type-fest/source/subtract.d.ts

A collection of essential TypeScript types

import type {NumberAbsolute, BuildTuple, ReverseSign} from './internal';
import type {PositiveInfinity, NegativeInfinity, IsNegative} from './numeric';
import type {LessThan} from './less-than';

/**
Returns the difference between two numbers.

Note:
- A or B can only support `-999` ~ `999`.

@example
```
import type {Subtract} from 'type-fest';

Subtract<333, 222>;
//=> 111

Subtract<111, -222>;
//=> 333

Subtract<-111, 222>;
//=> -333

Subtract<18, 96>;
//=> -78

Subtract<PositiveInfinity, 9999>;
//=> PositiveInfinity

Subtract<PositiveInfinity, PositiveInfinity>;
//=> number
```

@category Numeric
*/
// TODO: Support big integer.
export type Subtract<A extends number, B extends number> =
	// Handle cases when A or B is the actual "number" type
	number extends A | B ? number
		// Handle cases when A and B are both +/- infinity
		: A extends B & (PositiveInfinity | NegativeInfinity) ? number
			// Handle cases when A is - infinity or B is + infinity
			: A extends NegativeInfinity ? NegativeInfinity : B extends PositiveInfinity ? NegativeInfinity
				// Handle cases when A is + infinity or B is - infinity
				: A extends PositiveInfinity ? PositiveInfinity : B extends NegativeInfinity ? PositiveInfinity
					// Handle case when numbers are equal to each other
					: A extends B ? 0
						// Handle cases when A or B is 0
						: A extends 0 ? ReverseSign<B> : B extends 0 ? A
							// Handle remaining regular cases
							: SubtractPostChecks<A, B>;

/**
Subtracts two numbers A and B, such that they are not equal and neither of them are 0, +/- infinity or the `number` type
*/
type SubtractPostChecks<A extends number, B extends number, AreNegative = [IsNegative<A>, IsNegative<B>]> =
	AreNegative extends [false, false]
		? SubtractPositives<A, B>
		: AreNegative extends [true, true]
			// When both numbers are negative we subtract the absolute values and then reverse the sign
			? ReverseSign<SubtractPositives<NumberAbsolute<A>, NumberAbsolute<B>>>
			// When the signs are different we can add the absolute values and then reverse the sign if A < B
			: [...BuildTuple<NumberAbsolute<A>>, ...BuildTuple<NumberAbsolute<B>>] extends infer R extends unknown[]
				? LessThan<A, B> extends true ? ReverseSign<R['length']> : R['length']
				: never;

/**
Subtracts two positive numbers.
*/
type SubtractPositives<A extends number, B extends number> =
	LessThan<A, B> extends true
		// When A < B we can reverse the result of B - A
		? ReverseSign<SubtractIfAGreaterThanB<B, A>>
		: SubtractIfAGreaterThanB<A, B>;

/**
Subtracts two positive numbers A and B such that A > B.
*/
type SubtractIfAGreaterThanB<A extends number, B extends number> =
	// This is where we always want to end up and do the actual subtraction
	BuildTuple<A> extends [...BuildTuple<B>, ...infer R]
		? R['length']
		: never;