import { v } from "../values/validator.js";
import type { Validator } from "../values/validators.js";
import type { Value } from "../values/value.js";

/**
 * An opaque identifier used for paginating a database query.
 *
 * Cursors are returned from {@link OrderedQuery.paginate} and represent the
 * point of the query where the page of results ended.
 *
 * To continue paginating, pass the cursor back into
 * {@link OrderedQuery.paginate} in the {@link PaginationOptions} object to
 * fetch another page of results.
 *
 * Note: Cursors can only be passed to _exactly_ the same database query that
 * they were generated from. You may not reuse a cursor between different
 * database queries.
 *
 * @public
 */
export type Cursor = string;

/**
 * The result of paginating using {@link OrderedQuery.paginate}.
 *
 * @public
 */
export interface PaginationResult<T> {
  /**
   * The page of results.
   */
  page: T[];

  /**
   * Have we reached the end of the results?
   */
  isDone: boolean;

  /**
   * A {@link Cursor} to continue loading more results.
   */
  continueCursor: Cursor;

  /**
   * A {@link Cursor} to split the page into two, so the page from
   * (cursor, continueCursor] can be replaced by two pages (cursor, splitCursor]
   * and (splitCursor, continueCursor].
   */
  splitCursor?: Cursor | null;

  /**
   * When a query reads too much data, it may return 'SplitRecommended' to
   * indicate that the page should be split into two with `splitCursor`.
   * When a query reads so much data that `page` might be incomplete, its status
   * becomes 'SplitRequired'.
   */
  pageStatus?: "SplitRecommended" | "SplitRequired" | null;
}

/**
 * The options passed to {@link OrderedQuery.paginate}.
 *
 * To use this type in [argument validation](https://docs.convex.dev/functions/validation),
 * use the {@link paginationOptsValidator}.
 *
 * @public
 */
export interface PaginationOptions {
  /**
   * Number of items to load in this page of results.
   *
   * Note: This is only an initial value!
   *
   * If you are running this paginated query in a reactive query function, you
   * may receive more or less items than this if items were added to or removed
   * from the query range.
   */
  numItems: number;

  /**
   * A {@link Cursor} representing the start of this page or `null` to start
   * at the beginning of the query results.
   */
  cursor: Cursor | null;

  /**
   * A {@link Cursor} representing the end of this page or `null | undefined` to
   * use `numItems` instead.
   *
   * This explicitly sets the range of documents the query will return, from
   * `cursor` to `endCursor`. It's used by reactive pagination clients to ensure
   * there are no gaps between pages when data changes, and to split pages when
   * `pageStatus` indicates a split is recommended or required.
   *
   * When splitting a page, use the returned `splitCursor` as `endCursor` for the
   * first half and as `cursor` for the second half.
   */
  endCursor?: Cursor | null;

  /**
   * The maximum number of rows to read from the database during pagination.
   *
   * This limits rows entering the query pipeline before filters are applied.
   * Use this when filtering for rare items, where low `numItems` won't bound
   * execution time because the query scans many rows to find matches.
   *
   * Currently this is not enforced for search queries.
   */
  maximumRowsRead?: number;

  /**
   * The maximum number of bytes to read from the database during pagination.
   *
   * This limits bytes entering the query pipeline before filters are applied.
   * Use this to control bandwidth usage when documents are large.
   * If the limit is reached, the query may return an incomplete page and
   * require a page split.
   *
   * Currently this is not enforced for search queries.
   */
  maximumBytesRead?: number;
}

/**
 * A {@link values.Validator} for {@link PaginationOptions}.
 *
 * Use this as the args validator in paginated query functions so that clients
 * can pass pagination options.
 *
 * @example
 * ```typescript
 * import { query } from "./_generated/server";
 * import { paginationOptsValidator } from "convex/server";
 * import { v } from "convex/values";
 *
 * export const listMessages = query({
 *   args: {
 *     channelId: v.id("channels"),
 *     paginationOpts: paginationOptsValidator,
 *   },
 *   handler: async (ctx, args) => {
 *     return await ctx.db
 *       .query("messages")
 *       .withIndex("by_channel", (q) => q.eq("channelId", args.channelId))
 *       .order("desc")
 *       .paginate(args.paginationOpts);
 *   },
 * });
 * ```
 *
 * On the client, use `usePaginatedQuery` from `"convex/react"`:
 * ```tsx
 * const { results, status, loadMore } = usePaginatedQuery(
 *   api.messages.listMessages,
 *   { channelId },
 *   { initialNumItems: 25 },
 * );
 * ```
 *
 * @see https://docs.convex.dev/database/pagination
 * @public
 */
export const paginationOptsValidator = v.object({
  numItems: v.number(),
  cursor: v.union(v.string(), v.null()),
  endCursor: v.optional(v.union(v.string(), v.null())),
  id: v.optional(v.number()),
  maximumRowsRead: v.optional(v.number()),
  maximumBytesRead: v.optional(v.number()),
});

/**
 * A {@link values.Validator} factory for {@link PaginationResult}.
 *
 * Create a validator for the result of calling {@link OrderedQuery.paginate}
 * with a given item validator.
 *
 * For example:
 * ```ts
 * const paginationResultValidator = paginationResultValidator(v.object({
 *   _id: v.id("users"),
 *   _creationTime: v.number(),
 *   name: v.string(),
 * }));
 * ```
 *
 * @param itemValidator - A validator for the items in the page
 * @returns A validator for the pagination result
 *
 * @public
 */
export function paginationResultValidator<
  T extends Validator<Value, "required", string>,
>(itemValidator: T) {
  return v.object({
    page: v.array(itemValidator),
    continueCursor: v.string(),
    isDone: v.boolean(),
    splitCursor: v.optional(v.union(v.string(), v.null())),
    pageStatus: v.optional(
      v.union(
        v.literal("SplitRecommended"),
        v.literal("SplitRequired"),
        v.null(),
      ),
    ),
  });
}