/**
 * LWW conflict resolution for the non-CRDT (REST) fields — TASK-0095 R1.
 *
 * The doc §5.5 "last-writer-wins" split (the FINAL rule, recorded against the
 * doc's plain LWW):
 *   - the PAGE TITLE (a CRDT-free field) = **LWW-with-consent** (strictly
 *     safer than the doc's plain LWW): when a title PATCH is replayed after a
 *     remote title change (the REST CAS 409 — the remote write won the field),
 *     the client does NOT silently overwrite. It refetches, shows a toast
 *     ("Title was changed by Ana — your change 'X' was not applied") + an
 *     "Apply anyway" action. Applying sends a fresh PATCH — an explicit user
 *     choice. (The doc says "LWW + a toast"; the consent variant still toasts
 *     and is strictly safer — recorded.)
 *   - a DATABASE PROPERTY VALUE (a record cell) = **plain LWW**: the newer
 *     timestamp wins, the toast says "Updated to the latest value" (no consent
 *     — a record cell is low-stakes and the doc's LWW is the right default).
 *
 * The two-field split is the load-bearing decision: the same 409 means
 * "ask the user" for a title but "apply silently (newer wins)" for a property.
 * This module is PURE (no UI, no fetch, no store) — the outbox worker + the
 * web UI call it. AGENTS rule 23 (domain logic independent from the UI).
 */

/** A field value + its write timestamp (+ who wrote it, for the toast). */
export interface FieldVersion {
  readonly value: string;
  /** The write's client/server timestamp (ms). */
  readonly ts: number;
  /** The remote author's display name (the toast's "changed by Ana"). */
  readonly author?: string;
}

export type ConflictField = "title" | "property";

/** The outcome of a 409 on a non-CRDT field (the outbox worker/UI act on it). */
export type LwwOutcome =
  /** Title: do NOT auto-apply. The UI refetches + toasts + offers "Apply anyway". */
  | {
      readonly kind: "consent";
      /** The toast text ("Title was changed by Ana — your change 'X' was not applied"). */
      readonly toast: string;
      /** The apply action (a fresh PATCH — the user's explicit choice). */
      readonly applyAction: "fresh-patch";
      /** The local value the user's change carried (the apply re-sends it). */
      readonly localValue: string;
    }
  /** Property (LWW, the local write is newer or tied): keep the local value. */
  | {
      readonly kind: "lww-local-wins";
      readonly value: string;
    }
  /** Property (LWW, the remote write is newer): take the remote value + toast. */
  | {
      readonly kind: "lww-remote-wins";
      readonly value: string;
      readonly toast: string;
    };

/**
 * Compare two write timestamps (the plain-LWW core). The LATER write wins; a
 * TIE resolves to the LOCAL write (the local edit is the user's in-front-of-
 * them intent — the conservative default). Returns the winning side.
 */
export function compareLwwTimestamps(localTs: number, remoteTs: number): "local" | "remote" {
  return localTs >= remoteTs ? "local" : "remote";
}

/**
 * Resolve a 409 on a non-CRDT field (the two-field split). `local` is the
 * user's in-flight write (the one the outbox is replaying); `remote` is the
 * current server state (the 409's fresh value — the one that won the CAS).
 */
export function resolveFieldConflict(
  field: ConflictField,
  local: FieldVersion,
  remote: FieldVersion,
): LwwOutcome {
  if (field === "title") {
    // Title = LWW-with-consent (NEVER auto-overwrite). The 409 means the
    // remote write won the field; the local ts does not auto-apply — the user
    // decides via the "Apply anyway" action.
    return {
      kind: "consent",
      toast: `Title was changed by ${remote.author ?? "someone"} — your change '${local.value}' was not applied`,
      applyAction: "fresh-patch",
      localValue: local.value,
    };
  }
  // Property = plain LWW: the newer timestamp wins.
  if (compareLwwTimestamps(local.ts, remote.ts) === "local") {
    return { kind: "lww-local-wins", value: local.value };
  }
  return { kind: "lww-remote-wins", value: remote.value, toast: "Updated to the latest value" };
}