import {
  type AuthSessionId,
  type BackgroundPolicySnapshot,
  type BackgroundScope,
  type ClientActivityLease,
  type ClientActivityReportInput,
  type HostPowerSnapshot,
  type RpcClientId,
} from "@t3tools/contracts";
import {
  getBackgroundActivityPresetSettings,
  resolveServerBackgroundActivitySettings,
  type ResolvedBackgroundActivitySettings,
} from "@t3tools/shared/backgroundActivitySettings";
import * as DateTime from "effect/DateTime";
import * as Context from "effect/Context";
import * as Effect from "effect/Effect";
import * as Layer from "effect/Layer";
import * as PubSub from "effect/PubSub";
import * as Ref from "effect/Ref";
import * as Stream from "effect/Stream";

import { ServerSettingsService } from "../serverSettings.ts";
import * as HostPowerMonitor from "./HostPowerMonitor.ts";

export interface BackgroundPolicyShape {
  readonly reportClientActivity: (
    sessionId: AuthSessionId,
    rpcClientId: RpcClientId,
    input: ClientActivityReportInput,
  ) => Effect.Effect<void>;
  readonly removeRpcClient: (rpcClientId: RpcClientId) => Effect.Effect<void>;
  readonly reportHostPowerState: (snapshot: HostPowerSnapshot) => Effect.Effect<void>;
  readonly snapshot: Effect.Effect<BackgroundPolicySnapshot>;
  readonly streamChanges: Stream.Stream<BackgroundPolicySnapshot>;
  readonly hasDemand: (scope: BackgroundScope) => Effect.Effect<boolean>;
  readonly shouldRunScopeWork: (scope: BackgroundScope) => Effect.Effect<boolean>;
  readonly shouldRunOpportunisticWork: Effect.Effect<boolean>;
}

export class BackgroundPolicy extends Context.Service<BackgroundPolicy, BackgroundPolicyShape>()(
  "t3/background/BackgroundPolicy",
) {}

const DEFAULT_LEASE_TTL_MS = 45_000;
const MAX_LEASE_TTL_MS = 120_000;

function scopeKey(scope: BackgroundScope): string {
  switch (scope.type) {
    case "server-config":
    case "diagnostics":
      return scope.type;
    case "provider-status":
      return scope.instanceId ? `${scope.type}:${scope.instanceId}` : scope.type;
    case "vcs-status":
    case "git-refs":
      return `${scope.type}:${scope.cwd}`;
    case "thread":
      return `${scope.type}:${scope.threadId}`;
  }
}

function isLeaseActive(lease: ClientActivityLease, now: DateTime.Utc): boolean {
  return DateTime.isGreaterThan(lease.expiresAt, now);
}

function isForegroundLease(lease: ClientActivityLease, now: DateTime.Utc): boolean {
  return isLeaseActive(lease, now) && lease.visible && lease.focused;
}

function leaseHasScope(lease: ClientActivityLease, scope: BackgroundScope): boolean {
  const key = scopeKey(scope);
  return lease.scopes.some((leaseScope) => scopeKey(leaseScope) === key);
}

function hasThermalPressure(hostPower: HostPowerSnapshot): boolean {
  return hostPower.thermalState === "serious" || hostPower.thermalState === "critical";
}

function isHostConstrained(
  hostPower: HostPowerSnapshot,
  settings: ResolvedBackgroundActivitySettings,
): boolean {
  if (hostPower.stale) return false;
  if (
    (settings.pauseWhenHostLocked && hostPower.locked === "true") ||
    hasThermalPressure(hostPower)
  ) {
    return true;
  }
  if (settings.pauseWhenHostLowPower && hostPower.lowPowerMode === "true") return true;
  return settings.pauseWhenOnBattery && hostPower.onBattery === "true";
}

function isClientConstrained(
  lease: ClientActivityLease,
  settings: ResolvedBackgroundActivitySettings,
): boolean {
  if (settings.pauseWhenClientLowPower && lease.lowPowerMode === "true") return true;
  return settings.pauseWhenOnBattery && lease.batteryState === "unplugged";
}

function leaseMayRunScopedWork(
  lease: ClientActivityLease,
  scope: BackgroundScope,
  now: DateTime.Utc,
  settings: ResolvedBackgroundActivitySettings,
): boolean {
  const activeWithScope = isLeaseActive(lease, now) && leaseHasScope(lease, scope);
  if (!activeWithScope || isClientConstrained(lease, settings)) {
    return false;
  }
  if (settings.profile === "performance") {
    return true;
  }
  return isForegroundLease(lease, now);
}

function computeSnapshot(input: {
  readonly hostPower: HostPowerSnapshot;
  readonly leases: ReadonlyMap<string, ClientActivityLease>;
  readonly now: DateTime.Utc;
  readonly settings: ResolvedBackgroundActivitySettings;
  readonly updatedAt: DateTime.Utc;
}): BackgroundPolicySnapshot {
  const activeLeases = [...input.leases.values()].filter((lease) =>
    isLeaseActive(lease, input.now),
  );
  const foregroundLeases = activeLeases.filter((lease) => isForegroundLease(lease, input.now));
  const activeScopeKeys = new Set<string>();
  for (const lease of activeLeases) {
    for (const scope of lease.scopes) {
      activeScopeKeys.add(scopeKey(scope));
    }
  }

  return {
    hostPower: input.hostPower,
    leases: activeLeases,
    activeForegroundLeaseCount: foregroundLeases.length,
    activeScopeKeys: [...activeScopeKeys].toSorted(),
    shouldRunOpportunisticWork:
      foregroundLeases.some((lease) => !isClientConstrained(lease, input.settings)) &&
      !isHostConstrained(input.hostPower, input.settings),
    updatedAt: input.updatedAt,
  };
}

export const make = Effect.fn("background.policy.make")(function* () {
  const hostPowerMonitor = yield* HostPowerMonitor.HostPowerMonitor;
  const serverSettings = yield* ServerSettingsService;
  const leasesRef = yield* Ref.make(new Map<string, ClientActivityLease>());
  const changes = yield* PubSub.sliding<BackgroundPolicySnapshot>(1);

  const backgroundActivitySettings = serverSettings.getSettings.pipe(
    Effect.map(resolveServerBackgroundActivitySettings),
    Effect.orElseSucceed(() => getBackgroundActivityPresetSettings("balanced")),
  );

  const snapshot = Effect.gen(function* () {
    const [hostPower, leases, now, settings] = yield* Effect.all([
      hostPowerMonitor.snapshot,
      Ref.get(leasesRef),
      DateTime.now,
      backgroundActivitySettings,
    ]);
    return computeSnapshot({ hostPower, leases, now, settings, updatedAt: now });
  });

  const publishSnapshot = snapshot.pipe(Effect.flatMap((next) => PubSub.publish(changes, next)));

  const reportClientActivity: BackgroundPolicyShape["reportClientActivity"] = (
    sessionId,
    rpcClientId,
    input,
  ) =>
    Effect.gen(function* () {
      const ttlMs = Math.min(
        Math.max(input.ttlMs ?? DEFAULT_LEASE_TTL_MS, 1_000),
        MAX_LEASE_TTL_MS,
      );
      const now = yield* DateTime.now;
      const expiresAt = DateTime.add(now, { milliseconds: ttlMs });
      const lease: ClientActivityLease = {
        sessionId,
        rpcClientId,
        clientId: input.clientId,
        clientKind: input.clientKind,
        visible: input.visible,
        focused: input.focused,
        recentlyInteracted: input.recentlyInteracted,
        ...(input.appState !== undefined ? { appState: input.appState } : {}),
        ...(input.lowPowerMode !== undefined ? { lowPowerMode: input.lowPowerMode } : {}),
        ...(input.batteryState !== undefined ? { batteryState: input.batteryState } : {}),
        ...(input.networkType !== undefined ? { networkType: input.networkType } : {}),
        scopes: input.scopes,
        updatedAt: now,
        expiresAt,
      };
      yield* Ref.update(leasesRef, (leases) => {
        const next = new Map(leases);
        next.set(`${rpcClientId}:${input.clientId}`, lease);
        return next;
      });
      yield* publishSnapshot;
    });

  const removeRpcClient: BackgroundPolicyShape["removeRpcClient"] = (rpcClientId) =>
    Ref.update(leasesRef, (leases) => {
      const next = new Map(leases);
      for (const key of next.keys()) {
        if (key.startsWith(`${rpcClientId}:`)) {
          next.delete(key);
        }
      }
      return next;
    }).pipe(Effect.andThen(publishSnapshot), Effect.asVoid);

  const hasDemand: BackgroundPolicyShape["hasDemand"] = (scope) =>
    Effect.map(snapshot, (current) => current.activeScopeKeys.includes(scopeKey(scope)));

  const shouldRunScopeWork: BackgroundPolicyShape["shouldRunScopeWork"] = (scope) =>
    Effect.gen(function* () {
      const [current, settings] = yield* Effect.all([snapshot, backgroundActivitySettings]);
      if (isHostConstrained(current.hostPower, settings)) {
        return false;
      }
      return current.leases.some((lease) =>
        leaseMayRunScopedWork(lease, scope, current.updatedAt, settings),
      );
    });

  const shouldRunOpportunisticWork = Effect.map(
    snapshot,
    (current) => current.shouldRunOpportunisticWork,
  );

  yield* Stream.runForEach(hostPowerMonitor.streamChanges, () => publishSnapshot).pipe(
    Effect.forkScoped,
  );

  yield* Effect.forever(
    Effect.sleep("15 seconds").pipe(
      Effect.andThen(
        Effect.gen(function* () {
          const now = yield* DateTime.now;
          yield* Ref.update(leasesRef, (leases) => {
            const next = new Map(leases);
            for (const [key, lease] of next) {
              if (!isLeaseActive(lease, now)) {
                next.delete(key);
              }
            }
            return next;
          });
        }),
      ),
      Effect.andThen(publishSnapshot),
    ),
  ).pipe(Effect.forkScoped);

  return BackgroundPolicy.of({
    reportClientActivity,
    removeRpcClient,
    reportHostPowerState: hostPowerMonitor.report,
    snapshot,
    streamChanges: Stream.fromPubSub(changes),
    hasDemand,
    shouldRunScopeWork,
    shouldRunOpportunisticWork,
  });
});

export const layer = Layer.effect(BackgroundPolicy, make());