export const MUTATION_INTENT_BOUNDS = Object.freeze({
  intentIdMaxBytes: 256,
  operationIdMaxBytes: 256,
  canonicalInputIdentityMaxBytes: 16_384,
  idempotencyKeyMaxBytes: 256,
} as const);

export type MutationIntent = Readonly<{
  intentId: string;
  operationId: string;
  canonicalInputIdentity: string;
  idempotencyKey?: string;
  createdAtMonotonicMs: number;
}>;

const UTF8 = new TextEncoder();

function validBoundedString(value: unknown, maxBytes: number): value is string {
  return (
    typeof value === "string" &&
    value.trim().length > 0 &&
    UTF8.encode(value).byteLength <= maxBytes
  );
}

/**
 * N-06. The single idempotency-key authority shared by the V2 compatibility
 * client and the V3 executor.
 *
 * A caller-supplied value is never trimmed, regenerated or silently dropped:
 * an invalid key is a contract violation, because replaying a keyed command
 * without its key is exactly the unsafe behaviour the key exists to prevent.
 */
export function isValidIdempotencyKey(value: unknown): value is string {
  if (
    !validBoundedString(
      value,
      MUTATION_INTENT_BOUNDS.idempotencyKeyMaxBytes,
    )
  ) {
    return false;
  }
  for (const character of value) {
    const codePoint = character.codePointAt(0) ?? 0;
    if (codePoint <= 0x1f || (codePoint >= 0x7f && codePoint <= 0x9f)) {
      return false;
    }
  }
  return true;
}

export function defineIdempotencyKey(value: unknown): string {
  if (!isValidIdempotencyKey(value)) {
    throw new TypeError("Idempotency key is invalid.");
  }
  return value;
}

export function defineMutationIntent(intent: MutationIntent): MutationIntent {
  if (
    !validBoundedString(
      intent.intentId,
      MUTATION_INTENT_BOUNDS.intentIdMaxBytes,
    ) ||
    !validBoundedString(
      intent.operationId,
      MUTATION_INTENT_BOUNDS.operationIdMaxBytes,
    ) ||
    !validBoundedString(
      intent.canonicalInputIdentity,
      MUTATION_INTENT_BOUNDS.canonicalInputIdentityMaxBytes,
    ) ||
    // OPT-NET-02. Intent definition and executor admission share one key
    // authority; a second, looser rule here is how a control character reaches
    // an `Idempotency-Key` header.
    (intent.idempotencyKey !== undefined &&
      !isValidIdempotencyKey(intent.idempotencyKey)) ||
    !Number.isFinite(intent.createdAtMonotonicMs) ||
    intent.createdAtMonotonicMs < 0
  ) {
    throw new TypeError("Mutation intent is invalid.");
  }
  return Object.freeze({
    intentId: intent.intentId,
    operationId: intent.operationId,
    canonicalInputIdentity: intent.canonicalInputIdentity,
    ...(intent.idempotencyKey === undefined
      ? {}
      : { idempotencyKey: intent.idempotencyKey }),
    createdAtMonotonicMs: intent.createdAtMonotonicMs,
  });
}