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// See http://bjoern.hoehrmann.de/utf-8/decoder/dfa/ and the sibling file
// utf8-decoder.h for details.
//
// This file decodes "generalized UTF-8", which is the same as UTF-8 except that
// it allows surrogates: https://simonsapin.github.io/wtf-8/#generalized-utf8
#include <stdint.h>
#ifndef __GENERALIZED_UTF8_DFA_DECODER_H
#define __GENERALIZED_UTF8_DFA_DECODER_H
struct GeneralizedUtf8DfaDecoder {
enum State : uint8_t {
kReject = 0,
kAccept = 11,
kTwoByte = 22,
kThreeByte = 33,
kFourByte = 44,
kFourByteLow = 55,
kThreeByteHigh = 66,
kFourByteMidHigh = 77,
};
static inline void Decode(uint8_t byte, State* state, uint32_t* buffer) {
// This first table maps bytes to character to a transition.
//
// The transition value takes a state to a new state, but it also determines
// the set of bits from the current byte that contribute to the decoded
// codepoint:
//
// Transition | Current byte bits that contribute to decoded codepoint
// -------------------------------------------------------------------
// 0, 1 | 0b01111111
// 2, 3 | 0b00111111
// 4, 5 | 0b00011111
// 6, 7 | 0b00001111
// 8, 9 | 0b00000111
// 10 | 0b00000011
//
// Given the WTF-8 encoding, we therefore have the following constraints:
// 1. The transition value for 1-byte encodings should have the value 0 or
// 1 so that we preserve all of the low 7 bits.
// 2. Continuation bytes (0x80 to 0xBF) are of the form 0b10xxxxxx, and
// therefore should have transition value between 0 and 3.
// 3. Leading bytes for 2-byte encodings are of the form 0b110yyyyy, and
// therefore the transition value can be between 2 and 5.
// 4. Leading bytes for 3-byte encodings (0b1110zzzz) need transition
// value between 4 and 7.
// 5. Leading bytes for 4-byte encodings (0b11110uuu) need transition
// value between 6 and 9.
// 6. We need more states to impose irregular constraints. Sometimes we
// can use the knowldege that e.g. some high significant bits of the
// xxxx in 0b1110xxxx are 0, then we can use a higher transition value.
// 7. Transitions to invalid states can use any transition value.
static constexpr uint8_t transitions[] = {
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 00-0F
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 10-1F
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 20-2F
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 30-3F
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 40-4F
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 50-5F
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 60-6F
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 70-7F
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, // 80-8F
2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, // 90-9F
3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, // A0-AF
3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, // B0-BF
8, 8, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, // C0-CF
4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, // D0-DF
9, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, // E0-EF
10, 6, 6, 6, 7, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, // F0-FF
};
// This second table maps a state to a new state when adding a transition.
// 00-7F
// | 80-8F
// | | 90-9F
// | | | A0-BF
// | | | | C2-DF
// | | | | | E1-EF
// | | | | | | F1-F3
// | | | | | | | F4
// | | | | | | | | C0, C1, F5-FF
// | | | | | | | | | E0
// | | | | | | | | | | F0
static constexpr uint8_t states[] = {
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // REJECT = 0
11, 0, 0, 0, 22, 33, 44, 55, 0, 66, 77, // ACCEPT = 11
0, 11, 11, 11, 0, 0, 0, 0, 0, 0, 0, // 2-byte = 22
0, 22, 22, 22, 0, 0, 0, 0, 0, 0, 0, // 3-byte = 33
0, 33, 33, 33, 0, 0, 0, 0, 0, 0, 0, // 4-byte = 44
0, 33, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 4-byte low = 55
0, 0, 0, 22, 0, 0, 0, 0, 0, 0, 0, // 3-byte high = 66
0, 0, 33, 33, 0, 0, 0, 0, 0, 0, 0, // 4-byte mid/high = 77
};
uint8_t type = transitions[byte];
*state = static_cast<State>(states[*state + type]);
*buffer = (*buffer << 6) | (byte & (0x7F >> (type >> 1)));
}
};
#endif // __GENERALIZED_UTF8_DFA_DECODER_H
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