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Fix encode zero bits on word boundary bug

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Bit stream writer was manifesting incorrect behaviour when the following
two conditions were met:
- writer was on 64-bit word boundary
- WriteBits was invoked with num_bits=0 (can happen when a Huffman codec has only one
value)

The bug was causing very rare sporadic corruption which was detected by
tests after a random experimental change in MARK-V model.
This commit is contained in:
Andrey Tuganov 2017-08-28 13:18:45 -04:00
parent 63e1e348b0
commit d41a52415a
2 changed files with 21 additions and 0 deletions
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2
source/util/bit_stream.cpp
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@ -300,6 +300,8 @@ void BitWriterWord64::WriteBits(uint64_t bits, size_t num_bits) {
assert(is_little_endian && "Big-endian architecture support not implemented"); assert(is_little_endian && "Big-endian architecture support not implemented");
if (!is_little_endian) return; if (!is_little_endian) return;
if (num_bits == 0) return;
bits = GetLowerBits(bits, num_bits); bits = GetLowerBits(bits, num_bits);
EmitSequence(bits, num_bits); EmitSequence(bits, num_bits);

19
test/bit_stream.cpp
View File

@ -565,6 +565,25 @@ TEST(BitWriterWord64, WriteBits) {
EXPECT_EQ(PadToWord<64>("110011100111001"), writer.GetStreamPadded64()); EXPECT_EQ(PadToWord<64>("110011100111001"), writer.GetStreamPadded64());
} }
TEST(BitWriterWord64, WriteZeroBits) {
BitWriterWord64 writer;
writer.WriteBits(0, 0);
writer.WriteBits(1, 0);
EXPECT_EQ(0u, writer.GetNumBits());
writer.WriteBits(1, 1);
writer.WriteBits(0, 0);
EXPECT_EQ(PadToWord<64>("1"), writer.GetStreamPadded64());
writer.WriteBits(0, 63);
EXPECT_EQ(64u, writer.GetNumBits());
writer.WriteBits(0, 0);
writer.WriteBits(7, 3);
writer.WriteBits(0, 0);
EXPECT_EQ(PadToWord<64>(
"1"
"000000000000000000000000000000000000000000000000000000000000000"
"111"), writer.GetStreamPadded64());
}
TEST(BitWriterWord64, ComparisonTestWriteLotsOfBits) { TEST(BitWriterWord64, ComparisonTestWriteLotsOfBits) {
BitWriterStringStream writer1; BitWriterStringStream writer1;
BitWriterWord64 writer2(16384); BitWriterWord64 writer2(16384);