diff --git a/snappy-internal.h b/snappy-internal.h index 0653dc6..c4d1f6d 100644 --- a/snappy-internal.h +++ b/snappy-internal.h @@ -144,6 +144,68 @@ static inline int FindMatchLength(const char* s1, } #endif +// Lookup tables for decompression code. Give --snappy_dump_decompression_table +// to the unit test to recompute char_table. + +enum { + LITERAL = 0, + COPY_1_BYTE_OFFSET = 1, // 3 bit length + 3 bits of offset in opcode + COPY_2_BYTE_OFFSET = 2, + COPY_4_BYTE_OFFSET = 3 +}; +static const int kMaximumTagLength = 5; // COPY_4_BYTE_OFFSET plus the actual offset. + +// Mapping from i in range [0,4] to a mask to extract the bottom 8*i bits +static const uint32 wordmask[] = { + 0u, 0xffu, 0xffffu, 0xffffffu, 0xffffffffu +}; + +// Data stored per entry in lookup table: +// Range Bits-used Description +// ------------------------------------ +// 1..64 0..7 Literal/copy length encoded in opcode byte +// 0..7 8..10 Copy offset encoded in opcode byte / 256 +// 0..4 11..13 Extra bytes after opcode +// +// We use eight bits for the length even though 7 would have sufficed +// because of efficiency reasons: +// (1) Extracting a byte is faster than a bit-field +// (2) It properly aligns copy offset so we do not need a <<8 +static const uint16 char_table[256] = { + 0x0001, 0x0804, 0x1001, 0x2001, 0x0002, 0x0805, 0x1002, 0x2002, + 0x0003, 0x0806, 0x1003, 0x2003, 0x0004, 0x0807, 0x1004, 0x2004, + 0x0005, 0x0808, 0x1005, 0x2005, 0x0006, 0x0809, 0x1006, 0x2006, + 0x0007, 0x080a, 0x1007, 0x2007, 0x0008, 0x080b, 0x1008, 0x2008, + 0x0009, 0x0904, 0x1009, 0x2009, 0x000a, 0x0905, 0x100a, 0x200a, + 0x000b, 0x0906, 0x100b, 0x200b, 0x000c, 0x0907, 0x100c, 0x200c, + 0x000d, 0x0908, 0x100d, 0x200d, 0x000e, 0x0909, 0x100e, 0x200e, + 0x000f, 0x090a, 0x100f, 0x200f, 0x0010, 0x090b, 0x1010, 0x2010, + 0x0011, 0x0a04, 0x1011, 0x2011, 0x0012, 0x0a05, 0x1012, 0x2012, + 0x0013, 0x0a06, 0x1013, 0x2013, 0x0014, 0x0a07, 0x1014, 0x2014, + 0x0015, 0x0a08, 0x1015, 0x2015, 0x0016, 0x0a09, 0x1016, 0x2016, + 0x0017, 0x0a0a, 0x1017, 0x2017, 0x0018, 0x0a0b, 0x1018, 0x2018, + 0x0019, 0x0b04, 0x1019, 0x2019, 0x001a, 0x0b05, 0x101a, 0x201a, + 0x001b, 0x0b06, 0x101b, 0x201b, 0x001c, 0x0b07, 0x101c, 0x201c, + 0x001d, 0x0b08, 0x101d, 0x201d, 0x001e, 0x0b09, 0x101e, 0x201e, + 0x001f, 0x0b0a, 0x101f, 0x201f, 0x0020, 0x0b0b, 0x1020, 0x2020, + 0x0021, 0x0c04, 0x1021, 0x2021, 0x0022, 0x0c05, 0x1022, 0x2022, + 0x0023, 0x0c06, 0x1023, 0x2023, 0x0024, 0x0c07, 0x1024, 0x2024, + 0x0025, 0x0c08, 0x1025, 0x2025, 0x0026, 0x0c09, 0x1026, 0x2026, + 0x0027, 0x0c0a, 0x1027, 0x2027, 0x0028, 0x0c0b, 0x1028, 0x2028, + 0x0029, 0x0d04, 0x1029, 0x2029, 0x002a, 0x0d05, 0x102a, 0x202a, + 0x002b, 0x0d06, 0x102b, 0x202b, 0x002c, 0x0d07, 0x102c, 0x202c, + 0x002d, 0x0d08, 0x102d, 0x202d, 0x002e, 0x0d09, 0x102e, 0x202e, + 0x002f, 0x0d0a, 0x102f, 0x202f, 0x0030, 0x0d0b, 0x1030, 0x2030, + 0x0031, 0x0e04, 0x1031, 0x2031, 0x0032, 0x0e05, 0x1032, 0x2032, + 0x0033, 0x0e06, 0x1033, 0x2033, 0x0034, 0x0e07, 0x1034, 0x2034, + 0x0035, 0x0e08, 0x1035, 0x2035, 0x0036, 0x0e09, 0x1036, 0x2036, + 0x0037, 0x0e0a, 0x1037, 0x2037, 0x0038, 0x0e0b, 0x1038, 0x2038, + 0x0039, 0x0f04, 0x1039, 0x2039, 0x003a, 0x0f05, 0x103a, 0x203a, + 0x003b, 0x0f06, 0x103b, 0x203b, 0x003c, 0x0f07, 0x103c, 0x203c, + 0x0801, 0x0f08, 0x103d, 0x203d, 0x1001, 0x0f09, 0x103e, 0x203e, + 0x1801, 0x0f0a, 0x103f, 0x203f, 0x2001, 0x0f0b, 0x1040, 0x2040 +}; + } // end namespace internal } // end namespace snappy diff --git a/snappy.cc b/snappy.cc index 9984b9e..d81e6b3 100644 --- a/snappy.cc +++ b/snappy.cc @@ -39,6 +39,14 @@ namespace snappy { +using internal::COPY_1_BYTE_OFFSET; +using internal::COPY_2_BYTE_OFFSET; +using internal::COPY_4_BYTE_OFFSET; +using internal::LITERAL; +using internal::char_table; +using internal::kMaximumTagLength; +using internal::wordmask; + // Any hash function will produce a valid compressed bitstream, but a good // hash function reduces the number of collisions and thus yields better // compression for compressible input, and more speed for incompressible @@ -76,14 +84,6 @@ size_t MaxCompressedLength(size_t source_len) { return 32 + source_len + source_len/6; } -enum { - LITERAL = 0, - COPY_1_BYTE_OFFSET = 1, // 3 bit length + 3 bits of offset in opcode - COPY_2_BYTE_OFFSET = 2, - COPY_4_BYTE_OFFSET = 3 -}; -static const int kMaximumTagLength = 5; // COPY_4_BYTE_OFFSET plus the actual offset. - // Copy "len" bytes from "src" to "op", one byte at a time. Used for // handling COPY operations where the input and output regions may // overlap. For example, suppose: @@ -493,162 +493,6 @@ char* CompressFragment(const char* input, // bool TryFastAppend(const char* ip, size_t available, size_t length); // }; -// ----------------------------------------------------------------------- -// Lookup table for decompression code. Generated by ComputeTable() below. -// ----------------------------------------------------------------------- - -// Mapping from i in range [0,4] to a mask to extract the bottom 8*i bits -static const uint32 wordmask[] = { - 0u, 0xffu, 0xffffu, 0xffffffu, 0xffffffffu -}; - -// Data stored per entry in lookup table: -// Range Bits-used Description -// ------------------------------------ -// 1..64 0..7 Literal/copy length encoded in opcode byte -// 0..7 8..10 Copy offset encoded in opcode byte / 256 -// 0..4 11..13 Extra bytes after opcode -// -// We use eight bits for the length even though 7 would have sufficed -// because of efficiency reasons: -// (1) Extracting a byte is faster than a bit-field -// (2) It properly aligns copy offset so we do not need a <<8 -static const uint16 char_table[256] = { - 0x0001, 0x0804, 0x1001, 0x2001, 0x0002, 0x0805, 0x1002, 0x2002, - 0x0003, 0x0806, 0x1003, 0x2003, 0x0004, 0x0807, 0x1004, 0x2004, - 0x0005, 0x0808, 0x1005, 0x2005, 0x0006, 0x0809, 0x1006, 0x2006, - 0x0007, 0x080a, 0x1007, 0x2007, 0x0008, 0x080b, 0x1008, 0x2008, - 0x0009, 0x0904, 0x1009, 0x2009, 0x000a, 0x0905, 0x100a, 0x200a, - 0x000b, 0x0906, 0x100b, 0x200b, 0x000c, 0x0907, 0x100c, 0x200c, - 0x000d, 0x0908, 0x100d, 0x200d, 0x000e, 0x0909, 0x100e, 0x200e, - 0x000f, 0x090a, 0x100f, 0x200f, 0x0010, 0x090b, 0x1010, 0x2010, - 0x0011, 0x0a04, 0x1011, 0x2011, 0x0012, 0x0a05, 0x1012, 0x2012, - 0x0013, 0x0a06, 0x1013, 0x2013, 0x0014, 0x0a07, 0x1014, 0x2014, - 0x0015, 0x0a08, 0x1015, 0x2015, 0x0016, 0x0a09, 0x1016, 0x2016, - 0x0017, 0x0a0a, 0x1017, 0x2017, 0x0018, 0x0a0b, 0x1018, 0x2018, - 0x0019, 0x0b04, 0x1019, 0x2019, 0x001a, 0x0b05, 0x101a, 0x201a, - 0x001b, 0x0b06, 0x101b, 0x201b, 0x001c, 0x0b07, 0x101c, 0x201c, - 0x001d, 0x0b08, 0x101d, 0x201d, 0x001e, 0x0b09, 0x101e, 0x201e, - 0x001f, 0x0b0a, 0x101f, 0x201f, 0x0020, 0x0b0b, 0x1020, 0x2020, - 0x0021, 0x0c04, 0x1021, 0x2021, 0x0022, 0x0c05, 0x1022, 0x2022, - 0x0023, 0x0c06, 0x1023, 0x2023, 0x0024, 0x0c07, 0x1024, 0x2024, - 0x0025, 0x0c08, 0x1025, 0x2025, 0x0026, 0x0c09, 0x1026, 0x2026, - 0x0027, 0x0c0a, 0x1027, 0x2027, 0x0028, 0x0c0b, 0x1028, 0x2028, - 0x0029, 0x0d04, 0x1029, 0x2029, 0x002a, 0x0d05, 0x102a, 0x202a, - 0x002b, 0x0d06, 0x102b, 0x202b, 0x002c, 0x0d07, 0x102c, 0x202c, - 0x002d, 0x0d08, 0x102d, 0x202d, 0x002e, 0x0d09, 0x102e, 0x202e, - 0x002f, 0x0d0a, 0x102f, 0x202f, 0x0030, 0x0d0b, 0x1030, 0x2030, - 0x0031, 0x0e04, 0x1031, 0x2031, 0x0032, 0x0e05, 0x1032, 0x2032, - 0x0033, 0x0e06, 0x1033, 0x2033, 0x0034, 0x0e07, 0x1034, 0x2034, - 0x0035, 0x0e08, 0x1035, 0x2035, 0x0036, 0x0e09, 0x1036, 0x2036, - 0x0037, 0x0e0a, 0x1037, 0x2037, 0x0038, 0x0e0b, 0x1038, 0x2038, - 0x0039, 0x0f04, 0x1039, 0x2039, 0x003a, 0x0f05, 0x103a, 0x203a, - 0x003b, 0x0f06, 0x103b, 0x203b, 0x003c, 0x0f07, 0x103c, 0x203c, - 0x0801, 0x0f08, 0x103d, 0x203d, 0x1001, 0x0f09, 0x103e, 0x203e, - 0x1801, 0x0f0a, 0x103f, 0x203f, 0x2001, 0x0f0b, 0x1040, 0x2040 -}; - -// In debug mode, allow optional computation of the table at startup. -// Also, check that the decompression table is correct. -#ifndef NDEBUG -DEFINE_bool(snappy_dump_decompression_table, false, - "If true, we print the decompression table at startup."); - -static uint16 MakeEntry(unsigned int extra, - unsigned int len, - unsigned int copy_offset) { - // Check that all of the fields fit within the allocated space - assert(extra == (extra & 0x7)); // At most 3 bits - assert(copy_offset == (copy_offset & 0x7)); // At most 3 bits - assert(len == (len & 0x7f)); // At most 7 bits - return len | (copy_offset << 8) | (extra << 11); -} - -static void ComputeTable() { - uint16 dst[256]; - - // Place invalid entries in all places to detect missing initialization - int assigned = 0; - for (int i = 0; i < 256; i++) { - dst[i] = 0xffff; - } - - // Small LITERAL entries. We store (len-1) in the top 6 bits. - for (unsigned int len = 1; len <= 60; len++) { - dst[LITERAL | ((len-1) << 2)] = MakeEntry(0, len, 0); - assigned++; - } - - // Large LITERAL entries. We use 60..63 in the high 6 bits to - // encode the number of bytes of length info that follow the opcode. - for (unsigned int extra_bytes = 1; extra_bytes <= 4; extra_bytes++) { - // We set the length field in the lookup table to 1 because extra - // bytes encode len-1. - dst[LITERAL | ((extra_bytes+59) << 2)] = MakeEntry(extra_bytes, 1, 0); - assigned++; - } - - // COPY_1_BYTE_OFFSET. - // - // The tag byte in the compressed data stores len-4 in 3 bits, and - // offset/256 in 5 bits. offset%256 is stored in the next byte. - // - // This format is used for length in range [4..11] and offset in - // range [0..2047] - for (unsigned int len = 4; len < 12; len++) { - for (unsigned int offset = 0; offset < 2048; offset += 256) { - dst[COPY_1_BYTE_OFFSET | ((len-4)<<2) | ((offset>>8)<<5)] = - MakeEntry(1, len, offset>>8); - assigned++; - } - } - - // COPY_2_BYTE_OFFSET. - // Tag contains len-1 in top 6 bits, and offset in next two bytes. - for (unsigned int len = 1; len <= 64; len++) { - dst[COPY_2_BYTE_OFFSET | ((len-1)<<2)] = MakeEntry(2, len, 0); - assigned++; - } - - // COPY_4_BYTE_OFFSET. - // Tag contents len-1 in top 6 bits, and offset in next four bytes. - for (unsigned int len = 1; len <= 64; len++) { - dst[COPY_4_BYTE_OFFSET | ((len-1)<<2)] = MakeEntry(4, len, 0); - assigned++; - } - - // Check that each entry was initialized exactly once. - if (assigned != 256) { - fprintf(stderr, "ComputeTable: assigned only %d of 256\n", assigned); - abort(); - } - for (int i = 0; i < 256; i++) { - if (dst[i] == 0xffff) { - fprintf(stderr, "ComputeTable: did not assign byte %d\n", i); - abort(); - } - } - - if (FLAGS_snappy_dump_decompression_table) { - printf("static const uint16 char_table[256] = {\n "); - for (int i = 0; i < 256; i++) { - printf("0x%04x%s", - dst[i], - ((i == 255) ? "\n" : (((i%8) == 7) ? ",\n " : ", "))); - } - printf("};\n"); - } - - // Check that computed table matched recorded table - for (int i = 0; i < 256; i++) { - if (dst[i] != char_table[i]) { - fprintf(stderr, "ComputeTable: byte %d: computed (%x), expect (%x)\n", - i, static_cast(dst[i]), static_cast(char_table[i])); - abort(); - } - } -} -#endif /* !NDEBUG */ // Helper class for decompression class SnappyDecompressor { diff --git a/snappy_unittest.cc b/snappy_unittest.cc index 4a80f2a..3081662 100644 --- a/snappy_unittest.cc +++ b/snappy_unittest.cc @@ -64,6 +64,9 @@ DEFINE_bool(write_compressed, false, DEFINE_bool(write_uncompressed, false, "Write uncompressed versions of each file to .uncomp"); +DEFINE_bool(snappy_dump_decompression_table, false, + "If true, we print the decompression table during tests."); + namespace snappy { @@ -1160,6 +1163,100 @@ TEST(Snappy, FindMatchLengthRandom) { } } +static uint16 MakeEntry(unsigned int extra, + unsigned int len, + unsigned int copy_offset) { + // Check that all of the fields fit within the allocated space + assert(extra == (extra & 0x7)); // At most 3 bits + assert(copy_offset == (copy_offset & 0x7)); // At most 3 bits + assert(len == (len & 0x7f)); // At most 7 bits + return len | (copy_offset << 8) | (extra << 11); +} + +// Check that the decompression table is correct, and optionally print out +// the computed one. +TEST(Snappy, VerifyCharTable) { + using snappy::internal::LITERAL; + using snappy::internal::COPY_1_BYTE_OFFSET; + using snappy::internal::COPY_2_BYTE_OFFSET; + using snappy::internal::COPY_4_BYTE_OFFSET; + using snappy::internal::char_table; + using snappy::internal::wordmask; + + uint16 dst[256]; + + // Place invalid entries in all places to detect missing initialization + int assigned = 0; + for (int i = 0; i < 256; i++) { + dst[i] = 0xffff; + } + + // Small LITERAL entries. We store (len-1) in the top 6 bits. + for (unsigned int len = 1; len <= 60; len++) { + dst[LITERAL | ((len-1) << 2)] = MakeEntry(0, len, 0); + assigned++; + } + + // Large LITERAL entries. We use 60..63 in the high 6 bits to + // encode the number of bytes of length info that follow the opcode. + for (unsigned int extra_bytes = 1; extra_bytes <= 4; extra_bytes++) { + // We set the length field in the lookup table to 1 because extra + // bytes encode len-1. + dst[LITERAL | ((extra_bytes+59) << 2)] = MakeEntry(extra_bytes, 1, 0); + assigned++; + } + + // COPY_1_BYTE_OFFSET. + // + // The tag byte in the compressed data stores len-4 in 3 bits, and + // offset/256 in 5 bits. offset%256 is stored in the next byte. + // + // This format is used for length in range [4..11] and offset in + // range [0..2047] + for (unsigned int len = 4; len < 12; len++) { + for (unsigned int offset = 0; offset < 2048; offset += 256) { + dst[COPY_1_BYTE_OFFSET | ((len-4)<<2) | ((offset>>8)<<5)] = + MakeEntry(1, len, offset>>8); + assigned++; + } + } + + // COPY_2_BYTE_OFFSET. + // Tag contains len-1 in top 6 bits, and offset in next two bytes. + for (unsigned int len = 1; len <= 64; len++) { + dst[COPY_2_BYTE_OFFSET | ((len-1)<<2)] = MakeEntry(2, len, 0); + assigned++; + } + + // COPY_4_BYTE_OFFSET. + // Tag contents len-1 in top 6 bits, and offset in next four bytes. + for (unsigned int len = 1; len <= 64; len++) { + dst[COPY_4_BYTE_OFFSET | ((len-1)<<2)] = MakeEntry(4, len, 0); + assigned++; + } + + // Check that each entry was initialized exactly once. + EXPECT_EQ(256, assigned) << "Assigned only " << assigned << " of 256"; + for (int i = 0; i < 256; i++) { + EXPECT_NE(0xffff, dst[i]) << "Did not assign byte " << i; + } + + if (FLAGS_snappy_dump_decompression_table) { + printf("static const uint16 char_table[256] = {\n "); + for (int i = 0; i < 256; i++) { + printf("0x%04x%s", + dst[i], + ((i == 255) ? "\n" : (((i%8) == 7) ? ",\n " : ", "))); + } + printf("};\n"); + } + + // Check that computed table matched recorded table. + for (int i = 0; i < 256; i++) { + EXPECT_EQ(dst[i], char_table[i]) << "Mismatch in byte " << i; + } +} + static void CompressFile(const char* fname) { string fullinput; CHECK_OK(file::GetContents(fname, &fullinput, file::Defaults()));