diff --git a/fcore/bin-search.sml b/fcore/bin-search.sml
new file mode 100644
index 0000000..c752672
--- /dev/null
+++ b/fcore/bin-search.sml
@@ -0,0 +1,23 @@
+structure BinSearch =
+struct
+  local
+    fun helpFind (findNum, vec, low, high) =
+      if high >= low then
+        let
+          val mid = low + ((high - low) div 2)
+          val curNum = Vector.sub (vec, mid)
+        in
+          if curNum = findNum then
+            mid
+          else if curNum < findNum then
+            helpFind (findNum, vec, mid + 1, high)
+          else
+            helpFind (findNum, vec, low, mid - 1)
+        end
+      else
+        ~1
+  in
+    fun find (findNum, vec) =
+      helpFind (findNum, vec, 0, Vector.length vec - 1)
+  end
+end
diff --git a/oms.mlb b/oms.mlb
index 54ec3af..6c0ccdd 100644
--- a/oms.mlb
+++ b/oms.mlb
@@ -2,6 +2,7 @@ $(SML_LIB)/basis/basis.mlb
 
 (* fcore *)
 fcore/quad-tree.sml
+fcore/bin-search.sml
 
 ann
   "allowVectorExps true"