/**
 * Smart-Merge-Diff zwischen zwei Versionen oder zwischen einer Version
 * und der Hash-Liste eines privaten geforkten Decks.
 *
 * Klassifikation:
 *   - **unchanged**: content_hash identisch zwischen alt und neu →
 *     unveränderte Karte, FSRS-State bleibt.
 *   - **added**: Hash nur in neu → komplett neue Karte.
 *   - **removed**: Hash nur in alt → Karte aus dem Deck entfernt.
 *   - **changed** (Heuristik): Karte am selben `ord` ist „added" und
 *     gleichzeitig an demselben `ord` eine „removed". Pull-Requests
 *     bringen später eine echte Karten-Lineage; bis dahin reicht die
 *     Position-Heuristik.
 *
 * Geport aus
 * `cards-decommission-base:services/cards-server/src/services/subscriptions.ts`
 * (`diffSince`-Funktion, ord-Pairing-Heuristik 1:1).
 */

export interface DiffCardForHash {
	contentHash: string;
	ord: number;
}

export interface DiffCardFull {
	contentHash: string;
	type: string;
	fields: Record<string, string>;
	ord: number;
}

export interface DiffPayload {
	from: { semver?: string; versionId?: string };
	to: { semver: string; versionId: string };
	added: DiffCardFull[];
	changed: { previous: { contentHash: string }; next: DiffCardFull }[];
	unchanged: { contentHash: string; ord: number }[];
	removed: { contentHash: string }[];
}

export interface ComputeDiffInput {
	from: DiffCardForHash[];
	to: DiffCardFull[];
	fromInfo: { semver?: string; versionId?: string };
	toInfo: { semver: string; versionId: string };
}

export function computeDiff(input: ComputeDiffInput): DiffPayload {
	const { from, to, fromInfo, toInfo } = input;
	const fromHashes = new Set(from.map((c) => c.contentHash));
	const toHashes = new Set(to.map((c) => c.contentHash));

	const unchanged: { contentHash: string; ord: number }[] = [];

	// Build ord → hash map of removed cards (in `from` but not in `to`).
	const removedByOrd = new Map<number, string>();
	for (const c of from) {
		if (!toHashes.has(c.contentHash)) removedByOrd.set(c.ord, c.contentHash);
	}

	const added: DiffCardFull[] = [];
	const changed: { previous: { contentHash: string }; next: DiffCardFull }[] = [];

	for (const c of to) {
		if (fromHashes.has(c.contentHash)) {
			unchanged.push({ contentHash: c.contentHash, ord: c.ord });
		} else if (removedByOrd.has(c.ord)) {
			const previousHash = removedByOrd.get(c.ord)!;
			removedByOrd.delete(c.ord);
			changed.push({ previous: { contentHash: previousHash }, next: c });
		} else {
			added.push(c);
		}
	}

	// Was in removedByOrd übrig bleibt = echte Removals (nicht ord-paired).
	const removed: { contentHash: string }[] = [...removedByOrd.values()].map((h) => ({
		contentHash: h,
	}));

	return {
		from: fromInfo,
		to: toInfo,
		added,
		changed,
		unchanged,
		removed,
	};
}