1//===-- runtime/utf.h -----------------------------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9// UTF-8 is the variant-width standard encoding of Unicode (ISO 10646)
10// code points.
11//
12// 7-bit values in [00 .. 7F] represent themselves as single bytes, so true
13// 7-bit ASCII is also valid UTF-8.
14//
15// Larger values are encoded with a start byte in [C0 .. FE] that carries
16// the length of the encoding and some of the upper bits of the value, followed
17// by one or more bytes in the range [80 .. BF].
18//
19// Specifically, the first byte holds two or more uppermost set bits,
20// a zero bit, and some payload; the second and later bytes each start with
21// their uppermost bit set, the next bit clear, and six bits of payload.
22// Payload parcels are in big-endian order. All bytes must be present in a
23// valid sequence; i.e., low-order sezo bits must be explicit. UTF-8 is
24// self-synchronizing on input as any byte value cannot be both a valid
25// first byte or trailing byte.
26//
27// 0xxxxxxx - 7 bit ASCII
28// 110xxxxx 10xxxxxx - 11-bit value
29// 1110xxxx 10xxxxxx 10xxxxxx - 16-bit value
30// 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx - 21-bit value
31// 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx - 26-bit value
32// 1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx - 31-bit value
33// 11111110 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx - 36-bit value
34//
35// Canonical UTF-8 sequences should be minimal, and our output is so, but
36// we do not reject non-minimal sequences on input. Unicode only defines
37// code points up to 0x10FFFF, so 21-bit (4-byte) UTF-8 is the actual
38// standard maximum. However, we support extended forms up to 32 bits so that
39// CHARACTER(KIND=4) can be abused to hold arbitrary 32-bit data.
40
41#ifndef FORTRAN_RUNTIME_UTF_H_
42#define FORTRAN_RUNTIME_UTF_H_
43
44#include <cstddef>
45#include <cstdint>
46#include <optional>
47
48namespace Fortran::runtime {
49
50// Derive the length of a UTF-8 character encoding from its first byte.
51// A zero result signifies an invalid encoding.
52extern const std::uint8_t UTF8FirstByteTable[256];
53static inline std::size_t MeasureUTF8Bytes(char first) {
54 return UTF8FirstByteTable[static_cast<std::uint8_t>(first)];
55}
56
57static constexpr std::size_t maxUTF8Bytes{7};
58
59// Ensure that all bytes are present in sequence in the input buffer
60// before calling; use MeasureUTF8Bytes(first byte) to count them.
61std::optional<char32_t> DecodeUTF8(const char *);
62
63// Ensure that at least maxUTF8Bytes remain in the output
64// buffer before calling.
65std::size_t EncodeUTF8(char *, char32_t);
66
67} // namespace Fortran::runtime
68#endif // FORTRAN_RUNTIME_UTF_H_
69

source code of flang/runtime/utf.h