Fix linear forwards backwards

lib/mod_list_troubleshooter.dart

@@ -15,6 +15,7 @@ import 'package:rimworld_modman/mod_list.dart';
 class ModListTroubleshooter {
   final ModList originalModList;
   ModList currentModList;
+  // These indices should ALWAYS represent the CURRENT selection of mods
   int _startIndex = 0;
   int _endIndex = 0;
 
@@ -48,19 +49,24 @@ class ModListTroubleshooter {
     return currentModList;
   }
 
+  // If the current selection is not equal to our proposed step size
+  // We do not MOVE but instead just return the correct amount of mods from the start
   ModList linearForward({int stepSize = 20}) {
-    final totalMods = originalModList.activeMods.length;
-    
-    if (_startIndex >= totalMods || _startIndex == _endIndex) {
-      int newStart = totalMods - stepSize;
-      if (newStart < 0) {
-        newStart = 0;
-      }
-      _startIndex = newStart;
+    // If we are not "in step"
+    if (_endIndex - _startIndex == stepSize) {
+      // Move the indices forward by the step size, step forward
+      _startIndex += stepSize;
+      _endIndex += stepSize;
+    } else {
+      // Correct the end index to be in step
+      _endIndex = _startIndex + stepSize;
+    }
+    if (_endIndex > originalModList.activeMods.length) {
+      // If we are at the end of the list, move the start index such that we return
+      // At most the step size amount of mods
+      _endIndex = originalModList.activeMods.length;
+      _startIndex = (_endIndex - stepSize).clamp(0, _endIndex);
     }
-
-    _startIndex = _startIndex.clamp(0, totalMods);
-    _endIndex = (_startIndex + stepSize).clamp(0, totalMods);
 
     final subset = originalModList.activeMods.keys.toList().sublist(
       _startIndex,
@@ -69,24 +75,20 @@ class ModListTroubleshooter {
 
     currentModList.disableAll();
     currentModList.enableMods(subset);
-    _startIndex = _endIndex;
-    
     return currentModList;
   }
 
   ModList linearBackward({int stepSize = 20}) {
-    final totalMods = originalModList.activeMods.length;
-
-    if (_endIndex <= 0 || _startIndex == _endIndex) {
-      int newEnd = stepSize;
-      if (newEnd > totalMods) {
-        newEnd = totalMods;
-      }
-      _endIndex = newEnd;
+    if (_endIndex - _startIndex == stepSize) {
+      _startIndex -= stepSize;
+      _endIndex -= stepSize;
+    } else {
+      _startIndex = _endIndex - stepSize;
+    }
+    if (_startIndex < 0) {
+      _startIndex = 0;
+      _endIndex = stepSize.clamp(0, originalModList.activeMods.length);
     }
-
-    _endIndex = _endIndex.clamp(0, totalMods);
-    _startIndex = (_endIndex - stepSize).clamp(0, _endIndex);
 
     final subset = originalModList.activeMods.keys.toList().sublist(
       _startIndex,
@@ -95,8 +97,6 @@ class ModListTroubleshooter {
 
     currentModList.disableAll();
     currentModList.enableMods(subset);
-    _endIndex = _startIndex;
-    
     return currentModList;
   }