export const SDK_STATE_VERSION = 1;

// The session-index snapshot carries its own format version, independent of the shared
// SDK_STATE_VERSION used by discovery, the lifecycle ledger, and per-event records.
// Version 2 snapshots relax the on-disk schema to allow gaps in indexSeq (compaction
// drops terminal+dead sessions and superseded heartbeats), so an older broker must
// hard-fail rather than silently fall back to a truncated log. Version 3 snapshots may
// additionally carry composite-identity events (hostIncarnation, activity checkpoints,
// session_deleted tombstones) and retention-compacted sessions; v2 (and v1) snapshots
// remain readable. Bumping only this constant keeps the broker protocol untouched.
export const SESSION_INDEX_SNAPSHOT_VERSION = 3;

export class UnsupportedStateVersionError extends Error {
	readonly code = "unsupported_state_version";

	constructor(
		readonly file: string,
		readonly version: number,
		readonly maximumSupportedVersion = SDK_STATE_VERSION,
	) {
		super(
			`Unsupported SDK state version ${version} in ${file}; maximum supported version is ${maximumSupportedVersion}.`,
		);
		this.name = "UnsupportedStateVersionError";
	}
}

export function assertSupportedStateVersion(file: string, value: unknown): void {
	if (!value || typeof value !== "object") return;
	const record = value as { version?: unknown; stateVersion?: unknown };
	for (const version of [record.version, record.stateVersion]) {
		if (typeof version === "number" && Number.isFinite(version) && version > SDK_STATE_VERSION) {
			throw new UnsupportedStateVersionError(file, version);
		}
	}
}

// Fences the session-index snapshot format independently of SDK_STATE_VERSION. An older
// broker (whose SESSION_INDEX_SNAPSHOT_VERSION is below 3) trips assertSupportedStateVersion
// on a version-3 snapshot and refuses to start; a current broker accepts the legacy
// contiguous format (version 1), the gapped version-2 format, and the version-3 format
// with composite-identity events.
export function assertSupportedSnapshotVersion(file: string, value: unknown): void {
	if (!value || typeof value !== "object") return;
	const record = value as { version?: unknown; stateVersion?: unknown };
	for (const version of [record.version, record.stateVersion]) {
		if (typeof version === "number" && Number.isFinite(version) && version > SESSION_INDEX_SNAPSHOT_VERSION) {
			throw new UnsupportedStateVersionError(file, version, SESSION_INDEX_SNAPSHOT_VERSION);
		}
	}
}