/* @license
 * Copyright 2019 Google LLC. All Rights Reserved.
 * Licensed under the Apache License, Version 2.0 (the 'License');
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an 'AS IS' BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

// Adapted from https://gist.github.com/gre/1650294
export const easeInOutQuad: TimingFunction = (t: number) =>
    t < .5 ? 2 * t * t : -1 + (4 - 2 * t) * t;

/**
 * A TimingFunction accepts a value from 0-1 and returns a corresponding
 * interpolated value
 */
export type TimingFunction = (time: number) => number;

/**
 * Creates a TimingFunction that uses a given ease to interpolate between
 * two configured number values.
 */
export const interpolate =
    (start: number, end: number, ease: TimingFunction = easeInOutQuad):
        TimingFunction => (time: number) => start + (end - start) * ease(time);

/**
 * Creates a TimingFunction that interpolates through a weighted list
 * of other TimingFunctions ("tracks"). Tracks are interpolated in order, and
 * allocated a percentage of the total time based on their relative weight.
 */
export const sequence =
    (tracks: Array<TimingFunction>, weights: Array<number>): TimingFunction => {
      const totalWeight = weights.reduce((total, weight) => total + weight, 0);
      const ratios: Array<number> = weights.map(weight => weight / totalWeight);

      return (time: number) => {
        let start: number = 0;
        let ratio: number = Infinity;
        let track: TimingFunction = () => 0;

        for (let i = 0; i < ratios.length; ++i) {
          ratio = ratios[i];
          track = tracks[i];

          if (time <= (start + ratio)) {
            break;
          }

          start += ratio;
        }

        return track!((time - start) / ratio!);
      }
    };

/**
 * A Keyframe groups a target value, the number of frames to interpolate towards
 * that value and an optional easing funnction to use for interpolation.
 */
export interface Keyframe {
  value: number;
  frames: number;
  ease?: TimingFunction;
}

/**
 * Creates a "timeline" TimingFunction out of an initial value and a series of
 * Keyframes. The timeline function accepts value from 0-1 and returns the
 * current value based on keyframe interpolation across the total number of
 * frames. Frames are only used to indicate the relative length of each keyframe
 * transition, so interpolated values will be computed for fractional frames.
 */
export const timeline =
    (initialValue: number, keyframes: Array<Keyframe>): TimingFunction => {
      const tracks: Array<TimingFunction> = [];
      const weights: Array<number> = [];

      let lastValue = initialValue;

      for (let i = 0; i < keyframes.length; ++i) {
        const keyframe = keyframes[i];
        const {value, frames} = keyframe;
        const ease = keyframe.ease || easeInOutQuad;
        const track = interpolate(lastValue, value, ease);

        tracks.push(track);
        weights.push(frames);
        lastValue = value;
      }

      return sequence(tracks, weights);
    };