Fixes potion data conversion from 1.8 to 1.9

src/main/java/us/myles/ViaVersion/protocols/protocol1_9to1_8/ItemRewriter.java

@@ -213,11 +213,9 @@ public class ItemRewriter {
                     item.setId((short) 438); // splash id
                     item.setData((short) (item.getData() - 8192));
                 }
-                if (POTION_ID_TO_NAME.containsKey((int) item.getData())) {
-                    String name = POTION_ID_TO_NAME.get((int) item.getData());
+                String name = potionNameFromDamage(item.getData());
                 StringTag potion = new StringTag("Potion", "minecraft:" + name);
                 tag.put(potion);
-                }
                 item.setTag(tag);
                 item.setData((short) 0);
             }
@@ -245,10 +243,78 @@ public class ItemRewriter {
         }
     }
 
+    public static String potionNameFromDamage(short damage) {
+        String cached = POTION_ID_TO_NAME.get((int) damage);
+        if(cached != null) {
+            return cached;
+        }
+
+        int effect = damage & 0xF;
+        int name = damage & 0x3F;
+        boolean enhanced = (damage & 0x20) > 0;
+        boolean extended = (damage & 0x40) > 0;
+        boolean drinkable = (damage & 0x2000) > 0;
+        //boolean splash = (damage & 0x4000) > 0;
+
+        boolean canEnhance = true;
+        boolean canExtend = true;
+
+        String id;
+        switch (effect) {
+            case 1: id="regeneration"; break;
+            case 2: id="swiftness"; break;
+            case 3: id="fire_resistance"; canEnhance = false; break;
+            case 4: id="poison"; break;
+            case 5: id="healing"; canExtend = false; break;
+            case 6: id="night_vision"; canEnhance = false; break;
+
+            case 8: id="weakness"; canEnhance = false; break;
+            case 9: id="strength"; break;
+            case 10: id="slowness"; canEnhance = false; break;
+            case 11: id="leaping"; break;
+            case 12: id="harming"; canExtend=false; break;
+            case 13: id="water_breathing"; canEnhance = false; break;
+            case 14: id="invisibility"; canEnhance = false; break;
+
+
+            default:
+                switch (name) {
+                    case 16: id="awkward"; break;
+                    case 32: id="thick"; break;
+                    default:
+                        if(drinkable) {
+                            id = "mundane";
+                        }
+                        else {
+                            id = "water";
+                        }
+                }
+        }
+
+        if(effect > 0) {
+            if(canEnhance && enhanced) {
+                id = "strong_" + id;
+            }
+            else if(canExtend && extended) {
+                id = "long_" + id;
+            }
+
+            //if(splash)
+            //    potion.splash();
+        }
+
+        return id;
+    }
+
     public static int getNewEffectID(int oldID) {
         if (oldID >= 16384) {
             oldID -= 8192;
         }
+        if(POTION_INDEX.containsKey(oldID)) {
+            return POTION_INDEX.get(oldID);
+        }
+
+        oldID = POTION_NAME_TO_ID.get(potionNameFromDamage((short)oldID));
         return POTION_INDEX.containsKey(oldID) ? POTION_INDEX.get(oldID) : 0;
     }