import {createEmptyActionSummary} from "app/common/ActionSummary";
import {ApiError} from 'app/common/ApiError';
import {BrowserSettings} from "app/common/BrowserSettings";
import {BulkColValues, ColValues, fromTableDataAction, TableColValues, TableRecordValue} from 'app/common/DocActions';
import {isRaisedException} from "app/common/gristTypes";
import {isAffirmative} from "app/common/gutil";
import {SortFunc} from 'app/common/SortFunc';
import {Sort} from 'app/common/SortSpec';
import {MetaRowRecord} from 'app/common/TableData';
import {DocReplacementOptions, DocState, DocStateComparison, DocStates, NEW_DOCUMENT_CODE} from 'app/common/UserAPI';
import {HomeDBManager, makeDocAuthResult} from 'app/gen-server/lib/HomeDBManager';
import * as Types from "app/plugin/DocApiTypes";
import DocApiTypesTI from "app/plugin/DocApiTypes-ti";
import GristDataTI from 'app/plugin/GristData-ti';
import {OpOptions} from "app/plugin/TableOperations";
import {
  handleSandboxErrorOnPlatform,
  TableOperationsImpl,
  TableOperationsPlatform
} from 'app/plugin/TableOperationsImpl';
import {concatenateSummaries, summarizeAction} from "app/server/lib/ActionSummary";
import {ActiveDoc, tableIdToRef} from "app/server/lib/ActiveDoc";
import {
  assertAccess,
  getOrSetDocAuth,
  getTransitiveHeaders,
  getUserId,
  isAnonymousUser,
  RequestWithLogin
} from 'app/server/lib/Authorizer';
import {DocManager} from "app/server/lib/DocManager";
import {docSessionFromRequest, makeExceptionalDocSession, OptDocSession} from "app/server/lib/DocSession";
import {DocWorker} from "app/server/lib/DocWorker";
import {IDocWorkerMap} from "app/server/lib/DocWorkerMap";
import {parseExportParameters} from "app/server/lib/Export";
import {downloadCSV, DownloadCSVOptions} from "app/server/lib/ExportCSV";
import {downloadXLSX, DownloadXLSXOptions} from "app/server/lib/ExportXLSX";
import {expressWrap} from 'app/server/lib/expressWrap';
import {filterDocumentInPlace} from "app/server/lib/filterUtils";
import {googleAuthTokenMiddleware} from "app/server/lib/GoogleAuth";
import {exportToDrive} from "app/server/lib/GoogleExport";
import {GristServer} from 'app/server/lib/GristServer';
import {HashUtil} from 'app/server/lib/HashUtil';
import {makeForkIds} from "app/server/lib/idUtils";
import {
  getDocId,
  getDocScope,
  getScope,
  integerParam,
  isParameterOn,
  optStringParam,
  sendOkReply,
  sendReply,
  stringParam
} from 'app/server/lib/requestUtils';
import {ServerColumnGetters} from 'app/server/lib/ServerColumnGetters';
import {localeFromRequest} from "app/server/lib/ServerLocale";
import {allowedEventTypes, isUrlAllowed, WebhookAction, WebHookSecret} from "app/server/lib/Triggers";
import {handleOptionalUpload, handleUpload} from "app/server/lib/uploads";
import * as assert from 'assert';
import * as contentDisposition from 'content-disposition';
import {Application, NextFunction, Request, RequestHandler, Response} from "express";
import * as _ from "lodash";
import * as LRUCache from 'lru-cache';
import * as moment from 'moment';
import fetch from 'node-fetch';
import * as path from 'path';
import * as t from "ts-interface-checker";
import {Checker} from "ts-interface-checker";
import * as uuidv4 from "uuid/v4";

// Cap on the number of requests that can be outstanding on a single document via the
// rest doc api.  When this limit is exceeded, incoming requests receive an immediate
// reply with status 429.
const MAX_PARALLEL_REQUESTS_PER_DOC = 10;

// This is NOT the number of docs that can be handled at a time.
// It's a very generous upper bound of what that number might be.
// If there are more docs than this for which API requests are being regularly made at any moment,
// then the _dailyUsage cache may become unreliable and users may be able to exceed their allocated requests.
const MAX_ACTIVE_DOCS_USAGE_CACHE = 1000;

type WithDocHandler = (activeDoc: ActiveDoc, req: RequestWithLogin, resp: Response) => Promise<void>;

// Schema validators for api endpoints that creates or updates records.
const {RecordsPatch, RecordsPost, RecordsPut} = t.createCheckers(DocApiTypesTI, GristDataTI);
RecordsPatch.setReportedPath("body");
RecordsPost.setReportedPath("body");
RecordsPut.setReportedPath("body");

/**
 * Middleware for validating request's body with a Checker instance.
 */
function validate(checker: Checker): RequestHandler {
  return (req, res, next) => {
    try {
      checker.check(req.body);
    } catch(err) {
      res.status(400).json({
        error : "Invalid payload",
        details: String(err)
      }).end();
      return;
    }
    next();
  };
}

export class DocWorkerApi {
  // Map from docId to number of requests currently being handled for that doc
  private _currentUsage = new Map<string, number>();

  // Map from (docId, time period) combination produced by docPeriodicApiUsageKey
  // to number of requests previously served for that combination.
  // We multiply by 5 because there are 5 relevant keys per doc at any time (current/next day/hour and current minute).
  private _dailyUsage = new LRUCache<string, number>({max: 5 * MAX_ACTIVE_DOCS_USAGE_CACHE});

  constructor(private _app: Application, private _docWorker: DocWorker,
              private _docWorkerMap: IDocWorkerMap, private _docManager: DocManager,
              private _dbManager: HomeDBManager, private _grist: GristServer) {}

  /**
   * Adds endpoints for the doc api.
   *
   * Note that it expects bodyParser, userId, and jsonErrorHandler middleware to be set up outside
   * to apply to these routes.
   */
  public addEndpoints() {

    // check document exists (not soft deleted) and user can view it
    const canView = expressWrap(this._assertAccess.bind(this, 'viewers', false));
    // check document exists (not soft deleted) and user can edit it
    const canEdit = expressWrap(this._assertAccess.bind(this, 'editors', false));
    const isOwner = expressWrap(this._assertAccess.bind(this, 'owners', false));
    // check user can edit document, with soft-deleted documents being acceptable
    const canEditMaybeRemoved = expressWrap(this._assertAccess.bind(this, 'editors', true));
    // converts google code to access token and adds it to request object
    const decodeGoogleToken = expressWrap(googleAuthTokenMiddleware.bind(null));

    // Middleware to limit number of outstanding requests per document.  Will also
    // handle errors like expressWrap would.
    const throttled = this._apiThrottle.bind(this);
    const withDoc = (callback: WithDocHandler) => throttled(this._requireActiveDoc(callback));

    // Apply user actions to a document.
    this._app.post('/api/docs/:docId/apply', canEdit, withDoc(async (activeDoc, req, res) => {
      const parseStrings = !isAffirmative(req.query.noparse);
      res.json(await activeDoc.applyUserActions(docSessionFromRequest(req), req.body, {parseStrings}));
    }));

    async function getTableData(activeDoc: ActiveDoc, req: RequestWithLogin, optTableId?: string) {
      const filters = req.query.filter ? JSON.parse(String(req.query.filter)) : {};
      // Option to skip waiting for document initialization.
      const immediate = isAffirmative(req.query.immediate);
      if (!Object.keys(filters).every(col => Array.isArray(filters[col]))) {
        throw new ApiError("Invalid query: filter values must be arrays", 400);
      }
      const tableId = optTableId || req.params.tableId;
      const session = docSessionFromRequest(req);
      const tableData = await handleSandboxError(tableId, [], activeDoc.fetchQuery(
        session, {tableId, filters}, !immediate));
      // For metaTables we don't need to specify columns, search will infer it from the sort expression.
      const isMetaTable = tableId.startsWith('_grist');
      const columns = isMetaTable ? null :
        await handleSandboxError('', [], activeDoc.getTableCols(session, tableId, true));
      const params = getQueryParameters(req);
      // Apply sort/limit parameters, if set.  TODO: move sorting/limiting into data engine
      // and sql.
      return applyQueryParameters(fromTableDataAction(tableData), params, columns);
    }

    async function getTableRecords(
      activeDoc: ActiveDoc, req: RequestWithLogin, optTableId?: string
    ): Promise<TableRecordValue[]> {
      const columnData = await getTableData(activeDoc, req, optTableId);
      const fieldNames = Object.keys(columnData)
        .filter(k => !(
          ["id", "manualSort"].includes(k)
          || k.startsWith("gristHelper_")
        ));
      return columnData.id.map((id, index) => {
        const result: TableRecordValue = {id, fields: {}};
        for (const key of fieldNames) {
          let value = columnData[key][index];
          if (isRaisedException(value)) {
            _.set(result, ["errors", key], (value as string[])[1]);
            value = null;
          }
          result.fields[key] = value;
        }
        return result;
      });
    }

    // Get the specified table in column-oriented format
    this._app.get('/api/docs/:docId/tables/:tableId/data', canView,
      withDoc(async (activeDoc, req, res) => {
        res.json(await getTableData(activeDoc, req));
      })
    );

    // Get the specified table in record-oriented format
    this._app.get('/api/docs/:docId/tables/:tableId/records', canView,
      withDoc(async (activeDoc, req, res) => {
        const records = await getTableRecords(activeDoc, req);
        res.json({records});
      })
    );

    async function getMetaTables(activeDoc: ActiveDoc, req: RequestWithLogin) {
      return await handleSandboxError("", [],
        activeDoc.fetchMetaTables(docSessionFromRequest(req)));
    }

    // Get the columns of the specified table in recordish format
    this._app.get('/api/docs/:docId/tables/:tableId/columns', canView,
      withDoc(async (activeDoc, req, res) => {
        const tableId = req.params.tableId;
        const columns = await handleSandboxError('', [],
          activeDoc.getTableCols(docSessionFromRequest(req), tableId));
        res.json({columns});
      })
    );

    // The upload should be a multipart post with an 'upload' field containing one or more files.
    // Returns the list of rowIds for the rows created in the _grist_Attachments table.
    this._app.post('/api/docs/:docId/attachments', canEdit, withDoc(async (activeDoc, req, res) => {
      const uploadResult = await handleUpload(req, res);
      res.json(await activeDoc.addAttachments(docSessionFromRequest(req), uploadResult.uploadId));
    }));

    // Select the fields from an attachment record that we want to return to the user,
    // and convert the timeUploaded from a number to an ISO string.
    function cleanAttachmentRecord(record: MetaRowRecord<"_grist_Attachments">) {
      const {fileName, fileSize, timeUploaded: time} = record;
      const timeUploaded = (typeof time === 'number') ? new Date(time).toISOString() : undefined;
      return {fileName, fileSize, timeUploaded};
    }

    // Returns cleaned metadata for all attachments in /records format.
    this._app.get('/api/docs/:docId/attachments', canView, withDoc(async (activeDoc, req, res) => {
      const rawRecords = await getTableRecords(activeDoc, req, "_grist_Attachments");
      const records = rawRecords.map(r => ({
        id: r.id,
        fields: cleanAttachmentRecord(r.fields as MetaRowRecord<"_grist_Attachments">),
      }));
      res.json({records});
    }));

    // Returns cleaned metadata for a given attachment ID (i.e. a rowId in _grist_Attachments table).
    this._app.get('/api/docs/:docId/attachments/:attId', canView, withDoc(async (activeDoc, req, res) => {
      const attRecord = activeDoc.getAttachmentMetadata(req.params.attId as string);
      res.json(cleanAttachmentRecord(attRecord));
    }));

    // Responds with attachment contents, with suitable Content-Type and Content-Disposition.
    this._app.get('/api/docs/:docId/attachments/:attId/download', canView, withDoc(async (activeDoc, req, res) => {
      const attRecord = activeDoc.getAttachmentMetadata(req.params.attId as string);
      const fileIdent = attRecord.fileIdent as string;
      const ext = path.extname(fileIdent);
      const origName = attRecord.fileName as string;
      const fileName = ext ? path.basename(origName, path.extname(origName)) + ext : origName;
      const fileData = await activeDoc.getAttachmentData(docSessionFromRequest(req), fileIdent);
      res.status(200)
        .type(ext)
        // Construct a content-disposition header of the form 'attachment; filename="NAME"'
        .set('Content-Disposition', contentDisposition(fileName, {type: 'attachment'}))
        .set('Cache-Control', 'private, max-age=3600')
        .send(fileData);
    }));

    // Mostly for testing
    this._app.post('/api/docs/:docId/attachments/updateUsed', canEdit, withDoc(async (activeDoc, req, res) => {
      await activeDoc.updateUsedAttachmentsIfNeeded();
      res.json(null);
    }));
    this._app.post('/api/docs/:docId/attachments/removeUnused', isOwner, withDoc(async (activeDoc, req, res) => {
      const expiredOnly = isAffirmative(req.query.expiredonly);
      const verifyFiles = isAffirmative(req.query.verifyfiles);
      await activeDoc.removeUnusedAttachments(expiredOnly);
      if (verifyFiles) {
        await verifyAttachmentFiles(activeDoc);
      }
      res.json(null);
    }));
    this._app.post('/api/docs/:docId/attachments/verifyFiles', isOwner, withDoc(async (activeDoc, req, res) => {
      await verifyAttachmentFiles(activeDoc);
      res.json(null);
    }));

    async function verifyAttachmentFiles(activeDoc: ActiveDoc) {
      assert.deepStrictEqual(
        await activeDoc.docStorage.all(`SELECT DISTINCT fileIdent AS ident FROM _grist_Attachments ORDER BY ident`),
        await activeDoc.docStorage.all(`SELECT                       ident FROM _gristsys_Files    ORDER BY ident`),
      );
    }

    // Adds records given in a column oriented format,
    // returns an array of row IDs
    this._app.post('/api/docs/:docId/tables/:tableId/data', canEdit,
      withDoc(async (activeDoc, req, res) => {
        const colValues = req.body as BulkColValues;
        const count = colValues[Object.keys(colValues)[0]].length;
        const op = getTableOperations(req, activeDoc);
        const ids = await op.addRecords(count, colValues);
        res.json(ids);
      })
    );

    // Adds records given in a record oriented format,
    // returns in the same format as GET /records but without the fields object for now
    this._app.post('/api/docs/:docId/tables/:tableId/records', canEdit, validate(RecordsPost),
      withDoc(async (activeDoc, req, res) => {
        const body = req.body as Types.RecordsPost;
        const ops = getTableOperations(req, activeDoc);
        const records = await ops.create(body.records);
        res.json({records});
      })
    );

    this._app.post('/api/docs/:docId/tables/:tableId/data/delete', canEdit, withDoc(async (activeDoc, req, res) => {
      const rowIds = req.body;
      const op = getTableOperations(req, activeDoc);
      await op.destroy(rowIds);
      res.json(null);
    }));

    // Download full document
    // TODO: look at download behavior if ActiveDoc is shutdown during call (cannot
    // use withDoc wrapper)
    this._app.get('/api/docs/:docId/download', canView, throttled(async (req, res) => {
      // We want to be have a way download broken docs that ActiveDoc may not be able
      // to load.  So, if the user owns the document, we unconditionally let them
      // download.
      if (await this._isOwner(req)) {
        try {
          // We carefully avoid creating an ActiveDoc for the document being downloaded,
          // in case it is broken in some way.  It is convenient to be able to download
          // broken files for diagnosis/recovery.
          return await this._docWorker.downloadDoc(req, res, this._docManager.storageManager);
        } catch (e) {
          if (e.message && e.message.match(/does not exist yet/)) {
            // The document has never been seen on file system / s3.  It may be new, so
            // we try again after having created an ActiveDoc for the document.
            await this._getActiveDoc(req);
            return this._docWorker.downloadDoc(req, res, this._docManager.storageManager);
          } else {
            throw e;
          }
        }
      } else {
        // If the user is not an owner, we load the document as an ActiveDoc, and then
        // check if the user has download permissions.
        const activeDoc = await this._getActiveDoc(req);
        if (!await activeDoc.canDownload(docSessionFromRequest(req))) {
          throw new ApiError('not authorized to download this document', 403);
        }
        return this._docWorker.downloadDoc(req, res, this._docManager.storageManager);
      }
    }));

    // Fork the specified document.
    this._app.post('/api/docs/:docId/fork', canView, withDoc(async (activeDoc, req, res) => {
      const result = await activeDoc.fork(docSessionFromRequest(req));
      res.json(result);
    }));

    // Initiate a fork.  Used internally to implement ActiveDoc.fork.  Only usable via a Permit.
    this._app.post('/api/docs/:docId/create-fork', canEdit, throttled(async (req, res) => {
      const docId = stringParam(req.params.docId, 'docId');
      const srcDocId = stringParam(req.body.srcDocId, 'srcDocId');
      if (srcDocId !== req.specialPermit?.otherDocId) { throw new Error('access denied'); }
      const fname = await this._docManager.storageManager.prepareFork(srcDocId, docId);
      await filterDocumentInPlace(docSessionFromRequest(req), fname);
      res.json({srcDocId, docId});
    }));

    // Update records given in column format
    // The records to update are identified by their id column.
    this._app.patch('/api/docs/:docId/tables/:tableId/data', canEdit,
      withDoc(async (activeDoc, req, res) => {
        const columnValues = req.body;
        const rowIds = columnValues.id;
        // sandbox expects no id column
        delete columnValues.id;
        const ops = getTableOperations(req, activeDoc);
        await ops.updateRecords(columnValues, rowIds);
        res.json(null);
      })
    );

    // Update records given in records format
    this._app.patch('/api/docs/:docId/tables/:tableId/records', canEdit, validate(RecordsPatch),
      withDoc(async (activeDoc, req, res) => {
        const body = req.body as Types.RecordsPatch;
        const ops = getTableOperations(req, activeDoc);
        await ops.update(body.records);
        res.json(null);
      })
    );

    // Add or update records given in records format
    this._app.put('/api/docs/:docId/tables/:tableId/records', canEdit, validate(RecordsPut),
      withDoc(async (activeDoc, req, res) => {
        const ops = getTableOperations(req, activeDoc);
        const body = req.body as Types.RecordsPut;
        const options = {
          add: !isAffirmative(req.query.noadd),
          update: !isAffirmative(req.query.noupdate),
          onMany: stringParam(req.query.onmany || "first", "onmany",
                              ["first", "none", "all"]) as 'first'|'none'|'all'|undefined,
          allowEmptyRequire: isAffirmative(req.query.allow_empty_require),
        };
        await ops.upsert(body.records, options);
        res.json(null);
      })
    );

    // Add a new webhook and trigger
    this._app.post('/api/docs/:docId/tables/:tableId/_subscribe', isOwner,
      withDoc(async (activeDoc, req, res) => {
        const {isReadyColumn, eventTypes, url} = req.body;

        if (!(Array.isArray(eventTypes) && eventTypes.length)) {
          throw new ApiError(`eventTypes must be a non-empty array`, 400);
        }
        if (!eventTypes.every(allowedEventTypes.guard)) {
          throw new ApiError(`Allowed values in eventTypes are: ${allowedEventTypes.values}`, 400);
        }
        if (!url) {
          throw new ApiError('Bad request: url required', 400);
        }
        if (!isUrlAllowed(url)) {
          throw new ApiError('Provided url is forbidden', 403);
        }

        const metaTables = await getMetaTables(activeDoc, req);
        const tableRef = tableIdToRef(metaTables, req.params.tableId);

        let isReadyColRef = 0;
        if (isReadyColumn) {
          const [, , colRefs, columnData] = metaTables._grist_Tables_column;
          const colRowIndex = columnData.colId.indexOf(isReadyColumn);
          if (colRowIndex === -1) {
            throw new ApiError(`Column not found "${isReadyColumn}"`, 404);
          }
          isReadyColRef = colRefs[colRowIndex];
        }

        const unsubscribeKey = uuidv4();
        const webhook: WebHookSecret = {unsubscribeKey, url};
        const secretValue = JSON.stringify(webhook);
        const webhookId = (await this._dbManager.addSecret(secretValue, activeDoc.docName)).id;

        const webhookAction: WebhookAction = {type: "webhook", id: webhookId};

        const sandboxRes = await handleSandboxError("_grist_Triggers", [], activeDoc.applyUserActions(
          docSessionFromRequest(req),
          [['AddRecord', "_grist_Triggers", null, {
            tableRef,
            isReadyColRef,
            eventTypes: ["L", ...eventTypes],
            actions: JSON.stringify([webhookAction])
          }]]));

        res.json({
          unsubscribeKey,
          triggerId: sandboxRes.retValues[0],
          webhookId,
        });
      })
    );

    // Remove webhook and trigger created above
    this._app.post('/api/docs/:docId/tables/:tableId/_unsubscribe', canEdit,
      withDoc(async (activeDoc, req, res) => {
        const metaTables = await getMetaTables(activeDoc, req);
        const tableRef = tableIdToRef(metaTables, req.params.tableId);
        const {triggerId, unsubscribeKey, webhookId} = req.body;

        // Validate combination of triggerId, webhookId, and tableRef.
        // This is overly strict, webhookId should be enough,
        // but it should be easy to relax that later if we want.
        const [, , triggerRowIds, triggerColData] = metaTables._grist_Triggers;
        const triggerRowIndex = triggerRowIds.indexOf(triggerId);
        if (triggerRowIndex === -1) {
          throw new ApiError(`Trigger not found "${triggerId}"`, 404);
        }
        if (triggerColData.tableRef[triggerRowIndex] !== tableRef) {
          throw new ApiError(`Wrong table`, 400);
        }
        const actions = JSON.parse(triggerColData.actions[triggerRowIndex] as string);
        if (!_.find(actions, {type: "webhook", id: webhookId})) {
          throw new ApiError(`Webhook not found "${webhookId}"`, 404);
        }

        // Validate unsubscribeKey before deleting trigger from document
        await this._dbManager.removeWebhook(webhookId, activeDoc.docName, unsubscribeKey);

        // TODO handle trigger containing other actions when that becomes possible
        await handleSandboxError("_grist_Triggers", [], activeDoc.applyUserActions(
          docSessionFromRequest(req),
          [['RemoveRecord', "_grist_Triggers", triggerId]]));

        res.json({success: true});
      })
    );

    // Reload a document forcibly (in fact this closes the doc, it will be automatically
    // reopened on use).
    this._app.post('/api/docs/:docId/force-reload', canEdit, throttled(async (req, res) => {
      const activeDoc = await this._getActiveDoc(req);
      await activeDoc.reloadDoc();
      res.json(null);
    }));

    this._app.post('/api/docs/:docId/recover', canEdit, throttled(async (req, res) => {
      const recoveryModeRaw = req.body.recoveryMode;
      const recoveryMode = (typeof recoveryModeRaw === 'boolean') ? recoveryModeRaw : undefined;
      if (!await this._isOwner(req)) { throw new Error('Only owners can control recovery mode'); }
      this._docManager.setRecovery(getDocId(req), recoveryMode ?? true);
      const activeDoc = await this._docManager.fetchDoc(docSessionFromRequest(req), getDocId(req), recoveryMode);
      res.json({
        recoveryMode: activeDoc.recoveryMode
      });
    }));

    // DELETE /api/docs/:docId
    // Delete the specified doc.
    this._app.delete('/api/docs/:docId', canEditMaybeRemoved, throttled(async (req, res) => {
      await this._removeDoc(req, res, true);
    }));

    // POST /api/docs/:docId/remove
    // Soft-delete the specified doc.  If query parameter "permanent" is set,
    // delete permanently.
    this._app.post('/api/docs/:docId/remove', canEditMaybeRemoved, throttled(async (req, res) => {
      await this._removeDoc(req, res, isParameterOn(req.query.permanent));
    }));

    this._app.get('/api/docs/:docId/snapshots', canView, withDoc(async (activeDoc, req, res) => {
      const {snapshots} = await activeDoc.getSnapshots(isAffirmative(req.query.raw));
      res.json({snapshots});
    }));

    this._app.post('/api/docs/:docId/snapshots/remove', isOwner, withDoc(async (activeDoc, req, res) => {
      const docSession = docSessionFromRequest(req);
      const snapshotIds = req.body.snapshotIds as string[];
      if (snapshotIds) {
        await activeDoc.removeSnapshots(docSession, snapshotIds);
        res.json({snapshotIds});
        return;
      }
      if (req.body.select === 'unlisted') {
        // Remove any snapshots not listed in inventory.  Ideally, there should be no
        // snapshots, and this undocumented feature is just for fixing up problems.
        const full = (await activeDoc.getSnapshots(true)).snapshots.map(s => s.snapshotId);
        const listed = new Set((await activeDoc.getSnapshots()).snapshots.map(s => s.snapshotId));
        const unlisted = full.filter(snapshotId => !listed.has(snapshotId));
        await activeDoc.removeSnapshots(docSession, unlisted);
        res.json({snapshotIds: unlisted});
        return;
      }
      if (req.body.select === 'past') {
        // Remove all but the latest snapshot.  Useful for sanitizing history if something
        // bad snuck into previous snapshots and they are not valuable to preserve.
        const past = (await activeDoc.getSnapshots(true)).snapshots.map(s => s.snapshotId);
        past.shift();  // remove current version.
        await activeDoc.removeSnapshots(docSession, past);
        res.json({snapshotIds: past});
        return;
      }
      throw new Error('please specify snapshotIds to remove');
    }));

    this._app.post('/api/docs/:docId/flush', canEdit, throttled(async (req, res) => {
      const activeDocPromise = this._getActiveDocIfAvailable(req);
      if (!activeDocPromise) {
        // Only need to flush if doc is actually open.
        res.json(false);
        return;
      }
      const activeDoc = await activeDocPromise;
      await activeDoc.flushDoc();
      res.json(true);
    }));

    // Administrative endpoint, that checks if a document is in the expected group,
    // and frees it for reassignment if not.  Has no effect if document is in the
    // expected group.  Does not require specific rights.  Returns true if the document
    // is freed up for reassignment, otherwise false.
    this._app.post('/api/docs/:docId/assign', canEdit, throttled(async (req, res) => {
      const docId = getDocId(req);
      const status = await this._docWorkerMap.getDocWorker(docId);
      if (!status) { res.json(false); return; }
      const workerGroup = await this._docWorkerMap.getWorkerGroup(status.docWorker.id);
      const docGroup = await this._docWorkerMap.getDocGroup(docId);
      if (docGroup === workerGroup) { res.json(false); return; }
      const activeDoc = await this._getActiveDoc(req);
      await activeDoc.flushDoc();
      // flushDoc terminates once there's no pending operation on the document.
      // There could still be async operations in progress.  We mute their effect,
      // as if they never happened.
      activeDoc.docClients.interruptAllClients();
      activeDoc.setMuted();
      await activeDoc.shutdown();
      await this._docWorkerMap.releaseAssignment(status.docWorker.id, docId);
      res.json(true);
    }));

    // This endpoint cannot use withDoc since it is expected behavior for the ActiveDoc it
    // starts with to become muted.
    this._app.post('/api/docs/:docId/replace', canEdit, throttled(async (req, res) => {
      const activeDoc = await this._getActiveDoc(req);
      const options: DocReplacementOptions = {};
      if (req.body.sourceDocId) {
        options.sourceDocId = await this._confirmDocIdForRead(req, String(req.body.sourceDocId));
        // We should make sure the source document has flushed recently.
        // It may not be served by the same worker, so work through the api.
        await fetch(this._grist.getHomeUrl(req, `/api/docs/${options.sourceDocId}/flush`), {
          method: 'POST',
          headers: {
            ...getTransitiveHeaders(req),
            'Content-Type': 'application/json',
          }
        });
      }
      if (req.body.snapshotId) {
        options.snapshotId = String(req.body.snapshotId);
      }
      await activeDoc.replace(options);
      res.json(null);
    }));

    this._app.get('/api/docs/:docId/states', canView, withDoc(async (activeDoc, req, res) => {
      const docSession = docSessionFromRequest(req);
      res.json(await this._getStates(docSession, activeDoc));
    }));

    this._app.post('/api/docs/:docId/states/remove', isOwner, withDoc(async (activeDoc, req, res) => {
      const docSession = docSessionFromRequest(req);
      const keep = integerParam(req.body.keep, 'keep');
      res.json(await activeDoc.deleteActions(docSession, keep));
    }));

    this._app.get('/api/docs/:docId/compare/:docId2', canView, withDoc(async (activeDoc, req, res) => {
      const showDetails = isAffirmative(req.query.detail);
      const docSession = docSessionFromRequest(req);
      const {states} = await this._getStates(docSession, activeDoc);
      const ref = await fetch(this._grist.getHomeUrl(req, `/api/docs/${req.params.docId2}/states`), {
        headers: {
          ...getTransitiveHeaders(req),
          'Content-Type': 'application/json',
        }
      });
      const states2: DocState[] = (await ref.json()).states;
      const left = states[0];
      const right = states2[0];
      if (!left || !right) {
        // This should not arise unless there's a bug.
        throw new Error('document with no history');
      }
      const rightHashes = new Set(states2.map(state => state.h));
      const parent = states.find(state => rightHashes.has(state.h )) || null;
      const leftChanged = parent && parent.h !== left.h;
      const rightChanged = parent && parent.h !== right.h;
      const summary = leftChanged ? (rightChanged ? 'both' : 'left') :
        (rightChanged ? 'right' : (parent ? 'same' : 'unrelated'));
      const comparison: DocStateComparison = {
        left, right, parent, summary
      };
      if (showDetails && parent) {
        // Calculate changes from the parent to the current version of this document.
        const leftChanges = (await this._getChanges(docSession, activeDoc, states, parent.h,
                                                    'HEAD')).details!.rightChanges;

        // Calculate changes from the (common) parent to the current version of the other document.
        const url = `/api/docs/${req.params.docId2}/compare?left=${parent.h}`;
        const rightChangesReq = await fetch(this._grist.getHomeUrl(req, url), {
          headers: {
            ...getTransitiveHeaders(req),
            'Content-Type': 'application/json',
          }
        });
        const rightChanges = (await rightChangesReq.json()).details!.rightChanges;

        // Add the left and right changes as details to the result.
        comparison.details = { leftChanges, rightChanges };
      }
      res.json(comparison);
    }));

    // Give details about what changed between two versions of a document.
    this._app.get('/api/docs/:docId/compare', canView, withDoc(async (activeDoc, req, res) => {
      // This could be a relatively slow operation if actions are large.
      const left = stringParam(req.query.left || 'HEAD', 'left');
      const right = stringParam(req.query.right || 'HEAD', 'right');
      const docSession = docSessionFromRequest(req);
      const {states} = await this._getStates(docSession, activeDoc);
      res.json(await this._getChanges(docSession, activeDoc, states, left, right));
    }));

    // Do an import targeted at a specific workspace. Although the URL fits ApiServer, this
    // endpoint is handled only by DocWorker, so is handled here. (Note: this does not handle
    // actual file uploads, so no worries here about large request bodies.)
    this._app.post('/api/workspaces/:wid/import', expressWrap(async (req, res) => {
      const userId = getUserId(req);
      const wsId = integerParam(req.params.wid, 'wid');
      const uploadId = integerParam(req.body.uploadId, 'uploadId');
      const result = await this._docManager.importDocToWorkspace(userId, uploadId, wsId, req.body.browserSettings);
      res.json(result);
    }));

    this._app.get('/api/docs/:docId/download/csv', canView, withDoc(async (activeDoc, req, res) => {
      // Query DB for doc metadata to get the doc title.
      const {name: docTitle} = await this._dbManager.getDoc(req);

      const params = parseExportParameters(req);
      const filename = docTitle + (params.tableId === docTitle ? '' : '-' + params.tableId);

      const options: DownloadCSVOptions = {
        ...params,
        filename,
      };

      await downloadCSV(activeDoc, req, res, options);
    }));

    this._app.get('/api/docs/:docId/download/xlsx', canView, withDoc(async (activeDoc, req, res) => {
      // Query DB for doc metadata to get the doc title (to use as the filename).
      const {name: filename} = await this._dbManager.getDoc(req);

      const options: DownloadXLSXOptions = {filename};

      await downloadXLSX(activeDoc, req, res, options);
    }));

    this._app.get('/api/docs/:docId/send-to-drive', canView, decodeGoogleToken, withDoc(exportToDrive));

    // Create a document.  When an upload is included, it is imported as the initial
    // state of the document.  Otherwise a fresh empty document is created.
    // A "timezone" option can be supplied.
    // Documents are created "unsaved".
    // TODO: support workspaceId option for creating regular documents, at which point
    // existing import endpoint and doc creation endpoint can share implementation
    // with this.
    // Returns the id of the created document.
    this._app.post('/api/docs', expressWrap(async (req, res) => {
      const userId = getUserId(req);
      let uploadId: number|undefined;
      let parameters: {[key: string]: any};
      if (req.is('multipart/form-data')) {
        const formResult = await handleOptionalUpload(req, res);
        if (formResult.upload) {
          uploadId = formResult.upload.uploadId;
        }
        parameters = formResult.parameters || {};
      } else {
        parameters = req.body;
      }
      if (parameters.workspaceId) { throw new Error('workspaceId not supported'); }
      const browserSettings: BrowserSettings = {};
      if (parameters.timezone) { browserSettings.timezone = parameters.timezone; }
      browserSettings.locale = localeFromRequest(req);
      if (uploadId !== undefined) {
        const result = await this._docManager.importDocToWorkspace(userId, uploadId, null,
                                                                   browserSettings);
        return res.json(result.id);
      }
      const isAnonymous = isAnonymousUser(req);
      const {docId} = makeForkIds({userId, isAnonymous, trunkDocId: NEW_DOCUMENT_CODE,
                                   trunkUrlId: NEW_DOCUMENT_CODE});
      await this._docManager.createNamedDoc(makeExceptionalDocSession('nascent', {
        req: req as RequestWithLogin,
        browserSettings
      }), docId);
      return res.status(200).json(docId);
    }));
  }

  /**
   * Check for read access to the given document, and return its
   * canonical docId.  Throws error if read access not available.
   * This method is used for documents that are not the main document
   * associated with the request, but are rather an extra source to be
   * read from, so the access information is not cached in the
   * request.
   */
  private async _confirmDocIdForRead(req: Request, urlId: string): Promise<string> {
    const docAuth = await makeDocAuthResult(this._dbManager.getDoc({...getScope(req), urlId}));
    if (docAuth.error) { throw docAuth.error; }
    assertAccess('viewers', docAuth);
    return docAuth.docId!;
  }

  private _getActiveDoc(req: RequestWithLogin): Promise<ActiveDoc> {
    return this._docManager.fetchDoc(docSessionFromRequest(req), getDocId(req));
  }

  private _getActiveDocIfAvailable(req: RequestWithLogin): Promise<ActiveDoc>|undefined {
    return this._docManager.getActiveDoc(getDocId(req));
  }

  /**
   * Middleware to track the number of requests outstanding on each document, and to
   * throw an exception when the maximum number of requests are already outstanding.
   * Also throws an exception if too many requests (based on the user's product plan)
   * have been made today for this document.
   * Access to a document must already have been authorized.
   */
  private _apiThrottle(callback: (req: RequestWithLogin,
                                  resp: Response,
                                  next: NextFunction) => void | Promise<void>): RequestHandler {
    return async (req, res, next) => {
      const docId = getDocId(req);
      try {
        const count = this._currentUsage.get(docId) || 0;
        this._currentUsage.set(docId, count + 1);
        if (count + 1 > MAX_PARALLEL_REQUESTS_PER_DOC) {
          throw new ApiError(`Too many backlogged requests for document ${docId} - ` +
            `try again later?`, 429);
        }

        if (await this._checkDailyDocApiUsage(req, docId)) {
          throw new ApiError(`Exceeded daily limit for document ${docId}`, 429);
        }

        await callback(req as RequestWithLogin, res, next);
      } catch (err) {
        next(err);
      } finally {
        const count = this._currentUsage.get(docId);
        if (count) {
          if (count === 1) {
            this._currentUsage.delete(docId);
          } else {
            this._currentUsage.set(docId, count - 1);
          }
        }
      }
    };
  }

  /**
   * Usually returns true if too many requests (based on the user's product plan)
   * have been made today for this document and the request should be rejected.
   * Access to a document must already have been authorized.
   * This is called frequently so it uses caches to check quickly in the common case,
   * which allows a few ways for users to exceed the limit slightly if the timing works out,
   * but these should be acceptable.
   */
  private async _checkDailyDocApiUsage(req: Request, docId: string): Promise<boolean> {
    // Use the cached doc to avoid a database call.
    // This leaves a small window (currently 5 seconds) for the user to bypass this limit after downgrading,
    // or to be wrongly rejected after upgrading.
    const doc = (req as RequestWithLogin).docAuth!.cachedDoc!;

    const max = doc.workspace.org.billingAccount?.product.features.baseMaxApiUnitsPerDocumentPerDay;
    if (!max) {
      // This doc has no associated product (happens to new unsaved docs)
      // or the product has no API limit. Allow the request through.
      return false;
    }

    // Check the counts in the dailyUsage cache rather than waiting for redis.
    // The cache will not have counts if this is the first request for this document served by this worker process
    // or if so many other documents have been served since then that the keys were evicted from the LRU cache.
    // Both scenarios are temporary and unlikely when usage has been exceeded.
    // Note that if the limits are exceeded then `keys` below will be undefined,
    // otherwise it will be an array of three keys corresponding to a day, hour, and minute.
    const m = moment.utc();
    const keys = getDocApiUsageKeysToIncr(docId, this._dailyUsage, max, m);
    if (!keys) {
      // The limit has been exceeded, reject the request.
      return true;
    }

    // If Redis isn't configured, this is as far as we can go with checks.
    if (!process.env.REDIS_URL) { return false; }

    // Note the increased API usage on redis and in our local cache.
    // Update redis in the background so that the rest of the request can continue without waiting for redis.
    const multi = this._docWorkerMap.getRedisClient().multi();
    for (let i = 0; i < keys.length; i++) {
      const key = keys[i];
      // Incrementing the local count immediately prevents many requests from being squeezed through every minute
      // before counts are received from redis.
      // But this cache is not 100% reliable and the count from redis may be higher.
      this._dailyUsage.set(key, (this._dailyUsage.get(key) ?? 0) + 1);
      const period = docApiUsagePeriods[i];
      // Expire the key just so that it cleans itself up and saves memory on redis.
      // Expire after two periods to handle 'next' buckets.
      const expiry = 2 * 24 * 60 * 60 / period.periodsPerDay;
      multi.incr(key).expire(key, expiry);
    }
    multi.execAsync().then(result => {
      for (let i = 0; i < keys.length; i++) {
        const key = keys[i];
        const newCount = Number(result![i * 2]);  // incrs are at even positions, expires at odd positions
        // Theoretically this could be overwritten by a lower count that was requested earlier
        // but somehow arrived after.
        // This doesn't really matter, and the count on redis will still increase reliably.
        this._dailyUsage.set(key, newCount);
      }
    }).catch(e => console.error(`Error tracking API usage for doc ${docId}`, e));

    // Allow the request through.
    return false;
  }

  private async _assertAccess(role: 'viewers'|'editors'|'owners'|null, allowRemoved: boolean,
                              req: Request, res: Response, next: NextFunction) {
    const scope = getDocScope(req);
    allowRemoved = scope.showAll || scope.showRemoved || allowRemoved;
    const docAuth = await getOrSetDocAuth(req as RequestWithLogin, this._dbManager, scope.urlId);
    if (role) { assertAccess(role, docAuth, {allowRemoved}); }
    next();
  }

  /**
   * Check if user is an owner of the document.
   */
  private async _isOwner(req: Request) {
    const scope = getDocScope(req);
    const docAuth = await getOrSetDocAuth(req as RequestWithLogin, this._dbManager, scope.urlId);
    return docAuth.access === 'owners';
  }

  // Helper to generate a 503 if the ActiveDoc has been muted.
  private _checkForMute(activeDoc: ActiveDoc|undefined) {
    if (activeDoc && activeDoc.muted) {
      throw new ApiError('Document in flux - try again later', 503);
    }
  }

  /**
   * Throws an error if, during processing, the ActiveDoc becomes "muted".  Also replaces any
   * other error that may have occurred if the ActiveDoc becomes "muted", since the document
   * shutting down during processing may have caused a variety of errors.
   *
   * Expects to be called within a handler that catches exceptions.
   */
  private _requireActiveDoc(callback: WithDocHandler): RequestHandler {
    return async (req, res) => {
      let activeDoc: ActiveDoc|undefined;
      try {
        activeDoc = await this._getActiveDoc(req as RequestWithLogin);
        await callback(activeDoc, req as RequestWithLogin, res);
        if (!res.headersSent) { this._checkForMute(activeDoc); }
      } catch (err) {
        this._checkForMute(activeDoc);
        throw err;
      }
    };
  }

  private async _getStates(docSession: OptDocSession, activeDoc: ActiveDoc): Promise<DocStates> {
    const states = await activeDoc.getRecentStates(docSession);
    return {
      states,
    };
  }

  /**
   *
   * Calculate changes between two document versions identified by leftHash and rightHash.
   * If rightHash is the latest version of the document, the ActionSummary for it will
   * contain a copy of updated and added rows.
   *
   * Currently will fail if leftHash is not an ancestor of rightHash (this restriction could
   * be lifted, but is adequate for now).
   *
   */
  private async _getChanges(docSession: OptDocSession, activeDoc: ActiveDoc, states: DocState[],
                            leftHash: string, rightHash: string): Promise<DocStateComparison> {
    const finder = new HashUtil(states);
    const leftOffset = finder.hashToOffset(leftHash);
    const rightOffset = finder.hashToOffset(rightHash);
    if (rightOffset > leftOffset) {
      throw new Error('Comparisons currently require left to be an ancestor of right');
    }
    const actionNums: number[] = states.slice(rightOffset, leftOffset).map(state => state.n);
    const actions = (await activeDoc.getActions(actionNums)).reverse();
    let totalAction = createEmptyActionSummary();
    for (const action of actions) {
      if (!action) { continue; }
      const summary = summarizeAction(action);
      totalAction = concatenateSummaries([totalAction, summary]);
    }
    const result: DocStateComparison = {
      left: states[leftOffset],
      right: states[rightOffset],
      parent: states[leftOffset],
      summary: (leftOffset === rightOffset) ? 'same' : 'right',
      details: {
        leftChanges: {tableRenames: [], tableDeltas: {}},
        rightChanges: totalAction
      }
    };
    return result;
  }

  private async _removeDoc(req: Request, res: Response, permanent: boolean) {
    const scope = getDocScope(req);
    const docId = getDocId(req);
    if (permanent) {
      // Soft delete the doc first, to de-list the document.
      await this._dbManager.softDeleteDocument(scope);
      // Delete document content from storage.
      await this._docManager.deleteDoc(null, docId, true);
      // Permanently delete from database.
      const query = await this._dbManager.deleteDocument(scope);
      this._dbManager.checkQueryResult(query);
      await sendReply(req, res, query);
    } else {
      await this._dbManager.softDeleteDocument(scope);
      await sendOkReply(req, res);
    }
    await this._dbManager.flushSingleDocAuthCache(scope, docId);
    await this._docManager.interruptDocClients(docId);
  }
}

export function addDocApiRoutes(
  app: Application, docWorker: DocWorker, docWorkerMap: IDocWorkerMap, docManager: DocManager, dbManager: HomeDBManager,
  grist: GristServer
) {
  const api = new DocWorkerApi(app, docWorker, docWorkerMap, docManager, dbManager, grist);
  api.addEndpoints();
}

/**
 * Options for returning results from a query about document data.
 * Currently these option don't affect the query itself, only the
 * results returned to the user.
 */
export interface QueryParameters {
  sort?: string[];  // Columns names to sort by (ascending order by default,
                    // prepend "-" for descending order, can contain flags,
                    // see more in Sort.SortSpec).
  limit?: number;   // Limit on number of rows to return.
}


/**
 * Extract a sort parameter from a request, if present.  Follows
 * https://jsonapi.org/format/#fetching-sorting for want of a better
 * standard - comma separated, defaulting to ascending order, keys
 * prefixed by "-" for descending order.
 *
 * The sort parameter can either be given as a query parameter, or
 * as a header.
 */
function getSortParameter(req: Request): string[]|undefined {
  const sortString: string|undefined = optStringParam(req.query.sort) || req.get('X-Sort');
  if (!sortString) { return undefined; }
  return sortString.split(',');
}

/**
 * Extract a limit parameter from a request, if present.  Should be a
 * simple integer.  The limit parameter can either be given as a query
 * parameter, or as a header.
 */
function getLimitParameter(req: Request): number|undefined {
  const limitString: string|undefined = optStringParam(req.query.limit) || req.get('X-Limit');
  if (!limitString) { return undefined; }
  const limit = parseInt(limitString, 10);
  if (isNaN(limit)) { throw new Error('limit is not a number'); }
  return limit;
}

/**
 * Extract sort and limit parameters from request, if they are present.
 */
function getQueryParameters(req: Request): QueryParameters {
  return {
    sort: getSortParameter(req),
    limit: getLimitParameter(req),
  };
}

/**
 * Sort table contents being returned.  Sort keys with a '-' prefix
 * are sorted in descending order, otherwise ascending.  Contents are
 * modified in place. Sort keys can contain sort options.
 * Columns can be either expressed as a colId (name string) or as colRef (rowId number).
 */
function applySort(
  values: TableColValues,
  sort: string[],
  _columns: TableRecordValue[]|null = null) {
  if (!sort) { return values; }

  // First we need to prepare column description in ColValue format (plain objects).
  // This format is used by ServerColumnGetters.
  let properColumns: ColValues[] = [];

  // We will receive columns information only for user tables, not for metatables. So
  // if this is the case, we will infer them from the result.
  if (!_columns) {
    _columns = Object.keys(values).map((col, index) => ({ id: col, fields: { colRef: index }}));
  }
  // For user tables, we will not get id column (as this column is not in the schema), so we need to
  // make sure the column is there.
  else {
    // This is enough information for ServerGetters
    _columns = [..._columns, { id : 'id', fields: {colRef: 0 }}];
  }

  // Once we have proper columns, we can convert them to format that ServerColumnGetters
  // understand.
  properColumns = _columns.map(c => ({
    ...c.fields,
    id : c.fields.colRef,
    colId: c.id
  }));

  // We will sort row indices in the values object, not rows ids.
  const rowIndices = values.id.map((__, i) => i);
  const getters = new ServerColumnGetters(rowIndices, values, properColumns);
  const sortFunc = new SortFunc(getters);
  const colIdToRef = new Map(properColumns.map(({id, colId}) => [colId as string, id as number]));
  sortFunc.updateSpec(Sort.parseNames(sort, colIdToRef));
  rowIndices.sort(sortFunc.compare.bind(sortFunc));

  // Sort resulting values according to the sorted index.
  for (const key of Object.keys(values)) {
    const col = values[key];
    values[key] = rowIndices.map(i => col[i]);
  }
  return values;
}

/**
 * Truncate columns to the first N values.  Columns are modified in place.
 */
function applyLimit(values: TableColValues, limit: number) {
  // for no limit, or 0 limit, do not apply any restriction
  if (!limit) { return values; }
  for (const key of Object.keys(values)) {
    values[key].splice(limit);
  }
  return values;
}

/**
 * Apply query parameters to table contents.  Contents are modified in place.
 */
export function applyQueryParameters(
  values: TableColValues,
  params: QueryParameters,
  columns: TableRecordValue[]|null = null): TableColValues {
  if (params.sort) { applySort(values, params.sort, columns); }
  if (params.limit) { applyLimit(values, params.limit); }
  return values;
}

function getErrorPlatform(tableId: string): TableOperationsPlatform {
  return {
    async getTableId() { return tableId; },
    throwError(verb, text, status) {
      throw new ApiError(verb + (verb ? ' ' : '') + text, status);
    },
    applyUserActions() {
      throw new Error('no document');
    }
  };
}

function getTableOperations(req: RequestWithLogin, activeDoc: ActiveDoc): TableOperationsImpl {
  const options: OpOptions = {
    parseStrings: !isAffirmative(req.query.noparse)
  };
  const platform: TableOperationsPlatform = {
    ...getErrorPlatform(req.params.tableId),
    applyUserActions(actions, opts) {
      if (!activeDoc) { throw new Error('no document'); }
      return activeDoc.applyUserActions(
        docSessionFromRequest(req),
        actions,
        opts
      );
    }
  };
  return new TableOperationsImpl(platform, options);
}

async function handleSandboxError<T>(tableId: string, colNames: string[], p: Promise<T>): Promise<T> {
  return handleSandboxErrorOnPlatform(tableId, colNames, p, getErrorPlatform(tableId));
}

export interface DocApiUsagePeriod {
  unit: 'day' | 'hour' | 'minute',
  format: string;
  periodsPerDay: number;
}

export const docApiUsagePeriods: DocApiUsagePeriod[] = [
  {
    unit: 'day',
    format: 'YYYY-MM-DD',
    periodsPerDay: 1,
  },
  {
    unit: 'hour',
    format: 'YYYY-MM-DDTHH',
    periodsPerDay: 24,
  },
  {
    unit: 'minute',
    format: 'YYYY-MM-DDTHH:mm',
    periodsPerDay: 24 * 60,
  },
];

/**
 * Returns a key used for redis and a local cache
 * which store the number of API requests made for the given document in the given period.
 * The key contains the current UTC date (and maybe hour and minute)
 * so that counts from previous periods are simply ignored and eventually evicted.
 * This means that the daily measured usage conceptually 'resets' at UTC midnight.
 * If `current` is false, returns a key for the next day/hour.
 */
export function docPeriodicApiUsageKey(docId: string, current: boolean, period: DocApiUsagePeriod, m: moment.Moment) {
  if (!current) {
    m = m.clone().add(1, period.unit);
  }
  return `doc-${docId}-periodicApiUsage-${m.format(period.format)}`;
}

/**
 * Checks whether the doc API usage fits within the daily maximum.
 * If so, returns an array of keys for each unit of time whose usage should be incremented.
 * If not, returns undefined.
 *
 * Description of the algorithm this is implementing:
 *
 * Maintain up to 5 buckets: current day, next day, current hour, next hour, current minute.
 * For each API request, check in order:
 * - if current_day < DAILY_LIMIT, allow; increment all 3 current buckets
 * - else if current_hour < DAILY_LIMIT/24, allow; increment next_day, current_hour, and current_minute buckets.
 * - else if current_minute < DAILY_LIMIT/24/60, allow; increment next_day, next_hour, and current_minute buckets.
 * - else reject.
 * I think it has pretty good properties:
 * - steady low usage may be maintained even if a burst exhausted the daily limit
 * - user could get close to twice the daily limit on the first day with steady usage after a burst,
 *   but would then be limited to steady usage the next day.
 */
export function getDocApiUsageKeysToIncr(
  docId: string, usage: LRUCache<string, number>, dailyMax: number, m: moment.Moment
): string[] | undefined {
  // Start with keys for the current day, minute, and hour
  const keys = docApiUsagePeriods.map(p => docPeriodicApiUsageKey(docId, true, p, m));
  for (let i = 0; i < docApiUsagePeriods.length; i++) {
    const period = docApiUsagePeriods[i];
    const key = keys[i];
    const periodMax = Math.ceil(dailyMax / period.periodsPerDay);
    const count = usage.get(key) || 0;
    if (count < periodMax) {
      return keys;
    }
    // Allocation for the current day/hour/minute has been exceeded, increment the next day/hour/minute instead.
    keys[i] = docPeriodicApiUsageKey(docId, false, period, m);
  }
  // Usage exceeded all the time buckets, so return undefined to reject the request.
}