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// Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
// See header file for description of DnsQueue class
#include "components/network_hints/renderer/dns_prefetch_queue.h"
#include "base/logging.h"
namespace network_hints {
DnsQueue::DnsQueue(BufferSize size)
: buffer_(new char[size + 2]),
buffer_size_(size + 1),
buffer_sentinel_(size + 1),
size_(0) {
CHECK(0 < static_cast<BufferSize>(size + 3)); // Avoid overflow worries.
buffer_[buffer_sentinel_] = '\0'; // Guard byte to help reading data.
readable_ = writeable_ = 0; // Buffer starts empty.
}
DnsQueue::~DnsQueue(void) {
}
void DnsQueue::Clear() {
size_ = 0;
readable_ = writeable_;
DCHECK(Validate());
}
// Push takes an unterminated string plus its length.
// The string must not contain a null terminator.
// Exactly length chars are written, or nothing is written.
// Returns true for success, false there was no room to push.
DnsQueue::PushResult DnsQueue::Push(const char* source,
const size_t unsigned_length) {
BufferSize length = static_cast<BufferSize>(unsigned_length);
if (0 > length+1) // Avoid overflows in conversion to signed.
return OVERFLOW_PUSH;
// To save on sites with a LOT of links to the SAME domain, we have a
// a compaction hack that removes duplicates when we try to push() a
// match with the last push.
if (0 < size_ && readable_ + length < buffer_sentinel_ &&
0 == strncmp(source, &buffer_[readable_], unsigned_length) &&
'\0' == buffer_[readable_ + unsigned_length]) {
// We already wrote this name to the queue, so we'll skip this repeat.
return REDUNDANT_PUSH;
}
// Calling convention precludes nulls.
DCHECK(!length || '\0' != source[length - 1]);
DCHECK(Validate());
BufferSize available_space = readable_ - writeable_;
if (0 >= available_space) {
available_space += buffer_size_;
}
if (length + 1 >= available_space)
return OVERFLOW_PUSH; // Not enough space to push.
BufferSize dest = writeable_;
BufferSize space_till_wrap = buffer_sentinel_ - dest;
if (space_till_wrap < length + 1) {
// Copy until we run out of room at end of buffer.
std::memcpy(&buffer_[dest], source, space_till_wrap);
// Ensure caller didn't have embedded '\0' and also
// ensure trailing sentinel was in place.
// Relies on sentinel.
DCHECK(static_cast<size_t>(space_till_wrap) == strlen(&buffer_[dest]));
length -= space_till_wrap;
source += space_till_wrap;
dest = 0; // Continue writing at start of buffer.
}
// Copy any remaining portion of source.
std::memcpy(&buffer_[dest], source, length);
DCHECK(dest + length < buffer_sentinel_);
buffer_[dest + length] = '\0'; // We need termination in our buffer.
// Preclude embedded '\0'.
DCHECK(static_cast<size_t>(length) == strlen(&buffer_[dest]));
dest += length + 1;
if (dest == buffer_sentinel_)
dest = 0;
writeable_ = dest;
size_++;
DCHECK(Validate());
return SUCCESSFUL_PUSH;
}
// Extracts the next available string from the buffer.
// The returned string is null terminated, and hence has length
// that is exactly one greater than the written string.
// If the buffer is empty, then the Pop and returns false.
bool DnsQueue::Pop(std::string* out_string) {
DCHECK(Validate());
// Sentinel will preclude memory reads beyond buffer's end.
DCHECK('\0' == buffer_[buffer_sentinel_]);
if (readable_ == writeable_) {
return false; // buffer was empty
}
// Constructor *may* rely on sentinel for null termination.
(*out_string) = &buffer_[readable_];
// Our sentinel_ at end of buffer precludes an overflow in cast.
BufferSize first_fragment_size = static_cast<BufferSize> (out_string->size());
BufferSize terminal_null;
if (readable_ + first_fragment_size >= buffer_sentinel_) {
// Sentinel was used, so we need the portion after the wrap.
out_string->append(&buffer_[0]); // Fragment at start of buffer.
// Sentinel precludes overflow in cast to signed type.
terminal_null = static_cast<BufferSize>(out_string->size())
- first_fragment_size;
} else {
terminal_null = readable_ + first_fragment_size;
}
DCHECK('\0' == buffer_[terminal_null]);
BufferSize new_readable = terminal_null + 1;
if (buffer_sentinel_ == new_readable)
new_readable = 0;
readable_ = new_readable;
size_--;
if (readable_ == writeable_ || 0 == size_) {
// Queue is empty, so reset to start of buffer to help with peeking.
readable_ = writeable_ = 0;
}
DCHECK(Validate());
return true;
}
bool DnsQueue::Validate() {
return (readable_ >= 0) &&
readable_ < buffer_sentinel_ &&
writeable_ >= 0 &&
writeable_ < buffer_sentinel_ &&
'\0' == buffer_[buffer_sentinel_] &&
((0 == size_) == (readable_ == writeable_));
}
} // namespace network_hints
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