/**
 * Extract JSON from AI's response using a tolerant parser.
 *
 * The function removes common wrappers such as markdown fences or variable
 * declarations and then attempts to parse the remaining text with
 * `jsonc-parser`.  If valid JSON (or JSONC) can be parsed, it is returned as a
 * string via `JSON.stringify`.  Otherwise the original text is returned.
 *
 * @param text - Raw text which may contain JSON
 * @returns A valid JSON string if extraction succeeds, otherwise the original text
 */
import { parse, type ParseError } from 'jsonc-parser';

export function extractJson(text: string): string {
	let content = text.trim();

	// Strip ```json or ``` fences
	const fenceMatch = /```(?:json)?\s*([\s\S]*?)\s*```/i.exec(content);
	if (fenceMatch) {
		content = fenceMatch[1];
	}

	// Strip variable declarations like `const foo =` or `let foo =`
	const varMatch = /^\s*(?:const|let|var)\s+\w+\s*=\s*([\s\S]*)/i.exec(content);
	if (varMatch) {
		content = varMatch[1];
		// Remove trailing semicolon if present
		if (content.trim().endsWith(';')) {
			content = content.trim().slice(0, -1);
		}
	}

	// Find the first opening bracket
	const firstObj = content.indexOf('{');
	const firstArr = content.indexOf('[');
	if (firstObj === -1 && firstArr === -1) {
		return text;
	}
	const start =
		firstArr === -1
			? firstObj
			: firstObj === -1
				? firstArr
				: Math.min(firstObj, firstArr);
	content = content.slice(start);

	// Try to parse the entire string with jsonc-parser
	const tryParse = (value: string): string | undefined => {
		const errors: ParseError[] = [];
		try {
			const result = parse(value, errors, { allowTrailingComma: true });
			if (errors.length === 0) {
				return JSON.stringify(result, null, 2);
			}
		} catch {
			// ignore
		}
		return undefined;
	};

	const parsed = tryParse(content);
	if (parsed !== undefined) {
		return parsed;
	}

	// If parsing the full string failed, use a more efficient approach
	// to find valid JSON boundaries
	const openChar = content[0];
	const closeChar = openChar === '{' ? '}' : ']';

	// Find all potential closing positions by tracking nesting depth
	const closingPositions: number[] = [];
	let depth = 0;
	let inString = false;
	let escapeNext = false;

	for (let i = 0; i < content.length; i++) {
		const char = content[i];

		if (escapeNext) {
			escapeNext = false;
			continue;
		}

		if (char === '\\') {
			escapeNext = true;
			continue;
		}

		if (char === '"' && !inString) {
			inString = true;
			continue;
		}

		if (char === '"' && inString) {
			inString = false;
			continue;
		}

		// Skip content inside strings
		if (inString) continue;

		if (char === openChar) {
			depth++;
		} else if (char === closeChar) {
			depth--;
			if (depth === 0) {
				closingPositions.push(i + 1);
			}
		}
	}

	// Try parsing at each valid closing position, starting from the end
	for (let i = closingPositions.length - 1; i >= 0; i--) {
		const attempt = tryParse(content.slice(0, closingPositions[i]));
		if (attempt !== undefined) {
			return attempt;
		}
	}

	// As a final fallback, try the original character-by-character approach
	// but only for the last 1000 characters to limit performance impact
	const searchStart = Math.max(0, content.length - 1000);
	for (let end = content.length - 1; end > searchStart; end--) {
		const attempt = tryParse(content.slice(0, end));
		if (attempt !== undefined) {
			return attempt;
		}
	}

	return text;
}