// Copyright 2015 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. #include "net/base/lookup_string_in_fixed_set.h" #include "base/logging.h" namespace net { namespace { // Read next offset from |pos|, increment |offset| by that amount, and increment // |pos| either to point to the start of the next encoded offset in its node, or // nullptr, if there are no remaining offsets. // // Returns true if an offset could be read; false otherwise. inline bool GetNextOffset(const unsigned char** pos, const unsigned char** offset) { if (*pos == nullptr) return false; size_t bytes_consumed; switch (**pos & 0x60) { case 0x60: // Read three byte offset *offset += (((*pos)[0] & 0x1F) << 16) | ((*pos)[1] << 8) | (*pos)[2]; bytes_consumed = 3; break; case 0x40: // Read two byte offset *offset += (((*pos)[0] & 0x1F) << 8) | (*pos)[1]; bytes_consumed = 2; break; default: *offset += (*pos)[0] & 0x3F; bytes_consumed = 1; } if ((**pos & 0x80) != 0) { *pos = nullptr; } else { *pos += bytes_consumed; } return true; } // Check if byte at |offset| is last in label. bool IsEOL(const unsigned char* offset) { return (*offset & 0x80) != 0; } // Check if byte at |offset| matches key. This version matches both end-of-label // chars and not-end-of-label chars. bool IsMatch(const unsigned char* offset, char key) { return (*offset & 0x7F) == key; } // Read return value at |offset|, if it is a return value. Returns true if a // return value could be read, false otherwise. bool GetReturnValue(const unsigned char* offset, int* return_value) { // Return values are always encoded as end-of-label chars (so the high bit is // set). So byte values in the inclusive range [0x80, 0x9F] encode the return // values 0 through 31 (though make_dafsa.py doesn't currently encode values // higher than 7). The following code does that translation. if ((*offset & 0xE0) == 0x80) { *return_value = *offset & 0x1F; return true; } return false; } } // namespace FixedSetIncrementalLookup::FixedSetIncrementalLookup(const unsigned char* graph, size_t length) : pos_(graph), end_(graph + length), pos_is_label_character_(false) {} FixedSetIncrementalLookup::FixedSetIncrementalLookup( const FixedSetIncrementalLookup& other) = default; FixedSetIncrementalLookup& FixedSetIncrementalLookup::operator=( const FixedSetIncrementalLookup& other) = default; FixedSetIncrementalLookup::~FixedSetIncrementalLookup() {} bool FixedSetIncrementalLookup::Advance(char input) { if (!pos_) { // A previous input exhausted the graph, so there are no possible matches. return false; } // Only ASCII printable chars are supported by the current DAFSA format -- the // high bit (values 0x80-0xFF) is reserved as a label-end signifier, and the // low values (values 0x00-0x1F) are reserved to encode the return values. So // values outside this range will never be in the dictionary. if (input >= 0x20) { if (pos_is_label_character_) { // Currently processing a label, so it is only necessary to check the byte // at |pos_| to see if it encodes a character matching |input|. bool is_last_char_in_label = IsEOL(pos_); bool is_match = IsMatch(pos_, input); if (is_match) { // If this is not the last character in the label, the next byte should // be interpreted as a character or return value. Otherwise, the next // byte should be interpreted as a list of child node offsets. ++pos_; DCHECK(pos_ < end_); pos_is_label_character_ = !is_last_char_in_label; return true; } } else { const unsigned char* offset = pos_; // Read offsets from |pos_| until the label of the child node at |offset| // matches |input|, or until there are no more offsets. while (GetNextOffset(&pos_, &offset)) { DCHECK(offset < end_); DCHECK((pos_ == nullptr) || (pos_ < end_)); // |offset| points to a DAFSA node that is a child of the original node. // // The low 7 bits of a node encodes a character value; the high bit // indicates whether it's the last character in the label. // // Note that |*offset| could also be a result code value, but these are // really just out-of-range ASCII values, encoded the same way as // characters. Since |input| was already validated as a printable ASCII // value ASCII value, IsMatch will never return true if |offset| is a // result code. bool is_last_char_in_label = IsEOL(offset); bool is_match = IsMatch(offset, input); if (is_match) { // If this is not the last character in the label, the next byte // should be interpreted as a character or return value. Otherwise, // the next byte should be interpreted as a list of child node // offsets. pos_ = offset + 1; DCHECK(pos_ < end_); pos_is_label_character_ = !is_last_char_in_label; return true; } } } } // If no match was found, then end of the DAFSA has been reached. pos_ = nullptr; pos_is_label_character_ = false; return false; } int FixedSetIncrementalLookup::GetResultForCurrentSequence() const { int value = kDafsaNotFound; // Look to see if there is a next character that's a return value. if (pos_is_label_character_) { // Currently processing a label, so it is only necessary to check the byte // at |pos_| to see if encodes a return value. GetReturnValue(pos_, &value); } else { // Otherwise, |pos_| is an offset list (or nullptr). Explore the list of // child nodes (given by their offsets) to find one whose label is a result // code. // // This search uses a temporary copy of |pos_|, since mutating |pos_| could // skip over a node that would be important to a subsequent Advance() call. const unsigned char* temp_pos = pos_; // Read offsets from |temp_pos| until either |temp_pos| is nullptr or until // the byte at |offset| contains a result code (encoded as an ASCII // character below 0x20). const unsigned char* offset = pos_; while (GetNextOffset(&temp_pos, &offset)) { DCHECK(offset < end_); DCHECK((temp_pos == nullptr) || temp_pos < end_); if (GetReturnValue(offset, &value)) break; } } return value; } int LookupStringInFixedSet(const unsigned char* graph, size_t length, const char* key, size_t key_length) { // Do an incremental lookup until either the end of the graph is reached, or // until every character in |key| is consumed. FixedSetIncrementalLookup lookup(graph, length); const char* key_end = key + key_length; while (key != key_end) { if (!lookup.Advance(*key)) return kDafsaNotFound; key++; } // The entire input was consumed without reaching the end of the graph. Return // the result code (if present) for the current position, or kDafsaNotFound. return lookup.GetResultForCurrentSequence(); } } // namespace net