More efficient BitReader::SkipBits for large numbers of bits.

media/base/bit_reader_unittest.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "media/base/bit_reader.h"
 
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace media {
 
+static void SetBit(uint8* buf, size_t size, size_t bit_pos) {
+  size_t byte_pos = bit_pos / 8;
+  bit_pos -= byte_pos * 8;
+  DCHECK_LT(byte_pos, size);
+  buf[byte_pos] |= (1 << (7 - bit_pos));
+}
+
 TEST(BitReaderTest, NormalOperationTest) {
   uint8 value8;
   uint64 value64;
   // 0101 0101 1001 1001 repeats 4 times
   uint8 buffer[] = {0x55, 0x99, 0x55, 0x99, 0x55, 0x99, 0x55, 0x99};
   BitReader reader1(buffer, 6);  // Initialize with 6 bytes only
 
   EXPECT_TRUE(reader1.ReadBits(1, &value8));
   EXPECT_EQ(value8, 0);
   EXPECT_TRUE(reader1.ReadBits(8, &value8));
@@ skipping 36 matching lines @@
   EXPECT_TRUE(reader1.ReadBits(8, &value8));
   EXPECT_EQ(value8, 3);
   EXPECT_TRUE(reader1.SkipBits(76));
   EXPECT_TRUE(reader1.ReadBits(4, &value8));
   EXPECT_EQ(value8, 13);
   EXPECT_FALSE(reader1.SkipBits(100));
   EXPECT_TRUE(reader1.SkipBits(0));
   EXPECT_FALSE(reader1.SkipBits(1));
 }
 
+TEST(BitReaderTest, VariableSkipBitsTest) {
+  uint8 buffer[256] = {0};
+
+  // The test alternates between ReadBits and SkipBits.
+  // The first number is the number of bits to read, the second one is the
+  // number of bits to skip. The number of bits to read was arbitrarily chosen
+  // while the number of bits to skip was chosen so as to cover from small skips
+  // to large skips.
+  const size_t pattern_read_skip[][2] = {
+    {  5,  17 },
+    {  4,  34 },
+    {  0,  44 },
+    {  3,   4 },   // Note: aligned read.
+    {  7,   7 },   // Note: both read&skip cross byte boundary.
+    { 17,  68 },
+    {  7, 102 },
+    {  9, 204 },
+    {  3, 408 } };
+
+  // Set bits to one only for the first and last bit of each read
+  // in the pattern.
+  size_t pos = 0;
+  for (size_t k = 0; k < arraysize(pattern_read_skip); ++k) {
+    const size_t read_bit_count = pattern_read_skip[k][0];
+    if (read_bit_count > 0) {
+      SetBit(buffer, sizeof(buffer), pos);
+      SetBit(buffer, sizeof(buffer), pos + read_bit_count - 1);
+      pos += read_bit_count;
+    }
+    pos += pattern_read_skip[k][1];
+  }
+
+  // Run the test.
+  BitReader bit_reader(buffer, sizeof(buffer));
+  EXPECT_EQ(bit_reader.bits_available(), static_cast<int>(sizeof(buffer) * 8));
+  for (size_t k = 0; k < arraysize(pattern_read_skip); ++k) {
+    const size_t read_bit_count = pattern_read_skip[k][0];
+    if (read_bit_count > 0) {
+      int value;
+      EXPECT_TRUE(bit_reader.ReadBits(read_bit_count, &value));
+      EXPECT_EQ(value, 1 | (1 << (read_bit_count - 1)));
+    }
+    EXPECT_TRUE(bit_reader.SkipBits(pattern_read_skip[k][1]));
+  }
+}
+
 TEST(BitReaderTest, BitsReadTest) {
   int value;
   bool flag;
   uint8 buffer[] = { 0x0a, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 };
   BitReader reader1(buffer, sizeof(buffer));
   EXPECT_EQ(reader1.bits_available(), 120);
 
   EXPECT_TRUE(reader1.SkipBits(2));
   EXPECT_EQ(reader1.bits_read(), 2);
   EXPECT_EQ(reader1.bits_available(), 118);
   EXPECT_TRUE(reader1.ReadBits(3, &value));
   EXPECT_EQ(reader1.bits_read(), 5);
   EXPECT_EQ(reader1.bits_available(), 115);
   EXPECT_TRUE(reader1.ReadFlag(&flag));
   EXPECT_EQ(reader1.bits_read(), 6);
   EXPECT_EQ(reader1.bits_available(), 114);
   EXPECT_TRUE(reader1.SkipBits(76));
   EXPECT_EQ(reader1.bits_read(), 82);
   EXPECT_EQ(reader1.bits_available(), 38);
 }
 
 }  // namespace media