lop/lib/lazy-iterators.js

var fromArray = exports.fromArray = function(array) {
    var index = 0;
    var hasNext = function() {
        return index < array.length;
    };
    return new LazyIterator({
        hasNext: hasNext,
        next: function() {
            if (!hasNext()) {
                throw new Error("No more elements");
            } else {
                return array[index++];
            }
        }
    });
};

var LazyIterator = function(iterator) {
    this._iterator = iterator;
};

LazyIterator.prototype.map = function(func) {
    var iterator = this._iterator;
    return new LazyIterator({
        hasNext: function() {
            return iterator.hasNext();
        },
        next: function() {
            return func(iterator.next());
        }
    });
};

LazyIterator.prototype.filter = function(condition) {
    var iterator = this._iterator;
    
    var moved = false;
    var hasNext = false;
    var next;
    var moveIfNecessary = function() {
        if (moved) {
            return;
        }
        moved = true;
        hasNext = false;
        while (iterator.hasNext() && !hasNext) {
            next = iterator.next();
            hasNext = condition(next);
        }
    };
    
    return new LazyIterator({
        hasNext: function() {
            moveIfNecessary();
            return hasNext;
        },
        next: function() {
            moveIfNecessary();
            var toReturn = next;
            moved = false;
            return toReturn;
        }
    });
};

LazyIterator.prototype.first = function() {
    var iterator = this._iterator;
    if (this._iterator.hasNext()) {
        return iterator.next();
    } else {
        return null;
    }
};

LazyIterator.prototype.toArray = function() {
    var result = [];
    while (this._iterator.hasNext()) {
        result.push(this._iterator.next());
    }
    return result;
};