import type {
	PrefsChangeHandler,
	PrefsDimension,
	PrefsEffectiveOf,
	PrefsEnvironment,
	PrefsIntentOf,
	PrefsSchema,
	PrefsSnapshot,
	PrefsUnsubscribe
} from '$libs/prefs';
import { createEnginePrefs } from './engine-prefs.ts';
import { PREFS_KIND } from './consts.ts';
import { PrefsReservedKeyError, reservedKeyErrorMessage } from './errors.ts';
import type { EnginePrefsOptions } from './types.ts';

/**
 * Per-dimension active surface. Every key in the schema becomes one of
 * these on the parent `ActivePrefs<S>`, addressable as
 * `App.prefs.<key>`. The verbs are uniform across every dimension: get
 * the effective value, set / clear user intent, listen for changes.
 */
export interface ActivePrefsDimension<TIntent, TEffective = TIntent> {
	/** Current effective value (intent → environment → default). */
	get(): TEffective;
	/**
	 * Set explicit user intent for this dimension. Validates first;
	 * throws `PrefsIntentInvalidError` on rejection.
	 */
	set(value: TIntent): void;
	/** Drop user intent. Falls back to environment / default. */
	clear(): void;
	/**
	 * Subscribe to commits where this dimension's effective value
	 * changed. Returns an unsubscribe function. Best-effort: a handler
	 * that throws does not block its peers.
	 */
	onChange(handler: (value: TEffective) => void): PrefsUnsubscribe;
	/** Optional capability catalog when the dimension exposes one. */
	catalog(): readonly TIntent[] | undefined;
}

/**
 * Reactive view of `EnginePrefs.state`. Backed by `$state` cells inside
 * `createActivePrefs` — accessing the getters inside a Svelte template
 * or `$derived` tracks dependencies the way you'd expect.
 *
 * `pending` and `lastError` are placeholders for async adapters (the
 * storage bridge in particular). With a sync-only engine they stay
 * `false` / `null`.
 */
export interface ActivePrefsState<S extends PrefsSchema> {
	readonly snapshot: PrefsSnapshot<S>;
	readonly effective: PrefsEffectiveOf<S>;
	readonly environment: PrefsEnvironment;
	readonly intent: PrefsIntentOf<S>;
	readonly version: number;
	readonly pending: boolean;
	readonly lastError: unknown;
}

/**
 * Reserved members of `ActivePrefs<S>`. A schema key that matches one
 * of these would shadow the active surface — the constructor throws
 * `PrefsReservedKeyError` so the misconfiguration fails fast.
 */
export const ACTIVE_PREFS_RESERVED_KEYS: readonly string[] = [
	'kind',
	'schema',
	'state',
	'snapshot',
	'environment',
	'intent',
	'effective',
	'resetIntent',
	'refreshEnvironment',
	'patchEnvironment',
	'subscribe',
	'dispose'
];

const RESERVED_SET = new Set(ACTIVE_PREFS_RESERVED_KEYS);

/**
 * Reactive Svelte adapter over `EnginePrefs<S>`. Adds the
 * `ActivePrefsState` block and exposes one `ActivePrefsDimension` per
 * schema key as a property — i.e. for `schema = { locale, theme }` the
 * returned object has `.locale.get()` / `.locale.set(...)` /
 * `.theme.get()` / etc., type-checked against each dimension's
 * `TIntent` and `TEffective`.
 */
/**
 * Top-level (non-dimension) surface. Always present regardless of the
 * schema. Service factories see this shape via `core.prefs` and read
 * specific dimensions defensively at runtime through their string key.
 */
export interface ActivePrefsBase<S extends PrefsSchema = PrefsSchema> {
	readonly kind: typeof PREFS_KIND;
	readonly schema: S;
	readonly state: ActivePrefsState<S>;

	snapshot(): PrefsSnapshot<S>;
	environment(): PrefsEnvironment;
	intent(): PrefsIntentOf<S>;
	effective(): PrefsEffectiveOf<S>;

	/**
	 * Low-level mutator. The recommended public API is
	 * `App.prefs.<dim>.set(value)` — this stays exposed only for
	 * adapters (storage bridge, devtools) that operate generically over
	 * dimension keys.
	 */
	setIntent<K extends keyof S>(key: K, value: unknown): PrefsSnapshot<S>;
	/** Low-level mutator; prefer `App.prefs.<dim>.clear()`. */
	clearIntent<K extends keyof S>(key: K): PrefsSnapshot<S>;
	resetIntent(next?: PrefsIntentOf<S>): PrefsSnapshot<S>;
	refreshEnvironment(next: PrefsEnvironment): PrefsSnapshot<S>;
	patchEnvironment(patch: Partial<PrefsEnvironment>): PrefsSnapshot<S>;
	subscribe(handler: PrefsChangeHandler<S>): PrefsUnsubscribe;
	dispose(): void;
}

/**
 * Dimension surface — one slot per schema key. Only meaningful when
 * `S` is a concrete schema literal. With the open `PrefsSchema`
 * (`Record<string, PrefsDimension>`) the mapped type would collide
 * with the index signature on the base, so we degrade to `unknown`
 * for the open case (`X & unknown = X`).
 *
 * Service factories that need typed dimensions cast through their own
 * schema generic; consumers that read `App.prefs.<key>` get the typed
 * surface because `App` flows the concrete `TPrefsSchema`.
 */
export type ActivePrefsDimensions<S extends PrefsSchema> = string extends keyof S
	? unknown
	: {
			readonly [K in keyof S]: S[K] extends PrefsDimension<infer TIntent, infer TEffective>
				? ActivePrefsDimension<TIntent, TEffective>
				: never;
		};

export type ActivePrefs<S extends PrefsSchema = PrefsSchema> = ActivePrefsBase<S> &
	ActivePrefsDimensions<S>;

export function createActivePrefs<S extends PrefsSchema>(
	options: EnginePrefsOptions<S>
): ActivePrefs<S> {
	for (const key of Object.keys(options.schema)) {
		if (RESERVED_SET.has(key)) {
			throw new PrefsReservedKeyError(key, reservedKeyErrorMessage(key));
		}
	}

	const engine = createEnginePrefs(options);

	let snapshotCell = $state<PrefsSnapshot<S>>(engine.snapshot());
	// `pending` / `lastError` are placeholders today (sync engine has
	// nothing async to track). Declared as `let` so async adapters
	// (storage bridge in particular) can flip them when wired in
	// without restructuring the rune layout.
	const pendingCell = $state(false);
	const lastErrorCell = $state<unknown>(null);

	const detachCommit = engine.subscribe((event) => {
		snapshotCell = event.next;
	});

	const state: ActivePrefsState<S> = {
		get snapshot() {
			return snapshotCell;
		},
		get effective() {
			return snapshotCell.effective;
		},
		get environment() {
			return snapshotCell.environment;
		},
		get intent() {
			return snapshotCell.intent;
		},
		get version() {
			return snapshotCell.version;
		},
		get pending() {
			return pendingCell;
		},
		get lastError() {
			return lastErrorCell;
		}
	};

	const dimensionMembers: Record<string, ActivePrefsDimension<unknown, unknown>> = {};
	for (const key of Object.keys(options.schema)) {
		dimensionMembers[key] = makeDimension(engine, key);
	}

	const base = {
		kind: PREFS_KIND as typeof PREFS_KIND,
		schema: options.schema,
		state,
		snapshot: () => engine.snapshot(),
		environment: () => engine.environment(),
		intent: () => engine.intent(),
		effective: () => engine.effective(),
		setIntent: <K extends keyof S>(key: K, value: unknown) => engine.setIntent(key, value),
		clearIntent: <K extends keyof S>(key: K) => engine.clearIntent(key),
		resetIntent: (next?: PrefsIntentOf<S>) => engine.resetIntent(next),
		refreshEnvironment: (next: PrefsEnvironment) => engine.refreshEnvironment(next),
		patchEnvironment: (patch: Partial<PrefsEnvironment>) => engine.patchEnvironment(patch),
		subscribe: (handler: PrefsChangeHandler<S>) => engine.subscribe(handler),
		dispose: () => {
			detachCommit();
			engine.dispose();
		}
	};

	return Object.assign(base, dimensionMembers) as unknown as ActivePrefs<S>;
}

function makeDimension(
	engine: ReturnType<typeof createEnginePrefs>,
	key: string
): ActivePrefsDimension<unknown, unknown> {
	return {
		get() {
			return (engine.snapshot().effective as Record<string, unknown>)[key];
		},
		set(value: unknown) {
			engine.setIntent(key, value);
		},
		clear() {
			engine.clearIntent(key);
		},
		onChange(handler: (value: unknown) => void) {
			return engine.subscribe((event) => {
				const diff = event.effectiveDiff as Record<string, unknown>;
				if (Object.prototype.hasOwnProperty.call(diff, key)) {
					handler(diff[key]);
				}
			});
		},
		catalog() {
			const dim = engine.schema[key];
			return dim?.catalog?.();
		}
	};
}