diff --git a/src/lib/array/timsort.ml b/src/lib/array/timsort.ml
index 254edb0..4d79015 100644
--- a/src/lib/array/timsort.ml
+++ b/src/lib/array/timsort.ml
@@ -63,63 +63,86 @@ let%test_module _ =
 
 let rec merge_lo
   cmp
-  dest ofs
-  src0 ofs0 len0
-  src1 ofs1 len1
+  dest beg
+  src0 beg0 end0 x0
+  src1 beg1 end1 x1
 =
-  assert (Array.length dest >= ofs + len0 + len1);
-  assert (Array.length src0 >= ofs0 + len0);
-  assert (Array.length src1 >= ofs1 + len1);
-  if len0 = 0 then
-    Array.blit src1 ofs1 dest ofs len1
-  else if len1 = 0 then
-    Array.blit src0 ofs0 dest ofs len0
-  else if cmp src0.(ofs0) src1.(ofs1) <= 0 then begin
-    dest.(ofs) <- src0.(ofs0);
-    merge_lo
-      cmp
-      dest (ofs + 1)
-      src0 (ofs0 + 1) (len0 - 1)
-      src1 ofs1 len1
+  assert (Array.length dest >= beg + (end0 - beg0 + 1) + (end1 - beg1 + 1));
+  assert (Array.length src0 > end0);
+  assert (Array.length src1 > end1);
+  assert (x0 = src0.(beg0));
+  assert (x1 = src1.(beg1));
+  assert (end0 >= beg0);
+  assert (end1 >= beg1);
+
+  (* This is used to optimise the case len0 = 1 below. *)
+  assert (dest == src1 && beg + (end0 - beg0 + 1) = beg1);
+
+  if cmp x0 x1 <= 0 then begin
+    dest.(beg) <- x0;
+    (* If this was the last element of run0 we stop. Besides, since we have
+     * [dest == src1], the remaining element of run1 are already where they
+     * are supposed to be and we can exit. *)
+    if beg0 < end0 then
+      merge_lo
+        cmp
+        dest (beg + 1)
+        src0 (beg0 + 1) end0 src0.(beg0 + 1)
+        src1 beg1 end1 x1
   end else begin
-    dest.(ofs) <- src1.(ofs1);
-    merge_lo
-      cmp
-      dest (ofs + 1)
-      src0 ofs0 len0
-      src1 (ofs1 + 1) (len1 - 1)
+    dest.(beg) <- x1;
+    (* If this was the last element of run1 we stop. There remains to move all
+     * the elements of run0 at the beginning of dest. *)
+    if beg1 < end1 then
+      merge_lo
+        cmp
+        dest (beg + 1)
+        src0 beg0 end0 x0
+        src1 (beg1 + 1) end1 src1.(beg1 + 1)
+    else
+      Array.blit src0 beg0 dest (beg + 1) (end0 - beg0 + 1)
   end
 
-
 let rec merge_hi
   cmp
-  dest ofs
-  src0 ofs0 len0
-  src1 ofs1 len1
+  dest end_
+  src0 beg0 end0 x0 (* run0 *)
+  src1 beg1 end1 x1 (* run1 *)
 =
-  assert (Array.length dest >= ofs + len0 + len1);
-  assert (Array.length src0 >= ofs0 + len0);
-  assert (Array.length src1 >= ofs1 + len1);
-  assert (len0 >= 0);
-  assert (len1 >= 0);
-  if len0 = 0 then
-    Array.blit src1 ofs1 dest ofs len1
-  else if len1 = 0 then
-    Array.blit src0 ofs0 dest ofs len0
-  else if cmp src0.(ofs0 + len0 - 1) src1.(ofs1 + len1 - 1) <= 0 then begin
-    dest.(ofs + len0 + len1 - 1) <- src1.(ofs1 + len1 - 1);
-    merge_hi
-      cmp
-      dest ofs
-      src0 ofs0 len0
-      src1 ofs1 (len1 - 1)
+  assert (Array.length dest > end_);
+  assert (Array.length src0 > end0);
+  assert (Array.length src1 > end1);
+  assert (x0 = src0.(end0));
+  assert (x1 = src1.(end1));
+  assert (end0 >= beg0);
+  assert (end1 >= beg1);
+  (* This is used to optimise the case len1 = 1 below. *)
+  assert (dest == src0 && end_ - (end1 - beg1 + 1) = end0);
+
+  if cmp x0 x1 <= 0 then begin
+    dest.(end_) <- x1;
+    (* If this was the last element of run1 we stop. Besides, since we have
+     * [dest == src0], the remaining element of run0 are already where they
+     * are supposed to be and we can exit. *)
+    if beg1 < end1 then
+      merge_hi
+        cmp
+        dest (end_ - 1)
+        src0 beg0 end0 x0
+        src1 beg1 (end1 - 1) src1.(end1 - 1)
   end else begin
-    dest.(ofs + len0 + len1 - 1) <- src0.(ofs0 + len0 - 1);
-    merge_hi
-      cmp
-      dest ofs
-      src0 ofs0 (len0 - 1)
-      src1 ofs1 len1
+    dest.(end_) <- x0;
+    (* If this was the last element of run0 we stop. There remains to move all
+     * the elements of run1 at the end of dest. *)
+    if beg0 < end0 then
+      merge_hi
+        cmp
+        dest (end_ - 1)
+        src0 beg0 (end0 - 1) src0.(end0 - 1)
+        src1 beg1 end1 x1
+    else
+      let len1 = end1 - beg1 + 1 in
+      Array.blit src1 beg1 dest (end_ - len1) len1
   end
 
 
@@ -131,15 +154,16 @@ let merge ~buffer cmp t (ofs0, len0) (ofs1, len1) =
     merge_lo
       cmp
       t ofs0
-      buffer 0 len0
-      t ofs1 len1
+      buffer 0 (len0 - 1) buffer.(0)
+      t ofs1 (ofs1 + len1 - 1) t.(ofs1)
   end else begin
     Array.blit t ofs1 buffer 0 len1;
+    let top0 = ofs0 + len0 - 1 in
     merge_hi
       cmp
-      t ofs0
-      t ofs0 len0
-      buffer 0 len1
+      t (top0 + len1)
+      t ofs0 top0 t.(top0)
+      buffer 0 (len1 - 1) buffer.(len1 - 1)
   end;
   ofs0, len0 + len1