import type { ScalingDecision, ScalingMember, ScalingPolicy, ScalingSnapshot } from '../../autoscaling.js';
import { parseServerName, zombieMaps } from '../../map-aliases.js';
import { ONLINE_MAX_MS } from '../../server-freshness.js';

export interface EmptyObservation { identity: string; observedAt: number | null; emptySince: number | null; evaluatedAt: number }
/** Only distinct, continuous evidence can advance an empty interval. */
export function emptyObservation(member: ScalingMember, previous: EmptyObservation | undefined, now: number): EmptyObservation {
    const stamp = member.observedAt;
    const empty = member.state === 'empty' && stamp !== null && stamp <= now && now - stamp <= ONLINE_MAX_MS;
    const continuous = empty && previous?.identity === member.identity && previous.emptySince !== null
        && (!member.emptyStartedAt || previous.emptySince >= member.emptyStartedAt)
        && previous.observedAt !== null && stamp >= previous.observedAt && stamp - previous.observedAt <= ONLINE_MAX_MS
        && now >= previous.evaluatedAt && now - previous.evaluatedAt <= ONLINE_MAX_MS;
    return { identity: member.identity, observedAt: stamp, emptySince: empty ? continuous ? previous!.emptySince : stamp : null, evaluatedAt: now };
}

export function planScaling(policy: ScalingPolicy, snapshot: ScalingSnapshot, now: number): ScalingDecision {
    if (policy.mapSlug === '*') {
        const maps = zombieMaps(policy.game);
        const decisions = maps.map(map => {
            const matches = (name: string) => (snapshot.members.find(m => m.name === name)?.mapSlug ?? parseServerName(name)?.mapSlug) === map.slug;
            return planScaling({ ...policy, mapSlug: map.slug }, {
                ...snapshot, members: snapshot.members.filter(m => matches(m.name)),
                reservedNames: snapshot.reservedNames.filter(matches)
            }, now);
        });
        // Serialize game-wide work and prefer replenishing any map before trimming another.
        const pending = snapshot.members.find(m => m.state === 'creating' || m.state === 'removing');
        const selected = decisions.find(d => d.action === 'create') ?? decisions.find(d => d.action === 'destroy')
            ?? decisions.find(d => d.deficit > 0 || d.unknown > 0) ?? decisions[0];
        const map = maps[decisions.indexOf(selected)];
        return { ...selected, action: pending ? 'wait' : selected.action, server: pending ? null : selected.server,
            reason: pending ? pending.blocker ?? `Waiting for ${pending.name}: ${pending.state}.` : `${map.friendly}: ${selected.reason}`,
            desired: decisions.reduce((n, d) => n + d.desired, 0), deficit: decisions.reduce((n, d) => n + d.deficit, 0), occupied: decisions.reduce((n, d) => n + d.occupied, 0),
            empty: decisions.reduce((n, d) => n + d.empty, 0), pending: decisions.reduce((n, d) => n + d.pending, 0), unknown: decisions.reduce((n, d) => n + d.unknown, 0) };
    }
    const { members } = snapshot;
    const occupied = members.filter(m => m.state === 'occupied').length, empty = members.filter(m => m.state === 'empty').length;
    const pending = members.filter(m => m.state === 'creating' || m.state === 'removing').length;
    const unknown = members.filter(m => m.state === 'unknown' || m.state === 'stopped').length;
    const desired = Math.min(policy.maximum, Math.max(policy.minimum, occupied + policy.spares));
    const result = (action: ScalingDecision['action'], reason: string, server: string | null = null): ScalingDecision =>
        ({ action, reason, server, desired, deficit: Math.max(0, policy.spares - empty), occupied, empty, pending, unknown });
    if (policy.mode === 'off') return result('wait', policy.pausedReason ?? 'Policy is off.');
    if (policy.pausedReason) return result('wait', policy.pausedReason);
    if (snapshot.blockers.length) return result('wait', snapshot.blockers[0]);
    const blocked = members.find(m => !['occupied', 'empty'].includes(m.state));
    if (blocked) return result('wait', blocked.blocker ?? `Waiting for ${blocked.name}: ${blocked.state}.`);
    if (members.some(m => m.observedAt === null || m.observedAt > now || now - m.observedAt > ONLINE_MAX_MS)) return result('wait', 'Waiting for fresh occupancy evidence.');
    if (occupied + empty < desired) {
        if (snapshot.reservedNames.length >= policy.maximum) return result('wait', 'Maximum lobby reservations reached.');
        if (!snapshot.keyPool.available) return result('wait', snapshot.keyPool.total
            ? `All ${policy.game.toUpperCase()} Plutonium keys are in use. Waiting for a free key.`
            : `No Plutonium keys are saved for ${policy.game.toUpperCase()}.`);
        const available = Array.from({ length: 20 }, (_, index) => `${policy.game}_zombie_${policy.mapSlug}_${index + 1}`)
            .find(name => !snapshot.reservedNames.includes(name));
        return available ? result('create', 'Replenish empty lobby capacity.', available) : result('wait', 'No unused lobby identity is available.');
    }
    if (occupied + empty > desired && empty > policy.spares && occupied + empty > policy.minimum) {
        // Protect the lowest-numbered spares before checking removal timers.
        // Lobby 1 must survive even when a higher lobby has only just emptied.
        const orderedEmpty = members.filter(m => m.state === 'empty').sort((a, b) =>
            (parseServerName(a.name)?.lobby ?? Number.MAX_SAFE_INTEGER) - (parseServerName(b.name)?.lobby ?? Number.MAX_SAFE_INTEGER)
            || a.name.localeCompare(b.name, 'en', { numeric: true }));
        const requiredEmpty = Math.max(policy.spares, policy.minimum - occupied);
        const candidates = orderedEmpty.slice(requiredEmpty).reverse()
            .filter(m => m.emptySince !== null && m.observedAt! - m.emptySince >= policy.emptyMinutes * 60_000);
        if (candidates.length) return result('destroy', 'Surplus lobby has remained empty.', candidates[0].name);
        return result('wait', 'Waiting for a continuous empty interval.');
    }
    return result('wait', empty < policy.spares ? 'Maximum reached; the empty spare target cannot be met.' : 'Empty spare target is satisfied.');
}