Adding arithmetic computation: Power.

README.md

@@ -39,6 +39,7 @@ $ yarn test
   - [Subtract](src/subtract/index.d.ts) - subtracts two numbers.
   - [Multiply](src/multiply/index.d.ts) - multiplies two numbers.
   - [Divide](src/divide/index.d.ts) - divides two numbers.
+  - [Power](src/power/index.d.ts) - computes A power B.
   - [Greater than or equal](src/gte/index.d.ts) - checks if a value is greater than or equal to another value.
   - [Less than or equal](src/lte/index.d.ts) - checks if a value is less than or equal to another value.
   - [Max](src/max/index.d.ts) - computes the maximum value of an array.

src/power/index.d.ts

@@ -0,0 +1,32 @@
+import { Multiply, Dec, Cast } from '..';
+
+// Power two numbers: https://lodash.com/docs/4.17.15#power.
+//
+type S = Power<2, 3>; // 8
+//
+// This type uses recursive (and not officially supported) type alias, see more:
+// https://github.com/microsoft/TypeScript/issues/26223#issuecomment-513187373.
+export type Power<
+  // The first number in a multiplication.
+  A extends number,
+  // The second number in a multiplication.
+  B extends number
+  > = {
+  // If A is 0: return 0
+  // If B is 0: return 1
+  zero: 0;
+  one: 1;
+  // If they're both not 1s or 0s, we call the recursion again, like this:
+  //
+  //   Multiply<A, Power<A, Dec<B>>>
+  //
+  // We multiply the value of the first number (A) to the result of calling Multiply
+  // again in which we decrease B's value by 1. This means that we call Multiply<A, A>
+  // B times.
+  //
+  // Computation is split into multiple steps with `infer`, see more:
+  // https://github.com/pirix-gh/medium/blob/master/types-curry-ramda/src/index.ts#L17.
+  next: Power<A, Dec<B>> extends infer G // Assign result to `G`
+    ? Multiply<A, Cast<G, number>>
+    : never;
+}[B extends 0 ? 'one' : A extends 0 ? 'zero' : A extends 1 ? 'one' : 'next'];

src/power/index.test-d.ts

@@ -0,0 +1,12 @@
+import { expectType } from 'tsd';
+import { Power } from '.';
+
+expectType<Power<2, 3>>(8);
+expectType<Power<3, 2>>(9);
+expectType<Power<0, 0>>(1);
+expectType<Power<0, 3>>(0);
+expectType<Power<4, 0>>(1);
+expectType<Power<1, 1>>(1);
+expectType<Power<1, 5>>(1);
+expectType<Power<4, 1>>(4);
+expectType<Power<3, 5>>(this as never);