add : TWEEN.Easing.generatePow function and test.

related tweenjs#116

src/Easing.ts

@@ -195,6 +195,30 @@ const Easing = {
 			return Easing.Bounce.Out(amount * 2 - 1) * 0.5 + 0.5
 		},
 	},
+	generatePow: function (
+		power = 4,
+	): {
+		In(amount: number): number
+		Out(amount: number): number
+		InOut(amount: number): number
+	} {
+		power = power < 1.0 ? 1.0 : power
+		power = power > 10000 ? 10000 : power
+		return {
+			In: function (amount: number): number {
+				return amount ** power
+			},
+			Out: function (amount: number): number {
+				return 1 - (1 - amount) ** power
+			},
+			InOut: function (amount: number): number {
+				if (amount < 0.5) {
+					return (amount * 2) ** power / 2
+				}
+				return (1 - (2 - amount * 2) ** power) / 2 + 0.5
+			},
+		}
+	},
 }
 
 export default Easing

src/tests.ts

@@ -1917,6 +1917,58 @@ export const tests = {
 
 		test.done()
 	},
+
+	'Test TWEEN.Easing.generatePow(1) equals Linear'(test: Test): void {
+		const ease1 = TWEEN.Easing.generatePow(1)
+		const easeMinus = TWEEN.Easing.generatePow(-1)
+
+		const compareWithLinear = (ease: EasingFunctionGroup, amount: number) => {
+			const linearResult = TWEEN.Easing.Linear.None(amount)
+			test.equal(linearResult, ease.In(amount))
+			test.equal(linearResult, ease.Out(amount))
+			test.equal(linearResult, ease.InOut(amount))
+		}
+		compareWithLinear(ease1, 0)
+		compareWithLinear(easeMinus, 0)
+		compareWithLinear(ease1, 0.25)
+		compareWithLinear(easeMinus, 0.25)
+		compareWithLinear(ease1, 0.5)
+		compareWithLinear(easeMinus, 0.5)
+		compareWithLinear(ease1, 0.75)
+		compareWithLinear(easeMinus, 0.75)
+		compareWithLinear(ease1, 1)
+		compareWithLinear(easeMinus, 1)
+
+		compareWithLinear(ease1, -1)
+		compareWithLinear(easeMinus, -1)
+		compareWithLinear(ease1, Infinity)
+		compareWithLinear(easeMinus, Infinity)
+
+		test.done()
+	},
+
+	'Test TWEEN.Easing.generatePow(n) should pass 0.0, 0.5, 1.0'(test: Test): void {
+		const checkEdgeValue = (ease: EasingFunctionGroup) => {
+			test.equal(ease.InOut(0.0), 0.0)
+			test.equal(ease.In(0.0), 0.0)
+			test.equal(ease.Out(0.0), 0.0)
+
+			test.equal(ease.InOut(0.5), 0.5)
+
+			test.equal(ease.InOut(1.0), 1.0)
+			test.equal(ease.In(1.0), 1.0)
+			test.equal(ease.Out(1.0), 1.0)
+		}
+		checkEdgeValue(TWEEN.Easing.generatePow(Number.NEGATIVE_INFINITY))
+		checkEdgeValue(TWEEN.Easing.generatePow(1))
+		checkEdgeValue(TWEEN.Easing.generatePow(Math.LOG2E))
+		checkEdgeValue(TWEEN.Easing.generatePow(Math.PI))
+		checkEdgeValue(TWEEN.Easing.generatePow())
+		checkEdgeValue(TWEEN.Easing.generatePow(6))
+		checkEdgeValue(TWEEN.Easing.generatePow(Number.POSITIVE_INFINITY))
+
+		test.done()
+	},
 }
 
 type Test = {
@@ -1926,3 +1978,9 @@ type Test = {
 	expect(n: number): void
 	done(): void
 }
+
+type EasingFunctionGroup = {
+	In(amount: number): number
+	Out(amount: number): number
+	InOut(amount: number): number
+}