/**
 * The order notes appear in inside a notebook.
 *
 * Kept as a pure function so the awkward part can be tested: the promise that
 * switching away to another order — or searching, which imposes its own — and
 * then switching back leaves the user's own arrangement exactly as they left it.
 *
 * That promise is only keepable if the manual order lives in the notes and is
 * never re-derived from whatever happens to be on screen. Hence `reindex`, which
 * takes the *whole* notebook and refuses to renumber a filtered subset: doing so
 * would give a hidden note the same position as a visible one and quietly
 * scramble everything that was not being looked at.
 */

import type { Chapter, NoteMeta, Separator, SortMode } from "../../types";

/**
 * What a notebook is in until it is told otherwise.
 *
 * The hand-made order, because that is where a notebook's own arrangement lives:
 * separators show, rows can be dragged into place, and nothing slides about on
 * its own. A notebook that has never been arranged loses nothing by it —
 * untouched rows all carry position 0 and fall back to newest first, which is
 * what the old default showed.
 */
export const DEFAULT_NOTEBOOK_SORT: SortMode = "manual";

/**
 * What a *view* is in — "All Notes", "Starred", the bin.
 *
 * Not the hand-made one: those are questions asked of every notebook at once, so
 * there is nothing for an arrangement to belong to and nowhere to keep it.
 */
export const DEFAULT_SORT: SortMode = "updated";

export const SORT_LABELS: Record<SortMode, string> = {
  manual: "My own order",
  updated: "Last changed",
  oldest: "Oldest first",
  created: "Newest created",
  title: "Title A–Z",
};

export const SORT_MODES = Object.keys(SORT_LABELS) as SortMode[];

/**
 * The orders a view can be put in.
 *
 * "All Notes" and "Starred" are not places, they are questions asked of every
 * notebook at once — so there is nothing for a hand-made arrangement to belong
 * to, and nowhere to keep it. Offering it there would be offering a setting that
 * cannot be honoured.
 */
export const sortModesFor = (isNotebook: boolean): SortMode[] =>
  isNotebook ? SORT_MODES : SORT_MODES.filter((m) => m !== "manual");

const time = (value: string | undefined) => {
  const t = Date.parse(value ?? "");
  return Number.isNaN(t) ? 0 : t;
};

const byTitle = (a: NoteMeta, b: NoteMeta) =>
  (a.title || "").localeCompare(b.title || "", "de", { sensitivity: "base", numeric: true });

/**
 * Sorts a list without mutating it.
 *
 * Every comparison ends in a tie-break that cannot itself tie: two notes created
 * in the same second, or two untouched imports that all carry position 0, must
 * still come out in the same order on every render, or the list shuffles under
 * the cursor as React re-renders.
 */
export function sortNotes(notes: NoteMeta[], mode: SortMode = DEFAULT_SORT): NoteMeta[] {
  const stable = (a: NoteMeta, b: NoteMeta) => a.id.localeCompare(b.id);

  const compare: Record<SortMode, (a: NoteMeta, b: NoteMeta) => number> = {
    // Notes that have never been dragged share position 0; newest first within
    // that group, so a fresh note appears at the top rather than at random.
    manual: (a, b) =>
      (a.sortOrder ?? 0) - (b.sortOrder ?? 0) || time(b.updatedAt) - time(a.updatedAt) || stable(a, b),
    updated: (a, b) => time(b.updatedAt) - time(a.updatedAt) || stable(a, b),
    oldest: (a, b) => time(a.updatedAt) - time(b.updatedAt) || stable(a, b),
    created: (a, b) => time(b.createdAt) - time(a.createdAt) || stable(a, b),
    title: (a, b) => byTitle(a, b) || stable(a, b),
  };

  return [...notes].sort(compare[mode] ?? compare[DEFAULT_SORT]);
}

/* ── separators ── */

/**
 * A line drawn between notes to group them.
 *
 * Separators live in the same order as the notes and are dragged the same way,
 * because a divider that cannot be moved with what it divides is worse than
 * none. They belong to the notebook rather than to any note: they are part of
 * the arrangement, and the arrangement is the notebook's.
 *
 * They exist only in the manual order. In "newest first" there is nothing for a
 * line to mean — the notes would slide past it on every edit — so the other
 * orders simply do not show them, and they are still there on the way back.
 */
export type Entry =
  | { kind: "note"; id: string; order: number; note: NoteMeta }
  | { kind: "chapter"; id: string; order: number; chapter: Chapter }
  | { kind: "separator"; id: string; order: number; separator: Separator };

/**
 * Notes and separators, interleaved into the one order the user arranged.
 *
 * A separator sitting at the same position as a note comes first, so "insert a
 * line above this note" lands where it was asked for rather than one row late.
 */
export function mergeEntries(
  notes: NoteMeta[],
  separators: Separator[] = [],
  chapters: Chapter[] = []
): Entry[] {
  /* Notes and chapters share one sequence.
   *
   * A chapter is a row in the list like a note is, and can be dragged among
   * them, so they cannot each be numbered from zero — they are ranked together
   * by the position each carries.
   *
   * Ranked, not placed by the stored number: rows that have never been dragged
   * all carry 0, and using the raw numbers would pile them at position 0 and
   * push every separator behind the lot. */
  const ranked = sortNotes(notes, "manual");
  const rows = [
    ...chapters.map((chapter, i) => ({
      kind: "chapter" as const,
      id: chapter.id,
      chapter,
      sort: chapter.sortOrder ?? 0,
      // Keeps the order within one kind settled when several share a position,
      // which every row does until it has been dragged once.
      tie: i,
    })),
    ...ranked.map((note, i) => ({ kind: "note" as const, id: note.id, note, sort: note.sortOrder ?? 0, tie: i })),
  ].sort(
    (a, b) =>
      a.sort - b.sort ||
      // A chapter shares a position with a note only before either has been
      // moved; putting chapters first there matches where they are created.
      (a.kind === b.kind ? a.tie - b.tie : a.kind === "chapter" ? -1 : 1)
  );

  const rowEntries: Entry[] = rows.map((row, rank) =>
    row.kind === "chapter"
      ? { kind: "chapter", id: row.id, order: rank, chapter: row.chapter }
      : { kind: "note", id: row.id, order: rank, note: row.note }
  );

  const separatorEntries: Entry[] = separators.map((separator) => ({
    kind: "separator" as const,
    id: separator.id,
    // Clamped, so a separator left over from a notebook that has since shrunk
    // still appears — at the end — rather than vanishing with its label.
    order: Math.max(0, Math.min(separator.order, rowEntries.length)),
    separator,
  }));

  return [...rowEntries, ...separatorEntries].sort((a, b) => {
    if (a.order !== b.order) return a.order - b.order;
    /* Two lines in the same gap keep the order they were written in.
     *
     * This used to say "a separator comes first" without asking what the other
     * one was — so for two separators it claimed *both* directions at once. A
     * comparator that contradicts itself has no defined result, and the sort
     * returned them in whichever order it happened to walk: a line dragged under
     * another sprang back above it, every time. */
    if (a.kind === "separator" && b.kind === "separator") return 0;
    // A separator sitting at the same position as a row comes first, so
    // "a line above this one" lands where it was asked for.
    return a.kind === "separator" ? -1 : 1;
  });
}

/** The result of a drag: what to write back for each kind of row. */
export interface Arrangement {
  /** Notes in their new order, with the position each should carry. */
  notes: Array<{ id: string; order: number }>;
  /** Chapters likewise — numbered from the *same* counter as the notes. */
  chapters: Array<{ id: string; order: number }>;
  separators: Separator[];
  /** The note ids alone, for callers that only reorder notes. */
  noteIds: string[];
}

/** Turns an ordered entry list back into storable positions. */
export function toArrangement(entries: Entry[]): Arrangement {
  const notes: Array<{ id: string; order: number }> = [];
  const chapters: Array<{ id: string; order: number }> = [];
  const separators: Separator[] = [];
  /* One counter for notes and chapters together. Numbering them separately
   * would leave two sequences that cannot be interleaved again, and a chapter
   * dragged between two notes would spring back. */
  let position = 0;
  for (const entry of entries) {
    if (entry.kind === "note") notes.push({ id: entry.id, order: position++ });
    else if (entry.kind === "chapter") chapters.push({ id: entry.id, order: position++ });
    // A separator's position is "how many rows come before me". Expressed that
    // way it stays put as the rows around it are renamed, edited or opened.
    else separators.push({ ...entry.separator, order: position });
  }
  return { notes, chapters, separators, noteIds: notes.map((n) => n.id) };
}

/**
 * The whole notebook rearranged after something was dragged onto something else.
 *
 * `notes` must be every note in the notebook, not the filtered view. Returns
 * null when the move changes nothing, which spares the caller a pointless write.
 *
 * A drop lands the row **under** the one it was released on, and only ever
 * there. Dropping above a row and dropping below the row before it are the same
 * move described twice, and having both meant every gap could be aimed at from
 * two sides — which is a hard thing to aim at and a harder thing to read. The
 * two strips, one above the first row and one below the last, are how the ends
 * are reached.
 */
export function reindexEntries(
  notes: NoteMeta[],
  separators: Separator[],
  dragId: string,
  targetId: string,
  chapters: Chapter[] = []
): Arrangement | null {
  if (dragId === targetId) return null;
  const entries = mergeEntries(notes, separators, chapters);
  const from = entries.findIndex((e) => e.id === dragId);
  if (from < 0) return null;

  const [moved] = entries.splice(from, 1);
  if (!moved) return null;

  if (targetId === START_ID) entries.unshift(moved);
  else if (targetId === END_ID) entries.push(moved);
  else {
    // Found after the row has been lifted out, so the index is the one in the
    // list the row is being dropped into.
    const to = entries.findIndex((e) => e.id === targetId);
    if (to < 0) return null;
    entries.splice(to + 1, 0, moved);
  }
  return toArrangement(entries);
}

/**
 * The whole list renumbered with one row put at the very top.
 *
 * For a note that has just been made. It used to be given no position at all and
 * left to the tie-break, which put it near the top and moved everything else:
 * a separator's position counts the rows above it, so a row appearing at the top
 * without anybody being renumbered left every line one row too high — the
 * notebook's arrangement came apart a little more with each new note.
 *
 * Renumbering the list settles that, and settles it for good: rows that were
 * never dragged all carried position 0 and were being ordered by how recently
 * they changed, so they slid about on their own. Afterwards each keeps the place
 * it is in — which is what a hand-made order is for.
 */
export function placeOnTop(
  notes: NoteMeta[],
  separators: Separator[],
  chapters: Chapter[],
  ids: string | string[]
): Arrangement | null {
  const wanted = Array.isArray(ids) ? ids : [ids];
  const arriving = wanted
    .map((id) => notes.find((n) => n.id === id))
    .filter((n): n is NoteMeta => !!n);
  if (arriving.length === 0) return null;
  /* Built from the list *without* the new notes, then those put in front.
   *
   * Merging them in first would be measuring the room after the furniture
   * moved: a new row already counts as a row, so every line reads as sitting
   * one place further down than it was drawn, and renumbering would write that
   * misreading back as the truth. */
  const held = new Set(arriving.map((n) => n.id));
  const entries = mergeEntries(
    notes.filter((n) => !held.has(n.id)),
    separators,
    chapters
  );
  entries.unshift(...arriving.map((note) => ({ kind: "note" as const, id: note.id, order: 0, note })));
  return toArrangement(entries);
}

/**
 * The whole list renumbered with one row taken out of it.
 *
 * For a note being thrown away or moved elsewhere. A row leaving is the same
 * problem as a row arriving, in reverse: a separator's position counts the rows
 * above it, so a note deleted above a line left that line one row too *low*, and
 * everything below the line appeared to jump up past it. Nothing here moves
 * except what is leaving.
 *
 * `notes` must still contain the row — it is the list as it is drawn, which is
 * what the lines were counted against.
 */
export function withoutRow(
  notes: NoteMeta[],
  separators: Separator[],
  chapters: Chapter[],
  id: string
): Arrangement | null {
  const entries = mergeEntries(notes, separators, chapters);
  const at = entries.findIndex((e) => e.id === id);
  if (at < 0) return null;
  entries.splice(at, 1);
  return toArrangement(entries);
}

/** Drop target standing for "after everything". */
export const END_ID = "__end__";

/** And its opposite: the one place a drop under a row cannot reach. */
export const START_ID = "__start__";

/** A new separator, added at the top where it can be seen and then dragged. */
export const withNewSeparator = (
  separators: Separator[],
  id: string,
  label = "",
  chapterId: string | null = null
): Separator[] => [{ id, label, order: 0, chapterId }, ...separators];

/**
 * The separators belonging to one list.
 *
 * A separator's `order` counts the notes before it, which only means anything
 * inside the list it was drawn in — the notebook itself, or one of its chapters.
 * Showing all of a notebook's separators in every chapter would put lines in
 * places nobody drew them.
 */
export const separatorsFor = (separators: Separator[], chapterId: string | null): Separator[] =>
  separators.filter((s) => (s.chapterId ?? null) === chapterId);

/**
 * The full notebook renumbered after a note was dragged onto another.
 *
 * Kept for the case with no separators; `reindexEntries` is the general one.
 */
export function reindex(all: NoteMeta[], dragId: string, targetId: string): string[] | null {
  return reindexEntries(all, [], dragId, targetId)?.noteIds ?? null;
}

/**
 * Whether the manual order may be changed right now.
 *
 * Dragging while a search is narrowing the list would reorder against a view the
 * hidden notes are not part of — so it is simply not offered, rather than being
 * offered and then producing an order nobody arranged.
 */
export const canReorder = (mode: SortMode, filtering: boolean) => mode === "manual" && !filtering;