chore: DRY the types

eslint.config.ts

@@ -41,7 +41,7 @@ export default defineConfig([
 			"@typescript-eslint/no-unused-vars": [
 				"error",
 				{
-					varsIgnorePattern: "^_",
+					varsIgnorePattern: "^_[^_]",
 					reportUsedIgnorePattern: true,
 				},
 			],

src/internal.ts

@@ -7,8 +7,6 @@ export interface Identity extends HKT {
 // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
 export const never = undefined as never;
 
-export type MaxDepth = 20;
-
 export class AssertionError extends Error {
 	public constructor(msg?: string) {
 		super(msg);

src/mixin/array.ts

@@ -2,6 +2,7 @@ import { makeFluent } from "../base";
 import type { HKT } from "../base/hkt";
 import { Mixin, type Input, type Return } from "../base/mixin";
 import { assert, assertType, type HidePrototype } from "../internal";
+import type { MaxDepth, At } from "../utility";
 
 type IterArgs<T extends readonly unknown[] = unknown[]> = [
 	element: T[number],
@@ -9,11 +10,7 @@ type IterArgs<T extends readonly unknown[] = unknown[]> = [
 	array: Readonly<T>,
 ];
 
-type MaxDepth = 20;
+type Unordered<T extends readonly unknown[]> = T[number][];
-
-type Unordered<T extends readonly unknown[]> = T extends unknown[]
-	? T[number][]
-	: readonly T[number][];
 
 type Map<
 	T extends readonly unknown[],
@@ -38,54 +35,6 @@ type Reverse<
 			? TOut
 			: T[number][];
 
-namespace At {
-	export type Dec<N extends number> = [
-		-1,
-		0,
-		1,
-		2,
-		3,
-		4,
-		5,
-		6,
-		7,
-		8,
-		9,
-		10,
-		11,
-		12,
-		13,
-		14,
-		15,
-		16,
-		17,
-		18,
-		19,
-		20,
-		21,
-	][N];
-
-	export type Select<
-		T extends readonly unknown[],
-		Skip extends number,
-	> = Skip extends 0
-		? T extends readonly [...unknown[], infer T]
-			? T
-			: undefined
-		: T extends readonly [...infer TRest, unknown]
-			? Select<TRest, Dec<Skip>>
-			: undefined;
-}
-
-type At<
-	T extends readonly unknown[],
-	K extends keyof T & number,
-> = number extends T["length"]
-	? T[K] | undefined
-	: `${K}` extends `-${infer N extends number}`
-		? At.Select<T, At.Dec<N>>
-		: T[K];
-
 type Repeat<
 	T extends readonly unknown[],
 	N extends number,
@@ -97,38 +46,6 @@ type Repeat<
 		? T[number][]
 		: Repeat<T, N, [...Acc, null], [...TOut, ...T]>;
 
-namespace Flat {
-	type Inc<
-		N extends number,
-		Acc extends unknown[] = [],
-	> = Acc["length"] extends N
-		? [...Acc, null]["length"]
-		: Inc<N, [...Acc, null]>;
-
-	export type Leaves<
-		T,
-		Depth extends number = 0,
-	> = Depth extends MaxDepth
-		? T
-		: T extends readonly unknown[]
-			? Leaves<T[number], Inc<Depth>>
-			: T;
-
-	export type Rebuild<
-		T extends readonly unknown[],
-		Depth extends number = 0,
-	> = Depth extends MaxDepth
-		? T
-		: T extends readonly [infer H, ...infer R]
-			? H extends readonly unknown[]
-				? [...Rebuild<H, Inc<Depth>>, ...Rebuild<R, Depth>]
-				: [H, ...Rebuild<R, Depth>]
-			: [];
-}
-
-export type Flat<T extends readonly unknown[]> =
-	number extends T["length"] ? Flat.Leaves<T>[] : Flat.Rebuild<T>;
-
 function isArray(v: unknown): v is readonly unknown[] {
 	return typeof v === "object" && globalThis.Array.isArray(v);
 }

src/mixin/math.ts

@@ -6,21 +6,10 @@ import {
 	type Props,
 	type Return,
 } from "../base/mixin";
-import { assertType, type MaxDepth } from "../internal";
+import { assertType } from "../internal";
+import type { Inc } from "../utility";
 import { applySwizzle, type Swizzle } from "./math/swizzle";
 
-type Vec<
-	T,
-	N extends number,
-	TOut extends T[] = [],
-> = TOut["length"] extends N
-	? TOut
-	: TOut["length"] extends MaxDepth
-		? T[]
-		: Vec<T, N, [...TOut, T]>;
-
-type Inc<N extends number> = [...Vec<null, N>, null]["length"];
-
 type Sign<T extends number> = T extends 0
 	? 0
 	: `${T}` extends `-${number}`

src/mixin/math/swizzle.ts

@@ -1,26 +1,13 @@
 import { makeFluent } from "../../base";
 import type { Props, Return } from "../../base/mixin";
-import { type MaxDepth } from "../../internal";
 import { Math } from "../math";
+import type { Vec, Dec, Inc as __Inc } from "../../utility";
 
 type Axis = typeof axis;
-type Vec<
-	T,
-	N extends number,
-	TOut extends T[] = [],
-> = TOut["length"] extends N
-	? TOut
-	: TOut["length"] extends MaxDepth
-		? T[]
-		: Vec<T, N, [...TOut, T]>;
 
 type Inc<N extends number | undefined> = N extends number
-	? [...Vec<null, N>, null]["length"]
+	? __Inc<N>
 	: 0;
-type Dec<N extends number> =
-	Vec<null, N> extends [...infer Rest extends null[], null]
-		? Rest["length"]
-		: -1;
 
 type Next<
 	N extends number,
@@ -52,13 +39,8 @@ type Key<
 type IsAscending<
 	State extends (number | undefined)[],
 	Prev extends number | undefined = undefined,
-> = State extends readonly [
+> = State extends readonly [Inc<Prev>, ...infer Rest extends number[]]
-	infer Current extends number,
-	...infer Rest extends number[],
-]
-	? Current extends Inc<Prev>
 	? IsAscending<Rest, Inc<Prev>>
-		: false
 	: State extends readonly []
 		? true
 		: false;

src/utility.ts

@@ -0,0 +1,252 @@
+export type MaxDepth = 50;
+
+export type Pretty<T> = { [K in keyof T]: T[K] };
+export type Constrain<T, U> = T extends U ? T : never;
+
+namespace Vec {
+	// prettier-ignore
+	type Lookup<T, N extends number> = N extends 0
+        ? []
+        : N extends 1
+            ? [T]
+            : N extends 2
+                ? [T, T]
+                : N extends 3
+                    ? [T, T, T]
+                    : N extends 4
+                        ? [T, T, T, T]
+                        : N extends 5
+                            ? [T, T, T, T, T]
+                            : N extends 6
+                                ? [T, T, T, T, T, T]
+                                : N extends 7
+                                    ? [T, T, T, T, T, T, T]
+                                    : N extends 8
+                                        ? [T, T, T, T, T, T, T, T]
+                                        : N extends 9
+                                            ? [T, T, T, T, T, T, T, T, T]
+                                            : N extends 10
+                                                ? [T, T, T, T, T, T, T, T, T, T]
+                                                : N extends 11
+                                                    ? [T, T, T, T, T, T, T, T, T, T, T]
+                                                    : N extends 12
+                                                        ? [T, T, T, T, T, T, T, T, T, T, T, T]
+                                                        : N extends 13
+                                                            ? [T, T, T, T, T, T, T, T, T, T, T, T, T]
+                                                            : N extends 14
+                                                                ? [T, T, T, T, T, T, T, T, T, T, T, T, T, T]
+                                                                : N extends 15
+                                                                    ? [T, T, T, T, T, T, T, T, T, T, T, T, T, T, T]
+                                                                    : N extends 16
+                                                                        ? [T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T]
+                                                                        : N extends 17
+                                                                            ? [T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T]
+                                                                            : N extends 18
+                                                                                ? [T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T]
+                                                                                : N extends 19
+                                                                                    ? [T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T]
+                                                                                    : N extends 20
+                                                                                        ? [T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T]
+                                                                                        : [T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T];
+
+	export type Recursive<
+		T,
+		N extends number,
+		TOut extends T[] = Lookup<T, N>,
+		TAcc extends never[] = [],
+	> = TOut["length"] extends N
+		? TOut
+		: TAcc["length"] extends MaxDepth
+			? T[]
+			: Recursive<T, N, [...TOut, T], [...TAcc, never]>;
+}
+export type Vec<
+	T,
+	N extends number,
+	Readonly extends boolean = false,
+> = number extends N
+	? Readonly extends true
+		? readonly T[]
+		: T[]
+	: `${N}` extends `${infer U extends number}${`.${number}` | ""}`
+		? `${N}` extends `-${number}`
+			? T[]
+			: Vec.Recursive<T, U> extends infer V
+				? Readonly extends true
+					? { readonly [K in keyof V]: V[K] }
+					: V
+				: never
+		: T[];
+
+export type Inc<N extends number> = Constrain<
+	number extends N ? number : [...Vec<never, N>, never]["length"],
+	number
+>;
+
+export type Dec<N extends number> = N extends 0
+	? -1
+	: `${N}` extends `-${infer U extends number}`
+		? `-${Inc<U>}` extends `${infer O extends number}`
+			? O
+			: never
+		: `${N}` extends `${number}`
+			? Vec<never, N> extends [never, ...infer Rest]
+				? Rest["length"]
+				: number
+			: number;
+
+namespace At {
+	export type FindBack<
+		T extends readonly unknown[],
+		N extends number,
+		TAcc extends never[] = [],
+	> = T extends readonly [...infer Rest, infer Last]
+		? TAcc["length"] extends N
+			? Last
+			: TAcc["length"] extends MaxDepth
+				? Rest[number]
+				: FindBack<Rest, N, [...TAcc, never]>
+		: T extends readonly []
+			? undefined
+			: never;
+
+	export type Sequence<
+		T extends readonly unknown[],
+		N extends readonly number[],
+		Fallback extends T[number][],
+		TOut extends T[number][] = [],
+	> = TOut["length"] extends MaxDepth
+		? Fallback
+		: N extends readonly [
+					infer Current extends number,
+					...infer Rest extends number[],
+			  ]
+			? Sequence<T, Rest, Fallback, [...TOut, At<T, Current>]>
+			: N extends readonly []
+				? TOut
+				: never;
+}
+export type At<
+	T extends readonly unknown[],
+	N extends number | readonly number[],
+> = N extends readonly number[]
+	? number extends N["length"]
+		? (T[number] | undefined)[]
+		: At.Sequence<T, N, Vec<T[number], N["length"]>>
+	: N extends number
+		? N extends `${number}.${number}`
+			? undefined
+			: number extends T["length"]
+				? T[number] | undefined
+				: `${N}` extends `-${infer U extends number}`
+					? At.FindBack<T, Dec<U>>
+					: T[N]
+		: never;
+
+namespace Flat {
+	export type Simple<
+		T,
+		Depth extends number = 0,
+	> = Depth extends MaxDepth
+		? T
+		: T extends readonly unknown[]
+			? Simple<T[number], Inc<Depth>>
+			: T;
+
+	export type Flatten<
+		T extends readonly unknown[],
+		TOrig extends readonly unknown[] = T,
+		TOut extends unknown[] = [],
+	> = TOut["length"] extends MaxDepth
+		? Flat.Simple<TOrig>[]
+		: T extends readonly [infer Current, ...infer Rest]
+			? Current extends readonly unknown[]
+				? number extends Current["length"]
+					? Flatten<Rest, TOrig, [...TOut, ...Current]>
+					: Flatten<
+							Rest,
+							TOrig,
+							[...TOut, ...Flat<Current>]
+						>
+				: Flatten<Rest, TOrig, [...TOut, Current]>
+			: T extends readonly []
+				? TOut
+				: never;
+	export type Flat<
+		T extends readonly unknown[],
+		TOrig extends readonly unknown[] = T,
+	> = number extends T["length"]
+		? Flat.Simple<T>[]
+		: Flat.Flatten<T, TOrig>;
+}
+export type Flat<T extends readonly unknown[]> = Flat.Flat<T>;
+
+if (import.meta.vitest) {
+	const { test, expectTypeOf } = import.meta.vitest;
+
+	test("Vec<T, N>", () => {
+		expectTypeOf<Vec<never, 0>>().toEqualTypeOf<[]>();
+
+		expectTypeOf<Vec<number, 4>>().toEqualTypeOf<
+			[number, number, number, number]
+		>();
+		expectTypeOf<Vec<number, 4, true>>().toEqualTypeOf<
+			readonly [number, number, number, number]
+		>();
+
+		expectTypeOf<Vec<number, typeof Infinity>>().toEqualTypeOf<
+			number[]
+		>();
+		expectTypeOf<Vec<number, number>>().toEqualTypeOf<number[]>();
+	});
+
+	test("Inc<N>", () => {
+		expectTypeOf<Inc<0>>().toEqualTypeOf<1>();
+		expectTypeOf<Inc<50>>().toEqualTypeOf<51>();
+
+		expectTypeOf<Inc<-5>>().toEqualTypeOf<number>();
+		expectTypeOf<Inc<typeof Infinity>>().toEqualTypeOf<number>();
+		expectTypeOf<Inc<number>>().toEqualTypeOf<number>();
+	});
+
+	test("Dec<N>", () => {
+		expectTypeOf<Dec<5>>().toEqualTypeOf<4>();
+		expectTypeOf<Dec<0>>().toEqualTypeOf<-1>();
+		expectTypeOf<Dec<-4>>().toEqualTypeOf<-5>();
+
+		expectTypeOf<Dec<typeof Infinity>>().toEqualTypeOf<number>();
+		expectTypeOf<Dec<number>>().toEqualTypeOf<number>();
+	});
+
+	test("At<T, N>", () => {
+		type Array = [0, 1, 2, 3, 4, 5];
+
+		expectTypeOf<At<Array, 0>>().toEqualTypeOf<Array[0]>();
+		expectTypeOf<At<Array, -1>>().toEqualTypeOf<Array[5]>();
+
+		expectTypeOf<At<Array, 10>>().toEqualTypeOf<undefined>();
+		expectTypeOf<At<Array, -10>>().toEqualTypeOf<undefined>();
+		expectTypeOf<At<number[], 0>>().toEqualTypeOf<
+			number | undefined
+		>();
+
+		expectTypeOf<At<Array, [0, -1]>>().toEqualTypeOf<[0, 5]>();
+		expectTypeOf<At<Readonly<Array>, [0]>>().toEqualTypeOf<[0]>();
+	});
+
+	test("Flat<T>", () => {
+		type Array = [0, [1, 2], [[3]]];
+
+		expectTypeOf<Flat<Array>>().toEqualTypeOf<[0, 1, 2, 3]>();
+		expectTypeOf<Flat<number[]>>().toEqualTypeOf<number[]>();
+
+		expectTypeOf<Flat<[...Array, boolean[]]>>().toEqualTypeOf<
+			[0, 1, 2, 3, ...boolean[]]
+		>();
+
+		expectTypeOf<Flat<number[][][]>>().toEqualTypeOf<number[]>();
+		expectTypeOf<
+			Flat<((number[] | number)[] | number)[]>
+		>().toEqualTypeOf<number[]>();
+	});
+}