import { findCommand, visibleCommandNames } from './commands';

export interface CompletionResult {
	input: string;
	options: string[];
}

function sharedPrefix(words: string[]): string {
	return words.reduce((prefix, word) => {
		let length = 0;
		while (length < prefix.length && prefix[length] === word[length]) length++;
		return prefix.slice(0, length);
	});
}

function candidatesFor(words: string[]): string[] {
	if (words.length <= 1) return visibleCommandNames();
	if (words.length > 2) return [];
	return findCommand(words[0])?.argumentOptions?.() ?? [];
}

export function complete(input: string): CompletionResult {
	const words = input.trimStart().split(/\s+/);
	const partial = words.at(-1) ?? '';
	const matches = [...new Set(candidatesFor(words))].filter((candidate) =>
		candidate.toLowerCase().startsWith(partial.toLowerCase())
	);

	if (matches.length === 0) return { input, options: [] };

	const before = input.slice(0, input.length - partial.length);
	if (matches.length === 1) return { input: `${before}${matches[0]} `, options: [] };

	const prefix = sharedPrefix(matches);
	const madeProgress = prefix.length > partial.length;
	return { input: `${before}${madeProgress ? prefix : partial}`, options: madeProgress ? [] : matches.sort() };
}