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20 lines
867 B
TypeScript
20 lines
867 B
TypeScript
/** length of const array or tuple */
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export type Length<T extends any[]> = T["length"];
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/** build a tuple with length L, and type T */
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export type Tuple<L extends number, T = any, R extends T[] = []> = R extends { length: L } ? R : Tuple<L, T, [...R, T]>;
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/** adds two constant positive whole number types */
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export type Add<A extends number, B extends number> = Length<[...Tuple<A>, ...Tuple<B>]>;
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/** subtracts one constant positive whole number type from another */
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export type Subtract<A extends number, B extends number> = Tuple<A> extends [...infer U, ...Tuple<B>]
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? Length<U>
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: never;
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/** type of the last element of a readonly array or tuple */
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export type Last<A extends readonly [...any]> = A[Subtract<Length<A>, 1>];
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/** type of the first element of a readonly array or tuple */
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export type First<A extends readonly [...any]> = A[Length<A>];
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