[flang] [llvm] [flang] Implement SPLIT intrinsic subroutine with tests (PR #185584)

[Peter Klausler via llvm-commits]

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@@ -992,6 +992,56 @@ static RT_API_ATTRS void TokenizePositionsImpl(Descriptor &first,
   }
 }
 
+// SPLIT - scans for the next separator character in STRING.
+// When BACK is false (or absent), returns the position of the leftmost
+// character in SET whose position in STRING is greater than POS, or
+// LEN(STRING)+1 if no such character exists.
+// When BACK is true, returns the position of the rightmost character in
+// SET whose position in STRING is less than POS, or 0 if no such
+// character exists.
+template <typename CHAR>
+static RT_API_ATTRS std::size_t SplitImpl(const CHAR *string,
+    std::size_t stringLen, const CHAR *set, std::size_t setLen, std::size_t pos,
+    bool back) {
+  if (back) {
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klausler wrote:

Always use braced initialization in the parts of flang-new, like the runtime, that are in more modern C++.  See `flang/docs/C++17.md`.  It's especially important for code like this that's messing with mixed integer types.

You could use a signed stride of 1 or -1 and avoid having two implementations.

`SPLIT` is basically the same as the older `SCAN`.  Can you have one implementation for both?  Or can you write `SPLIT` in terms of `SCAN`?

https://github.com/llvm/llvm-project/pull/185584