From 61ee601df10d267bb15ca6f68931e7f255d1ab2f Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Mon, 1 Sep 2014 16:23:46 +1000
Subject: [PATCH] bitwise: use lookup table for byte reverse

When used across large buffers an iterative reverse scheme is lesss
efficient. The small cache cost for single reverses is good enough to
just eat here.
---
 bitwise.cpp      | 25 +++++++++++++++++++++++++
 test/bitwise.cpp | 21 +++++++++++++++++++--
 2 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/bitwise.cpp b/bitwise.cpp
index 7e2c8bce..9419b883 100644
--- a/bitwise.cpp
+++ b/bitwise.cpp
@@ -20,3 +20,28 @@
 #include "bitwise.hpp"
 
 
+//-----------------------------------------------------------------------------
+template <>
+uint8_t
+reverse (uint8_t value) {
+    static const uint8_t LOOKUP[256] = {
+        0x00, 0x80, 0x40, 0xC0, 0x20, 0xA0, 0x60, 0xE0, 0x10, 0x90, 0x50, 0xD0, 0x30, 0xB0, 0x70, 0xF0,
+        0x08, 0x88, 0x48, 0xC8, 0x28, 0xA8, 0x68, 0xE8, 0x18, 0x98, 0x58, 0xD8, 0x38, 0xB8, 0x78, 0xF8,
+        0x04, 0x84, 0x44, 0xC4, 0x24, 0xA4, 0x64, 0xE4, 0x14, 0x94, 0x54, 0xD4, 0x34, 0xB4, 0x74, 0xF4,
+        0x0C, 0x8C, 0x4C, 0xCC, 0x2C, 0xAC, 0x6C, 0xEC, 0x1C, 0x9C, 0x5C, 0xDC, 0x3C, 0xBC, 0x7C, 0xFC,
+        0x02, 0x82, 0x42, 0xC2, 0x22, 0xA2, 0x62, 0xE2, 0x12, 0x92, 0x52, 0xD2, 0x32, 0xB2, 0x72, 0xF2,
+        0x0A, 0x8A, 0x4A, 0xCA, 0x2A, 0xAA, 0x6A, 0xEA, 0x1A, 0x9A, 0x5A, 0xDA, 0x3A, 0xBA, 0x7A, 0xFA,
+        0x06, 0x86, 0x46, 0xC6, 0x26, 0xA6, 0x66, 0xE6, 0x16, 0x96, 0x56, 0xD6, 0x36, 0xB6, 0x76, 0xF6,
+        0x0E, 0x8E, 0x4E, 0xCE, 0x2E, 0xAE, 0x6E, 0xEE, 0x1E, 0x9E, 0x5E, 0xDE, 0x3E, 0xBE, 0x7E, 0xFE,
+        0x01, 0x81, 0x41, 0xC1, 0x21, 0xA1, 0x61, 0xE1, 0x11, 0x91, 0x51, 0xD1, 0x31, 0xB1, 0x71, 0xF1,
+        0x09, 0x89, 0x49, 0xC9, 0x29, 0xA9, 0x69, 0xE9, 0x19, 0x99, 0x59, 0xD9, 0x39, 0xB9, 0x79, 0xF9,
+        0x05, 0x85, 0x45, 0xC5, 0x25, 0xA5, 0x65, 0xE5, 0x15, 0x95, 0x55, 0xD5, 0x35, 0xB5, 0x75, 0xF5,
+        0x0D, 0x8D, 0x4D, 0xCD, 0x2D, 0xAD, 0x6D, 0xED, 0x1D, 0x9D, 0x5D, 0xDD, 0x3D, 0xBD, 0x7D, 0xFD,
+        0x03, 0x83, 0x43, 0xC3, 0x23, 0xA3, 0x63, 0xE3, 0x13, 0x93, 0x53, 0xD3, 0x33, 0xB3, 0x73, 0xF3,
+        0x0B, 0x8B, 0x4B, 0xCB, 0x2B, 0xAB, 0x6B, 0xEB, 0x1B, 0x9B, 0x5B, 0xDB, 0x3B, 0xBB, 0x7B, 0xFB,
+        0x07, 0x87, 0x47, 0xC7, 0x27, 0xA7, 0x67, 0xE7, 0x17, 0x97, 0x57, 0xD7, 0x37, 0xB7, 0x77, 0xF7,
+        0x0F, 0x8F, 0x4F, 0xCF, 0x2F, 0xAF, 0x6F, 0xEF, 0x1F, 0x9F, 0x5F, 0xDF, 0x3F, 0xBF, 0x7F, 0xFF
+    };
+
+    return LOOKUP[value];
+}
diff --git a/test/bitwise.cpp b/test/bitwise.cpp
index 5b9d7627..9478d1c2 100644
--- a/test/bitwise.cpp
+++ b/test/bitwise.cpp
@@ -1,7 +1,8 @@
 #include "../bitwise.hpp"
 
-int
-main (int, char**) {
+
+static void
+test_rotate (void) {
     CHECK_EQ (rotatel (uint8_t (0x0F), 0), 0x0F);
     CHECK_EQ (rotatel (uint8_t (0x0F), 4), 0xF0);
     CHECK_EQ (rotatel (uint8_t (0xF0), 4), 0x0F);
@@ -29,6 +30,22 @@ main (int, char**) {
 
     CHECK_EQ (rotatel (uint64_t (0x1234567890ABCDEF), 12), 0x4567890ABCDEF123);
     CHECK_EQ (rotater (uint64_t (0x1234567890ABCDEF), 12), 0xDEF1234567890ABC);
+}
+
+
+void
+test_reverse (void) {
+    for (unsigned i = 0; i < 256; ++i) {
+        auto first = reverse<uint8_t> (i);
+        auto last  = reverse<uint8_t> (first);
+        CHECK_EQ (last, i);
+    }
+}
+
+int
+main (int, char**) {
+    test_rotate ();
+    test_reverse ();
 
     return 0;
 }