Add 64-bit version of BitBuffer::ReadBits

Bug: webrtc:11933
Change-Id: Ie935192d2c81d7c24b83561711d59a7eff53db04
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/215966
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33997}

rtc_base/bit_buffer.cc

@@ -141,10 +141,48 @@ bool BitBuffer::PeekBits(uint32_t* val, size_t bit_count) {
   return true;
 }
 
+bool BitBuffer::PeekBits(uint64_t* val, size_t bit_count) {
+  // TODO(nisse): Could allow bit_count == 0 and always return success. But
+  // current code reads one byte beyond end of buffer in the case that
+  // RemainingBitCount() == 0 and bit_count == 0.
+  RTC_DCHECK(bit_count > 0);
+  if (!val || bit_count > RemainingBitCount() || bit_count > 64) {
+    return false;
+  }
+  const uint8_t* bytes = bytes_ + byte_offset_;
+  size_t remaining_bits_in_current_byte = 8 - bit_offset_;
+  uint64_t bits = LowestBits(*bytes++, remaining_bits_in_current_byte);
+  // If we're reading fewer bits than what's left in the current byte, just
+  // return the portion of this byte that we need.
+  if (bit_count < remaining_bits_in_current_byte) {
+    *val = HighestBits(bits, bit_offset_ + bit_count);
+    return true;
+  }
+  // Otherwise, subtract what we've read from the bit count and read as many
+  // full bytes as we can into bits.
+  bit_count -= remaining_bits_in_current_byte;
+  while (bit_count >= 8) {
+    bits = (bits << 8) | *bytes++;
+    bit_count -= 8;
+  }
+  // Whatever we have left is smaller than a byte, so grab just the bits we need
+  // and shift them into the lowest bits.
+  if (bit_count > 0) {
+    bits <<= bit_count;
+    bits |= HighestBits(*bytes, bit_count);
+  }
+  *val = bits;
+  return true;
+}
+
 bool BitBuffer::ReadBits(uint32_t* val, size_t bit_count) {
   return PeekBits(val, bit_count) && ConsumeBits(bit_count);
 }
 
+bool BitBuffer::ReadBits(uint64_t* val, size_t bit_count) {
+  return PeekBits(val, bit_count) && ConsumeBits(bit_count);
+}
+
 bool BitBuffer::ConsumeBytes(size_t byte_count) {
   return ConsumeBits(byte_count * 8);
 }

rtc_base/bit_buffer.h

@@ -45,11 +45,13 @@ class BitBuffer {
   // Reads bit-sized values from the buffer. Returns false if there isn't enough
   // data left for the specified bit count.
   bool ReadBits(uint32_t* val, size_t bit_count);
+  bool ReadBits(uint64_t* val, size_t bit_count);
 
   // Peeks bit-sized values from the buffer. Returns false if there isn't enough
   // data left for the specified number of bits. Doesn't move the current
   // offset.
   bool PeekBits(uint32_t* val, size_t bit_count);
+  bool PeekBits(uint64_t* val, size_t bit_count);
 
   // Reads value in range [0, num_values - 1].
   // This encoding is similar to ReadBits(val, Ceil(Log2(num_values)),

rtc_base/bit_buffer_unittest.cc

@@ -142,6 +142,38 @@ TEST(BitBufferTest, ReadBits) {
   EXPECT_FALSE(buffer.ReadBits(&val, 1));
 }
 
+TEST(BitBufferTest, ReadBits64) {
+  const uint8_t bytes[] = {0x4D, 0x32, 0xAB, 0x54, 0x00, 0xFF, 0xFE, 0x01,
+                           0xAB, 0xCD, 0xEF, 0x01, 0x23, 0x45, 0x67, 0x89};
+  BitBuffer buffer(bytes, 16);
+  uint64_t val;
+
+  // Peek and read first 33 bits.
+  EXPECT_TRUE(buffer.PeekBits(&val, 33));
+  EXPECT_EQ(0x4D32AB5400FFFE01ull >> (64 - 33), val);
+  val = 0;
+  EXPECT_TRUE(buffer.ReadBits(&val, 33));
+  EXPECT_EQ(0x4D32AB5400FFFE01ull >> (64 - 33), val);
+
+  // Peek and read next 31 bits.
+  constexpr uint64_t kMask31Bits = (1ull << 32) - 1;
+  EXPECT_TRUE(buffer.PeekBits(&val, 31));
+  EXPECT_EQ(0x4D32AB5400FFFE01ull & kMask31Bits, val);
+  val = 0;
+  EXPECT_TRUE(buffer.ReadBits(&val, 31));
+  EXPECT_EQ(0x4D32AB5400FFFE01ull & kMask31Bits, val);
+
+  // Peek and read remaining 64 bits.
+  EXPECT_TRUE(buffer.PeekBits(&val, 64));
+  EXPECT_EQ(0xABCDEF0123456789ull, val);
+  val = 0;
+  EXPECT_TRUE(buffer.ReadBits(&val, 64));
+  EXPECT_EQ(0xABCDEF0123456789ull, val);
+
+  // Nothing more to read.
+  EXPECT_FALSE(buffer.ReadBits(&val, 1));
+}
+
 TEST(BitBufferDeathTest, SetOffsetValues) {
   uint8_t bytes[4] = {0};
   BitBufferWriter buffer(bytes, 4);