Issue 3827: Fixes target's side's ECM not effecting sensor spotting

megamek/src/megamek/common/Compute.java

@@ -5094,10 +5094,14 @@ public static int getSensorRangeBracket(Entity ae, Targetable target,
             if (ae.getGame().getOptions().booleanOption(OptionsConstants.ADVANCED_METAL_CONTENT)) {
                 check += sensor.getModForMetalContent(ae, te);
             }
+
+            check += sensor.getModForTargetECM(te, allECMInfo);
+
         }
         // ECM bubbles
         check += sensor.getModForECM(ae, allECMInfo);
 
+
         return Compute.getSensorBracket(check);
     }
 

megamek/src/megamek/common/ComputeECM.java

@@ -517,4 +517,29 @@ public static ArrayList<ECMInfo> computeAllEntitiesECMInfo(
         }
         return worstECMEffects;
     }
+
+    /**
+     * Returns the total friendly ECM effects on the supplied unit.
+     *
+     * @param affectedEntity The entity to check.
+     * @param allECMInfo
+     * @return
+     */
+    public static @Nullable ECMInfo getFriendlyECMEffects(Entity affectedEntity, @Nullable List<ECMInfo> allEcmInfo){
+        if (allEcmInfo == null) {
+            allEcmInfo = computeAllEntitiesECMInfo(affectedEntity.getGame().getEntitiesVector());
+        }
+
+        Coords entityPosition = affectedEntity.getPosition();
+
+        ECMInfo affectedInfo = new ECMInfo(0, 0, affectedEntity.getOwner(), entityPosition);
+        for (ECMInfo ecmInfo : allEcmInfo) {
+            // Is the ECMInfo in range of this position?
+            int dist = entityPosition.distance(ecmInfo.getPos());
+            if (dist <= ecmInfo.getRange()) {
+                affectedInfo.addAlliedECMEffects(ecmInfo);
+            }
+        }
+        return affectedInfo;
+    }
 }

megamek/src/megamek/common/Sensor.java

@@ -438,6 +438,27 @@ public int getModForECM(Entity en, List<ECMInfo> allECMInfo) {
         Coords pos = en.getPosition();
         ECMInfo ecmInfo = ComputeECM.getECMEffects(en, pos, pos, true,
                 allECMInfo);
+        return getECMSensorRangeModifier(ecmInfo);
+    }
+
+    /**
+     * Computes the sensor check modifier for ECM.
+     *
+     * @param targetEntity
+     * @param allECMInfo  A collection of ECMInfo for all entities, this value
+     *                    can be null and it will be computed when it's
+     *                    needed, however passing in the pre-computed
+     *                    collection is much faster
+     * @return
+     */
+    public int getModForTargetECM(Entity targetEntity, List<ECMInfo> allECMInfo) {
+        // how many ECM fields are affecting the target entity?
+        Coords pos = targetEntity.getPosition();
+        ECMInfo ecmInfo = ComputeECM.getFriendlyECMEffects(targetEntity, allECMInfo);
+        return getECMSensorRangeModifier(ecmInfo);
+    }
+
+    public int getECMSensorRangeModifier(ECMInfo ecmInfo){
         double ecm, ecmAngel;
         ecm = ecmAngel = 0;
         if (ecmInfo != null) {