'use strict';
/*global crypto*/

var Hashcash = module.exports;

var textEncoder = new TextEncoder();
Hashcash.solve = async function solveHc(hc) {
	var solution = 0;
	var parts = hc.split(':').slice(0, 6);
	if (parts.length < 6) {
		throw new Error('invalid Hashcash-Challenge: ' + hc);
	}

	var bits = parseInt(parts[1], 10) || -1;
	if (bits > 10 || bits < 0) {
		throw new Error('bad bit values');
	}
	console.log('bits:', bits);
	hc = parts.join(':') + ':';
	async function next() {
		var answer = hc + int52ToBase64(solution);
		var u8 = textEncoder.encode(answer);
		// REALLY SLOW due to async tasks and C++ context switch
		var hash = await crypto.subtle.digest('SHA-256', u8);
		hash = new Uint8Array(hash);
		if (checkHc(hash, bits)) {
			return answer;
		}
		solution += 1;
		return next();
	}

	return next();
};

function int52ToBase64(n) {
	var hex = n.toString(16);
	if (hex.length % 2) {
		hex = '0' + hex;
	}

	var bin = [];
	var i = 0;
	var d;
	var b;
	while (i < hex.length) {
		d = parseInt(hex.slice(i, i + 2), 16);
		b = String.fromCharCode(d);
		bin.push(b);
		i += 2;
	}

	return btoa(bin.join('')).replace(/=/g, '');
}

function checkHc(hash, bits) {
	var n = Math.floor(bits / 8);
	var m = bits % 8;
	var i;
	if (m > 0) {
		n += 1;
	}

	for (i = 0; i < n && i < hash.length; i += 1) {
		if (bits > 8) {
			bits -= 8;
			if (0 !== hash[i]) {
				return false;
			}
			continue;
		}

		if (0 !== hash[i] >> (8 - bits)) {
			return false;
		}

		return true;
	}
}