import { formatDate } from '../dateUtils';

// WHICH PERIOD MY HISTORY IS SPEAKING ABOUT, said on the screen rather than assumed by the reader.
//
// A populated list with no stated bound reads as "this is everything", and on the feature built for
// spotting an access that should not have happened that is the worst thing it could imply. So the line
// is always a date: never "the last 90 days" (a count the reader has to convert) and never
// "everything" (false on the tabs, which narrow the stream). A sentence, not a date picker: "Show older
// activity" widens the window and this line follows it.

/** The server's default. Ninety days is a floor on the window, never a ceiling. */
const DEFAULT_WINDOW_DAYS = 90;
const DAY_MS = 24 * 60 * 60 * 1000;

export interface HistoryWindow {
    /** True while the server may still have more behind the window. */
    mayHaveOlder: boolean;
    /** The server's cursor: the oldest moment it looked at. */
    olderThan?: string;
    /** The floor the server applied to a default-window search, when it said one. */
    earlierThan?: string;
    /** The earliest row on screen, for a server that sends no cursor. */
    oldest: string | undefined;
    /** Injected so the boundary can be tested rather than depended on. */
    now?: Date;
}

const earlier = (a: string, b: string): string => (new Date(a).getTime() <= new Date(b).getTime() ? a : b);

/** The first day of the window the server actually searched. */
export const historyWindowStart = ({ mayHaveOlder, olderThan, earlierThan, oldest, now = new Date() }: HistoryWindow): string => {
    // The server's own floor when it said one; the client's estimate of it otherwise.
    const floor = earlierThan ?? new Date(now.getTime() - DEFAULT_WINDOW_DAYS * DAY_MS).toISOString();
    // Nothing left behind it: the search reached at least the floor, and further if a row did.
    if (!mayHaveOlder) return [olderThan, oldest].reduce<string>((start, value) => (value ? earlier(start, value) : start), floor);
    // More remains: name the oldest loaded row, never the ninety-day floor. Claiming the floor
    // would say the screen already holds ninety days while Show older is still offered.
    if (oldest) return oldest;
    if (olderThan) return olderThan;
    return floor;
};

export const describeHistoryWindow = (window: HistoryWindow): string =>
    `Showing ${formatDate(historyWindowStart(window))} to today.`;

// An empty search speaks for the window searched, never for the whole record.
export const describeEmptyWindow = (window: HistoryWindow): string =>
    `No activity on your record since ${formatDate(historyWindowStart(window))}.`;

/** The door past the default window, offered once that window has nothing further. */
export const SEARCH_EARLIER_LABEL = 'Search earlier activity';

export const describeEarlierDoor = (window: HistoryWindow): string =>
    `Activity before ${formatDate(historyWindowStart(window))} has not been searched.`;

// The server has more but gave no cursor past it: stopping here is not the end of the record.
export const STALLED_HISTORY_LINE = 'Older activity could not be reached just now, so this is not everything. Try again.';