export const splitAt = (subject: string, index: number): readonly [string, string] => {
    if (index < 0) {
        return [subject, ''];
    }

    if (index > subject.length) {
        return [subject, ''];
    }

    return [subject.slice(0, index), subject.slice(index + 1)];
};

const decodeRegex = /(?<!\\)\\(t|b|n|r|f|'|"|u[0-9a-f]{1,4})/gi;
const decodeReplacer = (_: any, char: string) => ({
    t: '\t',
    b: '\b',
    n: '\n',
    r: '\r',
    f: '\f',
    "'": '\'',
    '"': '\"',
    u: String.fromCharCode(Number.parseInt(char.slice(1))),
}[char.charAt(0)] ?? '');
export const decode = (subject: string): string => subject.replace(decodeRegex, decodeReplacer);

export const deepCopy = <T>(original: T): T => {
    if (typeof original !== 'object' || original === null || original === undefined) {
        return original;
    }

    if (original instanceof Date) {
        return new Date(original.getTime()) as T;
    }

    if (original instanceof Array) {
        return original.map(item => deepCopy(item)) as T;
    }

    if (original instanceof Set) {
        return new Set(original.values().map(item => deepCopy(item))) as T;
    }

    if (original instanceof Map) {
        return new Map(original.entries().map(([key, value]) => [key, deepCopy(value)])) as T;
    }

    return Object.assign(
        Object.create(Object.getPrototypeOf(original)),
        Object.fromEntries(Object.entries(original).map(([key, value]) => [key, deepCopy(value)]))
    ) as T;
}

export enum MutarionKind {
    Create = 'created',
    Update = 'updated',
    Delete = 'deleted',
}
export type Created = { kind: MutarionKind.Create, key: string, value: any };
export type Updated = { kind: MutarionKind.Update, key: string, value: any, original: any };
export type Deleted = { kind: MutarionKind.Delete, key: string, original: any };
export type Mutation = Created | Updated | Deleted;

export function* deepDiff<T1 extends object, T2 extends object>(a: T1, b: T2, path: string[] = []): Generator<Mutation, void, unknown> {
    if (!isIterable(a) || !isIterable(b)) {
        console.error('Edge cases', a, b);

        return;
    }

    for (const [[keyA, valueA], [keyB, valueB]] of zip(entriesOf(a), entriesOf(b))) {
        if (keyA === undefined && keyB) {
            yield { key: path.concat(keyB.toString()).join('.'), kind: MutarionKind.Create, value: valueB };

            continue;
        }

        if (keyA && keyB === undefined) {
            yield { key: path.concat(keyA.toString()).join('.'), kind: MutarionKind.Delete, original: valueA };

            continue;
        }

        if (typeof valueA == 'object' && typeof valueB == 'object') {
            yield* deepDiff(valueA, valueB, path.concat(keyA!.toString()));

            continue;
        }

        if (valueA === valueB) {
            continue;
        }

        const key = path.concat(keyA!.toString()).join('.');

        yield ((): Mutation => {
            if (valueA === null || valueA === undefined) return { key, kind: MutarionKind.Create, value: valueB };
            if (valueB === null || valueB === undefined) return { key, kind: MutarionKind.Delete, original: valueA };

            return { key, kind: MutarionKind.Update, value: valueB, original: valueA };
        })();
    }
};

const isIterable = (subject: object): subject is Iterable<any> => ['boolean', 'undefined', 'null', 'number'].includes(typeof subject) === false;
const entriesOf = (subject: object): Iterable<readonly [string | number, any]> => {
    if (subject instanceof Array) {
        return subject.entries();
    }

    if (subject instanceof Map) {
        return subject.entries();
    }

    if (subject instanceof Set) {
        return subject.entries();
    }

    return Object.entries(subject);
};

type ZippedPair =
    | readonly [readonly [string | number, any], readonly [string | number, any]]
    | readonly [readonly [undefined, undefined], readonly [string | number, any]]
    | readonly [readonly [string | number, any], readonly [undefined, undefined]]
    ;
const zip = function* (a: Iterable<readonly [string | number, any]>, b: Iterable<readonly [string | number, any]>): Generator<ZippedPair, void, unknown> {
    const iterA = bufferredIterator(a);
    const iterB = bufferredIterator(b);

    const EMPTY = [undefined, undefined] as const;

    while (!iterA.done || !iterB.done) {
        // if we have a match on the keys of a and b we can simply consume and yield
        if (iterA.current.key === iterB.current.key) {
            // When we match keys it could have happened that
            // there are as many keys added as there are deleted, 
            // therefor we can now flush both a and b because we are aligned
            yield* iterA.flush().map(entry => [entry, EMPTY] as const);
            yield* iterB.flush().map(entry => [EMPTY, entry] as const);

            yield [iterA.consume(), iterB.consume()];
        }

        // key of a aligns with last key in buffer b
        // conclusion: a has key(s) that b does not
        else if (iterA.current.key === iterB.top.key) {
            yield* iterA.flush().map(entry => [entry, EMPTY] as const);
            yield [iterA.consume(), iterB.consume()];
        }

        // the reverse case, key of b is aligns with the last key in buffer a
        // conclusion: a is missing key(s) the b does have
        else if (iterB.current.key === iterA.top.key) {
            yield* iterB.flush().map(entry => [EMPTY, entry] as const);
            yield [iterA.consume(), iterB.consume()];
        }

        else if (iterA.done && !iterB.done) {
            yield [EMPTY, iterB.consume()];
        }

        else if (!iterA.done && iterB.done) {
            yield [iterA.consume(), EMPTY];
        }

        // Neiter of the above cases are hit.
        // conclusion: there still is no alignment.
        else {
            iterA.advance();
            iterB.advance();
        }
    }
};

const bufferredIterator = <T extends readonly [string | number, any]>(subject: Iterable<T>) => {
    const iterator = Iterator.from(subject);
    const buffer: T[] = [];
    let done = false;

    const next = () => {
        const res = iterator.next();
        done = res.done ?? false;

        if (!done) {
            buffer.push(res.value);
        }
    };

    next();

    return {
        advance() {
            next();
        },

        consume() {
            const value = buffer.shift()!;

            this.advance();

            return value;
        },

        flush(): T[] {
            const entries = buffer.splice(0, buffer.length - 1);

            return entries;
        },

        get done() {
            return done && buffer.length === 0;
        },

        get top() {
            const [key = undefined, value = undefined] = buffer.at(0) ?? [];

            return { key, value };
        },

        get current() {
            const [key = undefined, value = undefined] = buffer.at(-1) ?? [];

            return { key, value };
        },
    };
};

export interface filter {
    <T, S extends T>(subject: AsyncIterableIterator<T>, predicate: (value: T) => value is S): AsyncGenerator<S, void, unknown>;
    <T>(subject: AsyncIterableIterator<T>, predicate: (value: T) => unknown): AsyncGenerator<T, void, unknown>;
}

export const filter = async function*<T, S extends T>(subject: AsyncIterableIterator<T>, predicate: (value: T) => value is S): AsyncGenerator<S, void, unknown> {
    for await (const value of subject) {
        if (predicate(value)) {
            yield value;
        }
    }
};

export const map = async function*<TIn, TResult>(subject: AsyncIterableIterator<TIn>, predicate: (value: TIn) => TResult): AsyncGenerator<TResult, void, unknown> {
    for await (const value of subject) {
        yield predicate(value);
    }
};