summator.js

module.exports = (function () {
    var prev = function (x) {
        return x & (x - 1);
    }, next = function (x) {
        return (x | (x - 1)) + 1;
    }, Summator = function (length) {
        if (1 > length) {
            throw {
                message: 'Length is too small',
            };
        }
        this.length = length;
        this.container = [];
        for (var i = 0; i <= length; ++i) {
            this.container.push(0);
        }
    };

    Summator.prototype = {
        modify: function (position, value) {
            if (1 > position || this.length < position) {
                throw {
                    message: 'Invalid position "' + position + '" [1, length]'
                };
            }
            for (var x = position; x <= this.length; x = next(x)) {
                this.container[x] += value;
            }
        },
        findSum: function (left, right) {
            if (1 > left || 1 > right || this.length < left || this.length < right || right < left) {
                throw {
                    message: 'Invalid left or right argument "' + left + '", "' + right + '"'
                };
            }
            var result = 0;
            for (var x = right; x; x = prev(x)) {
                result += this.container[x];
            }
            for (var x = left - 1; x; x = prev(x)) {
                result -= this.container[x];
            }
            return result;
        }
    };

    return Summator;
})();

var Summator = require('./summator.js'),
    s = new Summator(10);
s.modify(1, 1);
s.modify(2, 1);
s.findSum(1, 2); // 2