Assignment 5 Done

assets/why3/assignment05/binary_search.mlw

@@ -18,6 +18,20 @@ module BinarySearch
     ensures  { forall i: int. result <= i < length a -> v <= a[i] }
   =
     (* IMPORTANT: DON'T MODIFY THE ABOVE LINES *)
-    0 (* TODO *)
+    let ref l = 0 in
+    let ref r = length a in
+    while l < r do
+        invariant { 0 <= l <= length a /\ 0 <= r <= length a }
+        invariant { forall i: int. 0 <= i < l -> a[i] < v }
+        invariant { forall i: int. r <= i < length a -> a[i] >= v }
+        variant { r - l }
+        let m = l + div (r - l) 2 in
+        assert { l <= m < r };
+        if a[m] < v then
+            l := m + 1
+        else
+            r := m
+    done;
+    l
 
 end