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// Copyright 2020 The Pigweed Authors
// 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
// 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.
#pragma once
#include <array>
#include <cstddef>
#include <cstdint>
#include <iterator>
namespace pw::tokenizer {
// Reads entries from a binary token string database. This class does not copy
// or modify the contents of the database.
// A binary token database is comprised of a 16-byte header followed by an array
// of 8-byte entries and a table of null-terminated strings. The header
// specifies the number of entries. Each entry contains information about a
// tokenized string: the token and removal date, if any. All fields are
// little-endian.
// Header
// ======
// Offset Size Field
// -----------------------------------
// 0 6 Magic number (TOKENS)
// 6 2 Version (00 00)
// 8 4 Entry count
// 12 4 Reserved
// Entry
// =====
// Offset Size Field
// -----------------------------------
// 0 4 Token
// 4 4 Removal date (d m yy)
// Entries are sorted by token. A string table with a null-terminated string for
// each entry in order follows the entries.
// Entries are accessed by iterating over the database. A O(n) Find function is
// also provided. In typical use, a TokenDatabase is preprocessed by a
// Detokenizer into a std::unordered_map.
class TokenDatabase {
// Internal struct that describes how the underlying binary token database
// stores entries. RawEntries generally should not be used directly. Instead,
// use an Entry, which contains a pointer to the entry's string.
struct RawEntry {
uint32_t token;
uint32_t date_removed;
static_assert(sizeof(RawEntry) == 8u);
// An entry in the token database. This struct adds the string to a RawEntry.
struct Entry {
// The token calculated for this string.
uint32_t token;
// The date the token and string was removed from the database, or
// 0xFFFFFFFF if it was never removed. Dates are encoded such that natural
// integer sorting sorts from oldest to newest dates. The day is stored an
// an 8-bit day, 8-bit month, and 16-bit year, packed into a little-endian
// uint32_t.
uint32_t date_removed;
// The null-terminated string represented by this token.
const char* string;
// Iterator for TokenDatabase values. Note that this is not a normal STL-style
// iterator, since * returns a value instead of a reference.
class Iterator {
constexpr Iterator(const RawEntry* raw_entry, const char* string)
: raw_(raw_entry), string_(string) {}
// Constructs a TokenDatabase::Entry for the entry this iterator refers to.
constexpr Entry entry() const {
return {raw_->token, raw_->date_removed, string_};
constexpr Iterator& operator++() {
raw_ += 1;
// Move string_ to the character beyond the next null terminator.
while (*string_++ != '\0') {
return *this;
constexpr Iterator operator++(int) { return operator++(); }
constexpr bool operator==(const Iterator& rhs) const {
return raw_ == rhs.raw_;
constexpr bool operator!=(const Iterator& rhs) const {
return raw_ != rhs.raw_;
// Derefencing a TokenDatabase::Iterator returns an Entry, not an Entry&.
constexpr Entry operator*() const { return entry(); }
// Point directly into the underlying RawEntry. Strings are not accessible
// via this operator.
constexpr const RawEntry* operator->() const { return raw_; }
constexpr ptrdiff_t operator-(const Iterator& rhs) const {
return raw_ - rhs.raw_;
const RawEntry* raw_;
const char* string_;
// A list of token entries returned from a Find operation. This object can be
// iterated over or indexed as an array.
class Entries {
constexpr Entries(const Iterator& begin, const Iterator& end)
: begin_(begin), end_(end) {}
// The number of entries in this list.
constexpr size_t size() const { return end_ - begin_; }
// True of the list is empty.
constexpr bool empty() const { return begin_ == end_; }
// Accesses the specified entry in this set. Returns an Entry object, which
// is constructed from the underlying raw entry. The index must be less than
// size(). This operation is O(n) in size().
Entry operator[](size_t index) const;
constexpr const Iterator& begin() const { return begin_; }
constexpr const Iterator& end() const { return end_; }
Iterator begin_;
Iterator end_;
// Returns true if the provided data is a valid token database. This checks
// the magic number ("TOKENS"), version (which must be 0), and that there is
// is one string for each entry in the database. A database with extra strings
// or other trailing data is considered valid.
template <typename ByteArray>
static constexpr bool IsValid(const ByteArray& bytes) {
return HasValidHeader(bytes) && EachEntryHasAString(bytes);
// Creates a TokenDatabase and checks if the provided data is valid at compile
// time. Accepts references to constexpr containers (array, span, string_view,
// etc.) with static storage duration. For example:
// constexpr char kMyData[] = ...;
// constexpr TokenDatabase db = TokenDatabase::Create<kMyData>();
template <const auto& kDatabaseBytes>
static constexpr TokenDatabase Create() {
"Databases must start with a 16-byte header that begins with TOKENS.");
"The database must have at least one string for each entry.");
return TokenDatabase(std::data(kDatabaseBytes));
// Creates a TokenDatabase from the provided byte array. The array may be a
// span, array, or other container type. If the data is not valid, returns a
// default-constructed database for which ok() is false.
// Prefer the Create overload that takes the data as a template parameter
// whenever possible, since that function checks the integrity of the data at
// compile time.
template <typename ByteArray>
static constexpr TokenDatabase Create(const ByteArray& database_bytes) {
return IsValid<ByteArray>(database_bytes)
? TokenDatabase(std::data(database_bytes))
: TokenDatabase(); // Invalid database.
// Creates a database with no data. ok() returns false.
constexpr TokenDatabase() : begin_{.data = nullptr}, end_{.data = nullptr} {}
// Returns all entries associated with this token. This is a O(n) operation.
Entries Find(uint32_t token) const;
// Returns the total number of entries (unique token-string pairs).
constexpr size_t size() const {
return ( - / sizeof(RawEntry);
// True if this database was constructed with valid data.
constexpr bool ok() const { return != nullptr; }
Iterator begin() const { return Iterator(begin_.entry,; }
Iterator end() const { return Iterator(end_.entry, nullptr); }
struct Header {
std::array<char, 6> magic;
uint16_t version;
uint32_t entry_count;
uint32_t reserved;
static_assert(sizeof(Header) == 2 * sizeof(RawEntry));
template <typename ByteArray>
static constexpr bool HasValidHeader(const ByteArray& bytes) {
static_assert(sizeof(*std::data(bytes)) == 1u);
if (std::size(bytes) < sizeof(Header)) {
return false;
// Check the magic number and version.
for (size_t i = 0; i < kMagicAndVersion.size(); ++i) {
if (bytes[i] != kMagicAndVersion[i]) {
return false;
return true;
template <typename ByteArray>
static constexpr bool EachEntryHasAString(const ByteArray& bytes) {
const size_t entries = ReadEntryCount(std::data(bytes));
// Check that the data is large enough to have a string table.
if (std::size(bytes) < StringTable(entries)) {
return false;
// Count the strings in the string table.
size_t string_count = 0;
for (auto i = std::begin(bytes) + StringTable(entries); i < std::end(bytes);
++i) {
string_count += (*i == '\0') ? 1 : 0;
// Check that there is at least one string for each entry.
return string_count >= entries;
// Reads the number of entries from a database header. Cast to the bytes to
// uint8_t to avoid sign extension if T is signed.
template <typename T>
static constexpr uint32_t ReadEntryCount(const T* header_bytes) {
const T* bytes = header_bytes + offsetof(Header, entry_count);
return static_cast<uint8_t>(bytes[0]) |
static_cast<uint8_t>(bytes[1]) << 8 |
static_cast<uint8_t>(bytes[2]) << 16 |
static_cast<uint8_t>(bytes[3]) << 24;
// Calculates the offset of the string table.
static constexpr size_t StringTable(size_t entries) {
return sizeof(Header) + entries * sizeof(RawEntry);
// The magic number that starts the table is "TOKENS". The version is encoded
// next as two bytes.
static constexpr std::array<char, 8> kMagicAndVersion = {
'T', 'O', 'K', 'E', 'N', 'S', '\0', '\0'};
template <typename Byte>
constexpr TokenDatabase(const Byte bytes[])
: TokenDatabase(bytes + sizeof(Header),
bytes + StringTable(ReadEntryCount(bytes))) {
static_assert(sizeof(Byte) == 1u);
// It is illegal to reinterpret_cast in constexpr functions, but acceptable to
// use unions. Instead of using a reinterpret_cast to change the byte pointer
// to a RawEntry pointer, have a separate overload for each byte pointer type
// and store them in a union.
constexpr TokenDatabase(const char* begin, const char* end)
: begin_{.data = begin}, end_{.data = end} {}
constexpr TokenDatabase(const unsigned char* begin, const unsigned char* end)
: begin_{.unsigned_data = begin}, end_{.unsigned_data = end} {}
constexpr TokenDatabase(const signed char* begin, const signed char* end)
: begin_{.signed_data = begin}, end_{.signed_data = end} {}
// Store the beginning and end pointers as a union to avoid breaking constexpr
// rules for reinterpret_cast.
union {
const RawEntry* entry;
const char* data;
const unsigned char* unsigned_data;
const signed char* signed_data;
} begin_, end_;
} // namespace pw::tokenizer