/* eslint-disable @typescript-eslint/no-non-null-assertion */
import { CancelFn, PromiseAndCancel } from '../links/types';

type BatchItem<TKey, TValue> = {
  aborted: boolean;
  key: TKey;
  resolve: (value: TValue) => void;
  reject: (error: Error) => void;
  batch: Batch<TKey, TValue> | null;
};
type Batch<TKey, TValue> = {
  items: BatchItem<TKey, TValue>[];
  cancel: CancelFn;
};
type BatchLoader<TKey, TValue> = {
  validate: (keys: TKey[]) => boolean;
  fetch: (keys: TKey[]) => {
    promise: Promise<TValue[]>;
    cancel: CancelFn;
  };
};

/**
 * A function that should never be called unless we messed something up.
 */
const throwFatalError = () => {
  throw new Error(
    'Something went wrong. Please submit an issue at https://github.com/trpc/trpc/issues/new',
  );
};

/**
 * Dataloader that's very inspired by https://github.com/graphql/dataloader
 * Less configuration, no caching, and allows you to cancel requests
 * When cancelling a single fetch the whole batch will be cancelled only when _all_ items are cancelled
 */
export function dataLoader<TKey, TValue>(
  batchLoader: BatchLoader<TKey, TValue>,
) {
  let pendingItems: BatchItem<TKey, TValue>[] | null = null;
  let dispatchTimer: ReturnType<typeof setTimeout> | null = null;

  const destroyTimerAndPendingItems = () => {
    clearTimeout(dispatchTimer as any);
    dispatchTimer = null;
    pendingItems = null;
  };

  /**
   * Iterate through the items and split them into groups based on the `batchLoader`'s validate function
   */
  function groupItems(items: BatchItem<TKey, TValue>[]) {
    const groupedItems: BatchItem<TKey, TValue>[][] = [[]];
    let index = 0;
    while (true) {
      const item = items[index];
      if (!item) {
        // we're done
        break;
      }
      const lastGroup = groupedItems[groupedItems.length - 1]!;

      if (item.aborted) {
        // Item was aborted before it was dispatched
        item.reject(new Error('Aborted'));
        index++;
        continue;
      }

      const isValid = batchLoader.validate(
        lastGroup.concat(item).map((it) => it.key),
      );

      if (isValid) {
        lastGroup.push(item);
        index++;
        continue;
      }

      if (lastGroup.length === 0) {
        item.reject(new Error('Input is too big for a single dispatch'));
        index++;
        continue;
      }
      // Create new group, next iteration will try to add the item to that
      groupedItems.push([]);
    }
    return groupedItems;
  }

  function dispatch() {
    const groupedItems = groupItems(pendingItems!);
    destroyTimerAndPendingItems();

    // Create batches for each group of items
    for (const items of groupedItems) {
      if (!items.length) {
        continue;
      }
      const batch: Batch<TKey, TValue> = {
        items,
        cancel: throwFatalError,
      };
      for (const item of items) {
        item.batch = batch;
      }
      const { promise, cancel } = batchLoader.fetch(
        batch.items.map((_item) => _item.key),
      );
      batch.cancel = cancel;

      promise
        .then((result) => {
          for (let i = 0; i < result.length; i++) {
            const value = result[i]!;
            const item = batch.items[i]!;
            item.resolve(value);
            item.batch = null;
          }
        })
        .catch((cause) => {
          for (const item of batch.items) {
            item.reject(cause);
            item.batch = null;
          }
        });
    }
  }
  function load(key: TKey): PromiseAndCancel<TValue> {
    const item: BatchItem<TKey, TValue> = {
      aborted: false,
      key,
      batch: null,
      resolve: throwFatalError,
      reject: throwFatalError,
    };

    const promise = new Promise<TValue>((resolve, reject) => {
      item.reject = reject;
      item.resolve = resolve;

      if (!pendingItems) {
        pendingItems = [];
      }
      pendingItems.push(item);
    });

    if (!dispatchTimer) {
      dispatchTimer = setTimeout(dispatch);
    }
    const cancel = () => {
      item.aborted = true;

      if (item.batch?.items.every((item) => item.aborted)) {
        // All items in the batch have been cancelled
        item.batch.cancel();
        item.batch = null;
      }
    };

    return { promise, cancel };
  }

  return {
    load,
  };
}