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<?php

declare(strict_types=1);

namespace GraphQL\Language;

use GraphQL\Error\SyntaxError;
use GraphQL\Utils\BlockString;
use GraphQL\Utils\Utils;
use function chr;
use function hexdec;
use function mb_convert_encoding;
use function ord;
use function pack;
use function preg_match;
use function substr;

/**
 * A Lexer is a stateful stream generator in that every time
 * it is advanced, it returns the next token in the Source. Assuming the
 * source lexes, the final Token emitted by the lexer will be of kind
 * EOF, after which the lexer will repeatedly return the same EOF token
 * whenever called.
 *
 * Algorithm is O(N) both on memory and time
 */
class Lexer
{
    private const TOKEN_BANG      = 33;
    private const TOKEN_HASH      = 35;
    private const TOKEN_DOLLAR    = 36;
    private const TOKEN_AMP       = 38;
    private const TOKEN_PAREN_L   = 40;
    private const TOKEN_PAREN_R   = 41;
    private const TOKEN_DOT       = 46;
    private const TOKEN_COLON     = 58;
    private const TOKEN_EQUALS    = 61;
    private const TOKEN_AT        = 64;
    private const TOKEN_BRACKET_L = 91;
    private const TOKEN_BRACKET_R = 93;
    private const TOKEN_BRACE_L   = 123;
    private const TOKEN_PIPE      = 124;
    private const TOKEN_BRACE_R   = 125;

    /** @var Source */
    public $source;

    /** @var bool[] */
    public $options;

    /**
     * The previously focused non-ignored token.
     *
     * @var Token
     */
    public $lastToken;

    /**
     * The currently focused non-ignored token.
     *
     * @var Token
     */
    public $token;

    /**
     * The (1-indexed) line containing the current token.
     *
     * @var int
     */
    public $line;

    /**
     * The character offset at which the current line begins.
     *
     * @var int
     */
    public $lineStart;

    /**
     * Current cursor position for UTF8 encoding of the source
     *
     * @var int
     */
    private $position;

    /**
     * Current cursor position for ASCII representation of the source
     *
     * @var int
     */
    private $byteStreamPosition;

    /**
     * @param bool[] $options
     */
    public function __construct(Source $source, array $options = [])
    {
        $startOfFileToken = new Token(Token::SOF, 0, 0, 0, 0, null);

        $this->source    = $source;
        $this->options   = $options;
        $this->lastToken = $startOfFileToken;
        $this->token     = $startOfFileToken;
        $this->line      = 1;
        $this->lineStart = 0;
        $this->position  = $this->byteStreamPosition = 0;
    }

    /**
     * @return Token
     */
    public function advance()
    {
        $this->lastToken = $this->token;

        return $this->token = $this->lookahead();
    }

    public function lookahead()
    {
        $token = $this->token;
        if ($token->kind !== Token::EOF) {
            do {
                $token = $token->next ?? ($token->next = $this->readToken($token));
            } while ($token->kind === Token::COMMENT);
        }

        return $token;
    }

    /**
     * @return Token
     *
     * @throws SyntaxError
     */
    private function readToken(Token $prev)
    {
        $bodyLength = $this->source->length;

        $this->positionAfterWhitespace();
        $position = $this->position;

        $line = $this->line;
        $col  = 1 + $position - $this->lineStart;

        if ($position >= $bodyLength) {
            return new Token(Token::EOF, $bodyLength, $bodyLength, $line, $col, $prev);
        }

        // Read next char and advance string cursor:
        [, $code, $bytes] = $this->readChar(true);

        switch ($code) {
            case self::TOKEN_BANG:
                return new Token(Token::BANG, $position, $position + 1, $line, $col, $prev);
            case self::TOKEN_HASH: // #
                $this->moveStringCursor(-1, -1 * $bytes);

                return $this->readComment($line, $col, $prev);
            case self::TOKEN_DOLLAR:
                return new Token(Token::DOLLAR, $position, $position + 1, $line, $col, $prev);
            case self::TOKEN_AMP:
                return new Token(Token::AMP, $position, $position + 1, $line, $col, $prev);
            case self::TOKEN_PAREN_L:
                return new Token(Token::PAREN_L, $position, $position + 1, $line, $col, $prev);
            case self::TOKEN_PAREN_R:
                return new Token(Token::PAREN_R, $position, $position + 1, $line, $col, $prev);
            case self::TOKEN_DOT: // .
                [, $charCode1] = $this->readChar(true);
                [, $charCode2] = $this->readChar(true);

                if ($charCode1 === self::TOKEN_DOT && $charCode2 === self::TOKEN_DOT) {
                    return new Token(Token::SPREAD, $position, $position + 3, $line, $col, $prev);
                }
                break;
            case self::TOKEN_COLON:
                return new Token(Token::COLON, $position, $position + 1, $line, $col, $prev);
            case self::TOKEN_EQUALS:
                return new Token(Token::EQUALS, $position, $position + 1, $line, $col, $prev);
            case self::TOKEN_AT:
                return new Token(Token::AT, $position, $position + 1, $line, $col, $prev);
            case self::TOKEN_BRACKET_L:
                return new Token(Token::BRACKET_L, $position, $position + 1, $line, $col, $prev);
            case self::TOKEN_BRACKET_R:
                return new Token(Token::BRACKET_R, $position, $position + 1, $line, $col, $prev);
            case self::TOKEN_BRACE_L:
                return new Token(Token::BRACE_L, $position, $position + 1, $line, $col, $prev);
            case self::TOKEN_PIPE:
                return new Token(Token::PIPE, $position, $position + 1, $line, $col, $prev);
            case self::TOKEN_BRACE_R:
                return new Token(Token::BRACE_R, $position, $position + 1, $line, $col, $prev);

            // A-Z
            case 65:
            case 66:
            case 67:
            case 68:
            case 69:
            case 70:
            case 71:
            case 72:
            case 73:
            case 74:
            case 75:
            case 76:
            case 77:
            case 78:
            case 79:
            case 80:
            case 81:
            case 82:
            case 83:
            case 84:
            case 85:
            case 86:
            case 87:
            case 88:
            case 89:
            case 90:
                // _
            case 95:
                // a-z
            case 97:
            case 98:
            case 99:
            case 100:
            case 101:
            case 102:
            case 103:
            case 104:
            case 105:
            case 106:
            case 107:
            case 108:
            case 109:
            case 110:
            case 111:
            case 112:
            case 113:
            case 114:
            case 115:
            case 116:
            case 117:
            case 118:
            case 119:
            case 120:
            case 121:
            case 122:
                return $this->moveStringCursor(-1, -1 * $bytes)
                    ->readName($line, $col, $prev);

            // -
            case 45:
                // 0-9
            case 48:
            case 49:
            case 50:
            case 51:
            case 52:
            case 53:
            case 54:
            case 55:
            case 56:
            case 57:
                return $this->moveStringCursor(-1, -1 * $bytes)
                    ->readNumber($line, $col, $prev);

            // "
            case 34:
                [, $nextCode]     = $this->readChar();
                [, $nextNextCode] = $this->moveStringCursor(1, 1)->readChar();

                if ($nextCode === 34 && $nextNextCode === 34) {
                    return $this->moveStringCursor(-2, (-1 * $bytes) - 1)
                        ->readBlockString($line, $col, $prev);
                }

                return $this->moveStringCursor(-2, (-1 * $bytes) - 1)
                    ->readString($line, $col, $prev);
        }

        throw new SyntaxError(
            $this->source,
            $position,
            $this->unexpectedCharacterMessage($code)
        );
    }

    private function unexpectedCharacterMessage($code)
    {
        // SourceCharacter
        if ($code < 0x0020 && $code !== 0x0009 && $code !== 0x000A && $code !== 0x000D) {
            return 'Cannot contain the invalid character ' . Utils::printCharCode($code);
        }

        if ($code === 39) {
            return "Unexpected single quote character ('), did you mean to use " .
                'a double quote (")?';
        }

        return 'Cannot parse the unexpected character ' . Utils::printCharCode($code) . '.';
    }

    /**
     * Reads an alphanumeric + underscore name from the source.
     *
     * [_A-Za-z][_0-9A-Za-z]*
     *
     * @param int $line
     * @param int $col
     *
     * @return Token
     */
    private function readName($line, $col, Token $prev)
    {
        $value         = '';
        $start         = $this->position;
        [$char, $code] = $this->readChar();

        while ($code !== null && (
                $code === 95 || // _
                ($code >= 48 && $code <= 57) || // 0-9
                ($code >= 65 && $code <= 90) || // A-Z
                ($code >= 97 && $code <= 122) // a-z
            )) {
            $value        .= $char;
            [$char, $code] = $this->moveStringCursor(1, 1)->readChar();
        }

        return new Token(
            Token::NAME,
            $start,
            $this->position,
            $line,
            $col,
            $prev,
            $value
        );
    }

    /**
     * Reads a number token from the source file, either a float
     * or an int depending on whether a decimal point appears.
     *
     * Int:   -?(0|[1-9][0-9]*)
     * Float: -?(0|[1-9][0-9]*)(\.[0-9]+)?((E|e)(+|-)?[0-9]+)?
     *
     * @param int $line
     * @param int $col
     *
     * @return Token
     *
     * @throws SyntaxError
     */
    private function readNumber($line, $col, Token $prev)
    {
        $value         = '';
        $start         = $this->position;
        [$char, $code] = $this->readChar();

        $isFloat = false;

        if ($code === 45) { // -
            $value        .= $char;
            [$char, $code] = $this->moveStringCursor(1, 1)->readChar();
        }

        // guard against leading zero's
        if ($code === 48) { // 0
            $value        .= $char;
            [$char, $code] = $this->moveStringCursor(1, 1)->readChar();

            if ($code >= 48 && $code <= 57) {
                throw new SyntaxError(
                    $this->source,
                    $this->position,
                    'Invalid number, unexpected digit after 0: ' . Utils::printCharCode($code)
                );
            }
        } else {
            $value        .= $this->readDigits();
            [$char, $code] = $this->readChar();
        }

        if ($code === 46) { // .
            $isFloat = true;
            $this->moveStringCursor(1, 1);

            $value        .= $char;
            $value        .= $this->readDigits();
            [$char, $code] = $this->readChar();
        }

        if ($code === 69 || $code === 101) { // E e
            $isFloat       = true;
            $value        .= $char;
            [$char, $code] = $this->moveStringCursor(1, 1)->readChar();

            if ($code === 43 || $code === 45) { // + -
                $value .= $char;
                $this->moveStringCursor(1, 1);
            }
            $value .= $this->readDigits();
        }

        return new Token(
            $isFloat ? Token::FLOAT : Token::INT,
            $start,
            $this->position,
            $line,
            $col,
            $prev,
            $value
        );
    }

    /**
     * Returns string with all digits + changes current string cursor position to point to the first char after digits
     */
    private function readDigits()
    {
        [$char, $code] = $this->readChar();

        if ($code >= 48 && $code <= 57) { // 0 - 9
            $value = '';

            do {
                $value        .= $char;
                [$char, $code] = $this->moveStringCursor(1, 1)->readChar();
            } while ($code >= 48 && $code <= 57); // 0 - 9

            return $value;
        }

        if ($this->position > $this->source->length - 1) {
            $code = null;
        }

        throw new SyntaxError(
            $this->source,
            $this->position,
            'Invalid number, expected digit but got: ' . Utils::printCharCode($code)
        );
    }

    /**
     * @param int $line
     * @param int $col
     *
     * @return Token
     *
     * @throws SyntaxError
     */
    private function readString($line, $col, Token $prev)
    {
        $start = $this->position;

        // Skip leading quote and read first string char:
        [$char, $code, $bytes] = $this->moveStringCursor(1, 1)->readChar();

        $chunk = '';
        $value = '';

        while ($code !== null &&
            // not LineTerminator
            $code !== 10 && $code !== 13
        ) {
            // Closing Quote (")
            if ($code === 34) {
                $value .= $chunk;

                // Skip quote
                $this->moveStringCursor(1, 1);

                return new Token(
                    Token::STRING,
                    $start,
                    $this->position,
                    $line,
                    $col,
                    $prev,
                    $value
                );
            }

            $this->assertValidStringCharacterCode($code, $this->position);
            $this->moveStringCursor(1, $bytes);

            if ($code === 92) { // \
                $value   .= $chunk;
                [, $code] = $this->readChar(true);

                switch ($code) {
                    case 34:
                        $value .= '"';
                        break;
                    case 47:
                        $value .= '/';
                        break;
                    case 92:
                        $value .= '\\';
                        break;
                    case 98:
                        $value .= chr(8);
                        break; // \b (backspace)
                    case 102:
                        $value .= "\f";
                        break;
                    case 110:
                        $value .= "\n";
                        break;
                    case 114:
                        $value .= "\r";
                        break;
                    case 116:
                        $value .= "\t";
                        break;
                    case 117:
                        $position = $this->position;
                        [$hex]    = $this->readChars(4, true);
                        if (! preg_match('/[0-9a-fA-F]{4}/', $hex)) {
                            throw new SyntaxError(
                                $this->source,
                                $position - 1,
                                'Invalid character escape sequence: \\u' . $hex
                            );
                        }

                        $code = hexdec($hex);

                        // UTF-16 surrogate pair detection and handling.
                        $highOrderByte = $code >> 8;
                        if (0xD8 <= $highOrderByte && $highOrderByte <= 0xDF) {
                            [$utf16Continuation] = $this->readChars(6, true);
                            if (! preg_match('/^\\\u[0-9a-fA-F]{4}$/', $utf16Continuation)) {
                                throw new SyntaxError(
                                    $this->source,
                                    $this->position - 5,
                                    'Invalid UTF-16 trailing surrogate: ' . $utf16Continuation
                                );
                            }
                            $surrogatePairHex = $hex . substr($utf16Continuation, 2, 4);
                            $value           .= mb_convert_encoding(pack('H*', $surrogatePairHex), 'UTF-8', 'UTF-16');
                            break;
                        }

                        $this->assertValidStringCharacterCode($code, $position - 2);

                        $value .= Utils::chr($code);
                        break;
                    default:
                        throw new SyntaxError(
                            $this->source,
                            $this->position - 1,
                            'Invalid character escape sequence: \\' . Utils::chr($code)
                        );
                }
                $chunk = '';
            } else {
                $chunk .= $char;
            }

            [$char, $code, $bytes] = $this->readChar();
        }

        throw new SyntaxError(
            $this->source,
            $this->position,
            'Unterminated string.'
        );
    }

    /**
     * Reads a block string token from the source file.
     *
     * """("?"?(\\"""|\\(?!=""")|[^"\\]))*"""
     */
    private function readBlockString($line, $col, Token $prev)
    {
        $start = $this->position;

        // Skip leading quotes and read first string char:
        [$char, $code, $bytes] = $this->moveStringCursor(3, 3)->readChar();

        $chunk = '';
        $value = '';

        while ($code !== null) {
            // Closing Triple-Quote (""")
            if ($code === 34) {
                // Move 2 quotes
                [, $nextCode]     = $this->moveStringCursor(1, 1)->readChar();
                [, $nextNextCode] = $this->moveStringCursor(1, 1)->readChar();

                if ($nextCode === 34 && $nextNextCode === 34) {
                    $value .= $chunk;

                    $this->moveStringCursor(1, 1);

                    return new Token(
                        Token::BLOCK_STRING,
                        $start,
                        $this->position,
                        $line,
                        $col,
                        $prev,
                        BlockString::value($value)
                    );
                }

                // move cursor back to before the first quote
                $this->moveStringCursor(-2, -2);
            }

            $this->assertValidBlockStringCharacterCode($code, $this->position);
            $this->moveStringCursor(1, $bytes);

            [, $nextCode]         = $this->readChar();
            [, $nextNextCode]     = $this->moveStringCursor(1, 1)->readChar();
            [, $nextNextNextCode] = $this->moveStringCursor(1, 1)->readChar();

            // Escape Triple-Quote (\""")
            if ($code === 92 &&
                $nextCode === 34 &&
                $nextNextCode === 34 &&
                $nextNextNextCode === 34
            ) {
                $this->moveStringCursor(1, 1);
                $value .= $chunk . '"""';
                $chunk  = '';
            } else {
                $this->moveStringCursor(-2, -2);
                $chunk .= $char;
            }

            [$char, $code, $bytes] = $this->readChar();
        }

        throw new SyntaxError(
            $this->source,
            $this->position,
            'Unterminated string.'
        );
    }

    private function assertValidStringCharacterCode($code, $position)
    {
        // SourceCharacter
        if ($code < 0x0020 && $code !== 0x0009) {
            throw new SyntaxError(
                $this->source,
                $position,
                'Invalid character within String: ' . Utils::printCharCode($code)
            );
        }
    }

    private function assertValidBlockStringCharacterCode($code, $position)
    {
        // SourceCharacter
        if ($code < 0x0020 && $code !== 0x0009 && $code !== 0x000A && $code !== 0x000D) {
            throw new SyntaxError(
                $this->source,
                $position,
                'Invalid character within String: ' . Utils::printCharCode($code)
            );
        }
    }

    /**
     * Reads from body starting at startPosition until it finds a non-whitespace
     * or commented character, then places cursor to the position of that character.
     */
    private function positionAfterWhitespace()
    {
        while ($this->position < $this->source->length) {
            [, $code, $bytes] = $this->readChar();

            // Skip whitespace
            // tab | space | comma | BOM
            if ($code === 9 || $code === 32 || $code === 44 || $code === 0xFEFF) {
                $this->moveStringCursor(1, $bytes);
            } elseif ($code === 10) { // new line
                $this->moveStringCursor(1, $bytes);
                $this->line++;
                $this->lineStart = $this->position;
            } elseif ($code === 13) { // carriage return
                [, $nextCode, $nextBytes] = $this->moveStringCursor(1, $bytes)->readChar();

                if ($nextCode === 10) { // lf after cr
                    $this->moveStringCursor(1, $nextBytes);
                }
                $this->line++;
                $this->lineStart = $this->position;
            } else {
                break;
            }
        }
    }

    /**
     * Reads a comment token from the source file.
     *
     * #[\u0009\u0020-\uFFFF]*
     *
     * @param int $line
     * @param int $col
     *
     * @return Token
     */
    private function readComment($line, $col, Token $prev)
    {
        $start = $this->position;
        $value = '';
        $bytes = 1;

        do {
            [$char, $code, $bytes] = $this->moveStringCursor(1, $bytes)->readChar();
            $value                .= $char;
        } while ($code !== null &&
        // SourceCharacter but not LineTerminator
        ($code > 0x001F || $code === 0x0009)
        );

        return new Token(
            Token::COMMENT,
            $start,
            $this->position,
            $line,
            $col,
            $prev,
            $value
        );
    }

    /**
     * Reads next UTF8Character from the byte stream, starting from $byteStreamPosition.
     *
     * @param bool $advance
     * @param int  $byteStreamPosition
     *
     * @return (string|int)[]
     */
    private function readChar($advance = false, $byteStreamPosition = null)
    {
        if ($byteStreamPosition === null) {
            $byteStreamPosition = $this->byteStreamPosition;
        }

        $code           = null;
        $utf8char       = '';
        $bytes          = 0;
        $positionOffset = 0;

        if (isset($this->source->body[$byteStreamPosition])) {
            $ord = ord($this->source->body[$byteStreamPosition]);

            if ($ord < 128) {
                $bytes = 1;
            } elseif ($ord < 224) {
                $bytes = 2;
            } elseif ($ord < 240) {
                $bytes = 3;
            } else {
                $bytes = 4;
            }

            $utf8char = '';
            for ($pos = $byteStreamPosition; $pos < $byteStreamPosition + $bytes; $pos++) {
                $utf8char .= $this->source->body[$pos];
            }
            $positionOffset = 1;
            $code           = $bytes === 1 ? $ord : Utils::ord($utf8char);
        }

        if ($advance) {
            $this->moveStringCursor($positionOffset, $bytes);
        }

        return [$utf8char, $code, $bytes];
    }

    /**
     * Reads next $numberOfChars UTF8 characters from the byte stream, starting from $byteStreamPosition.
     *
     * @param int  $charCount
     * @param bool $advance
     * @param null $byteStreamPosition
     *
     * @return (string|int)[]
     */
    private function readChars($charCount, $advance = false, $byteStreamPosition = null)
    {
        $result     = '';
        $totalBytes = 0;
        $byteOffset = $byteStreamPosition ?? $this->byteStreamPosition;

        for ($i = 0; $i < $charCount; $i++) {
            [$char, $code, $bytes] = $this->readChar(false, $byteOffset);
            $totalBytes           += $bytes;
            $byteOffset           += $bytes;
            $result               .= $char;
        }
        if ($advance) {
            $this->moveStringCursor($charCount, $totalBytes);
        }

        return [$result, $totalBytes];
    }

    /**
     * Moves internal string cursor position
     *
     * @param int $positionOffset
     * @param int $byteStreamOffset
     *
     * @return self
     */
    private function moveStringCursor($positionOffset, $byteStreamOffset)
    {
        $this->position           += $positionOffset;
        $this->byteStreamPosition += $byteStreamOffset;

        return $this;
    }
}