Improve the speed of toLower

This significantly improves the speed of my fuzzer,
by something like 15%.

apps/openmw_test_suite/misc/test_stringops.cpp

@@ -93,8 +93,8 @@ namespace
 
     using RawValue = const char[4];
 
-    constexpr RawValue foo = "foo";
-    constexpr RawValue fooUpper = "FOO";
+    constexpr RawValue foo = "f0#";
+    constexpr RawValue fooUpper = "F0#";
     constexpr RawValue bar = "bar";
 
     template <typename T>

components/misc/stringops.hpp

@@ -8,6 +8,25 @@
 #include <iterator>
 #include <functional>
 
+/* Mapping table to go from uppercase to lowercase for plain ASCII.*/
+static constexpr unsigned char tolowermap[256] = {
+    0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21,
+    22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41,
+    42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61,
+    62, 63, 64, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110,
+    111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 91, 92, 93, 94, 95,
+    96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111,
+    112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127,
+    128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143,
+    144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159,
+    160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175,
+    176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191,
+    192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207,
+    208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223,
+    224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239,
+    240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255
+};
+
 namespace Misc
 {
 class StringUtils
@@ -38,9 +57,9 @@ public:
     /// Plain and simple locale-unaware toLower. Anything from A to Z is lower-cased, multibyte characters are unchanged.
     /// Don't use std::tolower(char, locale&) because that is abysmally slow.
     /// Don't use tolower(int) because that depends on global locale.
-    static char toLower(char c)
+    static constexpr char toLower(char c)
     {
-        return (c >= 'A' && c <= 'Z') ? c + 'a' - 'A' : c;
+        return tolowermap[static_cast<unsigned char>(c)];
     }
 
     static bool ciLess(const std::string &x, const std::string &y) {