if only we have used purely functional data structures...

problem47.ml

@@ -0,0 +1,25 @@
+let sieve bound =
+  let bitmap = Array.make (bound+1) 0 in
+  let root = bound / 2 in 
+    for i = 2 to root do
+      if (bitmap.(i) == 0) then 
+        let j = ref (i*2) 
+        in   
+          while (!j <= bound) do
+            bitmap.(!j) <- bitmap.(!j) + 1;
+            j := !j + i; 
+          done;
+    done;
+    bitmap
+;;
+
+
+let solve =
+  let primes = sieve 1000000 in
+  let rec consecutive n acc =
+    match (primes.(n)) with
+      | 4 -> if ((acc+1)=4) then (n-3) else consecutive (n+1) (acc+1)
+      | _ -> consecutive (n+1) 0
+  in 
+    consecutive 2 0
+;;