<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
declare(strict_types=1);

namespace Magento\Framework\GraphQl\Query;

use GraphQL\Error\SyntaxError;
use GraphQL\Language\AST\DocumentNode;
use GraphQL\Language\Parser;
use GraphQL\Language\Source;
use Magento\Framework\GraphQl\Exception\GraphQlInputException;

/**
 * Wrapper for GraphQl query parser. It parses query string into a `GraphQL\Language\AST\DocumentNode`
 */
class QueryParser
{
    private const DEFAULT_MAX_NESTING_DEPTH = 200;

    /**
     * @var DocumentNode[]
     */
    private $parsedQueries = [];

    /**
     * @param int $maxNestingDepth
     */
    public function __construct(
        private readonly int $maxNestingDepth = self::DEFAULT_MAX_NESTING_DEPTH
    ) {
    }

    /**
     * Parse query string into a `GraphQL\Language\AST\DocumentNode`.
     *
     * @param string $query
     * @return DocumentNode
     * @throws GraphQlInputException
     * @throws SyntaxError
     * @throws \JsonException
     */
    public function parse(string $query): DocumentNode
    {
        $cacheKey = sha1($query);
        if (!isset($this->parsedQueries[$cacheKey])) {
            $this->validateNestingDepth($query);
            $this->parsedQueries[$cacheKey] = Parser::parse(new Source($query, 'GraphQL'));
        }
        return $this->parsedQueries[$cacheKey];
    }

    /**
     * Validates that the query does not exceed the maximum allowed nesting depth.
     *
     * This check runs before the recursive parser to prevent stack overflow from
     * deeply nested inline fragments or selection sets.
     *
     * @param string $query
     * @throws GraphQlInputException
     */
    private function validateNestingDepth(string $query): void
    {
        $state = [
            'depth' => 0,
            'maxDepth' => 0,
            'inString' => false,
            'escaped' => false,
        ];
        $len = strlen($query);

        for ($i = 0; $i < $len; $i++) {
            $char = $query[$i];

            if ($this->validateState($char, $state) || $state['inString']) {
                continue;
            }

            $this->assertDepth($char, $state);
        }
    }

    /**
     * Handles escape character state within string literals and string literal state transitions.
     *
     * @param string $char
     * @param array $state
     * @return bool
     */
    private function validateState(string $char, array &$state): bool
    {
        if ($state['escaped']) {
            $state['escaped'] = false;
            return true;
        }

        if ($char === '\\' && $state['inString']) {
            $state['escaped'] = true;
            return true;
        }

        if ($char === '"') {
            $state['inString'] = !$state['inString'];
            return true;
        }
        return false;
    }

    /**
     * Validates that the maximum nesting depth does not exceed the configured limit.
     *
     * Updates the current and maximum nesting depth based on structural characters.
     * Throws an exception if the query depth exceeds the allowed threshold.
     *
     * @param string $char
     * @param array $state
     * @return void
     * @throws GraphQlInputException
     */
    private function assertDepth(string $char, array &$state): void
    {
        if ($char === '{' || $char === '[') {
            $state['depth']++;
            $state['maxDepth'] = max($state['depth'], $state['maxDepth']);
        } elseif ($char === '}' || $char === ']') {
            $state['depth']--;
        }

        if ($state['maxDepth'] > $this->maxNestingDepth) {
            throw new GraphQlInputException(
                __('Query is too deep. Reduce the nesting level of your query.')
            );
        }
    }
}
