diff --git a/src/main/java/puzzle/SwedishGenerator.java b/src/main/java/puzzle/SwedishGenerator.java
index 02ffd12..90ff8e6 100644
--- a/src/main/java/puzzle/SwedishGenerator.java
+++ b/src/main/java/puzzle/SwedishGenerator.java
@@ -1,5 +1,6 @@
 package puzzle;
 
+import javax.naming.Context;
 import javax.xml.crypto.Data;
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
@@ -54,7 +55,7 @@ public record SwedishGenerator(int W, int H, int SIZE, int MAX_LEN, int[] buff)
       OFFSETS[6] = new nbrs_8(0, 1);
       STEPS[6]   = new nbrs_8(1, 0);
    }
-   final static nbrs_8[] nbrs8       = new nbrs_8[]{
+   final static nbrs_8[] nbrs8     = new nbrs_8[]{
            new nbrs_8(-1, -1),
            new nbrs_8(-1, 0),
            new nbrs_8(-1, 1),
@@ -64,15 +65,26 @@ public record SwedishGenerator(int W, int H, int SIZE, int MAX_LEN, int[] buff)
            new nbrs_8(1, 0),
            new nbrs_8(1, 1)
    };
-   static final nbrs_8[] nbrs4       = new nbrs_8[]{
+   static final nbrs_8[] nbrs4     = new nbrs_8[]{
            new nbrs_8(-1, 0),
            new nbrs_8(1, 0),
            new nbrs_8(0, -1),
            new nbrs_8(0, 1)
    };
-   static final char     FIRST_ABC   = 'A';
-   static final char     LAST_ABC    = 'Z';
-   static final char     FIRST_ARROW = '1', LAST_ARROW = '6', HOR_ARROW_1 = '2', HOR_ARROW_2 = '4';
+   static final char     FIRST_ABC = 'A';
+   static final char     LAST_ABC  = 'Z';
+   
+   static record Context(int[] covH,
+                         int[] covV,
+                         int[] cellCount,
+                         int[] stack,
+                         BitSet seen) {
+      
+      public Context() { this(new int[256], new int[256], new int[256], new int[256], new BitSet(256)); }
+   }
+   
+   static final ThreadLocal<Context> CTX = ThreadLocal.withInitial(Context::new);
+   
    static boolean isLetter(char ch) { return ch >= FIRST_ABC && ch <= LAST_ABC; }
    
    static final class Rng {
@@ -368,8 +380,11 @@ public record SwedishGenerator(int W, int H, int SIZE, int MAX_LEN, int[] buff)
       var slots = extractSlots(grid);
       if (slots.isEmpty()) return 1_000_000_000L;
       
-      var covH = new int[H][W];
-      var covV = new int[H][W];
+      var ctx  = CTX.get();
+      var covH = ctx.covH;
+      var covV = ctx.covV;
+      Arrays.fill(covH, 0, SIZE, 0);
+      Arrays.fill(covV, 0, SIZE, 0);
       
       for (var s : slots) {
          if (s.len < MIN_LEN) {
@@ -384,24 +399,27 @@ public record SwedishGenerator(int W, int H, int SIZE, int MAX_LEN, int[] buff)
          }
          
          for (var i = 0; i < s.len; i++) {
-            int r = s.r(i), c = s.c(i);
-            if (s.horiz()) covH[r][c] += 1;
-            else covV[r][c] += 1;
+            int r   = s.r(i), c = s.c(i);
+            int idx = grid.offset(r, c);
+            if (s.horiz()) covH[idx] += 1;
+            else covV[idx] += 1;
          }
       }
       
       for (var r = 0; r < H; r++)
          for (var c = 0; c < W; c++) {
             if (grid.isDigitAt(r, c)) continue;
-            int h = covH[r][c], v = covV[r][c];
+            int idx = grid.offset(r, c);
+            int h   = covH[idx], v = covV[idx];
             if (h == 0 && v == 0) penalty += 1500;
             else if (h > 0 && v > 0) { /* ok */ } else if (h + v == 1) penalty += 200;
             else penalty += 600;
          }
       
       // clue clustering (8-connected)
-      var seen  = new BitSet(8);
-      var stack = new int[SIZE];
+      var seen = ctx.seen;
+      seen.clear();
+      var stack = ctx.stack;
       int sp, idx;
       
       for (var r = 0; r < H; r++)
@@ -632,9 +650,9 @@ public record SwedishGenerator(int W, int H, int SIZE, int MAX_LEN, int[] buff)
       return pat;
    }
    
-   static int slotScore(int[][] cellCount, Slot s) {
+   static int slotScore(int[] cellCount, Slot s, Grid grid) {
       var cross = 0;
-      for (var i = 0; i < s.len; i++) cross += (cellCount[s.r(i)][s.c(i)] - 1);
+      for (var i = 0; i < s.len; i++) cross += (cellCount[grid.offset(s.r(i), s.c(i))] - 1);
       return cross * 10 + s.len;
    }
    
@@ -674,8 +692,10 @@ public record SwedishGenerator(int W, int H, int SIZE, int MAX_LEN, int[] buff)
       var used     = new BitSet();
       var assigned = new HashMap<Integer, Lemma>();
       
-      var cellCount = new int[H][W];
-      for (var s : slots) for (var i = 0; i < s.len; i++) cellCount[s.r(i)][s.c(i)]++;
+      var ctx       = CTX.get();
+      var cellCount = ctx.cellCount;
+      Arrays.fill(cellCount, 0, SIZE, 0);
+      for (var s : slots) for (var i = 0; i < s.len; i++) cellCount[grid.offset(s.r(i), s.c(i))]++;
       
       var       t0      = System.currentTimeMillis();
       final var lastLog = new java.util.concurrent.atomic.AtomicLong(t0);
@@ -727,7 +747,7 @@ public record SwedishGenerator(int W, int H, int SIZE, int MAX_LEN, int[] buff)
             
             if (best == null
                 || info.count < bestInfo.count
-                || (info.count == bestInfo.count && slotScore(cellCount, s) > slotScore(cellCount, best))) {
+                || (info.count == bestInfo.count && slotScore(cellCount, s, grid) > slotScore(cellCount, best, grid))) {
                best     = s;
                bestInfo = info;
                if (info.count <= 1) break;
@@ -740,7 +760,7 @@ public record SwedishGenerator(int W, int H, int SIZE, int MAX_LEN, int[] buff)
             return new Pick(best, bestInfo, false);
          }
       };
-      IO.println("hit");
+      System.out.println("hit");
       
       class Solver {
          
@@ -766,10 +786,6 @@ public record SwedishGenerator(int W, int H, int SIZE, int MAX_LEN, int[] buff)
             var pat   = patternForSlot(grid, s);
             
             Predicate<Lemma> tryWord = (Lemma w) -> {
-               if (w == null) {
-                  IO.println("tryword null Error");
-                  return false;
-               }
                if (used.get(w.index())) return false;
                
                for (var i = 0; i < pat.length; i++) {