process rol: precomputing single rotations in static code

Author: arekbulski

kaitai/kaitaistream.cpp

@@ -460,6 +460,19 @@ std::string kaitai::kstream::process_xor_many(std::string data, std::string key)
     return result;
 }
 
+// NOTE: perhaps it should be heap allocated?
+uint8_t precomputedSingleRotations[8][256];
+
+// NOTE: code based on StackOverflow answer at https://stackoverflow.com/a/34321324/2375119
+computeSingleRotations {
+    for (int amount = 1; amount < 8; amount++) {
+        int anti_amount = 8 - amount;
+        for (uint8_t i = 0; i < 256; i++) {
+            precomputedSingleRotations[amount][i] = (uint8_t)((i << amount) | (i >> anti_amount));
+        }
+    }
+}
+
 std::string kaitai::kstream::process_rotate_left(std::string data, int amount, int groupSize = 1) {
     if (groupSize < 1)
         throw std::runtime_error("process_rotate_left: groupSize must be at least 1");
@@ -473,11 +486,8 @@ std::string kaitai::kstream::process_rotate_left(std::string data, int amount, i
     std::string result(len, ' ');
 
     if (groupSize == 1) {
-        int anti_amount = 8 - amount;
-        uint8_t translate[256];
-        for (uint8_t i = 0; i < 256; i++) {
-            translate[i] = (uint8_t)((i << amount) | (i >> anti_amount));
-        }
+        // NOTE: perhaps its `amount * 256` in the index?
+        uint8_t *translate = &precomputedSingleRotations[amount];
 
         for (size_t i = 0; i < len; i++) {
             result[i] = translate[data[i]];