easing.js

function Transition(transition, params){
    params = NR.makeArray(params);
    var easeIn = function(pos){
        return transition(pos, params);
    };
    return NR.mix(easeIn, {
        easeIn: easeIn,
        easeOut: function(pos){
            return 1 - transition(1 - pos, params);
        },
        easeInOut: function(pos){
            return (pos <= 0.5 ? transition(2 * pos, params) : (2 - transition(2 * (1 - pos), params))) / 2;
        }
    });
};

var Easing = {
        linear: function(zero){
            return zero;
        }
    };


Easing.extend = function(transitions){
    for (var transition in transitions) Easing[transition] = new Transition(transitions[transition]);
};

Easing.extend({

    Pow: function(p, x){
        return Math.pow(p, x && x[0] || 6);
    },

    Expo: function(p){
        return Math.pow(2, 8 * (p - 1));
    },

    Circ: function(p){
        return 1 - Math.sin(Math.acos(p));
    },

    Sine: function(p){
        return 1 - Math.cos(p * Math.PI / 2);
    },

    Back: function(p, x){
        x = x && x[0] || 1.618;
        return Math.pow(p, 2) * ((x + 1) * p - x);
    },

    Bounce: function(p){
        var value;
        for (var a = 0, b = 1; 1; a += b, b /= 2){
            if (p >= (7 - 4 * a) / 11){
                value = b * b - Math.pow((11 - 6 * a - 11 * p) / 4, 2);
                break;
            }
        }
        return value;
    },

    Elastic: function(p, x){
        return Math.pow(2, 10 * --p) * Math.cos(20 * p * Math.PI * (x && x[0] || 1) / 3);
    }

});

['Quad', 'Cubic', 'Quart', 'Quint'].forEach(function(transition, i){
    Easing[transition] = new Transition(function(p){
        return Math.pow(p, i + 2);
    });
});

module.exports = Easing;