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rocksdb/snappy/snappy-1.0.5/snappy.cc
Dhruba Borthakur 21f2fe4419 Build snappy for leveldb. 
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2012-07-17 00:18:23 -07:00

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// Copyright 2005 Google Inc. All Rights Reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
// * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
// * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
// * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
#include "snappy.h"
#include "snappy-internal.h"
#include "snappy-sinksource.h"
#include <stdio.h>
#include <algorithm>
#include <string>
#include <vector>
namespace snappy {
// 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
// input. Of course, it doesn't hurt if the hash function is reasonably fast
// either, as it gets called a lot.
static inline uint32 HashBytes(uint32 bytes, int shift) {
uint32 kMul = 0x1e35a7bd;
return (bytes * kMul) >> shift;
}
static inline uint32 Hash(const char* p, int shift) {
return HashBytes(UNALIGNED_LOAD32(p), shift);
}
size_t MaxCompressedLength(size_t source_len) {
// Compressed data can be defined as:
// compressed := item* literal*
// item := literal* copy
//
// The trailing literal sequence has a space blowup of at most 62/60
// since a literal of length 60 needs one tag byte + one extra byte
// for length information.
//
// Item blowup is trickier to measure. Suppose the "copy" op copies
// 4 bytes of data. Because of a special check in the encoding code,
// we produce a 4-byte copy only if the offset is < 65536. Therefore
// the copy op takes 3 bytes to encode, and this type of item leads
// to at most the 62/60 blowup for representing literals.
//
// Suppose the "copy" op copies 5 bytes of data. If the offset is big
// enough, it will take 5 bytes to encode the copy op. Therefore the
// worst case here is a one-byte literal followed by a five-byte copy.
// I.e., 6 bytes of input turn into 7 bytes of "compressed" data.
//
// This last factor dominates the blowup, so the final estimate is:
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
};
// 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:
// src == "ab"
// op == src + 2
// len == 20
// After IncrementalCopy(src, op, len), the result will have
// eleven copies of "ab"
// ababababababababababab
// Note that this does not match the semantics of either memcpy()
// or memmove().
static inline void IncrementalCopy(const char* src, char* op, int len) {
DCHECK_GT(len, 0);
do {
*op++ = *src++;
} while (--len > 0);
}
// Equivalent to IncrementalCopy except that it can write up to ten extra
// bytes after the end of the copy, and that it is faster.
//
// The main part of this loop is a simple copy of eight bytes at a time until
// we've copied (at least) the requested amount of bytes. However, if op and
// src are less than eight bytes apart (indicating a repeating pattern of
// length < 8), we first need to expand the pattern in order to get the correct
// results. For instance, if the buffer looks like this, with the eight-byte
// <src> and <op> patterns marked as intervals:
//
// abxxxxxxxxxxxx
// [------] src
// [------] op
//
// a single eight-byte copy from <src> to <op> will repeat the pattern once,
// after which we can move <op> two bytes without moving <src>:
//
// ababxxxxxxxxxx
// [------] src
// [------] op
//
// and repeat the exercise until the two no longer overlap.
//
// This allows us to do very well in the special case of one single byte
// repeated many times, without taking a big hit for more general cases.
//
// The worst case of extra writing past the end of the match occurs when
// op - src == 1 and len == 1; the last copy will read from byte positions
// [0..7] and write to [4..11], whereas it was only supposed to write to
// position 1. Thus, ten excess bytes.
namespace {
const int kMaxIncrementCopyOverflow = 10;
} // namespace
static inline void IncrementalCopyFastPath(const char* src, char* op, int len) {
while (op - src < 8) {
UnalignedCopy64(src, op);
len -= op - src;
op += op - src;
}
while (len > 0) {
UnalignedCopy64(src, op);
src += 8;
op += 8;
len -= 8;
}
}
static inline char* EmitLiteral(char* op,
const char* literal,
int len,
bool allow_fast_path) {
int n = len - 1; // Zero-length literals are disallowed
if (n < 60) {
// Fits in tag byte
*op++ = LITERAL | (n << 2);
// The vast majority of copies are below 16 bytes, for which a
// call to memcpy is overkill. This fast path can sometimes
// copy up to 15 bytes too much, but that is okay in the
// main loop, since we have a bit to go on for both sides:
//
// - The input will always have kInputMarginBytes = 15 extra
// available bytes, as long as we're in the main loop, and
// if not, allow_fast_path = false.
// - The output will always have 32 spare bytes (see
// MaxCompressedLength).
if (allow_fast_path && len <= 16) {
UnalignedCopy64(literal, op);
UnalignedCopy64(literal + 8, op + 8);
return op + len;
}
} else {
// Encode in upcoming bytes
char* base = op;
int count = 0;
op++;
while (n > 0) {
*op++ = n & 0xff;
n >>= 8;
count++;
}
assert(count >= 1);
assert(count <= 4);
*base = LITERAL | ((59+count) << 2);
}
memcpy(op, literal, len);
return op + len;
}
static inline char* EmitCopyLessThan64(char* op, size_t offset, int len) {
DCHECK_LE(len, 64);
DCHECK_GE(len, 4);
DCHECK_LT(offset, 65536);
if ((len < 12) && (offset < 2048)) {
size_t len_minus_4 = len - 4;
assert(len_minus_4 < 8); // Must fit in 3 bits
*op++ = COPY_1_BYTE_OFFSET | ((len_minus_4) << 2) | ((offset >> 8) << 5);
*op++ = offset & 0xff;
} else {
*op++ = COPY_2_BYTE_OFFSET | ((len-1) << 2);
LittleEndian::Store16(op, offset);
op += 2;
}
return op;
}
static inline char* EmitCopy(char* op, size_t offset, int len) {
// Emit 64 byte copies but make sure to keep at least four bytes reserved
while (len >= 68) {
op = EmitCopyLessThan64(op, offset, 64);
len -= 64;
}
// Emit an extra 60 byte copy if have too much data to fit in one copy
if (len > 64) {
op = EmitCopyLessThan64(op, offset, 60);
len -= 60;
}
// Emit remainder
op = EmitCopyLessThan64(op, offset, len);
return op;
}
bool GetUncompressedLength(const char* start, size_t n, size_t* result) {
uint32 v = 0;
const char* limit = start + n;
if (Varint::Parse32WithLimit(start, limit, &v) != NULL) {
*result = v;
return true;
} else {
return false;
}
}
namespace internal {
uint16* WorkingMemory::GetHashTable(size_t input_size, int* table_size) {
// Use smaller hash table when input.size() is smaller, since we
// fill the table, incurring O(hash table size) overhead for
// compression, and if the input is short, we won't need that
// many hash table entries anyway.
assert(kMaxHashTableSize >= 256);
size_t htsize = 256;
while (htsize < kMaxHashTableSize && htsize < input_size) {
htsize <<= 1;
}
CHECK_EQ(0, htsize & (htsize - 1)) << ": must be power of two";
CHECK_LE(htsize, kMaxHashTableSize) << ": hash table too large";
uint16* table;
if (htsize <= ARRAYSIZE(small_table_)) {
table = small_table_;
} else {
if (large_table_ == NULL) {
large_table_ = new uint16[kMaxHashTableSize];
}
table = large_table_;
}
*table_size = htsize;
memset(table, 0, htsize * sizeof(*table));
return table;
}
} // end namespace internal
// For 0 <= offset <= 4, GetUint32AtOffset(GetEightBytesAt(p), offset) will
// equal UNALIGNED_LOAD32(p + offset). Motivation: On x86-64 hardware we have
// empirically found that overlapping loads such as
// UNALIGNED_LOAD32(p) ... UNALIGNED_LOAD32(p+1) ... UNALIGNED_LOAD32(p+2)
// are slower than UNALIGNED_LOAD64(p) followed by shifts and casts to uint32.
//
// We have different versions for 64- and 32-bit; ideally we would avoid the
// two functions and just inline the UNALIGNED_LOAD64 call into
// GetUint32AtOffset, but GCC (at least not as of 4.6) is seemingly not clever
// enough to avoid loading the value multiple times then. For 64-bit, the load
// is done when GetEightBytesAt() is called, whereas for 32-bit, the load is
// done at GetUint32AtOffset() time.
#ifdef ARCH_K8
typedef uint64 EightBytesReference;
static inline EightBytesReference GetEightBytesAt(const char* ptr) {
return UNALIGNED_LOAD64(ptr);
}
static inline uint32 GetUint32AtOffset(uint64 v, int offset) {
DCHECK_GE(offset, 0);
DCHECK_LE(offset, 4);
return v >> (LittleEndian::IsLittleEndian() ? 8 * offset : 32 - 8 * offset);
}
#else
typedef const char* EightBytesReference;
static inline EightBytesReference GetEightBytesAt(const char* ptr) {
return ptr;
}
static inline uint32 GetUint32AtOffset(const char* v, int offset) {
DCHECK_GE(offset, 0);
DCHECK_LE(offset, 4);
return UNALIGNED_LOAD32(v + offset);
}
#endif
// Flat array compression that does not emit the "uncompressed length"
// prefix. Compresses "input" string to the "*op" buffer.
//
// REQUIRES: "input" is at most "kBlockSize" bytes long.
// REQUIRES: "op" points to an array of memory that is at least
// "MaxCompressedLength(input.size())" in size.
// REQUIRES: All elements in "table[0..table_size-1]" are initialized to zero.
// REQUIRES: "table_size" is a power of two
//
// Returns an "end" pointer into "op" buffer.
// "end - op" is the compressed size of "input".
namespace internal {
char* CompressFragment(const char* input,
size_t input_size,
char* op,
uint16* table,
const int table_size) {
// "ip" is the input pointer, and "op" is the output pointer.
const char* ip = input;
CHECK_LE(input_size, kBlockSize);
CHECK_EQ(table_size & (table_size - 1), 0) << ": table must be power of two";
const int shift = 32 - Bits::Log2Floor(table_size);
DCHECK_EQ(static_cast<int>(kuint32max >> shift), table_size - 1);
const char* ip_end = input + input_size;
const char* base_ip = ip;
// Bytes in [next_emit, ip) will be emitted as literal bytes. Or
// [next_emit, ip_end) after the main loop.
const char* next_emit = ip;
const size_t kInputMarginBytes = 15;
if (PREDICT_TRUE(input_size >= kInputMarginBytes)) {
const char* ip_limit = input + input_size - kInputMarginBytes;
for (uint32 next_hash = Hash(++ip, shift); ; ) {
DCHECK_LT(next_emit, ip);
// The body of this loop calls EmitLiteral once and then EmitCopy one or
// more times. (The exception is that when we're close to exhausting
// the input we goto emit_remainder.)
//
// In the first iteration of this loop we're just starting, so
// there's nothing to copy, so calling EmitLiteral once is
// necessary. And we only start a new iteration when the
// current iteration has determined that a call to EmitLiteral will
// precede the next call to EmitCopy (if any).
//
// Step 1: Scan forward in the input looking for a 4-byte-long match.
// If we get close to exhausting the input then goto emit_remainder.
//
// Heuristic match skipping: If 32 bytes are scanned with no matches
// found, start looking only at every other byte. If 32 more bytes are
// scanned, look at every third byte, etc.. When a match is found,
// immediately go back to looking at every byte. This is a small loss
// (~5% performance, ~0.1% density) for compressible data due to more
// bookkeeping, but for non-compressible data (such as JPEG) it's a huge
// win since the compressor quickly "realizes" the data is incompressible
// and doesn't bother looking for matches everywhere.
//
// The "skip" variable keeps track of how many bytes there are since the
// last match; dividing it by 32 (ie. right-shifting by five) gives the
// number of bytes to move ahead for each iteration.
uint32 skip = 32;
const char* next_ip = ip;
const char* candidate;
do {
ip = next_ip;
uint32 hash = next_hash;
DCHECK_EQ(hash, Hash(ip, shift));
uint32 bytes_between_hash_lookups = skip++ >> 5;
next_ip = ip + bytes_between_hash_lookups;
if (PREDICT_FALSE(next_ip > ip_limit)) {
goto emit_remainder;
}
next_hash = Hash(next_ip, shift);
candidate = base_ip + table[hash];
DCHECK_GE(candidate, base_ip);
DCHECK_LT(candidate, ip);
table[hash] = ip - base_ip;
} while (PREDICT_TRUE(UNALIGNED_LOAD32(ip) !=
UNALIGNED_LOAD32(candidate)));
// Step 2: A 4-byte match has been found. We'll later see if more
// than 4 bytes match. But, prior to the match, input
// bytes [next_emit, ip) are unmatched. Emit them as "literal bytes."
DCHECK_LE(next_emit + 16, ip_end);
op = EmitLiteral(op, next_emit, ip - next_emit, true);
// Step 3: Call EmitCopy, and then see if another EmitCopy could
// be our next move. Repeat until we find no match for the
// input immediately after what was consumed by the last EmitCopy call.
//
// If we exit this loop normally then we need to call EmitLiteral next,
// though we don't yet know how big the literal will be. We handle that
// by proceeding to the next iteration of the main loop. We also can exit
// this loop via goto if we get close to exhausting the input.
EightBytesReference input_bytes;
uint32 candidate_bytes = 0;
do {
// We have a 4-byte match at ip, and no need to emit any
// "literal bytes" prior to ip.
const char* base = ip;
int matched = 4 + FindMatchLength(candidate + 4, ip + 4, ip_end);
ip += matched;
size_t offset = base - candidate;
DCHECK_EQ(0, memcmp(base, candidate, matched));
op = EmitCopy(op, offset, matched);
// We could immediately start working at ip now, but to improve
// compression we first update table[Hash(ip - 1, ...)].
const char* insert_tail = ip - 1;
next_emit = ip;
if (PREDICT_FALSE(ip >= ip_limit)) {
goto emit_remainder;
}
input_bytes = GetEightBytesAt(insert_tail);
uint32 prev_hash = HashBytes(GetUint32AtOffset(input_bytes, 0), shift);
table[prev_hash] = ip - base_ip - 1;
uint32 cur_hash = HashBytes(GetUint32AtOffset(input_bytes, 1), shift);
candidate = base_ip + table[cur_hash];
candidate_bytes = UNALIGNED_LOAD32(candidate);
table[cur_hash] = ip - base_ip;
} while (GetUint32AtOffset(input_bytes, 1) == candidate_bytes);
next_hash = HashBytes(GetUint32AtOffset(input_bytes, 2), shift);
++ip;
}
}
emit_remainder:
// Emit the remaining bytes as a literal
if (next_emit < ip_end) {
op = EmitLiteral(op, next_emit, ip_end - next_emit, false);
}
return op;
}
} // end namespace internal
// Signature of output types needed by decompression code.
// The decompression code is templatized on a type that obeys this
// signature so that we do not pay virtual function call overhead in
// the middle of a tight decompression loop.
//
// class DecompressionWriter {
// public:
// // Called before decompression
// void SetExpectedLength(size_t length);
//
// // Called after decompression
// bool CheckLength() const;
//
// // Called repeatedly during decompression
// bool Append(const char* ip, size_t length);
// bool AppendFromSelf(uint32 offset, size_t length);
//
// // The difference between TryFastAppend and Append is that TryFastAppend
// // is allowed to read up to <available> bytes from the input buffer,
// // whereas Append is allowed to read <length>.
// //
// // Also, TryFastAppend is allowed to return false, declining the append,
// // without it being a fatal error -- just "return false" would be
// // a perfectly legal implementation of TryFastAppend. The intention
// // is for TryFastAppend to allow a fast path in the common case of
// // a small append.
// //
// // NOTE(user): TryFastAppend must always return decline (return false)
// // if <length> is 61 or more, as in this case the literal length is not
// // decoded fully. In practice, this should not be a big problem,
// // as it is unlikely that one would implement a fast path accepting
// // this much data.
// 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
DCHECK_EQ(extra, extra & 0x7); // At most 3 bits
DCHECK_EQ(copy_offset, copy_offset & 0x7); // At most 3 bits
DCHECK_EQ(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.
CHECK_EQ(assigned, 256);
for (int i = 0; i < 256; i++) {
CHECK_NE(dst[i], 0xffff);
}
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++) {
CHECK_EQ(dst[i], char_table[i]);
}
}
#endif /* !NDEBUG */
// Helper class for decompression
class SnappyDecompressor {
private:
Source* reader_; // Underlying source of bytes to decompress
const char* ip_; // Points to next buffered byte
const char* ip_limit_; // Points just past buffered bytes
uint32 peeked_; // Bytes peeked from reader (need to skip)
bool eof_; // Hit end of input without an error?
char scratch_[5]; // Temporary buffer for PeekFast() boundaries
// Ensure that all of the tag metadata for the next tag is available
// in [ip_..ip_limit_-1]. Also ensures that [ip,ip+4] is readable even
// if (ip_limit_ - ip_ < 5).
//
// Returns true on success, false on error or end of input.
bool RefillTag();
public:
explicit SnappyDecompressor(Source* reader)
: reader_(reader),
ip_(NULL),
ip_limit_(NULL),
peeked_(0),
eof_(false) {
}
~SnappyDecompressor() {
// Advance past any bytes we peeked at from the reader
reader_->Skip(peeked_);
}
// Returns true iff we have hit the end of the input without an error.
bool eof() const {
return eof_;
}
// Read the uncompressed length stored at the start of the compressed data.
// On succcess, stores the length in *result and returns true.
// On failure, returns false.
bool ReadUncompressedLength(uint32* result) {
DCHECK(ip_ == NULL); // Must not have read anything yet
// Length is encoded in 1..5 bytes
*result = 0;
uint32 shift = 0;
while (true) {
if (shift >= 32) return false;
size_t n;
const char* ip = reader_->Peek(&n);
if (n == 0) return false;
const unsigned char c = *(reinterpret_cast<const unsigned char*>(ip));
reader_->Skip(1);
*result |= static_cast<uint32>(c & 0x7f) << shift;
if (c < 128) {
break;
}
shift += 7;
}
return true;
}
// Process the next item found in the input.
// Returns true if successful, false on error or end of input.
template <class Writer>
void DecompressAllTags(Writer* writer) {
const char* ip = ip_;
// We could have put this refill fragment only at the beginning of the loop.
// However, duplicating it at the end of each branch gives the compiler more
// scope to optimize the <ip_limit_ - ip> expression based on the local
// context, which overall increases speed.
#define MAYBE_REFILL() \
if (ip_limit_ - ip < 5) { \
ip_ = ip; \
if (!RefillTag()) return; \
ip = ip_; \
}
MAYBE_REFILL();
for ( ;; ) {
const unsigned char c = *(reinterpret_cast<const unsigned char*>(ip++));
if ((c & 0x3) == LITERAL) {
size_t literal_length = (c >> 2) + 1u;
if (writer->TryFastAppend(ip, ip_limit_ - ip, literal_length)) {
DCHECK_LT(literal_length, 61);
ip += literal_length;
MAYBE_REFILL();
continue;
}
if (PREDICT_FALSE(literal_length >= 61)) {
// Long literal.
const size_t literal_length_length = literal_length - 60;
literal_length =
(LittleEndian::Load32(ip) & wordmask[literal_length_length]) + 1;
ip += literal_length_length;
}
size_t avail = ip_limit_ - ip;
while (avail < literal_length) {
if (!writer->Append(ip, avail)) return;
literal_length -= avail;
reader_->Skip(peeked_);
size_t n;
ip = reader_->Peek(&n);
avail = n;
peeked_ = avail;
if (avail == 0) return; // Premature end of input
ip_limit_ = ip + avail;
}
if (!writer->Append(ip, literal_length)) {
return;
}
ip += literal_length;
MAYBE_REFILL();
} else {
const uint32 entry = char_table[c];
const uint32 trailer = LittleEndian::Load32(ip) & wordmask[entry >> 11];
const uint32 length = entry & 0xff;
ip += entry >> 11;
// copy_offset/256 is encoded in bits 8..10. By just fetching
// those bits, we get copy_offset (since the bit-field starts at
// bit 8).
const uint32 copy_offset = entry & 0x700;
if (!writer->AppendFromSelf(copy_offset + trailer, length)) {
return;
}
MAYBE_REFILL();
}
}
#undef MAYBE_REFILL
}
};
bool SnappyDecompressor::RefillTag() {
const char* ip = ip_;
if (ip == ip_limit_) {
// Fetch a new fragment from the reader
reader_->Skip(peeked_); // All peeked bytes are used up
size_t n;
ip = reader_->Peek(&n);
peeked_ = n;
if (n == 0) {
eof_ = true;
return false;
}
ip_limit_ = ip + n;
}
// Read the tag character
DCHECK_LT(ip, ip_limit_);
const unsigned char c = *(reinterpret_cast<const unsigned char*>(ip));
const uint32 entry = char_table[c];
const uint32 needed = (entry >> 11) + 1; // +1 byte for 'c'
DCHECK_LE(needed, sizeof(scratch_));
// Read more bytes from reader if needed
uint32 nbuf = ip_limit_ - ip;
if (nbuf < needed) {
// Stitch together bytes from ip and reader to form the word
// contents. We store the needed bytes in "scratch_". They
// will be consumed immediately by the caller since we do not
// read more than we need.
memmove(scratch_, ip, nbuf);
reader_->Skip(peeked_); // All peeked bytes are used up
peeked_ = 0;
while (nbuf < needed) {
size_t length;
const char* src = reader_->Peek(&length);
if (length == 0) return false;
uint32 to_add = min<uint32>(needed - nbuf, length);
memcpy(scratch_ + nbuf, src, to_add);
nbuf += to_add;
reader_->Skip(to_add);
}
DCHECK_EQ(nbuf, needed);
ip_ = scratch_;
ip_limit_ = scratch_ + needed;
} else if (nbuf < 5) {
// Have enough bytes, but move into scratch_ so that we do not
// read past end of input
memmove(scratch_, ip, nbuf);
reader_->Skip(peeked_); // All peeked bytes are used up
peeked_ = 0;
ip_ = scratch_;
ip_limit_ = scratch_ + nbuf;
} else {
// Pass pointer to buffer returned by reader_.
ip_ = ip;
}
return true;
}
template <typename Writer>
static bool InternalUncompress(Source* r,
Writer* writer,
uint32 max_len) {
// Read the uncompressed length from the front of the compressed input
SnappyDecompressor decompressor(r);
uint32 uncompressed_len = 0;
if (!decompressor.ReadUncompressedLength(&uncompressed_len)) return false;
return InternalUncompressAllTags(
&decompressor, writer, uncompressed_len, max_len);
}
template <typename Writer>
static bool InternalUncompressAllTags(SnappyDecompressor* decompressor,
Writer* writer,
uint32 uncompressed_len,
uint32 max_len) {
// Protect against possible DoS attack
if (static_cast<uint64>(uncompressed_len) > max_len) {
return false;
}
writer->SetExpectedLength(uncompressed_len);
// Process the entire input
decompressor->DecompressAllTags(writer);
return (decompressor->eof() && writer->CheckLength());
}
bool GetUncompressedLength(Source* source, uint32* result) {
SnappyDecompressor decompressor(source);
return decompressor.ReadUncompressedLength(result);
}
size_t Compress(Source* reader, Sink* writer) {
size_t written = 0;
size_t N = reader->Available();
char ulength[Varint::kMax32];
char* p = Varint::Encode32(ulength, N);
writer->Append(ulength, p-ulength);
written += (p - ulength);
internal::WorkingMemory wmem;
char* scratch = NULL;
char* scratch_output = NULL;
while (N > 0) {
// Get next block to compress (without copying if possible)
size_t fragment_size;
const char* fragment = reader->Peek(&fragment_size);
DCHECK_NE(fragment_size, 0) << ": premature end of input";
const size_t num_to_read = min(N, kBlockSize);
size_t bytes_read = fragment_size;
size_t pending_advance = 0;
if (bytes_read >= num_to_read) {
// Buffer returned by reader is large enough
pending_advance = num_to_read;
fragment_size = num_to_read;
} else {
// Read into scratch buffer
if (scratch == NULL) {
// If this is the last iteration, we want to allocate N bytes
// of space, otherwise the max possible kBlockSize space.
// num_to_read contains exactly the correct value
scratch = new char[num_to_read];
}
memcpy(scratch, fragment, bytes_read);
reader->Skip(bytes_read);
while (bytes_read < num_to_read) {
fragment = reader->Peek(&fragment_size);
size_t n = min<size_t>(fragment_size, num_to_read - bytes_read);
memcpy(scratch + bytes_read, fragment, n);
bytes_read += n;
reader->Skip(n);
}
DCHECK_EQ(bytes_read, num_to_read);
fragment = scratch;
fragment_size = num_to_read;
}
DCHECK_EQ(fragment_size, num_to_read);
// Get encoding table for compression
int table_size;
uint16* table = wmem.GetHashTable(num_to_read, &table_size);
// Compress input_fragment and append to dest
const int max_output = MaxCompressedLength(num_to_read);
// Need a scratch buffer for the output, in case the byte sink doesn't
// have room for us directly.
if (scratch_output == NULL) {
scratch_output = new char[max_output];
} else {
// Since we encode kBlockSize regions followed by a region
// which is <= kBlockSize in length, a previously allocated
// scratch_output[] region is big enough for this iteration.
}
char* dest = writer->GetAppendBuffer(max_output, scratch_output);
char* end = internal::CompressFragment(fragment, fragment_size,
dest, table, table_size);
writer->Append(dest, end - dest);
written += (end - dest);
N -= num_to_read;
reader->Skip(pending_advance);
}
delete[] scratch;
delete[] scratch_output;
return written;
}
// -----------------------------------------------------------------------
// Flat array interfaces
// -----------------------------------------------------------------------
// A type that writes to a flat array.
// Note that this is not a "ByteSink", but a type that matches the
// Writer template argument to SnappyDecompressor::DecompressAllTags().
class SnappyArrayWriter {
private:
char* base_;
char* op_;
char* op_limit_;
public:
inline explicit SnappyArrayWriter(char* dst)
: base_(dst),
op_(dst) {
}
inline void SetExpectedLength(size_t len) {
op_limit_ = op_ + len;
}
inline bool CheckLength() const {
return op_ == op_limit_;
}
inline bool Append(const char* ip, size_t len) {
char* op = op_;
const size_t space_left = op_limit_ - op;
if (space_left < len) {
return false;
}
memcpy(op, ip, len);
op_ = op + len;
return true;
}
inline bool TryFastAppend(const char* ip, size_t available, size_t len) {
char* op = op_;
const size_t space_left = op_limit_ - op;
if (len <= 16 && available >= 16 && space_left >= 16) {
// Fast path, used for the majority (about 95%) of invocations.
UnalignedCopy64(ip, op);
UnalignedCopy64(ip + 8, op + 8);
op_ = op + len;
return true;
} else {
return false;
}
}
inline bool AppendFromSelf(size_t offset, size_t len) {
char* op = op_;
const size_t space_left = op_limit_ - op;
if (op - base_ <= offset - 1u) { // -1u catches offset==0
return false;
}
if (len <= 16 && offset >= 8 && space_left >= 16) {
// Fast path, used for the majority (70-80%) of dynamic invocations.
UnalignedCopy64(op - offset, op);
UnalignedCopy64(op - offset + 8, op + 8);
} else {
if (space_left >= len + kMaxIncrementCopyOverflow) {
IncrementalCopyFastPath(op - offset, op, len);
} else {
if (space_left < len) {
return false;
}
IncrementalCopy(op - offset, op, len);
}
}
op_ = op + len;
return true;
}
};
bool RawUncompress(const char* compressed, size_t n, char* uncompressed) {
ByteArraySource reader(compressed, n);
return RawUncompress(&reader, uncompressed);
}
bool RawUncompress(Source* compressed, char* uncompressed) {
SnappyArrayWriter output(uncompressed);
return InternalUncompress(compressed, &output, kuint32max);
}
bool Uncompress(const char* compressed, size_t n, string* uncompressed) {
size_t ulength;
if (!GetUncompressedLength(compressed, n, &ulength)) {
return false;
}
// Protect against possible DoS attack
if ((static_cast<uint64>(ulength) + uncompressed->size()) >
uncompressed->max_size()) {
return false;
}
STLStringResizeUninitialized(uncompressed, ulength);
return RawUncompress(compressed, n, string_as_array(uncompressed));
}
// A Writer that drops everything on the floor and just does validation
class SnappyDecompressionValidator {
private:
size_t expected_;
size_t produced_;
public:
inline SnappyDecompressionValidator() : produced_(0) { }
inline void SetExpectedLength(size_t len) {
expected_ = len;
}
inline bool CheckLength() const {
return expected_ == produced_;
}
inline bool Append(const char* ip, size_t len) {
produced_ += len;
return produced_ <= expected_;
}
inline bool TryFastAppend(const char* ip, size_t available, size_t length) {
return false;
}
inline bool AppendFromSelf(size_t offset, size_t len) {
if (produced_ <= offset - 1u) return false; // -1u catches offset==0
produced_ += len;
return produced_ <= expected_;
}
};
bool IsValidCompressedBuffer(const char* compressed, size_t n) {
ByteArraySource reader(compressed, n);
SnappyDecompressionValidator writer;
return InternalUncompress(&reader, &writer, kuint32max);
}
void RawCompress(const char* input,
size_t input_length,
char* compressed,
size_t* compressed_length) {
ByteArraySource reader(input, input_length);
UncheckedByteArraySink writer(compressed);
Compress(&reader, &writer);
// Compute how many bytes were added
*compressed_length = (writer.CurrentDestination() - compressed);
}
size_t Compress(const char* input, size_t input_length, string* compressed) {
// Pre-grow the buffer to the max length of the compressed output
compressed->resize(MaxCompressedLength(input_length));
size_t compressed_length;
RawCompress(input, input_length, string_as_array(compressed),
&compressed_length);
compressed->resize(compressed_length);
return compressed_length;
}
} // end namespace snappy
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