fixup! Improve missing column error message in CS DSL

Author: koperagen

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/DataFrameReceiver.kt

@@ -59,19 +59,21 @@ internal open class DataFrameReceiver<T>(
             else -> this
         }
 
+    // Context:
     // it's strange that we have to reverse-search why the column is missing
     // would be nice to "fail fast" exactly where resolve failed, knowing the current path and parent.
     // but it's very unclear what to do with resolveSingle.
     // at first glance: a lot of changes.
+    @Suppress("FoldInitializerAndIfToElvis")
     private fun formatMissingColumnMessage(path: ColumnPath): String {
         val fullPath = path.joinToString()
 
-        for (index in path.indices) {
-            val currentPath = path.slice(0..index).toPath()
+        for (depth in path.indices) {
+            val currentPath = path.slice(0..depth).toPath()
             val currentPathString = currentPath.joinToString()
             val column = df.getColumnOrNull(currentPath)
             if (column == null) {
-                return if (index == 0) {
+                return if (depth == 0) {
                     "Column '$currentPathString' not found among ${df.columnNames()}."
                 } else {
                     val parentPath = currentPath.dropLast()
@@ -85,7 +87,7 @@ internal open class DataFrameReceiver<T>(
                 }
             }
 
-            if (index != path.lastIndex) {
+            if (depth != path.lastIndex) {
                 if (!column.isColumnGroup()) {
                     return "Column '$fullPath' cannot be resolved: '$currentPathString' is not a column group."
                 }