// Pure helpers for "unknown command" typo suggestions.
//
// Extracted from src/index.ts so they can be unit-tested in-process without
// spawning a subprocess. The CLI entrypoint (src/index.ts) wires
// suggestForUnknown() in front of program.parseAsync() to short-circuit
// unknown commands with a Levenshtein-based hint.

export function levenshtein(a: string, b: string): number {
	const m = a.length;
	const n = b.length;
	if (m === 0) return n;
	if (n === 0) return m;
	let prev: number[] = Array.from({ length: n + 1 }, (_, j) => j);
	for (let i = 1; i <= m; i++) {
		const curr: number[] = [i];
		for (let j = 1; j <= n; j++) {
			const cost = a[i - 1] === b[j - 1] ? 0 : 1;
			const left = curr[j - 1] ?? 0;
			const up = prev[j] ?? 0;
			const diag = prev[j - 1] ?? 0;
			curr.push(Math.min(left + 1, up + 1, diag + cost));
		}
		prev = curr;
	}
	return prev[n] ?? 0;
}

export interface UnknownCommandSuggestion {
	/** Closest known command name, or "" when the nearest match is too far. */
	suggestion: string;
	/** Pre-formatted human error message (no trailing newline). */
	errMsg: string;
}

/**
 * Given an unknown CLI command and the set of known command names, compute
 * the best suggestion (Levenshtein distance ≤ 2) and a human error string.
 */
export function suggestForUnknown(
	firstArg: string,
	knownNames: readonly string[],
): UnknownCommandSuggestion {
	let best = "";
	let bestDist = Number.POSITIVE_INFINITY;
	for (const name of knownNames) {
		const d = levenshtein(firstArg, name);
		if (d < bestDist) {
			bestDist = d;
			best = name;
		}
	}
	const suggestion = bestDist <= 2 ? best : "";
	const errMsg = suggestion
		? `Unknown command: ${firstArg}. Did you mean ${suggestion}?`
		: `Unknown command: ${firstArg}`;
	return { suggestion, errMsg };
}