feat: add MathPlus.ToNegative

.changeset/plenty-nails-study.md

@@ -0,0 +1,5 @@
+---
+'type-plus': minor
+---
+
+Add `MathPlus.ToNegative`

ts/index.ts

@@ -49,6 +49,7 @@ export type {
 	Max,
 	Subtract
 } from './math/index.js'
+export * as MathPlus from './math/math_plus.js'
 export type { IsNever, IsNotNever, Is_Never, NeverType, NotNeverType, Not_Never } from './never/never_type.js'
 export * from './nodejs/index.js'
 export * from './nominal/index.js'

ts/math/README.md

@@ -1,5 +1,22 @@
 # Math
 
+## MathPlus
+
+`MathPlus` contains all types and type utilities related to math.
+
+### [`MathPlus.ToNegative`](./math.to-negative.ts#L12)
+
+> `MathPlus.ToNegative<N>`
+
+Converts a number or bigint `N` to negative.
+If `N` is already negative, it returns itself.
+
+```ts
+MathPlus.ToNegative<5> // -5
+MathPlus.ToNegative<0> // 0
+MathPlus.ToNegative<-5> // -5
+```
+
 ## References
 
 - [Implementing Arithmetic Within TypeScript’s Type System](https://itnext.io/implementing-arithmetic-within-typescripts-type-system-a1ef140a6f6f)

ts/math/math_plus.to_negative.spec.ts

@@ -0,0 +1,23 @@
+import { it } from '@jest/globals'
+import { MathPlus, testType } from '../index.js'
+
+it('converts positive bigint to negative', () => {
+	testType.equal<MathPlus.ToNegative<1n>, -1n>(true)
+})
+
+it('converts positive number to negative', () => {
+	testType.equal<MathPlus.ToNegative<1>, -1>(true)
+})
+
+it('converts 0 to 0', () => {
+	testType.equal<MathPlus.ToNegative<0>, 0>(true)
+	testType.equal<MathPlus.ToNegative<0n>, 0n>(true)
+
+	testType.equal<MathPlus.ToNegative<-0>, 0>(true)
+	testType.equal<MathPlus.ToNegative<-0n>, 0n>(true)
+})
+
+it('returns N if N is negative', () => {
+	testType.equal<MathPlus.ToNegative<-1>, -1>(true)
+	testType.equal<MathPlus.ToNegative<-1n>, -1n>(true)
+})

ts/math/math_plus.to_negative.ts

@@ -0,0 +1,22 @@
+/**
+ * Converts a number or bigint `N` to negative.
+ * If `N` is already negative, it returns itself.
+ *
+ * @example
+ * ```ts
+ * ToNegative<5> // -5
+ * ToNegative<0> // 0
+ * ToNegative<-5> // -5
+ * ```
+ */
+export type ToNegative<N extends number | bigint> = N extends number
+	? N extends 0
+		? 0
+		: `-${N}` extends `${infer W extends number}`
+		? W
+		: N
+	: N extends 0n
+	? 0n
+	: `-${N}` extends `${infer W extends bigint}`
+	? W
+	: N

ts/math/math_plus.ts

@@ -0,0 +1 @@
+export type { ToNegative } from './math_plus.to_negative.js'