var pascalsTriangle = function () {
    "use strict";
    var tri = [[1]];
    var nextRow = function (prevRow) {
        var ary = [];

        // // more efficient for loop
        // prevRow = [0].concat(prevRow);
        // ary = new Array(prevRow.length);
        // for (var i = 0, j = ((prevRow.length + prevRow.length % 2) / 2) ; i < j ; i += 1 ) {
        //     ary[i] = prevRow[i] + prevRow[i + 1];
        // }
        // for (j = prevRow.length - 1 ; i <= j ; i += 1) {
        //     ary[i] = ary[j - i];
        // }

        ary = [0].concat(prevRow).map((v,i,a)=> v + (i + 1 < a.length ? a[i+1] : 0));
        return ary;
    };
    var triangle = function self(n) {
        if (n <= tri.length) {
            return tri.slice(0, n);
        }
        tri = self(n - 1);
        var prevRow = tri[tri.length - 1];
        tri.push(nextRow(prevRow));
        return tri;
    };
    var row = function (n) {
        if (tri.length < n) {
            tri = [].concat(triangle(n));
        }
        return tri[n - 1];
    };
    return {
        triangle: triangle,
        row: row,
        reset: function(){tri = [[1]];}
    };
};

var algorithm = pascalsTriangle();
var start_time;
var end_time;
var result;

start_time = performance.now();
result = algorithm.triangle(5);
end_time = performance.now();
console.log("calculate 5 rows", result[4], end_time - start_time);

test(100);

result = algorithm.triangle(999);
start_time = performance.now();
result = algorithm.triangle(999);
end_time = performance.now();
console.log("retrieve 999 rows from cache", result[4], end_time - start_time);

function rep(x,f){
    while (x > 0) {
        f();
        x -= 1;
    }
}

function test(x) {
    start_time = performance.now();
    rep(x, function(){ result = 0; result = algorithm.triangle(999); algorithm.reset(); });
    end_time = performance.now();
    console.log("Avg time to calculate 999 rows, tested " + x + " times", result[4], (end_time - start_time)/x);
}