Move StringMap's string has function into StringExtras.h

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@84344 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/StringExtras.h

@@ -16,6 +16,7 @@
 
 #include "llvm/Support/DataTypes.h"
 #include "llvm/ADT/APFloat.h"
+#include "llvm/ADT/StringRef.h"
 #include <cctype>
 #include <cstdio>
 #include <string>
@@ -225,6 +226,19 @@ void UnescapeString(std::string &Str);
 /// doesn't satisfy std::isprint into an escape sequence.
 void EscapeString(std::string &Str);
 
+/// HashString - Hash funtion for strings.
+///
+/// This is the Bernstein hash function.
+//
+// FIXME: Investigate whether a modified bernstein hash function performs
+// better: http://eternallyconfuzzled.com/tuts/algorithms/jsw_tut_hashing.aspx
+//   X*33+c -> X*33^c
+static inline unsigned HashString(StringRef Str, unsigned Result = 0) {
+  for (unsigned i = 0, e = Str.size(); i != e; ++i)
+    Result = Result * 33 + Str[i];
+  return Result;
+}
+
 } // End llvm namespace
 
 #endif