tiny-pattern-ts/src/primitive.ts

import type { Exact, ValueOf } from "type-fest";

type UnaryFn<T, R> = (shape: T) => R;

// The primitive universe a matcher can discriminate. `boolean` is admitted as
// the pair `true | false`; see README § Caveats for the unsupported members.
type Matchable = string | number | boolean | null | undefined;

// `boolean`, `null` and `undefined` cannot be property keys, so a mapped type
// over the universe keys each non-key member by its stringification. `Param`
// inverts that projection, so a handler callback still receives the *real*
// member (`true`, not `"true"`) — see README § Caveats for the limits.
type PatternKey<T> = T extends boolean
    ? T extends true
        ? "true"
        : "false"
    : T extends null
      ? "null"
      : T extends undefined
        ? "undefined"
        : T;
type PatternParam<K> = K extends "true"
    ? true
    : K extends "false"
      ? false
      : K extends "null"
        ? null
        : K extends "undefined"
          ? undefined
          : K;

// `Extract` drops optional handlers (`undefined`) so `PatternReturns` also works
// when `P`'s constraint has optional keys.
type PatternReturns<P> = ReturnType<
    Extract<ValueOf<P>, (...args: never[]) => unknown>
>;

type Handlers<T extends Matchable, R> = {
    [K in PatternKey<T>]: UnaryFn<PatternParam<K>, R>;
};

// The fallback is a *second argument*, not a property of the handler map,
// because its parameter is the remainder `Exclude<T, keyof Handled>` and TypeScript
// fixes a property's contextual type before it infers its sibling keys. A later
// argument, by contrast, is contextually typed from inference on an earlier
// one, so the split is what makes the remainder expressible at all.
// See development/library.md.
type Fallback<T extends Matchable, Handled, R> = UnaryFn<
    Exclude<T, PatternParam<keyof Handled>>,
    R
>;

// A fallback is redundant once the handler map covers `T`. The guard is folded
// into `Handled`'s own (self-referential) constraint so it is checked *after*
// inference; a conditional in the fallback's parameter type is evaluated while
// `Handled` is still its constraint and would reject context-sensitive partial
// maps. That placement also fixes where the diagnostic lands: the constraint
// failure is reported on the argument that inferred `Handled` (the handler
// map), so the required property is spelled as the message instead of relying
// on its position. See development/library.md.
interface RedundantFallback {
    readonly "every case is already handled, so the fallback is redundant": never;
}
type MustBePartial<T extends Matchable, Handled> =
    PatternKey<T> extends keyof Handled ? RedundantFallback : unknown;

// TypeScript does not apply the excess-property check to a generic constraint,
// so `Exact` restores it for the generic forms: a handler map can otherwise
// carry keys outside `T`.

// oxlint-disable typescript/unified-signatures
// Strict returns: one common `R`. Overload order is load-bearing:
//   #1 Handlers (first) -> the exhaustive form and the autocomplete popup
//   #2 Fallback (last)  -> accepts a partial handler map plus a fallback
interface MatcherStrict<T extends Matchable> {
    <R>(handlers: Handlers<T, R>): UnaryFn<T, R>;
    <
        R,
        Handled extends Exact<Partial<Handlers<T, R>>, Handled> &
            MustBePartial<T, Handled>,
    >(
        handlers: Handled & Partial<Handlers<T, R>>,
        fallback: Fallback<T, Handled, R>,
    ): UnaryFn<T, R>;
}
// oxlint-enable typescript/unified-signatures

// Widened returns: the union of every handler's return type. `P` is inferred
// from the whole handler map, whose closed constraint supplies the
// contextual/autocomplete type.
// oxlint-disable typescript/unified-signatures
interface MatcherWidening<T extends Matchable> {
    <P extends Exact<Handlers<T, unknown>, P>>(
        handlers: P,
    ): UnaryFn<T, PatternReturns<P>>;
    <
        R,
        Handled extends Exact<Partial<Handlers<T, unknown>>, Handled> &
            MustBePartial<T, Handled>,
    >(
        handlers: Handled,
        fallback: Fallback<T, Handled, R>,
    ): UnaryFn<T, PatternReturns<Handled> | R>;
}
// oxlint-enable typescript/unified-signatures

type HandlerMap = Record<string | number, UnaryFn<never, unknown> | undefined>;

const dispatch =
    (handlers: HandlerMap, fallback?: UnaryFn<never, unknown>) =>
    (shape: Matchable): unknown =>
        // `handlers[true]` already coerces to the `"true"` property at runtime,
        // identical to `handlers[String(shape)]`, so indexing with `shape`
        // directly is sound: `shape` is a facade-checked universe member and
        // `PatternKey` only ever produces valid property keys. The assertion is
        // needed solely because TypeScript forbids indexing with
        // `boolean`/`null`/`undefined` (TS2538); it buys the number fast path.
        (
            handlers[
                // oxlint-disable-next-line typescript/no-unsafe-type-assertion
                shape as string | number
            ] ??
            fallback ??
            (() => {
                throw new Error(`Unhandled shape: ${String(shape)}`);
            })
        )(
            // oxlint-disable-next-line typescript/no-unsafe-type-assertion
            shape as never,
        );

export const getMatcher = <T extends Matchable>(): MatcherStrict<T> => dispatch;
export const getMatcherW = <T extends Matchable>(): MatcherWidening<T> =>
    dispatch;