diff --git a/solvers/UParabolicReflectorDish.pas b/solvers/UParabolicReflectorDish.pas
index 5658d5b..9f20c03 100644
--- a/solvers/UParabolicReflectorDish.pas
+++ b/solvers/UParabolicReflectorDish.pas
@@ -204,27 +204,34 @@ end;
 procedure TPlatform.Tilt(constref AIntervals: TCubeRockIntervalsList; constref ASource, ATarget:
   TIntegersList; const AUp: Boolean);
 var
-  i, rock, nextFree, step: Integer;
-  interval: TCubeRockInterval;
+  i, j, rock, nextFree, direction: Integer;
 begin
   if AUp then
-    step := 1
+    direction := 1
   else
-    step := -1;
+    direction := -1;
   for i := 0 to AIntervals.Count - 1 do
   begin
-    for interval in AIntervals[i] do
+    j := 0;
+    if AUp then
+      nextFree := AIntervals[i][j].Start
+    else
+      nextFree := AIntervals[i][j].Stop;
+    for rock in ASource[i] do
     begin
-      if AUp then
-        nextFree := interval.Start
-      else
-        nextFree := interval.Stop;
-      for rock in ASource[i] do
-        if (interval.Start <= rock) and (rock <= interval.Stop) then
-        begin
-          ATarget[nextFree].Add(i);
-          Inc(nextFree, step);
-        end;
+      // Goes to next interval if rock is not in current interval.
+      while AIntervals[i][j].Stop < rock do
+      begin
+        Inc(j);
+        if AUp then
+          nextFree := AIntervals[i][j].Start
+        else
+          nextFree := AIntervals[i][j].Stop;
+      end;
+
+      // rock must now be in current interval.
+      ATarget[nextFree].Add(i);
+      Inc(nextFree, direction);
     end;
     ASource[i].Clear;
   end;