Correctly source param mappings on missing lvt

Codebook did not correctly respect the difference between parameter and
lvt indexing when sourcing parameter names from mappings on missing lvt
tables.

This caused parameter mappings to appear shifted after long/double
params.

Reviewed-on: #39

Author: lynxplay

codebook-lvt/src/main/java/io/papermc/codebook/lvt/LvtNamer.java

@@ -216,11 +216,10 @@ private void fillNames0(final MethodData method) throws IOException {
             }
 
             for (int i = 0; i < paramCount; i++) {
-                // always (i + 1) because abstract methods are never static
                 final int fi = i;
                 @Nullable
                 String paramName = methodMapping
-                        .flatMap(m -> m.getParameterMapping(fi + 1))
+                        .flatMap(m -> m.getParameterMapping(fromParamToLvtIndex(fi, method)))
                         .map(Mapping::getDeobfuscatedName)
                         .orElse(null);
 
@@ -483,4 +482,26 @@ private static int fromLvtToParamIndex(final int lvtIndex, final MethodData meth
 
         return -1;
     }
+
+    /*
+     * Transform the given param index into a lvt index.
+     */
+    private static int fromParamToLvtIndex(final int paramIndex, final MethodData method) {
+        int currentLvtIndex = method.isStatic() ? 0 : 1;
+        int currentParamIndex = 0;
+
+        for (final JvmType param : method.params()) {
+            if (currentParamIndex == paramIndex) {
+                return currentLvtIndex;
+            }
+
+            currentParamIndex++;
+            currentLvtIndex++;
+            if (param == PrimitiveType.LONG || param == PrimitiveType.DOUBLE) {
+                currentLvtIndex++;
+            }
+        }
+
+        return -1;
+    }
 }