (string& a, char x) {
-  a->resize (N (a) + 1);
-  a[N (a) - 1]= x;
+  int old_a_N= a->expand_or_shrink_by (1);
+  a[old_a_N] = x;
   return a;
 }
 
 string&
 operator<< (string& a, string b) {
-  int i, k1= N (a), k2= N (b);
-  a->resize (k1 + k2);
-  for (i= 0; i < k2; i++)
-    a[i + k1]= b[i];
+  int b_N    = N (b);
+  int old_a_N= a->expand_or_shrink_by (b_N);
+  for (int i= 0; i < b_N; i++)
+    a[i + old_a_N]= b[i];
   return a;
 }
 

Kernel/Types/string.hpp

@@ -24,15 +24,37 @@ class string;
 class string_rep : concrete_struct {
   int   n;
   char* a;
+  int   a_N;
 
 public:
-  inline string_rep () : n (0), a (NULL) {}
+  inline string_rep () : n (0), a_N (0), a (NULL) {}
   string_rep (int n);
   inline ~string_rep () {
-    if (n != 0) tm_delete_array (a);
+    if (a_N != 0) tm_delete_array (a);
   }
+  /**
+   * @brief expand (or shrink) string by delta, but do not release memory when
+   * string is shrinked.
+   *
+   * @return string length before expansion
+   */
+  int expand_or_shrink_by (int delta);
+
+  /**
+   * @brief expand (or shrink) string to given length n, and try to release
+   * memory when string is shrinked.
+   *
+   * @note expand_or_shrink_by may be faster if memory space is reserved
+   */
   void resize (int n);
 
+  /**
+   * @brief reserve memory space to contain at least n word in the whole string.
+   * Do not affect length of string, and do not release memory when n is smaller
+   * than current space.
+   */
+  void reserve (int n);
+
   friend class string;
   friend inline int N (string a);
 };

bench/Kernel/Types/string_bench.cpp

@@ -14,6 +14,7 @@ static ankerl::nanobench::Bench bench;
 int
 main () {
   lolly::init_tbox ();
+  bench.minEpochTime (std::chrono::milliseconds (20));
   bench.run ("construct string", [&] {
     string ("abc");
     string ();
@@ -43,7 +44,6 @@ main () {
     static string a ("abcdefgh");
     a (1, 6);
   });
-  bench.minEpochIterations (40000);
   bench.run ("concat string", [&] {
     static string a ("abc"), b ("de");
     a*            b;

tests/Kernel/Types/string_test.cpp

@@ -64,6 +64,81 @@ TEST_CASE ("test append") {
   CHECK_EQ (str == string ("xyz"), true);
 }
 
+TEST_CASE ("test append") {
+  auto str= string ();
+  str << 'x';
+  CHECK (str == "x");
+  str << string ("yz");
+  CHECK (str == "xyz");
+}
+
+TEST_CASE ("test reserve along with append") {
+
+  SUBCASE ("reserved more space") {
+    auto str= string ();
+    str->reserve (6);
+    str << 'x';
+    CHECK_EQ (str == "x", true);
+    str << string ("yz");
+    CHECK_EQ (str == "xyz", true);
+    str << string (": larger than reserved space");
+    CHECK_EQ (str == "xyz: larger than reserved space", true);
+  }
+  SUBCASE ("reserved the same space") {
+    auto str= string ("abc");
+    str->reserve (3);
+    CHECK_EQ (str == "abc", true);
+  }
+  SUBCASE ("reserved less space should take no effect") {
+    auto str= string ("abc");
+    str->reserve (2);
+    CHECK_EQ (str == "abc", true);
+  }
+}
+
+TEST_CASE ("test expand_or_shrink_by along with sub index") {
+
+  SUBCASE ("expand") {
+    auto str       = string ("abc");
+    int  previous_n= str->expand_or_shrink_by (1);
+    CHECK_EQ (previous_n, 3);
+    str[3]= 'd';
+    CHECK_EQ (str == "abcd", true);
+  }
+  SUBCASE ("shrink") {
+    auto str       = string ("abc");
+    int  previous_n= str->expand_or_shrink_by (-1);
+    CHECK_EQ (previous_n, 3);
+    CHECK_EQ (str == "ab", true);
+  }
+  SUBCASE ("delta 0 takes no effect") {
+    auto str       = string ("abc");
+    int  previous_n= str->expand_or_shrink_by (0);
+    CHECK_EQ (previous_n, 3);
+    CHECK_EQ (str == "abc", true);
+  }
+}
+
+TEST_CASE ("test resize") {
+
+  SUBCASE ("expand") {
+    auto str= string ("abc");
+    str->resize (4);
+    str[3]= 'd';
+    CHECK_EQ (str == "abcd", true);
+  }
+  SUBCASE ("shrink") {
+    auto str= string ("abc");
+    str->resize (2);
+    CHECK_EQ (str == "ab", true);
+  }
+  SUBCASE ("delta 0 takes no effect") {
+    auto str= string ("abc");
+    str->resize (3);
+    CHECK_EQ (str == "abc", true);
+  }
+}
+
 /******************************************************************************
  * Conversions
  ******************************************************************************/