Editorial: refactor IsStringPrefix and String.prototype.split to …

…use `StringIndexOf`; remove `SplitMatch` (tc39#2144)

Co-authored-by: Alexey Shvayka <[email protected]>
Co-authored-by: Jordan Harband <[email protected]>

spec.html

@@ -5877,10 +5877,11 @@ <h1>
         <dd>It determines if _p_ is a prefix of _q_.</dd>
       </dl>
       <emu-alg>
-        1. If _q_ can be the string-concatenation of _p_ and some other String _r_, return *true*. Otherwise, return *false*.
+        1. If ! StringIndexOf(_q_, _p_, 0) is 0, return *true*.
+        1. Else, return *false*.
       </emu-alg>
       <emu-note>
-        <p>Any String is a prefix of itself, because _r_ may be the empty String.</p>
+        <p>Any String is a prefix of itself.</p>
       </emu-note>
     </emu-clause>
 
@@ -33885,37 +33886,30 @@ <h1>String.prototype.split ( _separator_, _limit_ )</h1>
             1. If _splitter_ is not *undefined*, then
               1. Return ? Call(_splitter_, _separator_, &laquo; _O_, _limit_ &raquo;).
           1. Let _S_ be ? ToString(_O_).
-          1. Let _A_ be ! ArrayCreate(0).
-          1. Let _lengthA_ be 0.
           1. If _limit_ is *undefined*, let _lim_ be 2<sup>32</sup> - 1; else let _lim_ be ℝ(? ToUint32(_limit_)).
           1. Let _R_ be ? ToString(_separator_).
-          1. If _lim_ = 0, return _A_.
+          1. If _lim_ = 0, then
+            1. Return ! CreateArrayFromList(&laquo; &raquo;).
           1. If _separator_ is *undefined*, then
-            1. Perform ! CreateDataPropertyOrThrow(_A_, *"0"*, _S_).
-            1. Return _A_.
-          1. Let _s_ be the length of _S_.
-          1. If _s_ = 0, then
-            1. If _R_ is not the empty String, then
-              1. Perform ! CreateDataPropertyOrThrow(_A_, *"0"*, _S_).
-            1. Return _A_.
-          1. Let _p_ be 0.
-          1. Let _q_ be _p_.
-          1. Repeat, while _q_ &ne; _s_,
-            1. Let _e_ be SplitMatch(_S_, _q_, _R_).
-            1. If _e_ is ~not-matched~, set _q_ to _q_ + 1.
-            1. Else,
-              1. Assert: _e_ is a non-negative integer &le; _s_.
-              1. If _e_ = _p_, set _q_ to _q_ + 1.
-              1. Else,
-                1. Let _T_ be the substring of _S_ from _p_ to _q_.
-                1. Perform ! CreateDataPropertyOrThrow(_A_, ! ToString(𝔽(_lengthA_)), _T_).
-                1. Set _lengthA_ to _lengthA_ + 1.
-                1. If _lengthA_ = _lim_, return _A_.
-                1. Set _p_ to _e_.
-                1. Set _q_ to _p_.
-          1. Let _T_ be the substring of _S_ from _p_ to _s_.
-          1. Perform ! CreateDataPropertyOrThrow(_A_, ! ToString(𝔽(_lengthA_)), _T_).
-          1. Return _A_.
+            1. Return ! CreateArrayFromList(&laquo; _S_ &raquo;).
+          1. Let _separatorLength_ be the length of _R_.
+          1. If _separatorLength_ is 0, then
+            1. Let _head_ be the substring of _S_ from 0 to _lim_.
+            1. Let _codeUnits_ be a List consisting of the sequence of code units that are the elements of _head_.
+            1. Return ! CreateArrayFromList(_codeUnits_).
+          1. If _S_ is the empty String, return ! CreateArrayFromList(&laquo; _S_ &raquo;).
+          1. Let _substrings_ be a new empty List.
+          1. Let _i_ be 0.
+          1. Let _j_ be ! StringIndexOf(_S_, _R_, 0).
+          1. Repeat, while _j_ is not -1,
+            1. Let _T_ be the substring of _S_ from _i_ to _j_.
+            1. Append _T_ as the last element of _substrings_.
+            1. If the number of elements of _substrings_ is _lim_, return ! CreateArrayFromList(_substrings_).
+            1. Set _i_ to _j_ + _separatorLength_.
+            1. Set _j_ to ! StringIndexOf(_S_, _R_, _i_).
+          1. Let _T_ be the substring of _S_ from _i_.
+          1. Append _T_ to _substrings_.
+          1. Return ! CreateArrayFromList(_substrings_).
         </emu-alg>
         <emu-note>
           <p>The value of _separator_ may be an empty String. In this case, _separator_ does not match the empty <emu-not-ref>substring</emu-not-ref> at the beginning or end of the input String, nor does it match the empty <emu-not-ref>substring</emu-not-ref> at the end of the previous separator match. If _separator_ is the empty String, the String is split up into individual code unit elements; the length of the result array equals the length of the String, and each <emu-not-ref>substring</emu-not-ref> contains one code unit.</p>
@@ -33925,27 +33919,6 @@ <h1>String.prototype.split ( _separator_, _limit_ )</h1>
         <emu-note>
           <p>The `split` function is intentionally generic; it does not require that its *this* value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.</p>
         </emu-note>
-
-        <emu-clause id="sec-splitmatch" type="abstract operation">
-          <h1>
-            SplitMatch (
-              _S_: a String,
-              _q_: a non-negative integer,
-              _R_: a String,
-            )
-          </h1>
-          <dl class="header">
-            <dt>description</dt>
-            <dd>It returns either ~not-matched~ or the end index of a match.</dd>
-          </dl>
-          <emu-alg>
-            1. Let _r_ be the number of code units in _R_.
-            1. Let _s_ be the number of code units in _S_.
-            1. If _q_ + _r_ &gt; _s_, return ~not-matched~.
-            1. If there exists an integer _i_ between 0 (inclusive) and _r_ (exclusive) such that the code unit at index _q_ + _i_ within _S_ is different from the code unit at index _i_ within _R_, return ~not-matched~.
-            1. Return _q_ + _r_.
-          </emu-alg>
-        </emu-clause>
       </emu-clause>
 
       <emu-clause id="sec-string.prototype.startswith">