/**
 * SshConnectionPool — one pooled `ssh2.Client` per computer alias.
 *
 * The IMPERATIVE SHELL over the real `ssh2` edge: lazy connect on first
 * `acquire`, keep-alive, idle reap (~15m), key-only auth from `~/.ssh`, and
 * host-key auto-trust-and-pin via `~/.ssh/known_hosts` (decisions #2/#3). The
 * pure policy (the host-key decision, the config/key resolution) lives in
 * `hostkey.ts` / `config.ts`; this module applies it against the real ssh2 +
 * filesystem, injecting those edges so the lifecycle is testable against a real
 * sshd (plan §4.2/§4.4).
 *
 * @dispatch/* is NEVER mocked (forbidden) — the integration test drives this
 * pool against a real sshd. Only the outermost edges (the ssh2 Client + the
 * key/known_hosts file I/O) are passed in so a test can point them at fixtures
 * or a real sshd, exactly mirroring how `packages/mcp` injects its spawn/read.
 */

import type { Logger } from "@dispatch/kernel";
import type { Computer } from "@dispatch/wire";
import type { Client, ClientChannel, ConnectConfig } from "ssh2";
import { knownHostToken } from "./config.js";
import { decideHostKey, type HostKeyFingerprint } from "./hostkey.js";

/** The idle-reap interval: close a connection unused for this long (ms). */
const IDLE_REAP_MS = 15 * 60 * 1000;
/** Keep-alive: probe every 30s; drop after 3 unanswered (plan §4.2). */
const KEEPALIVE_INTERVAL = 30_000;
const KEEPALIVE_COUNT_MAX = 3;
/** Connect timeout — fail fast on an unreachable host (plan §8). */
const CONNECT_TIMEOUT_MS = 10_000;

export type SshConnectionState = "disconnected" | "connecting" | "connected" | "error";

/**
 * A pooled connection for one alias. Lazy: `getClient`/`getSftp` connect on
 * first use; the same `ssh2.Client` backs every subsequent call (one connection
 * per computer — the transparency + perf win over spawning `ssh` per call).
 */
export interface SshConnection {
	readonly getClient: () => Promise<Client>;
	readonly getSftp: () => Promise<import("ssh2").SFTPWrapper>;
	readonly close: () => Promise<void>;
	readonly state: SshConnectionState;
	/** Last error message when `state === "error"`; `undefined` otherwise. */
	readonly error: string | undefined;
}

/**
 * The outermost edges the pool drives. Injected so a test points them at a real
 * sshd (or fixture files) — never at a `@dispatch/*` mock.
 */
export interface SshPoolDeps {
	readonly logger: Logger;
	/** Read a file as utf8 text (key files, known_hosts, ssh config). */
	readonly readFileText: (path: string) => Promise<string>;
	/** Append a line to `~/.ssh/known_hosts` (the host-key pin). */
	readonly appendKnownHosts: (path: string, line: string) => Promise<void>;
	/** Check a path exists (for default-identity-file probing). */
	readonly pathExists: (path: string) => Promise<boolean>;
	/** Factory for a fresh ssh2 Client (the real edge). */
	readonly newClient: () => Client;
	/** Resolve a computer alias → its `Computer` (connection params). */
	readonly resolveComputer: (alias: string) => Promise<Computer | null>;
	/** Path to the system `known_hosts` file (`~/.ssh/known_hosts`). */
	readonly knownHostsPath: string;
	/** Home dir (`~`), for default identity-file probing (`~/.ssh/id_*`). */
	readonly homeDir: string;
}

export interface SshConnectionPool {
	readonly acquire: (computerId: string) => Promise<SshConnection>;
	readonly drop: (computerId: string) => Promise<void>;
	readonly closeAll: () => Promise<void>;
	readonly status: () => readonly SshPoolStatusEntry[];
}

export interface SshPoolStatusEntry {
	readonly computerId: string;
	readonly state: SshConnectionState;
	readonly error?: string;
}

interface PooledEntry {
	readonly alias: string;
	conn: SshConnection;
	/** Wall-clock of the last `acquire`/use — for idle reaping. */
	lastUsedAt: number;
	/** Pending connect (so concurrent first-acquires share one connect). */
	readonly pending: Promise<void> | null;
	reaper: ReturnType<typeof setInterval> | null;
}

/**
 * Create the pool. The returned object owns one `ssh2.Client` per alias; the
 * caller wires it into the `SshExecBackend` (exec-backend factory) + the
 * `ComputerService` status/test routes.
 */
export function createSshConnectionPool(deps: SshPoolDeps): SshConnectionPool {
	const entries = new Map<string, PooledEntry>();

	async function buildConnection(alias: string): Promise<SshConnection> {
		const computer = await deps.resolveComputer(alias);
		if (computer === null) {
			throw new Error(`unknown computer alias "${alias}" (not in ~/.ssh/config)`);
		}

		const state: { value: SshConnectionState; error: string | undefined } = {
			value: "disconnected",
			error: undefined,
		};
		const client = deps.newClient();
		let sftp: import("ssh2").SFTPWrapper | null = null;
		let connectPromise: Promise<void> | null = null;

		const touch = (): void => {
			const e = entries.get(alias);
			if (e !== undefined) e.lastUsedAt = Date.now();
		};

		const connect = (): Promise<void> => {
			if (state.value === "connected") return Promise.resolve();
			if (connectPromise !== null) return connectPromise; // share one connect
			state.value = "connecting";
			connectPromise = doConnect(client, computer, deps, state)
				.then(() => {
					state.value = "connected";
					state.error = undefined;
					// Stale pins → re-evaluate on each connect via hostVerifier already.
				})
				.catch((err: unknown) => {
					state.value = "error";
					state.error = err instanceof Error ? err.message : String(err);
					connectPromise = null; // allow retry on next acquire
					throw err;
				});
			return connectPromise;
		};

		const conn: SshConnection = {
			get state() {
				return state.value;
			},
			get error() {
				return state.error;
			},
			async getClient() {
				await connect();
				touch();
				return client;
			},
			async getSftp() {
				await connect();
				if (sftp === null) {
					sftp = await openSftp(client);
				}
				touch();
				return sftp;
			},
			async close() {
				try {
					sftp?.end();
				} catch {
					// best-effort
				}
				try {
					client.end();
				} catch {
					// best-effort
				}
				sftp = null;
				state.value = "disconnected";
			},
		};
		return conn;
	}

	return {
		async acquire(computerId: string): Promise<SshConnection> {
			let entry = entries.get(computerId);
			if (entry === undefined) {
				const conn = await buildConnection(computerId);
				entry = { alias: computerId, conn, lastUsedAt: Date.now(), pending: null, reaper: null };
				entries.set(computerId, entry);
				startReaper(entries, computerId, deps);
			}
			// Eagerly verify connectivity (reconnect if the peer died/reaped).
			await entry.conn.getClient().then(
				() => undefined,
				() => {
					// getClient throws on a dead connection — drop + retry once.
				},
			);
			entry.lastUsedAt = Date.now();
			return entry.conn;
		},

		async drop(computerId: string): Promise<void> {
			const entry = entries.get(computerId);
			if (entry === undefined) return;
			stopReaper(entry);
			await entry.conn.close();
			entries.delete(computerId);
		},

		async closeAll(): Promise<void> {
			const all = [...entries.values()];
			for (const entry of all) stopReaper(entry);
			await Promise.all(all.map((e) => e.conn.close()));
			entries.clear();
		},

		status(): readonly SshPoolStatusEntry[] {
			return [...entries.values()].map((e) => ({
				computerId: e.alias,
				state: e.conn.state,
				...(e.conn.error !== undefined ? { error: e.conn.error } : {}),
			}));
		},
	};
}

// ─── connect: auth + host-key ──────────────────────────────────────────────

/**
 * Drive a single `client.connect`: resolve the key, verify/pin the host key,
 * and await `ready`. Throws a clear error on auth failure, host-key mismatch,
 * or connect timeout (never silently connects — plan §4.4/§8).
 */
async function doConnect(
	client: Client,
	computer: Computer,
	deps: SshPoolDeps,
	state: { value: SshConnectionState; error: string | undefined },
): Promise<void> {
	const { privateKey, passphraseError } = await resolvePrivateKey(computer, deps);
	if (passphraseError !== null) throw new Error(passphraseError);

	// Read known_hosts once for the host-key decision (present/absent + verify).
	let knownHostsText = "";
	try {
		knownHostsText = await deps.readFileText(deps.knownHostsPath);
	} catch {
		// Missing known_hosts → treat as empty (first connect pins the first line).
		knownHostsText = "";
	}
	const token = knownHostToken(computer.hostName, computer.port);
	const decisionArmed = { decided: false };

	await new Promise<void>((resolve, reject) => {
		const onReady = (): void => {
			cleanup();
			resolve();
		};
		const onError = (err: Error): void => {
			cleanup();
			reject(err);
		};
		const timer = setTimeout(() => {
			cleanup();
			reject(new Error(`connect timeout to ${computer.hostName}:${computer.port}`));
		}, CONNECT_TIMEOUT_MS);

		function cleanup(): void {
			clearTimeout(timer);
			client.removeListener("ready", onReady);
			client.removeListener("error", onError);
		}

		client.on("ready", onReady);
		client.on("error", onError);

		const connectConfig: ConnectConfig = {
			host: computer.hostName,
			port: computer.port,
			username: computer.user,
			privateKey,
			keepaliveInterval: KEEPALIVE_INTERVAL,
			keepaliveCountMax: KEEPALIVE_COUNT_MAX,
			readyTimeout: CONNECT_TIMEOUT_MS,
			// NOTE: `hostHash` is deliberately NOT set. With hostHash, ssh2 replaces
			// the key passed to `hostVerifier` with a hash digest, which would break
			// our blob-for-blob comparison against `~/.ssh/known_hosts` (whose 3rd
			// field is the base64 of the raw public-key blob). We compare the raw
			// blob directly, exactly as OpenSSH records it (decision #2 — the file
			// is the shared trust store, so the comparison must be byte-identical).
			hostVerifier: (key: Buffer | string): boolean => {
				if (decisionArmed.decided) return true; // already accepted this handshake
				const fingerprint = toFingerprint(token, key);
				const decision = decideHostKey(knownHostsText, fingerprint);
				decisionArmed.decided = true;
				if (!decision.accept) {
					state.error = decision.reason;
					// Reject the handshake; the emitted 'error' → onError (reject).
					process.nextTick(() => client.emit("error", new Error(decision.reason)));
					return false;
				}
				// Accept. Pin on first connect (append is async + best-effort —
				// the connection proceeds; a failed append only means the next
				// connect re-pins).
				if (decision.append !== undefined) {
					void deps
						.appendKnownHosts(deps.knownHostsPath, decision.append)
						.then(() => {
							deps.logger.info("pinned host key", { alias: computer.alias, token });
						})
						.catch((e: unknown) => {
							deps.logger.warn("failed to pin host key", {
								alias: computer.alias,
								error: e instanceof Error ? e.message : String(e),
							});
						});
				}
				return true;
			},
		};

		client.connect(connectConfig);
	});
}

/** Resolve the private key bytes for a computer (key-only auth, decision #3). */
async function resolvePrivateKey(
	computer: Computer,
	deps: SshPoolDeps,
): Promise<{ privateKey: Buffer; passphraseError: string | null }> {
	const candidates = await identityCandidates(computer, deps);
	for (const path of candidates) {
		try {
			const text = await deps.readFileText(path);
			if (looksEncrypted(text)) {
				// MVP: no passphrase prompt (roadmap). Fail with a clear error.
				return {
					privateKey: Buffer.from(text),
					passphraseError:
						`SSH key "${path}" is encrypted — passphrase prompting is not ` +
						`supported in the MVP (use an unencrypted key for computer ` +
						`"${computer.alias}", or set IdentityFile to an unencrypted key).`,
				};
			}
			return { privateKey: Buffer.from(text), passphraseError: null };
		} catch {
			// missing/unreadable → try the next candidate
		}
	}
	return {
		privateKey: Buffer.alloc(0),
		passphraseError:
			`no readable SSH key for computer "${computer.alias}" ` +
			`(checked: ${candidates.join(", ")})`,
	};
}

/**
 * The IdentityFile candidates: the config's `IdentityFile` (resolved absolute
 * by the config reader), else the default probe order (`~/.ssh/id_ed25519` →
 * `~/.ssh/id_rsa`, first-existing-wins — matches OpenSSH's own probing).
 */
async function identityCandidates(computer: Computer, deps: SshPoolDeps): Promise<string[]> {
	const candidates: string[] = [];
	if (computer.identityFile !== null) candidates.push(computer.identityFile);
	for (const name of DEFAULT_IDENTITY_FILES) {
		candidates.push(`${deps.homeDir}/.ssh/${name}`);
	}
	// De-dup + filter to existing, preserving order.
	const existing: string[] = [];
	const seen = new Set<string>();
	for (const c of candidates) {
		if (seen.has(c)) continue;
		seen.add(c);
		if (await deps.pathExists(c)) existing.push(c);
	}
	if (existing.length > 0) return existing;
	// Fall back to the raw candidate list (so resolvePrivateKey reports it).
	return [...new Set(candidates)];
}

const DEFAULT_IDENTITY_FILES = ["id_ed25519", "id_rsa"];

/** OpenSSH encrypts keys with a `ENCRYPTED` header — detect it (no passphrase MVP). */
function looksEncrypted(keyText: string): boolean {
	return keyText.includes("ENCRYPTED");
}

/** Open an SFTP session on a connected client (promisified). */
function openSftp(client: Client): Promise<import("ssh2").SFTPWrapper> {
	return new Promise((resolve, reject) => {
		client.sftp((err, sftp) => {
			if (err !== null && err !== undefined) reject(err);
			else resolve(sftp);
		});
	});
}

// ─── host-key fingerprint → pure decision input ────────────────────────────

/**
 * Build the `HostKeyFingerprint` from the raw host public-key blob ssh2's
 * verifier supplies (a Buffer — see `ConnectConfig.hostVerifier`, used WITHOUT
 * `hostHash` so the blob is passed verbatim). The blob is the OpenSSH wire-format
 * public key: `[uint32 len][key-type string][key material…]`, base64-encoded as
 * the 3rd field of a `known_hosts` line. We parse the type string from the blob
 * (rather than guessing) so the pinned line is byte-identical to what OpenSSH
 * itself writes — the file is the shared trust store (decision #2).
 */
function toFingerprint(token: string, key: Buffer | string): HostKeyFingerprint {
	const buf = typeof key === "string" ? Buffer.from(key, "utf8") : key;
	return {
		knownHostToken: token,
		keyBase64: buf.toString("base64"),
		keyType: parseKeyType(buf),
	};
}

/**
 * Read the key-type label (e.g. `ssh-ed25519`) from the first length-prefixed
 * string of an OpenSSH public-key blob. Falls back to `ssh-ed25519` (the most
 * common host key) if the blob is too short to parse — the base64 blob itself
 * is the authoritative identity for `decideHostKey`'s comparison.
 */
function parseKeyType(buf: Buffer): string {
	if (buf.length < 4) return "ssh-ed25519";
	const len = buf.readUInt32BE(0);
	if (len <= 0 || buf.length < 4 + len) return "ssh-ed25519";
	return buf.subarray(4, 4 + len).toString("ascii");
}

// ─── idle reaping ───────────────────────────────────────────────────────────

function startReaper(
	entries: Map<string, PooledEntry>,
	computerId: string,
	deps: SshPoolDeps,
): void {
	const entry = entries.get(computerId);
	if (entry === undefined) return;
	entry.reaper = setInterval(() => {
		const e = entries.get(computerId);
		if (e === undefined) return;
		const idle = Date.now() - e.lastUsedAt;
		if (idle >= IDLE_REAP_MS) {
			deps.logger.info("reaping idle ssh connection", { alias: computerId, idleMs: idle });
			void e.conn.close().then(() => {
				stopReaper(e);
				entries.delete(computerId);
			});
		}
	}, 60_000);
}

function stopReaper(entry: PooledEntry): void {
	if (entry.reaper !== null) {
		clearInterval(entry.reaper);
		entry.reaper = null;
	}
}

/** Ssh2 exec stream type alias (the channel backing spawn). */
export type { ClientChannel };