Factor out advanceStringIndex

Summary:
The advanceStringIndex operation is used to skip over surrogate pairs.
Factor the implementation into a global function so it can be shared.

Reviewed By: avp

Differential Revision: D17413829

fbshipit-source-id: da339c0d4edc0b90145dfbe2e23475ca92324ac0

lib/VM/JSLib/JSLibInternal.h

@@ -315,6 +315,13 @@ CallResult<HermesValue> splitInternal(
     Handle<> limit,
     Handle<> separator);
 
+/// ES6.0 21.2.5.2.3
+/// If \p unicode is set and the character at \p index in \S is the start of a
+/// surrogate pair, \return index + 2. Otherwise \return index + 1.
+/// Note that this function does not allocate.
+uint64_t
+advanceStringIndex(const StringPrimitive *S, uint64_t index, bool unicode);
+
 /// Create and initialize the global Function constructor. Populate the methods
 /// of Function and Function.prototype.
 /// \return the global Function constructor.

lib/VM/JSLib/RegExp.cpp

@@ -44,13 +44,6 @@ static CallResult<HermesValue> regExpBuiltinExec(
     Handle<JSRegExp> R,
     Handle<StringPrimitive> S);
 
-/// ES6.0 21.2.5.2.3
-static uint64_t advanceStringIndex(
-    Runtime *runtime,
-    PseudoHandle<StringPrimitive> /* S */,
-    uint64_t index,
-    bool unicode);
-
 /// @}
 
 // Several RegExp accessors are defined to do particular things when passed the
@@ -567,11 +560,8 @@ static CallResult<HermesValue> regExpBuiltinExec(
 }
 
 /// ES6.0 21.2.5.2.3
-static uint64_t advanceStringIndex(
-    Runtime *runtime,
-    PseudoHandle<StringPrimitive> S,
-    uint64_t index,
-    bool unicode) {
+uint64_t
+advanceStringIndex(const StringPrimitive *S, uint64_t index, bool unicode) {
   if (unicode && index + 1 < S->getStringLength() &&
       isHighSurrogate(S->at(index)) && isLowSurrogate(S->at(index + 1))) {
     return index + 2;
@@ -1046,7 +1036,7 @@ regExpPrototypeSymbolMatch(void *, Runtime *runtime, NativeArgs args) {
       }
       // c. Let nextIndex be AdvanceStringIndex(S, thisIndex, fullUnicode).
       double nextIndex = advanceStringIndex(
-          runtime, S, thisIndex->getNumberAs<uint64_t>(), fullUnicode);
+          S.get(), thisIndex->getNumberAs<uint64_t>(), fullUnicode);
       // d. Let setStatus be Set(rx, "lastIndex", nextIndex, true).
       auto setStatus = setLastIndex(rx, runtime, nextIndex);
       // e. ReturnIfAbrupt(setStatus).
@@ -1391,7 +1381,7 @@ regExpPrototypeSymbolReplace(void *, Runtime *runtime, NativeArgs args) {
         }
         // c. Let nextIndex be AdvanceStringIndex(S, thisIndex, fullUnicode).
         nextIndex = HermesValue::encodeDoubleValue(advanceStringIndex(
-            runtime, S, thisIndex->getNumberAs<uint64_t>(), fullUnicode));
+            S.get(), thisIndex->getNumberAs<uint64_t>(), fullUnicode));
         // d. Let setStatus be Set(rx, "lastIndex", nextIndex, true).
         auto setStatus = setLastIndex(rx, runtime, *nextIndex);
         // e. ReturnIfAbrupt(setStatus).