// Copyright 2007 Google Inc.
// Author: Lincoln Smith
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// Implementation of the Address Cache and Address Encoding
// algorithms described in sections 5.1 - 5.4 of RFC 3284 -
// The VCDIFF Generic Differencing and Compression Data Format.
// The RFC text can be found at http://www.faqs.org/rfcs/rfc3284.html
//
// Assumptions:
// * The VCDAddress type is large enough to hold any offset within
// the source and target windows. The limit (for int32_t) is 2^31-1 bytes.
// The source (dictionary) should not approach this size limit;
// to compress a target file that is larger than
// INT_MAX - (dictionary size) bytes, the encoder must
// break it up into multiple target windows.
#include <config.h>
#include "addrcache.h"
#include "logging.h"
#include "varint_bigendian.h"
#include "vcdiff_defs.h" // RESULT_ERROR
namespace open_vcdiff {
// The constructor does not initialize near_addresses_ and same_addresses_.
// Therefore, Init() must be called before any other method can be used.
//
// Arguments:
// near_cache_size: Size of the NEAR cache (number of 4-byte integers)
// same_cache_size: Size of the SAME cache (number of blocks of
// 256 4-byte integers per block)
// Because the mode is expressed as a byte value,
// near_cache_size + same_cache_size should not exceed 254.
//
VCDiffAddressCache::VCDiffAddressCache(int near_cache_size,
int same_cache_size)
: near_cache_size_(near_cache_size),
same_cache_size_(same_cache_size),
next_slot_(0) { }
VCDiffAddressCache::VCDiffAddressCache()
: near_cache_size_(kDefaultNearCacheSize),
same_cache_size_(kDefaultSameCacheSize),
next_slot_(0) { }
// Sets up data structures needed to call other methods. Operations that may
// fail at runtime (for example, validating the provided near_cache_size_ and
// same_cache_size_ parameters against their maximum allowed values) are
// confined to this routine in order to guarantee that the class constructor
// will never fail. Other methods (except the destructor) cannot be invoked
// until this method has been called successfully. After the object has been
// initialized and used, Init() can be called again to reset it to its initial
// state.
//
// Return value: "true" if initialization succeeded, "false" if it failed.
// No other method except the destructor may be invoked if this function
// returns false. The caller is responsible for checking the return value
// and providing an exit path in case of error.
//
bool VCDiffAddressCache::Init() {
// The mode is expressed as a byte value, so there is only room for 256 modes,
// including the two non-cached modes (SELF and HERE). Do not allow a larger
// number of modes to be defined. We do a separate sanity check for
// near_cache_size_ and same_cache_size_ because adding them together can
// cause an integer overflow if each is set to, say, INT_MAX.
if ((near_cache_size_ > (VCD_MAX_MODES - 2)) || (near_cache_size_ < 0)) {
VCD_ERROR << "Near cache size " << near_cache_size_ << " is invalid"
<< VCD_ENDL;
return false;
}
if ((same_cache_size_ > (VCD_MAX_MODES - 2)) || (same_cache_size_ < 0)) {
VCD_ERROR << "Same cache size " << same_cache_size_ << " is invalid"
<< VCD_ENDL;
return false;
}
if ((near_cache_size_ + same_cache_size_) > VCD_MAX_MODES - 2) {
VCD_ERROR << "Using near cache size " << near_cache_size_
<< " and same cache size " << same_cache_size_
<< " would exceed maximum number of COPY modes ("
<< VCD_MAX_MODES << ")" << VCD_ENDL;
return false;
}
if (near_cache_size_ > 0) {
near_addresses_.assign(near_cache_size_, 0);
}
if (same_cache_size_ > 0) {
same_addresses_.assign(same_cache_size_ * 256, 0);
}
next_slot_ = 0; // in case Init() is called a second time to reinit
return true;
}
// This method will be called whenever an address is calculated for an
// encoded or decoded COPY instruction, and will update the contents
// of the SAME and NEAR caches. It is vital that the use of
// UpdateCache (called cache_update in the RFC examples) exactly match
// the RFC standard, and that the same caching logic be used in the
// decoder as in the encoder, in order for the decoded addresses to
// match.
//
// Argument:
// address: This must be a valid address between 0 and
// (source window size + target window size). It is assumed that
// these bounds have been checked before calling UpdateCache.
//
void VCDiffAddressCache::UpdateCache(VCDAddress address) {
if (near_cache_size_ > 0) {
near_addresses_[next_slot_] = address;
next_slot_ = (next_slot_ + 1) % near_cache_size_;
}
if (same_cache_size_ > 0) {
same_addresses_[address % (same_cache_size_ * 256)] = address;
}
}
// Determines the address mode that yields the most compact encoding
// of the given address value, writes the encoded address into the
// address stream, and returns the mode used. The most compact encoding
// is found by looking for the numerically lowest encoded address.
// The Init() function must already have been called.
//
// Arguments:
// address: The address to be encoded. Must be a non-negative integer
// between 0 and (here_address - 1).
// here_address: The current location in the target data (i.e., the
// position just after the last encoded value.) Must be non-negative.
// encoded_addr: Points to an VCDAddress that will be replaced
// with the encoded representation of address.
// If WriteAddressAsVarintForMode returns true when passed
// the return value, then encoded_addr should be written
// into the delta file as a variable-length integer (Varint);
// otherwise, it should be written as a byte (unsigned char).
//
// Return value: A mode value between 0 and 255. The mode will tell
// how to interpret the next value in the address stream.
// The values 0 and 1 correspond to SELF and HERE addressing.
//
// The function is guaranteed to succeed unless the conditions on the arguments
// have not been met, in which case a VCD_DFATAL message will be produced,
// 0 will be returned, and *encoded_addr will be replaced with 0.
//
unsigned char VCDiffAddressCache::EncodeAddress(VCDAddress address,
VCDAddress here_address,
VCDAddress* encoded_addr) {
if (address < 0) {
VCD_DFATAL << "EncodeAddress was passed a negative address: "
<< address << VCD_ENDL;
*encoded_addr = 0;
return 0;
}
if (address >= here_address) {
VCD_DFATAL << "EncodeAddress was called with address (" << address
<< ") < here_address (" << here_address << ")" << VCD_ENDL;
*encoded_addr = 0;
return 0;
}
// Try using the SAME cache. This method, if available, always
// results in the smallest encoding and takes priority over other modes.
if (same_cache_size() > 0) {
const VCDAddress same_cache_pos =
address % (same_cache_size() * 256);
if (SameAddress(same_cache_pos) == address) {
// This is the only mode for which an single byte will be written
// to the address stream instead of a variable-length integer.
UpdateCache(address);
*encoded_addr = same_cache_pos % 256;
return FirstSameMode() + (same_cache_pos / 256); // SAME mode
}
}
// Try SELF mode
unsigned char best_mode = VCD_SELF_MODE;
VCDAddress best_encoded_address = address;
// Try HERE mode
{
const VCDAddress here_encoded_address = here_address - address;
if (here_encoded_address < best_encoded_address) {
best_mode = VCD_HERE_MODE;
best_encoded_address = here_encoded_address;
}
}
// Try using the NEAR cache
for (int i = 0; i < near_cache_size(); ++i) {
const VCDAddress near_encoded_address = address - NearAddress(i);
if ((near_encoded_address >= 0) &&
(near_encoded_address < best_encoded_address)) {
best_mode = FirstNearMode() + i;
best_encoded_address = near_encoded_address;
}
}
UpdateCache(address);
*encoded_addr = best_encoded_address;
return best_mode;
}
// Increments *byte_pointer and returns the byte it pointed to before the
// increment. The caller must check bounds to ensure that *byte_pointer
// points to a valid address in memory.
static unsigned char ParseByte(const char** byte_pointer) {
unsigned char byte_value = static_cast<unsigned char>(**byte_pointer);
++(*byte_pointer);
return byte_value;
}
// Checks the given decoded address for validity. Returns true if the
// address is valid; otherwise, prints an error message to the log and
// returns false.
static bool IsDecodedAddressValid(VCDAddress decoded_address,
VCDAddress here_address) {
if (decoded_address < 0) {
VCD_ERROR << "Decoded address " << decoded_address << " is invalid"
<< VCD_ENDL;
return false;
} else if (decoded_address >= here_address) {
VCD_ERROR << "Decoded address (" << decoded_address
<< ") is beyond location in target file (" << here_address
<< ")" << VCD_ENDL;
return false;
}
return true;
}
// Interprets the next value in the address_stream using the provided mode,
// which may need to access the SAME or NEAR address cache. Returns the
// decoded address.
// The Init() function must already have been called.
//
// Arguments:
// here_address: The current location in the source + target data (i.e., the
// location into which the COPY instruction will copy.) By definition,
// all addresses between 0 and (here_address - 1) are valid, and
// any other address is invalid.
// mode: A byte value between 0 and (near_cache_size_ + same_cache_size_ + 1)
// which tells how to interpret the next value in the address stream.
// The values 0 and 1 correspond to SELF and HERE addressing.
// The validity of "mode" should already have been checked before
// calling this function.
// address_stream: Points to a pointer holding the position
// in the "Addresses section for COPYs" part of the input data.
// That section must already have been uncompressed
// using a secondary decompressor (if necessary.)
// This is an IN/OUT argument; the value of *address_stream will be
// incremented by the size of an integer, or (if the SAME cache
// was used) by the size of a byte (1).
// address_stream_end: Points to the position just after the end of
// the address stream buffer. All addresses between *address_stream
// and address_stream_end should contain valid address data.
//
// Return value: If the input conditions were met, and the address section
// of the input data contains properly encoded addresses that match
// the instructions section, then an integer between 0 and here_address - 1
// will be returned, representing the address from which data should
// be copied from the source or target window into the output stream.
// If an invalid address value is found in address_stream, then
// RESULT_ERROR will be returned. If the limit address_stream_end
// is reached before the address can be decoded, then
// RESULT_END_OF_DATA will be returned. If more streamed data
// is expected, this means that the consumer should block and wait
// for more data before continuing to decode. If no more data is expected,
// this return value signals an error condition.
//
VCDAddress VCDiffAddressCache::DecodeAddress(VCDAddress here_address,
unsigned char mode,
const char** address_stream,
const char* address_stream_end) {
if (here_address < 0) {
VCD_DFATAL << "DecodeAddress was passed a negative value"
" for here_address: " << here_address << VCD_ENDL;
return RESULT_ERROR;
}
const char* new_address_pos = *address_stream;
if (new_address_pos >= address_stream_end) {
return RESULT_END_OF_DATA;
}
VCDAddress decoded_address;
if (IsSameMode(mode)) {
// SAME mode expects a byte value as the encoded address
unsigned char encoded_address = ParseByte(&new_address_pos);
decoded_address = DecodeSameAddress(mode, encoded_address);
} else {
// All modes except SAME mode expect a VarintBE as the encoded address
int32_t encoded_address = VarintBE<int32_t>::Parse(address_stream_end,
&new_address_pos);
switch (encoded_address) {
case RESULT_ERROR:
VCD_ERROR << "Found invalid variable-length integer "
"as encoded address value" << VCD_ENDL;
return RESULT_ERROR;
case RESULT_END_OF_DATA:
return RESULT_END_OF_DATA;
default:
break;
}
if (IsSelfMode(mode)) {
decoded_address = DecodeSelfAddress(encoded_address);
} else if (IsHereMode(mode)) {
decoded_address = DecodeHereAddress(encoded_address, here_address);
} else if (IsNearMode(mode)) {
decoded_address = DecodeNearAddress(mode, encoded_address);
} else {
VCD_DFATAL << "Invalid mode value (" << static_cast<int>(mode)
<< ") passed to DecodeAddress; maximum mode value = "
<< static_cast<int>(LastMode()) << VCD_ENDL;
return RESULT_ERROR;
}
}
// Check for an out-of-bounds address (corrupt/malicious data)
if (!IsDecodedAddressValid(decoded_address, here_address)) {
return RESULT_ERROR;
}
*address_stream = new_address_pos;
UpdateCache(decoded_address);
return decoded_address;
}
} // namespace open_vcdiff