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- // 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.
- #ifndef NET_BASE_LOOKUP_STRING_IN_FIXED_SET_H_
- #define NET_BASE_LOOKUP_STRING_IN_FIXED_SET_H_
- #include <stddef.h>
- #include "base/strings/string_piece.h"
- #include "net/base/net_export.h"
- namespace net {
- enum {
- kDafsaNotFound = -1, // key is not in set
- kDafsaFound = 0, // key is in set
- // The following return values are used by the implementation of
- // GetDomainAndRegistry() and are probably not generally useful.
- kDafsaExceptionRule = 1, // key excluded from set via exception
- kDafsaWildcardRule = 2, // key matched a wildcard rule
- kDafsaPrivateRule = 4, // key matched a private rule
- };
- // Looks up the string |key| with length |key_length| in a fixed set of
- // strings. The set of strings must be known at compile time. It is converted to
- // a graph structure named a DAFSA (Deterministic Acyclic Finite State
- // Automaton) by the script make_dafsa.py during compilation. This permits
- // efficient (in time and space) lookup. The graph generated by make_dafsa.py
- // takes the form of a constant byte array which should be supplied via the
- // |graph| and |length| parameters. The return value is kDafsaNotFound,
- // kDafsaFound, or a bitmap consisting of one or more of kDafsaExceptionRule,
- // kDafsaWildcardRule and kDafsaPrivateRule ORed together.
- //
- // TODO(nick): Replace this with FixedSetIncrementalLookup everywhere.
- NET_EXPORT int LookupStringInFixedSet(const unsigned char* graph,
- size_t length,
- const char* key,
- size_t key_length);
- // Looks up the longest matching suffix for |host| with length |length| in a
- // reversed DAFSA. Partial matches must begin at a new component, i.e. host
- // itself could match or a host part starting after a dot could match.
- // If no match was found a value of 0 is written to |suffix_length| and the
- // value kDafsaNotFound is returned, otherwise the length of the longest match
- // is written to |suffix_length| and the type of the longest match is returned.
- int LookupSuffixInReversedSet(const unsigned char* graph,
- size_t length,
- bool include_private,
- base::StringPiece host,
- size_t* suffix_length);
- // FixedSetIncrementalLookup provides efficient membership and prefix queries
- // against a fixed set of strings. The set of strings must be known at compile
- // time. The set is converted to a graph structure named a DAFSA (Deterministic
- // Acyclic Finite State Automaton) by the script //net/tools/dafsa/make_dafsa.py
- // during compilation. The conversion generates a C++ header file defining the
- // encoded graph as a constant byte array. This class provides a fast, constant-
- // space lookup operation against such byte arrays.
- //
- // The lookup proceeds incrementally, with input characters provided one at a
- // time. This approach allow queries of the form: "given an input string, which
- // prefixes of that string that appear in the fixed set?" As the matching
- // prefixes (and their result codes) are enumerated, the most suitable match
- // among them can be selected in a single pass.
- //
- // This class can also be used to perform suffix queries (instead of prefix
- // queries) against a fixed set, so long as the DAFSA is constructed on reversed
- // values, and the input is provided in reverse order.
- //
- // Example usage for simple membership query; |input| is a std::string:
- //
- // FixedSetIncrementalLookup lookup(kDafsa, sizeof(kDafsa));
- // for (char c : input) {
- // if (!lookup.Advance(c))
- // return false;
- // }
- // return lookup.GetResultForCurrentSequence() != kDafsaNotFound;
- //
- // Example usage for 'find longest prefix in set with result code == 3' query:
- //
- // FixedSetIncrementalLookup prefix_lookup(kDafsa, sizeof(kDafsa));
- // size_t longest_match_end = 0;
- // for (size_t i = 0; i < input.length(); ++i) {
- // if (!prefix_lookup.Advance(input[i]))
- // break;
- // if (prefix_lookup.GetResultForCurrentSequence() == 3)
- // longest_match_end = (i + 1);
- // }
- // return input.substr(0, longest_match_end);
- //
- class NET_EXPORT FixedSetIncrementalLookup {
- public:
- // Begin a lookup against the provided fixed set. |graph| and |length|
- // describe a byte buffer generated by the make_dafsa.py script, as described
- // in the class comment.
- //
- // FixedSetIncrementalLookup is initialized to a state corresponding to the
- // empty input sequence. Calling GetResultForCurrentSequence() in the initial
- // state would indicate whether the empty string appears in the fixed set.
- // Characters can be added to the sequence by calling Advance(), and the
- // lookup result can be checked after each addition by calling
- // GetResultForCurrentSequence().
- FixedSetIncrementalLookup(const unsigned char* graph, size_t length);
- // FixedSetIncrementalLookup is copyable so that callers can save/restore
- // their position in the search. This is for cases where branching or
- // backtracking might be required (e.g. to probe for the presence of a
- // designated wildcard character).
- FixedSetIncrementalLookup(const FixedSetIncrementalLookup&);
- FixedSetIncrementalLookup& operator=(const FixedSetIncrementalLookup&);
- ~FixedSetIncrementalLookup();
- // Advance the query by adding a character to the input sequence. |input| can
- // be any char value, but only ASCII characters will ever result in matches,
- // since the fixed set itself is limited to ASCII strings.
- //
- // Returns true if the resulting input sequence either appears in the fixed
- // set itself, or is a prefix of some longer string in the fixed set. Returns
- // false otherwise, implying that the graph is exhausted and
- // GetResultForCurrentSequence() will return kDafsaNotFound.
- //
- // Once Advance() has returned false, the caller can safely stop feeding more
- // characters, as subsequent calls to Advance() will return false and have no
- // effect.
- bool Advance(char input);
- // Returns the result code corresponding to the input sequence provided thus
- // far to Advance().
- //
- // If the sequence does not appear in the fixed set, the return value is
- // kDafsaNotFound. Otherwise, the value is a non-negative integer (currently
- // limited to 0-7) corresponding to the result code for that string, as listed
- // in the .gperf file from which the DAFSA was generated. For
- // GetDomainAndRegistry DAFSAs, these values should be interpreted as a
- // bitmask of kDafsaExceptionRule, kDafsaWildcardRule, and kDafsaPrivateRule.
- //
- // It is okay to call this function, and then extend the sequence further by
- // calling Advance().
- int GetResultForCurrentSequence() const;
- private:
- // Pointer to the current position in the graph indicating the current state
- // of the automaton, or nullptr if the graph is exhausted.
- const unsigned char* pos_;
- // Pointer just past the end of the graph. |pos_| should never get here. This
- // is used only in DCHECKs.
- const unsigned char* end_;
- // Contains the current decoder state. If true, |pos_| points to a label
- // character or a return code. If false, |pos_| points to a sequence of
- // offsets that indicate the child nodes of the current state.
- bool pos_is_label_character_ = false;
- };
- } // namespace net
- #endif // NET_BASE_LOOKUP_STRING_IN_FIXED_SET_H_
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