Inside Bitcoin

1// Copyright 2016 Wladimir J. van der Laan
2// Distributed under the MIT software license, see the accompanying
3// file COPYING or https://opensource.org/licenses/mit-license.php.
4#ifndef BITCOIN_UNIVALUE_INCLUDE_UNIVALUE_UTFFILTER_H
5#define BITCOIN_UNIVALUE_INCLUDE_UNIVALUE_UTFFILTER_H
6
7#include <string>
8
9/**
10 * Filter that generates and validates UTF-8, as well as collates UTF-16
11 * surrogate pairs as specified in RFC4627.
12 */
13class JSONUTF8StringFilter
14{
15public:
16    explicit JSONUTF8StringFilter(std::string& s)
17        : str(s)
18    {
19    }
20    // Write single 8-bit char (may be part of UTF-8 sequence)
21    void push_back(unsigned char ch)
22    {
23        if (state == 0) {
24            if (ch < 0x80) // 7-bit ASCII, fast direct pass-through
25                str.push_back(ch);
26            else if (ch < 0xc0) // Mid-sequence character, invalid in this state
27                is_valid = false;
28            else if (ch < 0xe0) { // Start of 2-byte sequence
29                codepoint = (ch & 0x1f) << 6;
30                state = 6;
31            } else if (ch < 0xf0) { // Start of 3-byte sequence
32                codepoint = (ch & 0x0f) << 12;
33                state = 12;
34            } else if (ch < 0xf8) { // Start of 4-byte sequence
35                codepoint = (ch & 0x07) << 18;
36                state = 18;
37            } else // Reserved, invalid
38                is_valid = false;
39        } else {
40            if ((ch & 0xc0) != 0x80) // Not a continuation, invalid
41                is_valid = false;
42            state -= 6;
43            codepoint |= (ch & 0x3f) << state;
44            if (state == 0)
45                push_back_u(codepoint);
46        }
47    }
48    // Write codepoint directly, possibly collating surrogate pairs
49    void push_back_u(unsigned int codepoint_)
50    {
51        if (state) // Only accept full codepoints in open state
52            is_valid = false;
53        if (codepoint_ >= 0xD800 && codepoint_ < 0xDC00) { // First half of surrogate pair
54            if (surpair) // Two subsequent surrogate pair openers - fail
55                is_valid = false;
56            else
57                surpair = codepoint_;
58        } else if (codepoint_ >= 0xDC00 && codepoint_ < 0xE000) { // Second half of surrogate pair
59            if (surpair) { // Open surrogate pair, expect second half
60                // Compute code point from UTF-16 surrogate pair
61                append_codepoint(0x10000 | ((surpair - 0xD800)<<10) | (codepoint_ - 0xDC00));
62                surpair = 0;
63            } else // Second half doesn't follow a first half - fail
64                is_valid = false;
65        } else {
66            if (surpair) // First half of surrogate pair not followed by second - fail
67                is_valid = false;
68            else
69                append_codepoint(codepoint_);
70        }
71    }
72    // Check that we're in a state where the string can be ended
73    // No open sequences, no open surrogate pairs, etc
74    bool finalize()
75    {
76        if (state || surpair)
77            is_valid = false;
78        return is_valid;
79    }
80private:
81    std::string &str;
82    bool is_valid{true};
83    // Current UTF-8 decoding state
84    unsigned int codepoint{0};
85    int state{0}; // Top bit to be filled in for next UTF-8 byte, or 0
86
87    // Keep track of the following state to handle the following section of
88    // RFC4627:
89    //
90    //    To escape an extended character that is not in the Basic Multilingual
91    //    Plane, the character is represented as a twelve-character sequence,
92    //    encoding the UTF-16 surrogate pair.  So, for example, a string
93    //    containing only the G clef character (U+1D11E) may be represented as
94    //    "\uD834\uDD1E".
95    //
96    //  Two subsequent \u.... may have to be replaced with one actual codepoint.
97    unsigned int surpair{0}; // First half of open UTF-16 surrogate pair, or 0
98
99    void append_codepoint(unsigned int codepoint_)
100    {
101        if (codepoint_ <= 0x7f)
102            str.push_back((char)codepoint_);
103        else if (codepoint_ <= 0x7FF) {
104            str.push_back((char)(0xC0 | (codepoint_ >> 6)));
105            str.push_back((char)(0x80 | (codepoint_ & 0x3F)));
106        } else if (codepoint_ <= 0xFFFF) {
107            str.push_back((char)(0xE0 | (codepoint_ >> 12)));
108            str.push_back((char)(0x80 | ((codepoint_ >> 6) & 0x3F)));
109            str.push_back((char)(0x80 | (codepoint_ & 0x3F)));
110        } else if (codepoint_ <= 0x1FFFFF) {
111            str.push_back((char)(0xF0 | (codepoint_ >> 18)));
112            str.push_back((char)(0x80 | ((codepoint_ >> 12) & 0x3F)));
113            str.push_back((char)(0x80 | ((codepoint_ >> 6) & 0x3F)));
114            str.push_back((char)(0x80 | (codepoint_ & 0x3F)));
115        }
116    }
117};
118
119#endif // BITCOIN_UNIVALUE_INCLUDE_UNIVALUE_UTFFILTER_H
120