/**
 * Validation for the PIN settings form.
 *
 * Pure, and separate from the component, so the rules can be tested without
 * mounting anything — the same reason `episodeStrip.ts` exists.
 *
 * This is *not* the security boundary. Rust validates the PIN shape again in
 * `pin::validate_pin` and is the only thing that ever compares one. What lives
 * here is the difference between a form that tells you what is wrong before you
 * submit and one that bounces you off a backend error.
 *
 * TRACES: UR-083 | DR-268, DR-276
 */

/** Mirrors `pin::MAX_ATTEMPTS`-adjacent shape rules in Rust. */
export const PIN_MIN_LENGTH = 4;
export const PIN_MAX_LENGTH = 8;

export type PinIntent = "set" | "change" | "clear";

export interface PinFormState {
  intent: PinIntent;
  /** Whether the profile currently has a PIN — decides if `currentPin` is required. */
  hasPin: boolean;
  currentPin: string;
  newPin: string;
  confirmPin: string;
}

/**
 * The reason this form cannot be submitted yet, or `null` when it can.
 *
 * Returns one message rather than a list: a PIN form has at most one thing
 * wrong with it worth saying, and stacking "too short" under "doesn't match"
 * reads as nagging.
 *
 * TRACES: UR-083 | DR-276
 */
export function validatePinForm(state: PinFormState): string | null {
  if (state.hasPin && state.currentPin.length === 0) {
    return "Enter your current PIN.";
  }

  if (state.intent === "clear") {
    return null;
  }

  if (state.newPin.length === 0) {
    return "Choose a PIN.";
  }
  if (!/^[0-9]+$/.test(state.newPin)) {
    return "A PIN can only contain digits.";
  }
  if (state.newPin.length < PIN_MIN_LENGTH || state.newPin.length > PIN_MAX_LENGTH) {
    return `A PIN must be ${PIN_MIN_LENGTH}–${PIN_MAX_LENGTH} digits.`;
  }
  if (state.confirmPin.length === 0) {
    return "Confirm your new PIN.";
  }
  if (state.newPin !== state.confirmPin) {
    return "The two PINs do not match.";
  }
  if (state.intent === "change" && state.currentPin === state.newPin) {
    return "The new PIN is the same as the current one.";
  }

  return null;
}

/**
 * What the button should say. Derived rather than hardcoded per branch so the
 * three intents cannot drift apart in the markup.
 *
 * TRACES: UR-083 | DR-276
 */
export function pinActionLabel(intent: PinIntent): string {
  switch (intent) {
    case "set":
      return "Set PIN";
    case "change":
      return "Change PIN";
    case "clear":
      return "Turn off PIN";
  }
}