/* @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.
 */

import {clamp} from '../utilities';

// 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 cumulativeSum = ((sum: number) => (value: number) => sum += value);
      const times = weights.map(cumulativeSum(0));

      return (time: number) => {
        time = clamp(time, 0, 1);
        time *= times[times.length - 1];
        const i = times.findIndex((val) => val >= time);

        const start = i < 1 ? 0 : times[i - 1];
        const end = times[i];

        return tracks[i]((time - start) / (end - start));
      }
    };

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

export interface Path {
  initialValue: number;
  keyframes: Frame[];
}

/**
 * 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 = (path: Path): TimingFunction => {
  const tracks: Array<TimingFunction> = [];
  const weights: Array<number> = [];

  let lastValue = path.initialValue;

  for (let i = 0; i < path.keyframes.length; ++i) {
    const keyframe = path.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);
};