summaryrefslogtreecommitdiff
path: root/snappy/snappy.cc
diff options
context:
space:
mode:
Diffstat (limited to 'snappy/snappy.cc')
-rw-r--r--snappy/snappy.cc1119
1 files changed, 1119 insertions, 0 deletions
diff --git a/snappy/snappy.cc b/snappy/snappy.cc
new file mode 100644
index 00000000..1230321f
--- /dev/null
+++ b/snappy/snappy.cc
@@ -0,0 +1,1119 @@
+// 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) {
+ assert(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) {
+ assert(len <= 64);
+ assert(len >= 4);
+ assert(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;
+ }
+
+ 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) {
+ assert(offset >= 0);
+ assert(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) {
+ assert(offset >= 0);
+ assert(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;
+ assert(input_size <= kBlockSize);
+ assert((table_size & (table_size - 1)) == 0); // table must be power of two
+ const int shift = 32 - Bits::Log2Floor(table_size);
+ assert(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); ; ) {
+ assert(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;
+ assert(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];
+ assert(candidate >= base_ip);
+ assert(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."
+ assert(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;
+ assert(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
+ 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<int>(dst[i]), static_cast<int>(char_table[i]));
+ abort();
+ }
+ }
+}
+#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) {
+ assert(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)) {
+ assert(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
+ assert(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'
+ assert(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);
+ }
+ assert(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);
+ assert(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);
+ }
+ assert(bytes_read == num_to_read);
+ fragment = scratch;
+ fragment_size = num_to_read;
+ }
+ assert(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
+
generated by cgit v1.2.3 (git 2.25.1) at 2024-11-11 15:06:52 +0000