import React from 'react';
import { useUiLayouts } from '@/lib/krowHooks';
import { useSkillSections } from '@/components/skills/SkillSurface';
import { skillNodesByPlacement } from '@/lib/ui/skillNodes';
import { composePage } from '@/lib/ui/composition';
import { applyOperation } from '@/lib/ui/operations';
import { clearOps, emptyPatch, popOp, pushOp } from '@/lib/ui/patch';
import { nodeRegistry } from '@/lib/ui/registry';

/**
 * Preview, Apply, and the line between them.
 *
 * Three states of a UI change live here, and keeping them apart is the whole
 * job of this module:
 *
 *   - **Preview** is in React state. It is what the person is trying. It never
 *     reaches the network, and a reload discards it.
 *   - **Saved** is the person's own stored operation list, read from and
 *     written to their account preferences. It survives a reload and a new
 *     session.
 *   - **Source** — the page's registered composition and the Markdown skill
 *     files — is never written by anything here. Not on preview, not on apply.
 *
 * A proposed operation is validated *before* it becomes a preview, so a change
 * that could not be saved is never shown as though it could. Preview and apply
 * then run the identical merge through `composePage` and the identical
 * renderer, which is what makes a preview honest: there is no second code path
 * for the applied state that could disagree with it.
 */

const UiEditingContext = React.createContext(null);

/** The editing session for the page around it. Null outside a provider. */
export const useUiEditing = () => React.useContext(UiEditingContext);

export function UiEditingProvider({ page, role = null, registry = nodeRegistry, children }) {
  const { layouts, save, saving } = useUiLayouts();

  /**
   * What the person is trying, not yet theirs.
   *
   * Held per page so that navigating away and back does not carry an
   * unfinished experiment onto a different surface.
   */
  const [preview, setPreview] = React.useState(() => emptyPatch(page));
  const [problems, setProblems] = React.useState([]);

  /* A page change is a new editing session. Anything unsaved was about the page
     that is no longer on screen. */
  React.useEffect(() => {
    setPreview(emptyPatch(page));
    setProblems([]);
  }, [page]);

  const saved = layouts[page] || emptyPatch(page);

  /**
   * What this page's definitions contribute, as nodes.
   *
   * Resolved here because it needs the account's custom skills and disabled
   * list, and handed to `composePage` so that module stays a pure function.
   * The result is that a Board skill's card is a node in the same tree as the
   * page's own sections — addressable by the same operations, and hidden or
   * moved by a patch rather than by editing the definition.
   */
  const sections = useSkillSections(page);
  const skillNodes = React.useMemo(() => skillNodesByPlacement(sections), [sections]);

  /**
   * The tree on screen: what the application ships, with what the person saved,
   * with what they are trying, in that order.
   */
  const composed = React.useMemo(
    () => composePage(page, { patch: saved, preview, registry, role, skillNodes }),
    [page, saved, preview, registry, role, skillNodes]
  );

  /**
   * Try an operation.
   *
   * Validated against the tree as it currently stands — saved changes included
   * — so an operation is judged against what the person is actually looking at.
   * A refusal returns the reasons and changes nothing; there is no partially
   * applied preview.
   */
  const propose = React.useCallback((op) => {
    const result = applyOperation(composed.tree, op, { registry, role });
    if (!result.ok) {
      setProblems(result.problems);
      return result;
    }
    setProblems([]);
    setPreview((current) => pushOp(current, op));
    return result;
  }, [composed.tree, registry, role]);

  /** Throw the experiment away. Nothing was stored, so nothing is undone. */
  const discard = React.useCallback(() => {
    setPreview(emptyPatch(page));
    setProblems([]);
  }, [page]);

  /**
   * Keep it.
   *
   * The preview's operations are appended to what was already saved and written
   * as one list. The preview is only cleared once the write resolves, so a
   * failed save leaves the person looking at the change they asked for rather
   * than watching it disappear with an error beside it.
   */
  const apply = React.useCallback(async () => {
    if (!preview.ops.length) return { ok: true, saved };
    const next = { ...saved, page, ops: [...saved.ops, ...preview.ops], updatedAt: new Date().toISOString() };
    const result = await save(page, next);
    if (result?.persisted === false) {
      setProblems([{ at: null, message: result.error || 'That change could not be saved.' }]);
      return { ok: false, saved };
    }
    setPreview(emptyPatch(page));
    return { ok: true, saved: next };
  }, [preview, saved, page, save]);

  /**
   * Undo one step.
   *
   * The most recent thing first: an unsaved operation if there is one, and only
   * then a saved one. Undoing a saved change is a write, because the saved list
   * is the record of what the person chose.
   */
  const undo = React.useCallback(async () => {
    if (preview.ops.length) {
      setPreview((current) => popOp(current));
      return { ok: true };
    }
    if (!saved.ops.length) return { ok: true };
    await save(page, popOp(saved));
    return { ok: true };
  }, [preview, saved, page, save]);

  /** Back to the page as the application ships it. Clears both tiers. */
  const reset = React.useCallback(async () => {
    setPreview(emptyPatch(page));
    setProblems([]);
    if (saved.ops.length) await save(page, clearOps(saved));
    return { ok: true };
  }, [page, saved, save]);

  const value = React.useMemo(() => ({
    page,
    tree: composed.tree,
    /* Operations that no longer apply — a saved change naming a section a
       release has since removed. Surfaced so a page can say so quietly rather
       than leaving the person wondering why nothing happened. */
    skipped: composed.skipped,
    saved,
    preview,
    problems,
    previewing: preview.ops.length > 0,
    customised: saved.ops.length > 0,
    saving,
    propose,
    discard,
    apply,
    undo,
    reset,
  }), [
    page, composed, saved, preview, problems, saving, propose, discard, apply, undo, reset,
  ]);

  return <UiEditingContext.Provider value={value}>{children}</UiEditingContext.Provider>;
}