import { uuidv7 } from "@earendil-works/pi-ai";
import {
	type LeafEntry,
	SessionError,
	type SessionMetadata,
	type SessionStorage,
	type SessionTreeEntry,
} from "../types.ts";

function updateLabelCache(labelsById: Map<string, string>, entry: SessionTreeEntry): void {
	if (entry.type !== "label") return;
	const label = entry.label?.trim();
	if (label) {
		labelsById.set(entry.targetId, label);
	} else {
		labelsById.delete(entry.targetId);
	}
}

function buildLabelsById(entries: SessionTreeEntry[]): Map<string, string> {
	const labelsById = new Map<string, string>();
	for (const entry of entries) {
		updateLabelCache(labelsById, entry);
	}
	return labelsById;
}

function generateEntryId(byId: { has(id: string): boolean }): string {
	for (let i = 0; i < 100; i++) {
		// The uuidv7 prefix is timestamp-derived and nearly constant between calls,
		// so short ids must come from the random tail.
		const id = uuidv7().slice(-8);
		if (!byId.has(id)) return id;
	}
	return uuidv7();
}

function leafIdAfterEntry(entry: SessionTreeEntry): string | null {
	return entry.type === "leaf" ? entry.targetId : entry.id;
}

export class InMemorySessionStorage<TMetadata extends SessionMetadata = SessionMetadata>
	implements SessionStorage<TMetadata>
{
	private readonly metadata: TMetadata;
	private entries: SessionTreeEntry[];
	private byId: Map<string, SessionTreeEntry>;
	private labelsById: Map<string, string>;
	private leafId: string | null;

	constructor(options?: { entries?: SessionTreeEntry[]; metadata?: TMetadata }) {
		this.entries = options?.entries ? [...options.entries] : [];
		this.byId = new Map(this.entries.map((entry) => [entry.id, entry]));
		this.labelsById = buildLabelsById(this.entries);
		this.leafId = null;
		for (const entry of this.entries) this.leafId = leafIdAfterEntry(entry);
		if (this.leafId !== null && !this.byId.has(this.leafId)) {
			throw new SessionError("invalid_session", `Entry ${this.leafId} not found`);
		}
		this.metadata = options?.metadata ?? ({ id: uuidv7(), createdAt: new Date().toISOString() } as TMetadata);
	}

	async getMetadata(): Promise<TMetadata> {
		return this.metadata;
	}

	async getLeafId(): Promise<string | null> {
		if (this.leafId !== null && !this.byId.has(this.leafId)) {
			throw new SessionError("invalid_session", `Entry ${this.leafId} not found`);
		}
		return this.leafId;
	}

	async setLeafId(leafId: string | null): Promise<void> {
		if (leafId !== null && !this.byId.has(leafId)) {
			throw new SessionError("not_found", `Entry ${leafId} not found`);
		}
		const entry: LeafEntry = {
			type: "leaf",
			id: generateEntryId(this.byId),
			parentId: this.leafId,
			timestamp: new Date().toISOString(),
			targetId: leafId,
		};
		this.entries.push(entry);
		this.byId.set(entry.id, entry);
		this.leafId = leafId;
	}

	async createEntryId(): Promise<string> {
		return generateEntryId(this.byId);
	}

	async appendEntry(entry: SessionTreeEntry): Promise<void> {
		this.entries.push(entry);
		this.byId.set(entry.id, entry);
		updateLabelCache(this.labelsById, entry);
		this.leafId = leafIdAfterEntry(entry);
	}

	async getEntry(id: string): Promise<SessionTreeEntry | undefined> {
		return this.byId.get(id);
	}

	async findEntries<TType extends SessionTreeEntry["type"]>(
		type: TType,
	): Promise<Array<Extract<SessionTreeEntry, { type: TType }>>> {
		return this.entries.filter((entry): entry is Extract<SessionTreeEntry, { type: TType }> => entry.type === type);
	}

	async getLabel(id: string): Promise<string | undefined> {
		return this.labelsById.get(id);
	}

	async getPathToRoot(leafId: string | null): Promise<SessionTreeEntry[]> {
		if (leafId === null) return [];
		const path: SessionTreeEntry[] = [];
		let current = this.byId.get(leafId);
		if (!current) throw new SessionError("not_found", `Entry ${leafId} not found`);
		while (current) {
			path.unshift(current);
			if (!current.parentId) break;
			const parent = this.byId.get(current.parentId);
			if (!parent) throw new SessionError("invalid_session", `Entry ${current.parentId} not found`);
			current = parent;
		}
		return path;
	}

	async getEntries(): Promise<SessionTreeEntry[]> {
		return [...this.entries];
	}
}