Added a BitBufferWriter subclass that contains methods for writing bit and byte-sized data, along with exponential golomb encoded data.
This pattern (read-only base, writable subclass) was picked to maintain a *Buffer option that doesn't copy the source bits when parsing. ByteBuffer and Buffer both copy. I'm open to discussion on what the type relationship would be, though :)
Tests have been added to ensure the symmetric nature of read/write operations.
BUG=
R=bcornell@google.com, pthatcher@webrtc.org
Review URL: https://webrtc-codereview.appspot.com/45259005
Cr-Commit-Position: refs/heads/master@{#9107}
diff --git a/webrtc/base/bitbuffer.cc b/webrtc/base/bitbuffer.cc
index f41962a..7530838 100644
--- a/webrtc/base/bitbuffer.cc
+++ b/webrtc/base/bitbuffer.cc
@@ -10,6 +10,7 @@
#include "webrtc/base/bitbuffer.h"
+#include <algorithm>
#include <limits>
#include "webrtc/base/checks.h"
@@ -19,8 +20,7 @@
// Returns the lowest (right-most) |bit_count| bits in |byte|.
uint8 LowestBits(uint8 byte, size_t bit_count) {
DCHECK_LE(bit_count, 8u);
- uint8 mask_shift = 8 - static_cast<uint8>(bit_count);
- return byte & (0xFF >> mask_shift);
+ return byte & ((1 << bit_count) - 1);
}
// Returns the highest (left-most) |bit_count| bits in |byte|, shifted to the
@@ -32,6 +32,41 @@
return (byte & mask) >> shift;
}
+// Returns the highest byte of |val| in a uint8.
+uint8 HighestByte(uint64 val) {
+ return static_cast<uint8>(val >> 56);
+}
+
+// Returns the result of writing partial data from |source|, of
+// |source_bit_count| size in the highest bits, to |target| at
+// |target_bit_offset| from the highest bit.
+uint8 WritePartialByte(uint8 source, size_t source_bit_count,
+ uint8 target, size_t target_bit_offset) {
+ DCHECK(target_bit_offset < 8);
+ DCHECK(source_bit_count < 9);
+ DCHECK(source_bit_count <= (8 - target_bit_offset));
+ // Generate a mask for just the bits we're going to overwrite, so:
+ uint8 mask =
+ // The number of bits we want, in the most significant bits...
+ static_cast<uint8>(0xFF << (8 - source_bit_count))
+ // ...shifted over to the target offset from the most signficant bit.
+ >> target_bit_offset;
+
+ // We want the target, with the bits we'll overwrite masked off, or'ed with
+ // the bits from the source we want.
+ return (target & ~mask) | (source >> target_bit_offset);
+}
+
+// Counts the number of bits used in the binary representation of val.
+size_t CountBits(uint64 val) {
+ size_t bit_count = 0;
+ while (val != 0) {
+ bit_count++;
+ val >>= 1;
+ }
+ return bit_count;
+}
+
} // namespace
namespace rtc {
@@ -143,12 +178,104 @@
// read the value.
size_t value_bit_count = zero_bit_count + 1;
if (value_bit_count > 32 || !ReadBits(val, value_bit_count)) {
- byte_offset_ = original_byte_offset;
- bit_offset_ = original_bit_offset;
+ CHECK(Seek(original_byte_offset, original_bit_offset));
return false;
}
*val -= 1;
return true;
}
+void BitBuffer::GetCurrentOffset(
+ size_t* out_byte_offset, size_t* out_bit_offset) {
+ CHECK(out_byte_offset != NULL);
+ CHECK(out_bit_offset != NULL);
+ *out_byte_offset = byte_offset_;
+ *out_bit_offset = bit_offset_;
+}
+
+bool BitBuffer::Seek(size_t byte_offset, size_t bit_offset) {
+ if (byte_offset > byte_count_ || bit_offset > 7 ||
+ (byte_offset == byte_count_ && bit_offset > 0)) {
+ return false;
+ }
+ byte_offset_ = byte_offset;
+ bit_offset_ = bit_offset;
+ return true;
+}
+
+BitBufferWriter::BitBufferWriter(uint8* bytes, size_t byte_count)
+ : BitBuffer(bytes, byte_count), writable_bytes_(bytes) {
+}
+
+bool BitBufferWriter::WriteUInt8(uint8 val) {
+ return WriteBits(val, sizeof(uint8) * 8);
+}
+
+bool BitBufferWriter::WriteUInt16(uint16 val) {
+ return WriteBits(val, sizeof(uint16) * 8);
+}
+
+bool BitBufferWriter::WriteUInt32(uint32 val) {
+ return WriteBits(val, sizeof(uint32) * 8);
+}
+
+bool BitBufferWriter::WriteBits(uint64 val, size_t bit_count) {
+ if (bit_count > RemainingBitCount()) {
+ return false;
+ }
+ size_t total_bits = bit_count;
+
+ // For simplicity, push the bits we want to read from val to the highest bits.
+ val <<= (sizeof(uint64) * 8 - bit_count);
+
+ uint8* bytes = writable_bytes_ + byte_offset_;
+
+ // The first byte is relatively special; the bit offset to write to may put us
+ // in the middle of the byte, and the total bit count to write may require we
+ // save the bits at the end of the byte.
+ size_t remaining_bits_in_current_byte = 8 - bit_offset_;
+ size_t bits_in_first_byte =
+ std::min(bit_count, remaining_bits_in_current_byte);
+ *bytes = WritePartialByte(
+ HighestByte(val), bits_in_first_byte, *bytes, bit_offset_);
+ if (bit_count <= remaining_bits_in_current_byte) {
+ // Nothing left to write, so quit early.
+ return ConsumeBits(total_bits);
+ }
+
+ // Subtract what we've written from the bit count, shift it off the value, and
+ // write the remaining full bytes.
+ val <<= bits_in_first_byte;
+ bytes++;
+ bit_count -= bits_in_first_byte;
+ while (bit_count >= 8) {
+ *bytes++ = HighestByte(val);
+ val <<= 8;
+ bit_count -= 8;
+ }
+
+ // Last byte may also be partial, so write the remaining bits from the top of
+ // val.
+ if (bit_count > 0) {
+ *bytes = WritePartialByte(HighestByte(val), bit_count, *bytes, 0);
+ }
+
+ // All done! Consume the bits we've written.
+ return ConsumeBits(total_bits);
+}
+
+bool BitBufferWriter::WriteExponentialGolomb(uint32 val) {
+ // We don't support reading UINT32_MAX, because it doesn't fit in a uint32
+ // when encoded, so don't support writing it either.
+ if (val == std::numeric_limits<uint32>::max()) {
+ return false;
+ }
+ uint64 val_to_encode = static_cast<uint64>(val) + 1;
+
+ // We need to write CountBits(val+1) 0s and then val+1. Since val (as a
+ // uint64) has leading zeros, we can just write the total golomb encoded size
+ // worth of bits, knowing the value will appear last.
+ return WriteBits(val_to_encode, CountBits(val_to_encode) * 2 - 1);
+}
+
} // namespace rtc
diff --git a/webrtc/base/bitbuffer.h b/webrtc/base/bitbuffer.h
index 356a817..5aa1f13 100644
--- a/webrtc/base/bitbuffer.h
+++ b/webrtc/base/bitbuffer.h
@@ -16,14 +16,20 @@
namespace rtc {
// A class, similar to ByteBuffer, that can parse bit-sized data out of a set of
-// bytes. Has a similar API to the read-only parts of ByteBuffer, plus methods
-// for reading bit-sized data and processing exponential golomb encoded data.
+// bytes. Has a similar API to ByteBuffer, plus methods for reading bit-sized
+// and exponential golomb encoded data. For a writable version, use
+// BitBufferWriter. Unlike ByteBuffer, this class doesn't make a copy of the
+// source bytes, so it can be used on read-only data.
// Sizes/counts specify bits/bytes, for clarity.
// Byte order is assumed big-endian/network.
class BitBuffer {
public:
BitBuffer(const uint8* bytes, size_t byte_count);
+ // Gets the current offset, in bytes/bits, from the start of the buffer. The
+ // bit offset is the offset into the current byte, in the range [0,7].
+ void GetCurrentOffset(size_t* out_byte_offset, size_t* out_bit_offset);
+
// The remaining bits in the byte buffer.
uint64 RemainingBitCount() const;
@@ -34,14 +40,15 @@
bool ReadUInt32(uint32* val);
// Reads bit-sized values from the buffer. Returns false if there isn't enough
- // data left for the specified type.
+ // data left for the specified bit count..
bool ReadBits(uint32* val, size_t bit_count);
// Peeks bit-sized values from the buffer. Returns false if there isn't enough
- // data left for the specified type. Doesn't move the current read offset.
+ // data left for the specified number of bits. Doesn't move the current
+ // offset.
bool PeekBits(uint32* val, size_t bit_count);
- // Reads the exponential golomb encoded value at the current bit offset.
+ // Reads the exponential golomb encoded value at the current offset.
// Exponential golomb values are encoded as:
// 1) x = source val + 1
// 2) In binary, write [countbits(x) - 1] 0s, then x
@@ -58,7 +65,11 @@
// there aren't enough bits left in the buffer.
bool ConsumeBits(size_t bit_count);
- private:
+ // Sets the current offset to the provied byte/bit offsets. The bit
+ // offset is from the given byte, in the range [0,7].
+ bool Seek(size_t byte_offset, size_t bit_offset);
+
+ protected:
const uint8* const bytes_;
// The total size of |bytes_|.
size_t byte_count_;
@@ -70,6 +81,35 @@
DISALLOW_COPY_AND_ASSIGN(BitBuffer);
};
+// A BitBuffer API for write operations. Supports symmetric write APIs to the
+// reading APIs of BitBuffer. Note that the read/write offset is shared with the
+// BitBuffer API, so both reading and writing will consume bytes/bits.
+class BitBufferWriter : public BitBuffer {
+ public:
+ // Constructs a bit buffer for the writable buffer of |bytes|.
+ BitBufferWriter(uint8* bytes, size_t byte_count);
+
+ // Writes byte-sized values from the buffer. Returns false if there isn't
+ // enough data left for the specified type.
+ bool WriteUInt8(uint8 val);
+ bool WriteUInt16(uint16 val);
+ bool WriteUInt32(uint32 val);
+
+ // Writes bit-sized values to the buffer. Returns false if there isn't enough
+ // room left for the specified number of bits.
+ bool WriteBits(uint64 val, size_t bit_count);
+
+ // Writes the exponential golomb encoded version of the supplied value.
+ // Returns false if there isn't enough room left for the value.
+ bool WriteExponentialGolomb(uint32 val);
+
+ private:
+ // The buffer, as a writable array.
+ uint8* const writable_bytes_;
+
+ DISALLOW_COPY_AND_ASSIGN(BitBufferWriter);
+};
+
} // namespace rtc
#endif // WEBRTC_BASE_BITBUFFER_H_
diff --git a/webrtc/base/bitbuffer_unittest.cc b/webrtc/base/bitbuffer_unittest.cc
index 143ea6f..21cffd6 100644
--- a/webrtc/base/bitbuffer_unittest.cc
+++ b/webrtc/base/bitbuffer_unittest.cc
@@ -135,6 +135,39 @@
EXPECT_FALSE(buffer.ReadBits(&val, 1));
}
+TEST(BitBufferTest, SetOffsetValues) {
+ uint8 bytes[4] = {0};
+ BitBufferWriter buffer(bytes, 4);
+
+ size_t byte_offset, bit_offset;
+ // Bit offsets are [0,7].
+ EXPECT_TRUE(buffer.Seek(0, 0));
+ EXPECT_TRUE(buffer.Seek(0, 7));
+ buffer.GetCurrentOffset(&byte_offset, &bit_offset);
+ EXPECT_EQ(0u, byte_offset);
+ EXPECT_EQ(7u, bit_offset);
+ EXPECT_FALSE(buffer.Seek(0, 8));
+ buffer.GetCurrentOffset(&byte_offset, &bit_offset);
+ EXPECT_EQ(0u, byte_offset);
+ EXPECT_EQ(7u, bit_offset);
+ // Byte offsets are [0,length]. At byte offset length, the bit offset must be
+ // 0.
+ EXPECT_TRUE(buffer.Seek(0, 0));
+ EXPECT_TRUE(buffer.Seek(2, 4));
+ buffer.GetCurrentOffset(&byte_offset, &bit_offset);
+ EXPECT_EQ(2u, byte_offset);
+ EXPECT_EQ(4u, bit_offset);
+ EXPECT_TRUE(buffer.Seek(4, 0));
+ EXPECT_FALSE(buffer.Seek(5, 0));
+ buffer.GetCurrentOffset(&byte_offset, &bit_offset);
+ EXPECT_EQ(4u, byte_offset);
+ EXPECT_EQ(0u, bit_offset);
+ EXPECT_FALSE(buffer.Seek(4, 1));
+
+ // Passing a NULL out parameter is death.
+ EXPECT_DEATH(buffer.GetCurrentOffset(&byte_offset, NULL), "");
+}
+
uint64 GolombEncoded(uint32 val) {
val++;
uint32 bit_counter = val;
@@ -146,19 +179,23 @@
return static_cast<uint64>(val) << (64 - (bit_count * 2 - 1));
}
-TEST(BitBufferTest, GolombString) {
- char test_string[] = "my precious";
- for (size_t i = 0; i < ARRAY_SIZE(test_string); ++i) {
- uint64 encoded_val = GolombEncoded(test_string[i]);
- // Use ByteBuffer to convert to bytes, to account for endianness (BitBuffer
- // requires network order).
- ByteBuffer byteBuffer;
+TEST(BitBufferTest, GolombUint32Values) {
+ ByteBuffer byteBuffer;
+ byteBuffer.Resize(16);
+ BitBuffer buffer(reinterpret_cast<const uint8*>(byteBuffer.Data()),
+ byteBuffer.Capacity());
+ // Test over the uint32 range with a large enough step that the test doesn't
+ // take forever. Around 20,000 iterations should do.
+ const int kStep = std::numeric_limits<uint32>::max() / 20000;
+ for (uint32 i = 0; i < std::numeric_limits<uint32>::max() - kStep;
+ i += kStep) {
+ uint64 encoded_val = GolombEncoded(i);
+ byteBuffer.Clear();
byteBuffer.WriteUInt64(encoded_val);
- BitBuffer buffer(reinterpret_cast<const uint8*>(byteBuffer.Data()),
- byteBuffer.Length());
uint32 decoded_val;
+ EXPECT_TRUE(buffer.Seek(0, 0));
EXPECT_TRUE(buffer.ReadExponentialGolomb(&decoded_val));
- EXPECT_EQ(test_string[i], static_cast<char>(decoded_val));
+ EXPECT_EQ(i, decoded_val);
}
}
@@ -180,4 +217,87 @@
EXPECT_EQ(0x01FEu, decoded_val);
}
+TEST(BitBufferWriterTest, SymmetricReadWrite) {
+ uint8 bytes[16] = {0};
+ BitBufferWriter buffer(bytes, 4);
+
+ // Write some bit data at various sizes.
+ EXPECT_TRUE(buffer.WriteBits(0x2u, 3));
+ EXPECT_TRUE(buffer.WriteBits(0x1u, 2));
+ EXPECT_TRUE(buffer.WriteBits(0x53u, 7));
+ EXPECT_TRUE(buffer.WriteBits(0x0u, 2));
+ EXPECT_TRUE(buffer.WriteBits(0x1u, 1));
+ EXPECT_TRUE(buffer.WriteBits(0x1ABCDu, 17));
+ // That should be all that fits in the buffer.
+ EXPECT_FALSE(buffer.WriteBits(1, 1));
+
+ EXPECT_TRUE(buffer.Seek(0, 0));
+ uint32 val;
+ EXPECT_TRUE(buffer.ReadBits(&val, 3));
+ EXPECT_EQ(0x2u, val);
+ EXPECT_TRUE(buffer.ReadBits(&val, 2));
+ EXPECT_EQ(0x1u, val);
+ EXPECT_TRUE(buffer.ReadBits(&val, 7));
+ EXPECT_EQ(0x53u, val);
+ EXPECT_TRUE(buffer.ReadBits(&val, 2));
+ EXPECT_EQ(0x0u, val);
+ EXPECT_TRUE(buffer.ReadBits(&val, 1));
+ EXPECT_EQ(0x1u, val);
+ EXPECT_TRUE(buffer.ReadBits(&val, 17));
+ EXPECT_EQ(0x1ABCDu, val);
+ // And there should be nothing left.
+ EXPECT_FALSE(buffer.ReadBits(&val, 1));
+}
+
+TEST(BitBufferWriterTest, SymmetricBytesMisaligned) {
+ uint8 bytes[16] = {0};
+ BitBufferWriter buffer(bytes, 16);
+
+ // Offset 3, to get things misaligned.
+ EXPECT_TRUE(buffer.ConsumeBits(3));
+ EXPECT_TRUE(buffer.WriteUInt8(0x12u));
+ EXPECT_TRUE(buffer.WriteUInt16(0x3456u));
+ EXPECT_TRUE(buffer.WriteUInt32(0x789ABCDEu));
+
+ buffer.Seek(0, 3);
+ uint8 val8;
+ uint16 val16;
+ uint32 val32;
+ EXPECT_TRUE(buffer.ReadUInt8(&val8));
+ EXPECT_EQ(0x12u, val8);
+ EXPECT_TRUE(buffer.ReadUInt16(&val16));
+ EXPECT_EQ(0x3456u, val16);
+ EXPECT_TRUE(buffer.ReadUInt32(&val32));
+ EXPECT_EQ(0x789ABCDEu, val32);
+}
+
+TEST(BitBufferWriterTest, SymmetricGolomb) {
+ char test_string[] = "my precious";
+ uint8 bytes[64] = {0};
+ BitBufferWriter buffer(bytes, 64);
+ for (size_t i = 0; i < ARRAY_SIZE(test_string); ++i) {
+ EXPECT_TRUE(buffer.WriteExponentialGolomb(test_string[i]));
+ }
+ buffer.Seek(0, 0);
+ for (size_t i = 0; i < ARRAY_SIZE(test_string); ++i) {
+ uint32 val;
+ EXPECT_TRUE(buffer.ReadExponentialGolomb(&val));
+ EXPECT_LE(val, std::numeric_limits<uint8>::max());
+ EXPECT_EQ(test_string[i], static_cast<char>(val));
+ }
+}
+
+TEST(BitBufferWriterTest, WriteClearsBits) {
+ uint8 bytes[] = {0xFF, 0xFF};
+ BitBufferWriter buffer(bytes, 2);
+ EXPECT_TRUE(buffer.ConsumeBits(3));
+ EXPECT_TRUE(buffer.WriteBits(0, 1));
+ EXPECT_EQ(0xEFu, bytes[0]);
+ EXPECT_TRUE(buffer.WriteBits(0, 3));
+ EXPECT_EQ(0xE1u, bytes[0]);
+ EXPECT_TRUE(buffer.WriteBits(0, 2));
+ EXPECT_EQ(0xE0u, bytes[0]);
+ EXPECT_EQ(0x7F, bytes[1]);
+}
+
} // namespace rtc
