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AI solution for TypeScript puzzle on BFE.dev
45. implement Slice<A, S, E>
Here is a possible implementation of Slice<A, S, E>
:
type Slice<A extends any[], S extends number, E extends number = A['length']> = A extends [...infer Result, ...infer Rest] ? Rest extends [] ? Result : S extends 0 ? E extends 0 ? [] : E extends 1 ? [Result[0]] : [Result[0], ...Slice<Rest, 0, E-1>] : Slice<Rest, S-1, E-1> : []type A = Slice<[1,2,3,4], 0, 2> // [1, 2]type B = Slice<[1,2,3,4], 2> // [3, 4]type C = Slice<[number, boolean, bigint], 2, 5> // [bigint]type D = Slice<[string, boolean], 0, 1> // [string]type E = Slice<[number, boolean, bigint], 5, 6> // []
Here's how it works:
Slice
is a generic type that takes an array typeA
, and two numbersS
(the start index) andE
(the end index).- It first checks if
A
is an array type by trying to destructure it with a rest pattern[...infer Result, ...infer Rest]
. IfA
is not an array type, it returns an empty array[]
. - If
A
is an array type, it checks ifRest
is empty. If it is, it means we have reached the end of the array, so it returnsResult
. IfRest
is not empty, we need to keep slicing the array. - If
S
is 0, it means we have reached the start index. Now we need to start building the slice. We first check ifE
is 0. If it is, we return an empty array[]
. IfE
is 1, we return an array with just the first element ofResult
. - If
E
is greater than 1, we need to keep slicingRest
until we reach the end index. We do this by making a recursive call toSlice
withRest
, the start index decreased by 1 (S-1
) and the end index decreased by 1 (E-1
). We then prepend the first element ofResult
to the result of this recursive call to build the final slice. - If
S
is greater than 0, it means we haven't reached the start index yet, so we need to keep slicingRest
. We do this by making a recursive call toSlice
withRest
, the start index decreased by 1 (S-1
) and the end index decreased by 1 (E-1
).
Note that we use default type parameter E extends number = A['length']
to allow users omitting the end index. If E
is not provided, E
defaults to A['length']
, which means the slice goes from S
to the end of the array.