Improve reduce perf, non-sparse arrays

0-reduce-no-sparse-array.js

// Dropping sparse array support makes this code much simpler
function reduce(promises, f) {
	var hasArg = arguments.length > 2;
	if((promises.length>>>0) === 0) {
		return hasArg ? when.promise(arguments[2]) : when.reject(new TypeError('Cannot reduce empty array with no initial value'));
	}
	
	return hasArg ? runReduce(0, promises, f, arguments[2])
		: runReduce(1, promises, f, promises[0]);
}

function runReduce(i, promises, f, z) {
	if (i === promises.length) {
		return z;
	}

	return when.resolve(promises[i]).fold(function(z, x) {
		return runReduce(i + 1, promises, f, f(z, x));
	}, z);
}

1-reduce.js

// With sparse array support
// See https://github.com/cujojs/when/pull/391

function reduce(a, f) {
	var hasArg = arguments.length > 2;
	if((a.length>>>0) === 0) {
		return hasArg ? toPromise(arguments[2]) : reject(new TypeError('Cannot reduce empty array with no initial value'));
	}

	var s = findFirst(a);
	return hasArg ? runReduce(s, a, f, arguments[2])
		: runReduce(s + 1, a, f, a[s]);
}

function runReduce(i, a, f, z) {
	if (i === a.length) {
		return z;
	}

	var x = a[i];
	if(x === void 0 && !(i in a)) {
		return runReduce(i + 1, a, f, z);
	}

	return toPromise(a[i]).fold(function (z, x) {
		return runReduce(i + 1, a, f, f(z, x, i));
	}, z);
}

function findFirst(a) {
	for(var i=0; i<a.length; ++i) {
		if(i in a) { break; }
	}
	return i;
}