memo.ts

// A decorator that memoizes a function's arguments and return value.

type Primitive = string | number | bigint | boolean | symbol | null | undefined

export function memo<This, P extends Primitive, R>(
    value: (this: This, arg: P) => R,
    _context: ClassMethodDecoratorContext<This, (this: This, arg: P) => R>,
): (this: This, arg: P) => R {
    const memo = new Map<P, R>()

    return function (arg) {
        if (memo.has(arg)) {
            return memo.get(arg)!
        }

        const output = value.call(this, arg)

        memo.set(arg, output)

        return output
    }
}

class Fibonacci {
    @memo
    static of(value: number): number {
        if (value == 0 || value == 1) {
            return value
        }

        return Fibonacci.of(value - 1) + Fibonacci.of(value - 2)
    }
}