Make base64 decode significantly faster.

Use a reverse lookup table instead of a call to string.find for
each input byte, much speedier.

diff --git a/simgear/misc/strutils.cxx b/simgear/misc/strutils.cxx
index 47f0b4ef82132ee7b5b82836e3727429960498b0..34d930745cae6c02bc2213a709825041b6431dee 100644 (file)
--- a/simgear/misc/strutils.cxx
+++ b/simgear/misc/strutils.cxx
@@ -344,7 +344,24 @@ static const std::string base64_chars =
 "abcdefghijklmnopqrstuvwxyz"
 "0123456789+/";
 
-
+static const unsigned char base64_decode_map[128] =
+{
+    127, 127, 127, 127, 127, 127, 127, 127, 127, 127,
+    127, 127, 127, 127, 127, 127, 127, 127, 127, 127,
+    127, 127, 127, 127, 127, 127, 127, 127, 127, 127,
+    127, 127, 127, 127, 127, 127, 127, 127, 127, 127,
+    127, 127, 127,  62, 127, 127, 127,  63,  52,  53,
+    54,  55,  56,  57,  58,  59,  60,  61, 127, 127,
+    127,  64, 127, 127, 127,   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, 127, 127, 127, 127, 127, 127,  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, 127, 127, 127, 127, 127
+};
+ 
+        
 static inline bool is_base64(unsigned char c) {
   return (isalnum(c) || (c == '+') || (c == '/'));
 }
@@ -375,7 +392,7 @@ std::string decodeBase64(const std::string& encoded_string)
     char_array_4[i++] = encoded_string[in_]; in_++;
     if (i ==4) {
       for (i = 0; i <4; i++)
-        char_array_4[i] = base64_chars.find(char_array_4[i]);
+        char_array_4[i] = base64_decode_map[char_array_4[i]];
       
       char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
       char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);
@@ -392,7 +409,7 @@ std::string decodeBase64(const std::string& encoded_string)
       char_array_4[j] = 0;
     
     for (j = 0; j <4; j++)
-      char_array_4[j] = base64_chars.find(char_array_4[j]);
+      char_array_4[j] = base64_decode_map[char_array_4[j]];
     
     char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
     char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);