add compile-time option to store precomputed index->cursor tables for strings

eval.c

@@ -1947,6 +1947,8 @@ void sexp_string_utf8_set (sexp ctx, sexp str, sexp index, sexp ch) {
     sexp_string_size(str) += new_len - old_len;
   }
   sexp_utf8_encode_char(p, new_len, c);
+  if (old_len != new_len)
+    sexp_update_string_index_lookup(ctx, str);
 }
 
 sexp sexp_string_utf8_index_set (sexp ctx, sexp self, sexp_sint_t n, sexp str, sexp i, sexp ch) {

include/chibi/features.h

@@ -210,12 +210,14 @@
 /*   Making them immutable allows for packed UTF-8 strings. */
 /* #define SEXP_USE_MUTABLE_STRINGS 0 */
 
-/* uncomment this to make string cursors just fixnum offsets */
+/* uncomment this to enable precomputed index->cursor tables for strings */
-/*   The default when using UTF-8 is to have a disjoint string */
+/*   This makes string-ref faster at the expensive of making string */
-/*   cursor type.  This is an immediate type with no loss in  */
+/*   construction (including string-append and I/O) slower. */
-/*   performance, and prevents confusion mixing indexes and */
+/*   You can configure with SEXP_STRING_INDEX_TABLE_CHUNK_SIZE below, */
-/*   cursors. */
+/*   the default is caching every 64th index (<=12.5% string overhead). */
-/* #define SEXP_USE_DISJOINT_STRING_CURSORS 0 */
+/*   With a minimum of 1 you'd have up to 8x string overhead, and */
+/*   string-ref would still be slightly slower than string-cursors. */
+/* #define SEXP_USE_STRING_INDEX_TABLE 1 */
 
 /* uncomment this to disable automatic closing of ports */
 /*   If enabled, the underlying FILE* for file ports will be */
@@ -647,6 +649,18 @@
 #define SEXP_USE_PACKED_STRINGS 1
 #endif
 
+#if SEXP_USE_PACKED_STRINGS
+#define SEXP_USE_STRING_INDEX_TABLE 0
+#endif
+#ifndef SEXP_USE_STRING_INDEX_TABLE
+#define SEXP_USE_STRING_INDEX_TABLE 0
+#endif
+
+/* for every chunk_size indexes store the precomputed offset */
+#ifndef SEXP_STRING_INDEX_TABLE_CHUNK_SIZE
+#define SEXP_STRING_INDEX_TABLE_CHUNK_SIZE 64
+#endif
+
 #ifndef SEXP_USE_DISJOINT_STRING_CURSORS
 #define SEXP_USE_DISJOINT_STRING_CURSORS SEXP_USE_UTF8_STRINGS
 #endif

include/chibi/sexp.h

@@ -443,6 +443,9 @@ struct sexp_struct {
 #else
       sexp_uint_t offset, length;
       sexp bytes;
+#if SEXP_USE_STRING_INDEX_TABLE
+      sexp charlens;
+#endif
 #endif
     } string;
     struct {
@@ -1123,6 +1126,7 @@ enum sexp_uniform_vector_type {
 #define sexp_bit_set(u1v, i, x) (x ? (sexp_uvector_data(u1v)[i/8]|=(1<<(i%8))) : (sexp_uvector_data(u1v)[i/8]&=~(1<<(i%8))))
 
 #define sexp_string_size(x)     (sexp_field(x, string, SEXP_STRING, length))
+#define sexp_string_charlens(x) (sexp_field(x, string, SEXP_STRING, charlens))
 #if SEXP_USE_PACKED_STRINGS
 #define sexp_string_data(x)   (sexp_field(x, string, SEXP_STRING, data))
 #define sexp_string_bytes(x)  (x)
@@ -1722,6 +1726,12 @@ SEXP_API int sexp_write_utf8_char (sexp ctx, int c, sexp out);
 #define sexp_substring_cursor(ctx, s, i, j) sexp_substring_op(ctx, NULL, 3, s, i, j)
 #endif
 
+#if SEXP_USE_STRING_INDEX_TABLE
+SEXP_API void sexp_update_string_index_lookup(sexp ctx, sexp s);
+#else
+#define sexp_update_string_index_lookup(ctx, s)
+#endif
+
 #if SEXP_USE_GREEN_THREADS
 SEXP_API int sexp_maybe_block_port (sexp ctx, sexp in, int forcep);
 SEXP_API void sexp_maybe_unblock_port (sexp ctx, sexp in);

sexp.c

@@ -262,7 +262,7 @@ static struct sexp_type_struct _sexp_type_specs[] = {
 #if SEXP_USE_PACKED_STRINGS
   {SEXP_STRING, 0, 0, 0, 0, 0, sexp_sizeof(string)+1, sexp_offsetof(string, length), 1, 0, 0, 0, 0, 0, 0, (sexp)"String", SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, NULL, NULL, NULL, NULL},
 #else
-  {SEXP_STRING, sexp_offsetof(string, bytes), 1, 1, 0, 0, sexp_sizeof(string), 0, 0, 0, 0, 0, 0, 0, 0, (sexp)"String", SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, NULL, NULL, NULL, NULL},
+  {SEXP_STRING, sexp_offsetof(string, bytes), 1, 1+SEXP_USE_STRING_INDEX_TABLE, 0, 0, sexp_sizeof(string), 0, 0, 0, 0, 0, 0, 0, 0, (sexp)"String", SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, NULL, NULL, NULL, NULL},
 #endif
   {SEXP_VECTOR, sexp_offsetof(vector, data), 0, 0, sexp_offsetof(vector, length), 1, sexp_sizeof(vector), sexp_offsetof(vector, length), sizeof(sexp), 0, 0, 0, 0, 0, 0, (sexp)"Vector", SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, NULL, NULL, NULL, NULL},
   {SEXP_FLONUM, 0, 0, 0, 0, 0, sexp_sizeof(flonum), 0, 0, 0, 0, 0, 0, 0, 0, (sexp)"Flonum", SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, NULL, NULL, NULL, NULL},
@@ -1198,13 +1198,31 @@ void sexp_utf8_encode_char (unsigned char* p, int len, int c) {
 }
 
 sexp sexp_string_index_to_cursor (sexp ctx, sexp self, sexp_sint_t n, sexp str, sexp index) {
+#if SEXP_USE_STRING_INDEX_TABLE
+  sexp charlens;
+  sexp_sint_t* chunklens;
+  sexp_sint_t chunk;
+#endif
   sexp_sint_t i, j, limit;
   unsigned char *p;
   sexp_assert_type(ctx, sexp_stringp, SEXP_STRING, str);
   sexp_assert_type(ctx, sexp_fixnump, SEXP_FIXNUM, index);
   p = (unsigned char*)sexp_string_data(str);
   limit = sexp_string_size(str);
-  for (j=0, i=sexp_unbox_fixnum(index); i>0 && j<limit; i--)
+  i = sexp_unbox_fixnum(index);
+  j = 0;
+#if SEXP_USE_STRING_INDEX_TABLE
+  if (i > SEXP_STRING_INDEX_TABLE_CHUNK_SIZE) {
+    charlens = sexp_string_charlens(str);
+    if (charlens) {
+      chunklens = (sexp_sint_t*)sexp_bytes_data(charlens);
+      chunk = i / SEXP_STRING_INDEX_TABLE_CHUNK_SIZE - 1;
+      j = chunklens[chunk];
+      i -= (chunk+1) * SEXP_STRING_INDEX_TABLE_CHUNK_SIZE;
+    }
+  }
+#endif
+  for ( ; i>0 && j<limit; i--)
     j += sexp_utf8_initial_byte_count(p[j]);
   if (i != 0)
     return sexp_user_exception(ctx, self, "string-index->cursor: index out of range", index);
@@ -1227,6 +1245,36 @@ sexp sexp_string_cursor_offset (sexp ctx, sexp self, sexp_sint_t n, sexp cur) {
 
 #endif
 
+#if SEXP_USE_STRING_INDEX_TABLE
+void sexp_update_string_index_lookup(sexp ctx, sexp s) {
+  char *p;
+  sexp_sint_t numchunks, len, i, *chunks;
+  sexp_gc_var1(tmp);
+  if (sexp_string_size(s) < SEXP_STRING_INDEX_TABLE_CHUNK_SIZE*1.2) {
+    sexp_string_charlens(s) = NULL; /* don't build table for just a few chars */
+    return;
+  }
+  sexp_gc_preserve1(ctx, tmp);
+  tmp = s;
+  len = sexp_string_utf8_length((unsigned char*) sexp_string_data(s), sexp_string_size(s));
+  numchunks = ((len + SEXP_STRING_INDEX_TABLE_CHUNK_SIZE - 1) / SEXP_STRING_INDEX_TABLE_CHUNK_SIZE) - 1;
+  sexp_string_charlens(s) =
+    sexp_make_bytes_op(ctx, NULL, 2, sexp_make_fixnum(numchunks * sizeof(sexp_sint_t)), SEXP_VOID);
+  chunks = (sexp_sint_t*)sexp_bytes_data(sexp_string_charlens(s));
+  p = sexp_string_data(s);
+  i = 0;
+  while (1) {
+    p += sexp_utf8_initial_byte_count(*p);
+    if (++i % SEXP_STRING_INDEX_TABLE_CHUNK_SIZE == 0) {
+      chunks[i/SEXP_STRING_INDEX_TABLE_CHUNK_SIZE - 1] = p - sexp_string_data(s);
+      if (i / SEXP_STRING_INDEX_TABLE_CHUNK_SIZE >= numchunks-1)
+        break;
+    }
+  }
+  sexp_gc_release1(ctx);
+}
+#endif
+
 sexp sexp_make_string_op (sexp ctx, sexp self, sexp_sint_t n, sexp len, sexp ch)
 {
   sexp i = (sexp_charp(ch) ? sexp_make_fixnum(sexp_unbox_character(ch)) : ch);
@@ -1259,6 +1307,7 @@ sexp sexp_make_string_op (sexp ctx, sexp self, sexp_sint_t n, sexp len, sexp ch)
   sexp_string_bytes(s) = b;
   sexp_string_offset(s) = 0;
   sexp_string_size(s) = sexp_bytes_length(b);
+  sexp_update_string_index_lookup(ctx, s);
   sexp_gc_release2(ctx);
   return s;
 #endif
@@ -1273,6 +1322,7 @@ sexp sexp_c_string (sexp ctx, const char *str, sexp_sint_t slen) {
   if (sexp_exceptionp(s)) return s;
   memcpy(sexp_string_data(s), str, len);
   sexp_string_data(s)[len] = '\0';
+  sexp_update_string_index_lookup(ctx, s);
   return s;
 }
 
@@ -1294,6 +1344,7 @@ sexp sexp_substring_op (sexp ctx, sexp self, sexp_sint_t n, sexp str, sexp start
          sexp_string_data(str)+sexp_unbox_string_cursor(start),
          sexp_string_size(res));
   sexp_string_data(res)[sexp_string_size(res)] = '\0';
+  sexp_update_string_index_lookup(ctx, res);
   return res;
 }
 
@@ -1360,6 +1411,7 @@ sexp sexp_string_concatenate_op (sexp ctx, sexp self, sexp_sint_t n, sexp str_ls
     }
   }
   *p = '\0';
+  sexp_update_string_index_lookup(ctx, res);
   return res;
 }
 

tests/build/build-opts.txt

@@ -33,5 +33,6 @@ CPPFLAGS=-DSEXP_USE_UTF8_STRINGS=0
 CPPFLAGS=-DSEXP_USE_DISJOINT_STRING_CURSORS=0
 CFLAGS=-DSEXP_USE_STATIC_LIBS_NO_INCLUDE=0;CPPFLAGS=-DSEXP_USE_STATIC_LIBS=1
 CPPFLAGS=-DSEXP_USE_MUTABLE_STRINGS=0
+CPPFLAGS=-DSEXP_USE_STRING_INDEX_TABLE=1
 CPPFLAGS=-DSEXP_USE_STRICT_TOPLEVEL_BINDINGS=1
 CPPFLAGS=-DSEXP_USE_NO_FEATURES=1