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// Copyright 2016 Wladimir J. van der Laan
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// Distributed under the MIT software license, see the accompanying
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// file COPYING or https://opensource.org/licenses/mit-license.php.
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#ifndef BITCOIN_UNIVALUE_INCLUDE_UNIVALUE_UTFFILTER_H
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#define BITCOIN_UNIVALUE_INCLUDE_UNIVALUE_UTFFILTER_H
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#include <string>
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/**
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 * Filter that generates and validates UTF-8, as well as collates UTF-16
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 * surrogate pairs as specified in RFC4627.
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 */
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class JSONUTF8StringFilter
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{
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public:
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    explicit JSONUTF8StringFilter(std::string &s):
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        str(s), is_valid(true), codepoint(0), state(0), surpair(0)
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    {
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    }
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    // Write single 8-bit char (may be part of UTF-8 sequence)
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    void push_back(unsigned char ch)
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    {
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        if (state == 0) {
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            if (ch < 0x80) // 7-bit ASCII, fast direct pass-through
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                str.push_back(ch);
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            else if (ch < 0xc0) // Mid-sequence character, invalid in this state
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                is_valid = false;
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            else if (ch < 0xe0) { // Start of 2-byte sequence
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                codepoint = (ch & 0x1f) << 6;
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                state = 6;
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            } else if (ch < 0xf0) { // Start of 3-byte sequence
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                codepoint = (ch & 0x0f) << 12;
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                state = 12;
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            } else if (ch < 0xf8) { // Start of 4-byte sequence
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                codepoint = (ch & 0x07) << 18;
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                state = 18;
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            } else // Reserved, invalid
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                is_valid = false;
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        } else {
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            if ((ch & 0xc0) != 0x80) // Not a continuation, invalid
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                is_valid = false;
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            state -= 6;
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            codepoint |= (ch & 0x3f) << state;
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            if (state == 0)
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                push_back_u(codepoint);
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        }
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    }
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    // Write codepoint directly, possibly collating surrogate pairs
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    void push_back_u(unsigned int codepoint_)
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    {
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        if (state) // Only accept full codepoints in open state
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            is_valid = false;
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        if (codepoint_ >= 0xD800 && codepoint_ < 0xDC00) { // First half of surrogate pair
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            if (surpair) // Two subsequent surrogate pair openers - fail
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                is_valid = false;
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            else
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                surpair = codepoint_;
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        } else if (codepoint_ >= 0xDC00 && codepoint_ < 0xE000) { // Second half of surrogate pair
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            if (surpair) { // Open surrogate pair, expect second half
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                // Compute code point from UTF-16 surrogate pair
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                append_codepoint(0x10000 | ((surpair - 0xD800)<<10) | (codepoint_ - 0xDC00));
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                surpair = 0;
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            } else // Second half doesn't follow a first half - fail
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                is_valid = false;
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        } else {
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            if (surpair) // First half of surrogate pair not followed by second - fail
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                is_valid = false;
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            else
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                append_codepoint(codepoint_);
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        }
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    }
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    // Check that we're in a state where the string can be ended
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    // No open sequences, no open surrogate pairs, etc
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    bool finalize()
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    {
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        if (state || surpair)
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            is_valid = false;
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        return is_valid;
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    }
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private:
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    std::string &str;
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    bool is_valid;
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    // Current UTF-8 decoding state
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    unsigned int codepoint;
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    int state; // Top bit to be filled in for next UTF-8 byte, or 0
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    // Keep track of the following state to handle the following section of
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    // RFC4627:
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    //
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    //    To escape an extended character that is not in the Basic Multilingual
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    //    Plane, the character is represented as a twelve-character sequence,
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    //    encoding the UTF-16 surrogate pair.  So, for example, a string
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    //    containing only the G clef character (U+1D11E) may be represented as
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    //    "\uD834\uDD1E".
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    //
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    //  Two subsequent \u.... may have to be replaced with one actual codepoint.
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    unsigned int surpair; // First half of open UTF-16 surrogate pair, or 0
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    void append_codepoint(unsigned int codepoint_)
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    {
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        if (codepoint_ <= 0x7f)
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            str.push_back((char)codepoint_);
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        else if (codepoint_ <= 0x7FF) {
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            str.push_back((char)(0xC0 | (codepoint_ >> 6)));
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            str.push_back((char)(0x80 | (codepoint_ & 0x3F)));
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        } else if (codepoint_ <= 0xFFFF) {
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            str.push_back((char)(0xE0 | (codepoint_ >> 12)));
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            str.push_back((char)(0x80 | ((codepoint_ >> 6) & 0x3F)));
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            str.push_back((char)(0x80 | (codepoint_ & 0x3F)));
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        } else if (codepoint_ <= 0x1FFFFF) {
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            str.push_back((char)(0xF0 | (codepoint_ >> 18)));
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            str.push_back((char)(0x80 | ((codepoint_ >> 12) & 0x3F)));
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            str.push_back((char)(0x80 | ((codepoint_ >> 6) & 0x3F)));
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            str.push_back((char)(0x80 | (codepoint_ & 0x3F)));
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        }
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    }
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};
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#endif // BITCOIN_UNIVALUE_INCLUDE_UNIVALUE_UTFFILTER_H