/**
 * Renders the weekly digest and the monthly recap, as the drafts approved
 * them: the weekly one has four sections (what happened to the reader, what
 * the people and playlists they follow posted, the period's top dumps, and
 * the community), the monthly one only the last two. `digest-service.ts`
 * gathers a {@link DigestContent} and sends what this returns, drawn in the
 * reader's own style and scheme.
 *
 * @module
 */

import { EMAIL_LOCALE, OG_SITE_NAME } from "../config.ts";
import type { DigestCadence } from "../../shared/settings.ts";
import { i18n } from "./i18n.ts";
import {
  EmailKit,
  type EmailLook,
  escMd,
  mdLink,
  type RenderedEmail,
} from "./layout.ts";

/** The kinds of activity a weekly digest counts. */
export type ActivityKind =
  | "upvotes"
  | "followers"
  | "comments"
  | "mentions"
  | "likes"
  | "playlistFollows";

/** What a highlight of the weekly digest says happened. */
export type HighlightKind =
  | "mentionComment"
  | "mentionDump"
  | "mentionPlaylist"
  | "mentionChat"
  | "comment"
  | "playlistFollow"
  | "userFollow"
  | "commentLike";

/** A dump in a digest's lists, with its links made absolute. */
export interface DigestDump {
  title: string;
  url: string;
  poster: string;
  /** The link's host, or null for an upload. */
  domain: string | null;
  /** An absolute image URL, or null to show no image. */
  thumbnail: string | null;
  votes: number;
  comments: number;
  /** Posted by the reader, which the lists mark. */
  mine: boolean;
  /** The followed playlist it was added to, when that's why it's listed. */
  addedTo?: string;
}

/** One line of the weekly digest's "For you", made from a notification. */
export interface DigestHighlight {
  kind: HighlightKind;
  /** Who did it. */
  who: string;
  /**
   * What they did it on, absent for a follow of the reader or the chat. Its
   * `url` is null once it is deleted, and the title then shows unlinked.
   */
  target?: { title: string; url: string | null };
  /** The comment or message, flattened and shortened. */
  quote?: string;
  at: Date;
}

/**
 * Passed to {@link renderDigestEmail} by `digest-service.ts`. Each section is
 * null when it has nothing to show, and is left out; the service sends
 * nothing when all of them are.
 */
export interface DigestContent {
  cadence: DigestCadence;
  /** The period covered, from `from` to `to`. */
  from: Date;
  to: Date;
  reader: string;
  /** Weekly only. */
  activity: {
    counts: Partial<Record<ActivityKind, number>>;
    highlights: DigestHighlight[];
    /** Every notification of the period, which the link to them counts. */
    total: number;
  } | null;
  /** Weekly only. */
  network: { dumps: DigestDump[]; more: number } | null;
  top: DigestDump[];
  community: {
    members: { username: string; invitedBy: string | null }[];
    membersMore: number;
    playlists: { title: string; url: string; owner: string; dumps: number }[];
    playlistsMore: number;
    chatMessages: number;
    totals: { dumps: number; upvotes: number; comments: number };
  } | null;
  links: {
    notifications: string;
    followed: string;
    chat: string;
    settings: string;
    /** The one-click page that turns this cadence off. */
    unsubscribe: string;
  };
}

const ACTIVITY_ORDER: ActivityKind[] = [
  "upvotes",
  "followers",
  "comments",
  "mentions",
  "likes",
  "playlistFollows",
];

/** Returns the label of `n` of a kind of activity, as in "12 upvotes". */
function countLabel(kind: ActivityKind, n: number): string {
  switch (kind) {
    case "upvotes":
      return i18n._("email.digest.count.upvotes", { n });
    case "followers":
      return i18n._("email.digest.count.followers", { n });
    case "comments":
      return i18n._("email.digest.count.comments", { n });
    case "mentions":
      return i18n._("email.digest.count.mentions", { n });
    case "likes":
      return i18n._("email.digest.count.likes", { n });
    case "playlistFollows":
      return i18n._("email.digest.count.playlistFollows", { n });
  }
}

/**
 * Returns the sentence of a highlight, `who` and `target` already formatted
 * for the part it goes in (HTML or Markdown).
 */
function highlightSentence(
  kind: HighlightKind,
  who: string,
  target: string,
): string {
  const values = { who, target };
  switch (kind) {
    case "mentionComment":
      return i18n._("email.digest.hl.mentionComment", values);
    case "mentionDump":
    case "mentionChat":
      return i18n._("email.digest.hl.mentionIn", values);
    case "mentionPlaylist":
      return i18n._("email.digest.hl.mentionPlaylist", values);
    case "comment":
      return i18n._("email.digest.hl.comment", values);
    case "playlistFollow":
      return i18n._("email.digest.hl.playlistFollow", values);
    case "userFollow":
      return i18n._("email.digest.hl.userFollow", values);
    case "commentLike":
      return i18n._("email.digest.hl.commentLike", values);
  }
}

const byLine = (username: string) => i18n._("email.digest.by", { username });
const addedToLine = (playlist: string) =>
  i18n._("email.digest.addedTo", { playlist });
const uploadLabel = () => i18n._("email.digest.upload");
const invitedLine = (inviter: string, reader: string) =>
  inviter === reader
    ? i18n._("email.digest.invitedByYou")
    : i18n._("email.digest.invitedBy", { username: inviter });
const andMoreLine = (n: number) => i18n._("email.digest.andMore", { n });
const playlistMeta = (owner: string, n: number) =>
  i18n._("email.digest.playlistMeta", { username: owner, n });

/**
 * Returns the digest `content` describes, written as Markdown and drawn in
 * `look`, with its subject and hidden preheader.
 */
export function renderDigestEmail(
  content: DigestContent,
  look: EmailLook,
): RenderedEmail {
  const monthly = content.cadence === "monthly";
  const nf = new Intl.NumberFormat(EMAIL_LOCALE);
  const range = (year: boolean) =>
    new Intl.DateTimeFormat(
      EMAIL_LOCALE,
      year
        ? { month: "short", day: "numeric", year: "numeric" }
        : { month: "short", day: "numeric" },
    ).formatRange(content.from, content.to);
  const day = (d: Date) =>
    new Intl.DateTimeFormat(EMAIL_LOCALE, { weekday: "short" }).format(d);
  const figure = (n: number, label: string) =>
    `**${nf.format(n)}** ${escMd(label)}`;
  const yours = i18n._("email.digest.yours");
  /** A dump as a list item: its thumbnail, title, pill and detail. */
  const dumpItem = (d: DigestDump, bullet: string, image = true) =>
    `${bullet} ${image && d.thumbnail ? `![](${d.thumbnail}) ` : ""}${
      mdLink(d.title, d.url)
    }${d.mine ? ` *${escMd(yours)}*` : ""} · ${
      escMd(
        `${d.addedTo ? addedToLine(d.addedTo) : byLine(d.poster)}${
          d.addedTo ? "" : ` · ${d.domain ?? uploadLabel()}`
        } · ${nf.format(d.votes)} ${countLabel("upvotes", d.votes)} · ${
          nf.format(d.comments)
        } ${countLabel("comments", d.comments)}`,
      )
    }`;

  const md: string[] = [
    `*${
      escMd(
        `${monthly ? i18n._("digest.monthly") : i18n._("digest.weekly")} · ${
          range(true)
        }`,
      )
    }*`,
    `# ${
      escMd(
        monthly
          ? i18n._("email.digest.monthly.title", { site: OG_SITE_NAME })
          : i18n._("email.digest.weekly.title", { username: content.reader }),
      )
    }`,
  ];

  const { activity, network, community } = content;
  if (activity) {
    md.push(
      `## ${escMd(i18n._("email.digest.forYou"))}`,
      ACTIVITY_ORDER.filter((k) => (activity.counts[k] ?? 0) > 0)
        .map((k) =>
          figure(activity.counts[k]!, countLabel(k, activity.counts[k]!))
        )
        .join(" · "),
      activity.highlights.map((h) => {
        const target = h.kind === "mentionChat"
          ? mdLink(
            i18n._("email.digest.theChat"),
            content.links.chat,
          )
          : h.target?.url
          ? mdLink(h.target.title, h.target.url)
          : h.target
          ? escMd(h.target.title)
          : "";
        const line = `- ${
          highlightSentence(h.kind, `**@${escMd(h.who)}**`, target)
        } *${escMd(day(h.at))}*`;
        return h.quote ? `${line}\n  > ${escMd(h.quote)}` : line;
      }).join("\n"),
      `${
        mdLink(
          i18n._("email.digest.seeAll", { n: activity.total }),
          content.links.notifications,
        )
      } →`,
    );
  }

  if (network) {
    md.push(
      `## ${escMd(i18n._("email.digest.network"))}`,
      network.dumps.map((d) => dumpItem(d, "-")).join("\n"),
    );
    if (network.more > 0) {
      md.push(`${
        mdLink(
          i18n._("email.digest.more", { n: network.more }),
          content.links.followed,
        )
      } →`);
    }
  }

  if (content.top.length > 0) {
    const [first] = content.top;
    md.push(
      `## ${
        escMd(
          monthly
            ? i18n._("email.digest.topMonth")
            : i18n._("email.digest.topWeek"),
        )
      }`,
    );
    // The recap gives its first dump a wide image and the next two a thumbnail.
    if (monthly && first.thumbnail) md.push(`![](${first.thumbnail})`);
    md.push(
      content.top.map((d, i) =>
        dumpItem(d, `${i + 1}.`, monthly ? i === 1 || i === 2 : true)
      ).join("\n"),
    );
  }

  if (community) {
    const { dumps, upvotes, comments } = community.totals;
    md.push(
      `## ${escMd(i18n._("email.digest.community"))}`,
    );
    if (monthly) {
      md.push(
        [
          figure(
            dumps,
            i18n._("email.digest.stat.dumps", { n: dumps }),
          ),
          figure(upvotes, countLabel("upvotes", upvotes)),
          figure(comments, countLabel("comments", comments)),
          figure(
            community.chatMessages,
            i18n._("email.digest.stat.chat", { n: community.chatMessages }),
          ),
        ].join(" · "),
      );
    }
    if (community.members.length > 0) {
      md.push(
        `### ${
          escMd(
            i18n._("email.digest.newMembers"),
          )
        }`,
        [
          ...community.members.map((m) =>
            `- **@${escMd(m.username)}**${
              m.invitedBy
                ? ` ${escMd(invitedLine(m.invitedBy, content.reader))}`
                : ""
            }`
          ),
          ...(community.membersMore > 0
            ? [`- ${escMd(andMoreLine(community.membersMore))}`]
            : []),
        ].join("\n"),
      );
    }
    if (community.playlists.length > 0) {
      md.push(
        `### ${escMd(i18n._("email.digest.newPlaylists"))}`,
        [
          ...community.playlists.map((p) =>
            `- ${mdLink(p.title, p.url)} · ${
              escMd(playlistMeta(p.owner, p.dumps))
            }`
          ),
          ...(community.playlistsMore > 0
            ? [`- ${escMd(andMoreLine(community.playlistsMore))}`]
            : []),
        ].join("\n"),
      );
    }
    if (!monthly && community.chatMessages > 0) {
      md.push(
        `### ${escMd(i18n._("email.digest.inChat"))}`,
        `${
          escMd(i18n._("email.digest.chatWeek", { n: community.chatMessages }))
        } · ${
          mdLink(
            i18n._("email.digest.openChat"),
            content.links.chat,
          )
        }`,
      );
    }
    if (!monthly && dumps + upvotes + comments > 0) {
      md.push(
        `*${
          escMd(i18n._("email.digest.totals", { dumps, upvotes, comments }))
        }*`,
      );
    }
  }

  md.push(
    "---",
    escMd(
      monthly
        ? i18n._("email.digest.monthly.reason")
        : i18n._("email.digest.weekly.reason"),
    ),
    `${
      mdLink(
        monthly ? i18n._("digest.monthly.off") : i18n._("digest.weekly.off"),
        content.links.unsubscribe,
      )
    } · ${
      mdLink(
        i18n._("email.settings"),
        content.links.settings,
      )
    }`,
  );

  return new EmailKit(look).render({
    subject: monthly
      ? i18n._("email.digest.monthly.subject", {
        site: OG_SITE_NAME,
        range: range(false),
      })
      : i18n._("email.digest.weekly.subject", {
        site: OG_SITE_NAME,
        range: range(false),
      }),
    preheader: preheader(content),
    markdown: md.join("\n\n"),
  });
}

/**
 * Returns the line an inbox shows beside the subject: the counts that make
 * the digest worth opening, at most three of them.
 */
function preheader(content: DigestContent): string {
  const parts: string[] = [];
  if (content.cadence === "weekly") {
    for (const k of ["upvotes", "followers", "comments"] as const) {
      const n = content.activity?.counts[k] ?? 0;
      if (n > 0) parts.push(`${n} ${countLabel(k, n)}`);
    }
    const n = (content.network?.dumps.length ?? 0) +
      (content.network?.more ?? 0);
    if (n > 0) {
      parts.push(i18n._("email.digest.pre.network", { n }));
    }
  } else {
    const members = (content.community?.members.length ?? 0) +
      (content.community?.membersMore ?? 0);
    const playlists = (content.community?.playlists.length ?? 0) +
      (content.community?.playlistsMore ?? 0);
    if (content.top.length > 0) {
      parts.push(
        i18n._("email.digest.pre.topMonth", { n: content.top.length }),
      );
    }
    if (members > 0) {
      parts.push(i18n._("email.digest.pre.members", { n: members }));
    }
    if (playlists > 0) {
      parts.push(i18n._("email.digest.pre.playlists", { n: playlists }));
    }
  }
  if (parts.length === 0 && content.top.length > 0) {
    parts.push(i18n._("email.digest.pre.topWeek", { n: content.top.length }));
  }
  if (parts.length === 0) return "";
  const line = new Intl.ListFormat(EMAIL_LOCALE, { type: "conjunction" })
    .format(parts.slice(0, 3));
  return `${line.charAt(0).toUpperCase()}${line.slice(1)}.`;
}