Implement String indexOf(needle, startIndex)

src/vm/wren_core.c

@@ -869,7 +869,7 @@ DEF_PRIMITIVE(string_contains)
   ObjString* string = AS_STRING(args[0]);
   ObjString* search = AS_STRING(args[1]);
 
-  RETURN_BOOL(wrenStringFind(string, search) != UINT32_MAX);
+  RETURN_BOOL(wrenStringFind(string, search, 0) != UINT32_MAX);
 }
 
 DEF_PRIMITIVE(string_endsWith)
@@ -893,7 +893,19 @@ DEF_PRIMITIVE(string_indexOf)
   ObjString* string = AS_STRING(args[0]);
   ObjString* search = AS_STRING(args[1]);
 
-  uint32_t index = wrenStringFind(string, search);
+  uint32_t index = wrenStringFind(string, search, 0);
+  RETURN_NUM(index == UINT32_MAX ? -1 : (int)index);
+}
+
+DEF_PRIMITIVE(string_indexOf_with_startIndex)
+{
+  if (!validateString(vm, args[1], "Argument")) return false;
+
+  ObjString* string = AS_STRING(args[0]);
+  ObjString* search = AS_STRING(args[1]);
+  uint32_t startIndex = AS_NUM(args[2]);
+
+  uint32_t index = wrenStringFind(string, search, startIndex);
   RETURN_NUM(index == UINT32_MAX ? -1 : (int)index);
 }
 
@@ -1252,6 +1264,7 @@ void wrenInitializeCore(WrenVM* vm)
   PRIMITIVE(vm->stringClass, "contains(_)", string_contains);
   PRIMITIVE(vm->stringClass, "endsWith(_)", string_endsWith);
   PRIMITIVE(vm->stringClass, "indexOf(_)", string_indexOf);
+  PRIMITIVE(vm->stringClass, "indexOf(_,_)", string_indexOf_with_startIndex);
   PRIMITIVE(vm->stringClass, "iterate(_)", string_iterate);
   PRIMITIVE(vm->stringClass, "iterateByte_(_)", string_iterateByte);
   PRIMITIVE(vm->stringClass, "iteratorValue(_)", string_iteratorValue);

src/vm/wren_value.c

@@ -854,13 +854,16 @@ Value wrenStringCodePointAt(WrenVM* vm, ObjString* string, uint32_t index)
 }
 
 // Uses the Boyer-Moore-Horspool string matching algorithm.
-uint32_t wrenStringFind(ObjString* haystack, ObjString* needle)
+uint32_t wrenStringFind(ObjString* haystack, ObjString* needle, uint32_t startIndex)
 {
   // Corner case, an empty needle is always found.
   if (needle->length == 0) return 0;
 
   // If the needle is longer than the haystack it won't be found.
-  if (needle->length > haystack->length) return UINT32_MAX;
+  if (needle->length > (haystack->length - startIndex)) return UINT32_MAX;
+
+  // If the startIndex is too far it also won't be found.
+  if (startIndex >= haystack->length) return UINT32_MAX;
 
   // Pre-calculate the shift table. For each character (8-bit value), we
   // determine how far the search window can be advanced if that character is
@@ -890,18 +893,18 @@ uint32_t wrenStringFind(ObjString* haystack, ObjString* needle)
   // Slide the needle across the haystack, looking for the first match or
   // stopping if the needle goes off the end.
   char lastChar = needle->value[needleEnd];
-  uint32_t range = haystack->length - needle->length;
+  uint32_t range = (haystack->length - startIndex) - needle->length;
 
   for (uint32_t index = 0; index <= range; )
   {
     // Compare the last character in the haystack's window to the last character
     // in the needle. If it matches, see if the whole needle matches.
-    char c = haystack->value[index + needleEnd];
+    char c = haystack->value[startIndex + (index + needleEnd)];
     if (lastChar == c &&
-        memcmp(haystack->value + index, needle->value, needleEnd) == 0)
+        memcmp(haystack->value + startIndex + index, needle->value, needleEnd) == 0)
     {
       // Found a match.
-      return index;
+      return index + startIndex;
     }
 
     // Otherwise, slide the needle forward.

src/vm/wren_value.h

@@ -728,7 +728,7 @@ Value wrenStringCodePointAt(WrenVM* vm, ObjString* string, uint32_t index);
 // Search for the first occurence of [needle] within [haystack] and returns its
 // zero-based offset. Returns `UINT32_MAX` if [haystack] does not contain
 // [needle].
-uint32_t wrenStringFind(ObjString* haystack, ObjString* needle);
+uint32_t wrenStringFind(ObjString* haystack, ObjString* needle, uint32_t startIndex);
 
 // Creates a new open upvalue pointing to [value] on the stack.
 ObjUpvalue* wrenNewUpvalue(WrenVM* vm, Value* value);

test/core/string/index_of.wren

@@ -5,6 +5,12 @@ System.print("abcd".indexOf("abcd")) // expect: 0
 System.print("abcd".indexOf("abcde")) // expect: -1
 System.print("abab".indexOf("ab")) // expect: 0
 
+System.print("abcd".indexOf("cd", 0)) // expect: 2
+System.print("abcd".indexOf("cd", 1)) // expect: 2
+System.print("abcd".indexOf("cd", 2)) // expect: 2
+System.print("abcd".indexOf("cd", 3)) // expect: -1
+System.print("abcd".indexOf("cd", 10)) // expect: -1
+
 // More complex cases.
 System.print("abcdefabcdefg".indexOf("defg")) // expect: 9
 System.print("abcdabcdabcd".indexOf("dab")) // expect: 3