From 781c01f42188093af1bd887805fd9b48b2e5dd87 Mon Sep 17 00:00:00 2001
From: Vikas Arora <vikasa@google.com>
Date: Fri, 27 Apr 2012 05:51:13 +0000
Subject: [PATCH] Simple Huffman code changes.

No empty trees are codified with the simple Huffman code. The simple Huffman
code is simplified to be either a 1-bit code or 8-bit code for symbols.

Change-Id: I3e2813027b5a643862729339303d80197c497aff
---
 src/dec/vp8l.c | 30 +++++++++++-------------------
 src/enc/vp8l.c | 25 ++++++++++++-------------
 2 files changed, 23 insertions(+), 32 deletions(-)

diff --git a/src/dec/vp8l.c b/src/dec/vp8l.c
index 5be21965..d73ffe27 100644
--- a/src/dec/vp8l.c
+++ b/src/dec/vp8l.c
@@ -238,25 +238,17 @@ static int ReadHuffmanCode(int alphabet_size, VP8LDecoder* const dec,
     int symbols[2];
     int codes[2];
     int code_lengths[2];
-    const int nbits = VP8LReadBits(br, 3);
-    const int num_symbols = 1 + ((nbits == 0) ? 0 : VP8LReadBits(br, 1));
-
-    if (nbits == 0) {
-      symbols[0] = 0;
-      codes[0] = 0;
-      code_lengths[0] = 0;
-    } else {
-      const int num_bits = (nbits - 1) * 2 + 4;
-      int i;
-      for (i = 0; i < num_symbols; ++i) {
-        symbols[i] = VP8LReadBits(br, num_bits);
-        if (symbols[i] >= alphabet_size) {
-          dec->status_ = VP8_STATUS_BITSTREAM_ERROR;
-          return 0;
-        }
-        codes[i] = i;
-        code_lengths[i] = num_symbols - 1;
-      }
+    const int num_symbols = VP8LReadBits(br, 1) + 1;
+    const int first_symbol_len_code = VP8LReadBits(br, 1);
+    // The first code is either 1 bit or 8 bit code.
+    symbols[0] = VP8LReadBits(br, (first_symbol_len_code == 0) ? 1 : 8);
+    codes[0] = 0;
+    code_lengths[0] = num_symbols - 1;
+    // The second code (if present), is always 8 bit long.
+    if (num_symbols == 2) {
+      symbols[1] = VP8LReadBits(br, 8);
+      codes[1] = 1;
+      code_lengths[1] = num_symbols - 1;
     }
     ok = HuffmanTreeBuildExplicit(tree, code_lengths, codes,
                                   symbols, num_symbols);
diff --git a/src/enc/vp8l.c b/src/enc/vp8l.c
index 50b20f36..c666d7ea 100644
--- a/src/enc/vp8l.c
+++ b/src/enc/vp8l.c
@@ -531,20 +531,19 @@ static int StoreHuffmanCode(VP8LBitWriter* const bw,
       ++count;
     }
   }
-  if (count <= 2) {
-    int num_bits = 4;
-    // 0, 1 or 2 symbols to encode.
-    VP8LWriteBits(bw, 1, 1);
-    if (count == 0) {
-      VP8LWriteBits(bw, 3, 0);
-      ok = 1;
-      goto End;
-    }
-    while (symbols[count - 1] >= (1 << num_bits)) num_bits += 2;
-    VP8LWriteBits(bw, 3, (num_bits - 4) / 2 + 1);
+  if (count == 0) count = 1;
+  if (count <= 2 && symbols[0] < 256 && symbols[1] < 256) {
+    VP8LWriteBits(bw, 1, 1);  // Small tree marker to encode 1 or 2 symbols.
     VP8LWriteBits(bw, 1, count - 1);
-    for (i = 0; i < count; ++i) {
-      VP8LWriteBits(bw, num_bits, symbols[i]);
+    if (symbols[0] <= 1) {
+      VP8LWriteBits(bw, 1, 0);  // Code bit for small (1 bit) symbol value.
+      VP8LWriteBits(bw, 1, symbols[0]);
+    } else {
+      VP8LWriteBits(bw, 1, 1);
+      VP8LWriteBits(bw, 8, symbols[0]);
+    }
+    if (count == 2) {
+      VP8LWriteBits(bw, 8, symbols[1]);
     }
     ok = 1;
     goto End;