hytale-gravity-flip/src/main/java/com/mythlane/gravityflip/physics/FallDamageGuard.java

package com.mythlane.gravityflip.physics;

import com.mythlane.gravityflip.region.GravityFlipRegion;

import java.util.UUID;
import java.util.concurrent.ConcurrentHashMap;

/**
 * Thread-safe tracker that decides whether fall damage should be suppressed for a given UUID
 * based on (a) current in-region membership, (b) post-exit grace window.
 *
 * <p>Pure-data service : no Hytale runtime dependency (no ECS, no PhysicsValues), injected
 * via constructor into {@link GravityApplier} (populator) and
 * {@link FallDamageSuppressorSystem} (consumer) — no static mutable state.
 *
 * <p><b>Precedence rule (multi-region) :</b> when {@link #markInRegion} is called, the LAST
 * call for a given UUID within a single tick wins. {@link GravityApplier} takes care of
 * passing the FIRST matched region (iteration order of {@code enabledRegions}), so for an
 * entity simultaneously in N regions the first-match region's {@code FallDamage}
 * and {@code GracePeriodMs} drive suppression.
 *
 * <p><b>State machine :</b>
 * <ul>
 *   <li>{@code markInRegion(uuid, region)} → currentRegion[uuid]=region ; grace entries cleared.</li>
 *   <li>{@code markExit(uuid, region, nowMs)} → currentRegion[uuid] removed ; grace entry stored
 *       (region + timestamp).</li>
 *   <li>{@code shouldSuppressFallDamage(uuid, nowMs)} :
 *     <ol>
 *       <li>If current region != null AND current region {@code FallDamage==false} → true.</li>
 *       <li>Else if grace entry present AND grace region {@code FallDamage==false} AND
 *           {@code nowMs - exitMs <= gracePeriodMs} → true.</li>
 *       <li>Else → false.</li>
 *     </ol>
 *   </li>
 * </ul>
 */
public final class FallDamageGuard {

    /** Entity currently inside a region (cleared on exit). */
    private final ConcurrentHashMap<UUID, GravityFlipRegion> currentRegionByUuid =
            new ConcurrentHashMap<>();

    /** Timestamp (ms) at which the entity last exited a region. */
    private final ConcurrentHashMap<UUID, Long> exitTimestampByUuid =
            new ConcurrentHashMap<>();

    /** Region referenced at the moment of exit (used to read FallDamage + GracePeriodMs). */
    private final ConcurrentHashMap<UUID, GravityFlipRegion> regionAtExitByUuid =
            new ConcurrentHashMap<>();

    public void markInRegion(UUID uuid, GravityFlipRegion region) {
        if (uuid == null || region == null) return;
        currentRegionByUuid.put(uuid, region);
        // Re-entry ⇒ discard any stale grace entry (grace is a post-exit concept).
        exitTimestampByUuid.remove(uuid);
        regionAtExitByUuid.remove(uuid);
    }

    public void markExit(UUID uuid, GravityFlipRegion region, long nowMs) {
        if (uuid == null || region == null) return;
        currentRegionByUuid.remove(uuid);
        exitTimestampByUuid.put(uuid, nowMs);
        regionAtExitByUuid.put(uuid, region);
    }

    public boolean shouldSuppressFallDamage(UUID uuid, long nowMs) {
        if (uuid == null) return false;

        GravityFlipRegion current = currentRegionByUuid.get(uuid);
        if (current != null) {
            return !current.isFallDamage();
        }

        Long exitMs = exitTimestampByUuid.get(uuid);
        GravityFlipRegion exitRegion = regionAtExitByUuid.get(uuid);
        if (exitMs == null || exitRegion == null) return false;
        if (exitRegion.isFallDamage()) return false;  // region allowed fall damage → no grace
        return (nowMs - exitMs) <= exitRegion.getGracePeriodMs();
    }

    // ---- Test hooks / diagnostics (package-private) ----

    int trackedInRegionCount()  { return currentRegionByUuid.size(); }
    int trackedGraceCount()     { return exitTimestampByUuid.size(); }
}