import * as Automerge from '@automerge/automerge';
import * as diffLib from 'diff';

/**
 * Character-level CRDT text utilities, ported from pushwork
 * (`pushwork/src/utils/text-diff.ts`) to keep Automerge document behaviour —
 * and therefore the on-the-wire change ops — matching pushwork as closely as
 * possible.
 */

/**
 * Read content from an Automerge document field, normalizing legacy
 * ImmutableString values to plain strings for backwards compatibility.
 *
 * Returns `string | Uint8Array | null` (binary content is passed through).
 */
export function readDocContent(content: unknown): string | Uint8Array | null {
  if (content == null) return null;
  if (typeof content === 'string') return content;
  if (content instanceof Uint8Array) return content;
  // Legacy ImmutableString — convert to plain string.
  if (Automerge.isImmutableString(content)) return content.toString();
  return null;
}

/**
 * Update text content on an Automerge document property inside a change
 * callback.
 *
 * If the existing value is already a collaborative text string, we diff and
 * splice for minimal CRDT operations. If the existing value is a legacy
 * ImmutableString (or undefined) we can't splice into it, so we assign the
 * whole string, which converts the field to a collaborative text CRDT.
 */
export function updateTextContent(
  // eslint-disable-next-line @typescript-eslint/no-explicit-any
  doc: any,
  path: Automerge.Prop[],
  newContent: string,
): void {
  // eslint-disable-next-line @typescript-eslint/no-explicit-any
  const target = path.reduce((obj: any, key) => obj?.[key], doc);

  if (typeof target === 'string') {
    // Already a collaborative text string — diff and splice.
    spliceText(doc, path, target, newContent);
  } else {
    // Legacy ImmutableString, undefined, or other — assign directly.
    // This converts the field to a collaborative text CRDT.
    // eslint-disable-next-line @typescript-eslint/no-explicit-any
    let obj: any = doc;
    for (let i = 0; i < path.length - 1; i++) {
      obj = obj[path[i]!];
    }
    obj[path[path.length - 1]!] = newContent;
  }
}

/**
 * Apply a character-level diff between `oldContent` and `newContent` as
 * Automerge splice operations on the given property path, preserving the
 * collaborative text CRDT structure.
 */
export function spliceText(
  // eslint-disable-next-line @typescript-eslint/no-explicit-any
  doc: any,
  path: Automerge.Prop[],
  oldContent: string,
  newContent: string,
): void {
  if (oldContent === newContent) return;

  // Fast path: old empty — insert everything.
  if (oldContent === '') {
    Automerge.splice(doc, path, 0, 0, newContent);
    return;
  }

  // Fast path: new empty — delete everything.
  if (newContent === '') {
    Automerge.splice(doc, path, 0, oldContent.length);
    return;
  }

  const changes = diffLib.diffChars(oldContent, newContent);

  let pos = 0;
  for (const part of changes) {
    if (part.removed) {
      Automerge.splice(doc, path, pos, part.value.length);
      // Don't advance pos — text shifted left after deletion.
    } else if (part.added) {
      Automerge.splice(doc, path, pos, 0, part.value);
      pos += part.value.length;
    } else {
      // Unchanged — advance the cursor.
      pos += part.value.length;
    }
  }
}