react-spring/packages/shared/src/easings.ts

import { EasingFunction } from '@react-spring/types'

import { clamp } from './clamp'

export type Direction = 'start' | 'end'

type StepsEasing = (steps: number, direction?: Direction) => EasingFunction

const steps: StepsEasing =
  (steps: number, direction: Direction = 'end') =>
  (progress: number) => {
    progress =
      direction === 'end'
        ? Math.min(progress, 0.999)
        : Math.max(progress, 0.001)
    const expanded = progress * steps
    const rounded =
      direction === 'end' ? Math.floor(expanded) : Math.ceil(expanded)

    return clamp(0, 1, rounded / steps)
  }

/**
 * With thanks to ai easings.net
 * https://github.com/ai/easings.net/blob/master/src/easings/easingsFunctions.ts
 */
interface EasingDictionary {
  linear: (t: number) => number
  easeInQuad: (t: number) => number
  easeOutQuad: (t: number) => number
  easeInOutQuad: (t: number) => number
  easeInCubic: (t: number) => number
  easeOutCubic: (t: number) => number
  easeInOutCubic: (t: number) => number
  easeInQuart: (t: number) => number
  easeOutQuart: (t: number) => number
  easeInOutQuart: (t: number) => number
  easeInQuint: (t: number) => number
  easeOutQuint: (t: number) => number
  easeInOutQuint: (t: number) => number
  easeInSine: (t: number) => number
  easeOutSine: (t: number) => number
  easeInOutSine: (t: number) => number
  easeInExpo: (t: number) => number
  easeOutExpo: (t: number) => number
  easeInOutExpo: (t: number) => number
  easeInCirc: (t: number) => number
  easeOutCirc: (t: number) => number
  easeInOutCirc: (t: number) => number
  easeInBack: (t: number) => number
  easeOutBack: (t: number) => number
  easeInOutBack: (t: number) => number
  easeInElastic: (t: number) => number
  easeOutElastic: (t: number) => number
  easeInOutElastic: (t: number) => number
  easeInBounce: (t: number) => number
  easeOutBounce: (t: number) => number
  easeInOutBounce: (t: number) => number
  steps: StepsEasing
}

const c1 = 1.70158
const c2 = c1 * 1.525
const c3 = c1 + 1
const c4 = (2 * Math.PI) / 3
const c5 = (2 * Math.PI) / 4.5

const bounceOut: EasingFunction = x => {
  const n1 = 7.5625
  const d1 = 2.75

  if (x < 1 / d1) {
    return n1 * x * x
  } else if (x < 2 / d1) {
    return n1 * (x -= 1.5 / d1) * x + 0.75
  } else if (x < 2.5 / d1) {
    return n1 * (x -= 2.25 / d1) * x + 0.9375
  } else {
    return n1 * (x -= 2.625 / d1) * x + 0.984375
  }
}

export const easings: EasingDictionary = {
  linear: x => x,
  easeInQuad: x => x * x,
  easeOutQuad: x => 1 - (1 - x) * (1 - x),
  easeInOutQuad: x => (x < 0.5 ? 2 * x * x : 1 - Math.pow(-2 * x + 2, 2) / 2),
  easeInCubic: x => x * x * x,
  easeOutCubic: x => 1 - Math.pow(1 - x, 3),
  easeInOutCubic: x =>
    x < 0.5 ? 4 * x * x * x : 1 - Math.pow(-2 * x + 2, 3) / 2,
  easeInQuart: x => x * x * x * x,
  easeOutQuart: x => 1 - Math.pow(1 - x, 4),
  easeInOutQuart: x =>
    x < 0.5 ? 8 * x * x * x * x : 1 - Math.pow(-2 * x + 2, 4) / 2,
  easeInQuint: x => x * x * x * x * x,
  easeOutQuint: x => 1 - Math.pow(1 - x, 5),
  easeInOutQuint: x =>
    x < 0.5 ? 16 * x * x * x * x * x : 1 - Math.pow(-2 * x + 2, 5) / 2,
  easeInSine: x => 1 - Math.cos((x * Math.PI) / 2),
  easeOutSine: x => Math.sin((x * Math.PI) / 2),
  easeInOutSine: x => -(Math.cos(Math.PI * x) - 1) / 2,
  easeInExpo: x => (x === 0 ? 0 : Math.pow(2, 10 * x - 10)),
  easeOutExpo: x => (x === 1 ? 1 : 1 - Math.pow(2, -10 * x)),
  easeInOutExpo: x =>
    x === 0
      ? 0
      : x === 1
        ? 1
        : x < 0.5
          ? Math.pow(2, 20 * x - 10) / 2
          : (2 - Math.pow(2, -20 * x + 10)) / 2,
  easeInCirc: x => 1 - Math.sqrt(1 - Math.pow(x, 2)),
  easeOutCirc: x => Math.sqrt(1 - Math.pow(x - 1, 2)),
  easeInOutCirc: x =>
    x < 0.5
      ? (1 - Math.sqrt(1 - Math.pow(2 * x, 2))) / 2
      : (Math.sqrt(1 - Math.pow(-2 * x + 2, 2)) + 1) / 2,
  easeInBack: x => c3 * x * x * x - c1 * x * x,
  easeOutBack: x => 1 + c3 * Math.pow(x - 1, 3) + c1 * Math.pow(x - 1, 2),
  easeInOutBack: x =>
    x < 0.5
      ? (Math.pow(2 * x, 2) * ((c2 + 1) * 2 * x - c2)) / 2
      : (Math.pow(2 * x - 2, 2) * ((c2 + 1) * (x * 2 - 2) + c2) + 2) / 2,
  easeInElastic: x =>
    x === 0
      ? 0
      : x === 1
        ? 1
        : -Math.pow(2, 10 * x - 10) * Math.sin((x * 10 - 10.75) * c4),
  easeOutElastic: x =>
    x === 0
      ? 0
      : x === 1
        ? 1
        : Math.pow(2, -10 * x) * Math.sin((x * 10 - 0.75) * c4) + 1,
  easeInOutElastic: x =>
    x === 0
      ? 0
      : x === 1
        ? 1
        : x < 0.5
          ? -(Math.pow(2, 20 * x - 10) * Math.sin((20 * x - 11.125) * c5)) / 2
          : (Math.pow(2, -20 * x + 10) * Math.sin((20 * x - 11.125) * c5)) / 2 +
            1,
  easeInBounce: x => 1 - bounceOut(1 - x),
  easeOutBounce: bounceOut,
  easeInOutBounce: x =>
    x < 0.5 ? (1 - bounceOut(1 - 2 * x)) / 2 : (1 + bounceOut(2 * x - 1)) / 2,
  steps,
} as const