From 12eb9bfd705b8c1dde0fd592055258e461cd5e5f Mon Sep 17 00:00:00 2001
From: Zoltan Szabadka <szabadka@google.com>
Date: Thu, 7 May 2015 17:40:00 +0200
Subject: [PATCH] Align distance code meaning in the brotli encoder.

Two different definitions (offset by 1) were used in
command.h and hash.h. Now they have been made the same,
also consistent with the spec (e.g. 0 means use previous dist, etc...)
---
 enc/backward_references.cc | 15 ++++++++-------
 enc/command.h              |  6 +++---
 enc/prefix.h               |  1 -
 3 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/enc/backward_references.cc b/enc/backward_references.cc
index 97d74af..e762f9b 100644
--- a/enc/backward_references.cc
+++ b/enc/backward_references.cc
@@ -172,16 +172,17 @@ void CreateBackwardReferences(size_t num_bytes,
       apply_random_heuristics =
           i + 2 * best_len + random_heuristics_window_size;
       max_distance = std::min(i + i_diff, max_backward_limit);
-      int distance_code = best_dist + 16;
+      // The first 16 codes are special shortcodes, and the minimum offset is 1.
+      int distance_code = best_dist + 15;
       if (best_dist <= max_distance) {
         if (best_dist == dist_cache[0]) {
-          distance_code = 1;
+          distance_code = 0;
         } else if (best_dist == dist_cache[1]) {
-          distance_code = 2;
+          distance_code = 1;
         } else if (best_dist == dist_cache[2]) {
-          distance_code = 3;
+          distance_code = 2;
         } else if (best_dist == dist_cache[3]) {
-          distance_code = 4;
+          distance_code = 3;
         } else if (quality > 3 && best_dist >= 6) {
           for (int k = 4; k < kNumDistanceShortCodes; ++k) {
             int idx = kDistanceCacheIndex[k];
@@ -191,12 +192,12 @@ void CreateBackwardReferences(size_t num_bytes,
                                              11, 11, 11, 11,
                                              12, 12, 12, 12 };
             if (best_dist == candidate && best_dist >= kLimits[k]) {
-              distance_code = k + 1;
+              distance_code = k;
               break;
             }
           }
         }
-        if (distance_code > 1) {
+        if (distance_code > 0) {
           dist_cache[3] = dist_cache[2];
           dist_cache[2] = dist_cache[1];
           dist_cache[1] = dist_cache[0];
diff --git a/enc/command.h b/enc/command.h
index 5600455..05eb9d0 100644
--- a/enc/command.h
+++ b/enc/command.h
@@ -24,7 +24,6 @@ namespace brotli {
 
 static inline void GetDistCode(int distance_code,
                                uint16_t* code, uint32_t* extra) {
-  distance_code -= 1;
   if (distance_code < 16) {
     *code = distance_code;
     *extra = 0;
@@ -106,6 +105,7 @@ static inline void GetLengthCode(int insertlen, int copylen, int distancecode,
 struct Command {
   Command() {}
 
+  // distance_code is e.g. 0 for same-as-last short code, or 16 for offset 1.
   Command(int insertlen, int copylen, int copylen_code, int distance_code)
       : insert_len_(insertlen), copy_len_(copylen) {
     GetDistCode(distance_code, &dist_prefix_, &dist_extra_);
@@ -120,12 +120,12 @@ struct Command {
 
   int DistanceCode() const {
     if (dist_prefix_ < 16) {
-      return dist_prefix_ + 1;
+      return dist_prefix_;
     }
     int nbits = dist_extra_ >> 24;
     int extra = dist_extra_ & 0xffffff;
     int prefix = dist_prefix_ - 12 - 2 * nbits;
-    return (prefix << nbits) + extra + 13;
+    return (prefix << nbits) + extra + 12;
   }
 
   int DistanceContext() const {
diff --git a/enc/prefix.h b/enc/prefix.h
index 693a26f..552788f 100644
--- a/enc/prefix.h
+++ b/enc/prefix.h
@@ -62,7 +62,6 @@ inline void PrefixEncodeCopyDistance(int distance_code,
                                      int postfix_bits,
                                      uint16_t* code,
                                      uint32_t* extra_bits) {
-  distance_code -= 1;
   if (distance_code < kNumDistanceShortCodes + num_direct_codes) {
     *code = distance_code;
     *extra_bits = 0;