webmail/lib/list-step.ts

import type { Email } from '@/lib/jmap/types';
import { emailKeyFor } from '@/lib/thread-utils';

/** One row of the message list: its own key and the keys it stands for. */
export interface ListStepRow {
  key: string;
  members: string[];
}

/**
 * The message j / k open: the one after (`direction` 1) or before (-1) the
 * message `fromKey` names, staying put at either end. `fromKey` is the open
 * message, or the last one opened when the reader was closed, so j after
 * Escape goes on from there. Without a place in the list, j starts at the
 * top and k at the bottom.
 *
 * With `rows` (the list's rows in screen order) a conversation counts once,
 * and opens at its newest message, as a click on its row does: the reading
 * pane shows the rest of it.
 */
export function stepListEmail(
  emails: readonly Email[],
  fromKey: string | null | undefined,
  direction: 1 | -1,
  rows?: readonly ListStepRow[] | null,
): Email | null {
  const byKey = new Map(emails.map((e) => [emailKeyFor(e), e] as const));
  const order: readonly ListStepRow[] = rows && rows.length > 0
    ? rows.filter((row) => byKey.has(row.key))
    : emails.map((e) => ({ key: emailKeyFor(e), members: [] }));
  if (order.length === 0) return null;
  const at = fromKey
    ? order.findIndex((row) => row.key === fromKey || row.members.includes(fromKey))
    : -1;
  const target = at < 0
    ? (direction === 1 ? 0 : order.length - 1)
    : Math.min(order.length - 1, Math.max(0, at + direction));
  return byKey.get(order[target].key) ?? null;
}