"use strict";

function main(formName) {
	let result = document.getElementById("result");
	
	let arr = document.forms[formName].elements[0].value;
	arr = arr.trim().split(" ");
	let checkResult = arr.some( elem => isNotNumber(elem) );
	if (checkResult) {
		result.textContent = "Ошибка: arr должен содержать только целые числа, разделенные пробелом!";
		return;
	} else {
		arr = arr.map( elem => Number(elem) );
	}

	let graph = createGraph(arr);
	let path = findPath(graph);
	
	if (path == undefined) {
		result.textContent = "Результат: путь не найден";
		return;
	}
	
	path[0] = "Start";
	path[path.length - 1] = "End";
	result.textContent = "Результат: " + path.join(" > ") + ". Длина пути: " + path.length;
}

function createGraph(arr) {
	let result = [];
	
	// Добавляем узлы берегов
	arr.unshift(1);
	arr.push(0);

	length = arr.length;
	arr.forEach( (elem, index) => {
			result[index] = {
				prev: undefined,
				dist: Number.POSITIVE_INFINITY,
				visited: false,
				childs: []
			};

			if (elem <= index) {
				result[index].childs.push(index - elem);
			}

			let forwardIndex = elem >= (length - index) ? length - 1 : index + elem;
			result[index].childs.push(forwardIndex);
		}
	);

	return result;
}

function findPath(graph) {
	
	let queue = [];
	
	queue.push(0);
	graph[0].dist = 0;

	mainLoop: while(queue.length > 0) {
		let v = queue.shift()
		for (let child of graph[v].childs) {
			if (!graph[child].visited) {
				queue.push(child)
				graph[v].visited = true;
				
				let dist = graph[v].dist + 1;
				if ( dist < graph[child].dist ) {
					graph[child].prev = v;
					graph[child].dist = dist;
				}

				if (child == graph.length - 1) {
					break mainLoop;
				}
			}
		} 
	}
	
	if (graph[graph.length - 1].dist == Number.POSITIVE_INFINITY) {
		return undefined;
	}

	let path = [];
	let index = graph.length - 1;
	while (index >= 0) {
		path.push(index);
		index = graph[index].prev;
	}

	return path.reverse();
}

function isNotNumber(val) {
	return val.replace(/\s/g, '').length === 0 || isNaN(val) || !Number.isInteger(Number(val));
}