/**
 * Cross-context cache invalidation is a best-effort hint protocol. The wire
 * event intentionally carries neither cached data nor a concrete query key.
 * Receivers resolve the allowlisted topic through their local policy.
 */
export const CACHE_INVALIDATION_PROTOCOL_VERSION = 1 as const;

export const CACHE_INVALIDATION_WIRE_LIMITS = Object.freeze({
  maxWireBytes: 2_048,
  maxOpaqueIdentifierLength: 128,
  maxTopicLength: 64,
  maxEventTtlMs: 5 * 60 * 1_000,
  maxFutureClockSkewMs: 30_000,
});

export type CacheInvalidationTopicDefinition = Readonly<{
  topic: string;
  topicVersion: number;
}>;

export type CacheInvalidationWireEvent = Readonly<{
  protocolVersion: typeof CACHE_INVALIDATION_PROTOCOL_VERSION;
  eventId: string;
  sourceId: string;
  sourceEpoch: string;
  sequence: number;
  cacheEpoch: string;
  topic: string;
  topicVersion: number;
  emittedAt: number;
  expiresAt: number;
}>;

export type CacheInvalidationParseFailureReason =
  | "CACHE_EPOCH_MISMATCH"
  | "EXPIRED"
  | "INVALID_ENVELOPE"
  | "MALFORMED_JSON"
  | "OVERSIZED"
  | "PROTOCOL_MISMATCH"
  | "TOPIC_REJECTED";

export type CacheInvalidationParseResult =
  | Readonly<{ ok: true; value: CacheInvalidationWireEvent }>
  | Readonly<{
      ok: false;
      reason: CacheInvalidationParseFailureReason;
    }>;

export type CacheInvalidationParsePolicy = Readonly<{
  cacheEpoch: string;
  topicVersions: Readonly<Record<string, number>>;
  nowEpochMilliseconds: number;
}>;

const WIRE_KEYS = Object.freeze([
  "cacheEpoch",
  "emittedAt",
  "eventId",
  "expiresAt",
  "protocolVersion",
  "sequence",
  "sourceEpoch",
  "sourceId",
  "topic",
  "topicVersion",
] as const);

const OPAQUE_IDENTIFIER = /^[A-Za-z0-9][A-Za-z0-9._:-]*$/u;
const TOPIC = /^[a-z][a-z0-9.-]*$/u;

export function isCacheInvalidationOpaqueIdentifier(
  value: unknown,
): value is string {
  return (
    typeof value === "string" &&
    value.length >= 1 &&
    value.length <=
      CACHE_INVALIDATION_WIRE_LIMITS.maxOpaqueIdentifierLength &&
    OPAQUE_IDENTIFIER.test(value)
  );
}

export function isCacheInvalidationTopic(
  value: unknown,
): value is string {
  return (
    typeof value === "string" &&
    value.length >= 1 &&
    value.length <= CACHE_INVALIDATION_WIRE_LIMITS.maxTopicLength &&
    TOPIC.test(value)
  );
}

export function cacheInvalidationWireByteLength(value: string): number {
  return new TextEncoder().encode(value).byteLength;
}

export function decodeCacheInvalidationWireEvent(
  raw: string,
  policy: CacheInvalidationParsePolicy,
): CacheInvalidationParseResult {
  if (
    typeof raw !== "string" ||
    raw.length > CACHE_INVALIDATION_WIRE_LIMITS.maxWireBytes ||
    cacheInvalidationWireByteLength(raw) >
      CACHE_INVALIDATION_WIRE_LIMITS.maxWireBytes
  ) {
    return failure("OVERSIZED");
  }

  let parsed: unknown;
  try {
    parsed = JSON.parse(raw);
  } catch {
    return failure("MALFORMED_JSON");
  }
  return parseCacheInvalidationWireEvent(parsed, policy);
}

export function parseCacheInvalidationWireEvent(
  input: unknown,
  policy: CacheInvalidationParsePolicy,
): CacheInvalidationParseResult {
  try {
    return parseCacheInvalidationWireEventUnsafe(input, policy);
  } catch {
    return failure("INVALID_ENVELOPE");
  }
}

function parseCacheInvalidationWireEventUnsafe(
  input: unknown,
  policy: CacheInvalidationParsePolicy,
): CacheInvalidationParseResult {
  if (!isExactWireRecord(input)) {
    return failure("INVALID_ENVELOPE");
  }

  let serialized: string;
  try {
    serialized = JSON.stringify(input);
  } catch {
    return failure("INVALID_ENVELOPE");
  }
  if (
    cacheInvalidationWireByteLength(serialized) >
    CACHE_INVALIDATION_WIRE_LIMITS.maxWireBytes
  ) {
    return failure("OVERSIZED");
  }

  if (input.protocolVersion !== CACHE_INVALIDATION_PROTOCOL_VERSION) {
    return failure("PROTOCOL_MISMATCH");
  }
  if (
    !isCacheInvalidationOpaqueIdentifier(input.eventId) ||
    !isCacheInvalidationOpaqueIdentifier(input.sourceId) ||
    !isCacheInvalidationOpaqueIdentifier(input.sourceEpoch) ||
    !isCacheInvalidationOpaqueIdentifier(input.cacheEpoch) ||
    !isCacheInvalidationTopic(input.topic) ||
    !isPositiveSafeInteger(input.sequence) ||
    !isPositiveSafeInteger(input.topicVersion) ||
    !isEpochMilliseconds(input.emittedAt) ||
    !isEpochMilliseconds(input.expiresAt) ||
    input.expiresAt <= input.emittedAt ||
    input.expiresAt - input.emittedAt >
      CACHE_INVALIDATION_WIRE_LIMITS.maxEventTtlMs ||
    !isEpochMilliseconds(policy.nowEpochMilliseconds)
  ) {
    return failure("INVALID_ENVELOPE");
  }
  if (input.cacheEpoch !== policy.cacheEpoch) {
    return failure("CACHE_EPOCH_MISMATCH");
  }
  if (
    !Object.hasOwn(policy.topicVersions, input.topic) ||
    policy.topicVersions[input.topic] !== input.topicVersion
  ) {
    return failure("TOPIC_REJECTED");
  }
  if (
    input.expiresAt <= policy.nowEpochMilliseconds ||
    input.emittedAt >
      policy.nowEpochMilliseconds +
        CACHE_INVALIDATION_WIRE_LIMITS.maxFutureClockSkewMs
  ) {
    return failure("EXPIRED");
  }

  return {
    ok: true,
    value: Object.freeze({
      protocolVersion: CACHE_INVALIDATION_PROTOCOL_VERSION,
      eventId: input.eventId,
      sourceId: input.sourceId,
      sourceEpoch: input.sourceEpoch,
      sequence: input.sequence,
      cacheEpoch: input.cacheEpoch,
      topic: input.topic,
      topicVersion: input.topicVersion,
      emittedAt: input.emittedAt,
      expiresAt: input.expiresAt,
    }),
  };
}

function isExactWireRecord(
  value: unknown,
): value is Readonly<Record<(typeof WIRE_KEYS)[number], unknown>> {
  if (!value || typeof value !== "object" || Array.isArray(value)) {
    return false;
  }
  const keys = Object.keys(value).sort();
  return (
    keys.length === WIRE_KEYS.length &&
    keys.every((key, index) => key === WIRE_KEYS[index])
  );
}

function isPositiveSafeInteger(value: unknown): value is number {
  return (
    typeof value === "number" &&
    Number.isSafeInteger(value) &&
    value >= 1
  );
}

function isEpochMilliseconds(value: unknown): value is number {
  return (
    typeof value === "number" &&
    Number.isSafeInteger(value) &&
    value >= 0
  );
}

function failure(
  reason: CacheInvalidationParseFailureReason,
): Extract<CacheInvalidationParseResult, { ok: false }> {
  return Object.freeze({ ok: false, reason });
}