<?php declare(strict_types=1);

namespace YOOtheme\GraphQL\Language;

use YOOtheme\GraphQL\Error\SyntaxError;
use YOOtheme\GraphQL\Utils\Utils;

/**
 * A lexer is a stateful stream generator, it returns the next token in the Source when advanced.
 * Assuming the source is valid, the final returned token will be EOF,
 * after which the lexer will repeatedly return the same EOF token whenever called.
 *
 * Algorithm is O(N) both on memory and time.
 *
 * @phpstan-import-type ParserOptions from Parser
 *
 * @see \GraphQL\Tests\Language\LexerTest
 */
class Lexer
{
    // https://spec.graphql.org/October2021/#sec-Punctuators
    private const TOKEN_BANG = 33;
    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;

    public Source $source;

    /** @phpstan-var ParserOptions */
    public array $options;

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

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

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

    /** The character offset at which the current line begins. */
    public int $lineStart = 0;

    /** Current cursor position for UTF8 encoding of the source. */
    private int $position = 0;

    /** Current cursor position for ASCII representation of the source. */
    private int $byteStreamPosition = 0;

    /** @phpstan-param ParserOptions $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;
    }

    /**
     * @throws \JsonException
     * @throws SyntaxError
     */
    public function advance(): Token
    {
        $this->lastToken = $this->token;

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

    /**
     * @throws \JsonException
     * @throws SyntaxError
     */
    public function lookahead(): Token
    {
        $token = $this->token;
        if ($token->kind !== Token::EOF) {
            do {
                $token = $token->next ?? ($token->next = $this->readToken($token));
            } while ($token->kind === Token::COMMENT);
        }

        return $token;
    }

    /**
     * @throws \JsonException
     * @throws SyntaxError
     */
    private function readToken(Token $prev): Token
    {
        $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 35: // #
                $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));
    }

    /** @throws \JsonException */
    private function unexpectedCharacterMessage(?int $code): string
    {
        // SourceCharacter
        if ($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]*
     */
    private function readName(int $line, int $col, Token $prev): Token
    {
        $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]+)?
     *
     * @throws \JsonException
     * @throws SyntaxError
     */
    private function readNumber(int $line, int $col, Token $prev): Token
    {
        $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.
     *
     * @throws \JsonException
     * @throws SyntaxError
     */
    private function readDigits(): string
    {
        [$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));
    }

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

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

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

        while (
            $code !== null
            && $code !== 10 && $code !== 13 // not LineTerminator
        ) {
            if ($code === 34) { // Closing Quote (")
                $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); // \b (backspace)
                        break;
                    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);
                        if (preg_match('/[0-9a-fA-F]{4}/', $hex) !== 1) {
                            throw new SyntaxError($this->source, $position - 1, "Invalid character escape sequence: \\u{$hex}");
                        }

                        $code = hexdec($hex);
                        assert(is_int($code), 'Since only a single char is read');

                        // UTF-16 surrogate pair detection and handling.
                        $highOrderByte = $code >> 8;
                        if ($highOrderByte >= 0xD8 && $highOrderByte <= 0xDF) {
                            [$utf16Continuation] = $this->readChars(6);
                            if (preg_match('/^\\\u[0-9a-fA-F]{4}$/', $utf16Continuation) !== 1) {
                                throw new SyntaxError($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;
                        // null means EOF, will delegate to general handling of unterminated strings
                    case null:
                        continue 2;
                    default:
                        $chr = Utils::chr($code);
                        throw new SyntaxError($this->source, $this->position - 1, "Invalid character escape sequence: \\{$chr}");
                }

                $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.
     *
     * """("?"?(\\"""|\\(?!=""")|[^"\\]))*"""
     *
     * @throws \JsonException
     * @throws SyntaxError
     */
    private function readBlockString(int $line, int $col, Token $prev): Token
    {
        $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,
                        $prev,
                        BlockString::dedentBlockStringLines($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 {
                // move cursor back to before the first quote
                $this->moveStringCursor(-2, -2);

                if ($code === 10) { // new line
                    ++$this->line;
                    $this->lineStart = $this->position;
                }

                $chunk .= $char;
            }

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

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

    /**
     * @throws \JsonException
     * @throws SyntaxError
     */
    private function assertValidStringCharacterCode(int $code, int $position): void
    {
        // SourceCharacter
        if ($code < 0x0020 && $code !== 0x0009) {
            $char = Utils::printCharCode($code);
            throw new SyntaxError($this->source, $position, "Invalid character within String: {$char}");
        }
    }

    /**
     * @throws \JsonException
     * @throws SyntaxError
     */
    private function assertValidBlockStringCharacterCode(int $code, int $position): void
    {
        // SourceCharacter
        if ($code < 0x0020 && $code !== 0x0009 && $code !== 0x000A && $code !== 0x000D) {
            $char = Utils::printCharCode($code);
            throw new SyntaxError($this->source, $position, "Invalid character within String: {$char}");
        }
    }

    /**
     * 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(): void
    {
        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]*
     */
    private function readComment(int $line, int $col, Token $prev): Token
    {
        $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.
     *
     * @return array{string, int|null, int}
     */
    private function readChar(bool $advance = false, ?int $byteStreamPosition = null): array
    {
        if ($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;
            }

            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.
     *
     * @return array{string, int}
     */
    private function readChars(int $charCount): array
    {
        $result = '';
        $totalBytes = 0;
        $byteOffset = $this->byteStreamPosition;

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

        $this->moveStringCursor($charCount, $totalBytes);

        return [$result, $totalBytes];
    }

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

        return $this;
    }
}
