perf: optimize single byte codepoints computation

It is 25% faster than v1.4.0.

ext/jaro_winkler/codepoints.c

@@ -5,31 +5,57 @@
 #include <stdlib.h>
 #include <string.h>
 
+// this function is copied from string.c
+static inline int single_byte_optimizable(VALUE str) {
+  rb_encoding *enc;
+
+  /* Conservative.  It may be ENC_CODERANGE_UNKNOWN. */
+  if (ENC_CODERANGE(str) == ENC_CODERANGE_7BIT)
+    return 1;
+
+  enc = rb_enc_get(str);
+  if (rb_enc_mbmaxlen(enc) == 1)
+    return 1;
+
+  /* Conservative.  Possibly single byte.
+   * "\xa1" in Shift_JIS for example. */
+  return 0;
+}
+
 void codepoints_init(CodePoints *codepoints, VALUE str) {
+  size_t i, length;
   int32_t n;
   uint32_t c;
   const char *ptr, *end;
   rb_encoding *enc;
 
-  codepoints->length = 0;
-  codepoints->size = 32;
-  codepoints->data = malloc(codepoints->size * sizeof(*codepoints->data));
-  str = rb_str_new_frozen(str);
-  ptr = RSTRING_PTR(str);
-  end = RSTRING_END(str);
-  enc = rb_enc_get(str);
+  if (single_byte_optimizable(str)) {
+    length = RSTRING_LEN(str);
+    ptr = RSTRING_PTR(str);
+    codepoints->data = malloc(length * sizeof(*codepoints->data));
+    for (i = 0, codepoints->length = 0; i < length; i++, codepoints->length++)
+      codepoints->data[i] = ptr[i] & 0xff;
+  } else {
+    codepoints->length = 0;
+    codepoints->size = 32;
+    codepoints->data = malloc(codepoints->size * sizeof(*codepoints->data));
+    str = rb_str_new_frozen(str);
+    ptr = RSTRING_PTR(str);
+    end = RSTRING_END(str);
+    enc = rb_enc_get(str);
 
-  while (ptr < end) {
-    c = rb_enc_codepoint_len(ptr, end, &n, enc);
-    if (codepoints->length == codepoints->size) {
-      codepoints->size *= 2;
-      codepoints->data = realloc(codepoints->data,
-                                 sizeof(*codepoints->data) * codepoints->size);
+    while (ptr < end) {
+      c = rb_enc_codepoint_len(ptr, end, &n, enc);
+      if (codepoints->length == codepoints->size) {
+        codepoints->size *= 2;
+        codepoints->data = realloc(codepoints->data, sizeof(*codepoints->data) *
+                                                         codepoints->size);
+      }
+      codepoints->data[codepoints->length++] = c;
+      ptr += n;
     }
-    codepoints->data[codepoints->length++] = c;
-    ptr += n;
+    RB_GC_GUARD(str);
   }
-  RB_GC_GUARD(str);
 }
 
 void codepoints_free(CodePoints *codepoints) { free(codepoints->data); }