Fixing overlapping intervals bug (#138)

why:
  did not properly merge

stew/interval_set.nim

@@ -426,14 +426,15 @@ proc merge[P,S](ds: Desc[P,S]; iv: Segm[P,S]): Segm[P,S] =
     #     iv:     ...----------------)
     #     p:      ...-----)
     #
-    p.blk.size += iv.len # update database
-    ds.ptsCount += iv.len   # update database
+    let extend = iv.right - p.right
+    p.blk.size += extend   # update database
+    ds.ptsCount += extend  # update database
     #
     #     iv:     ...----------------)
     #     p:      ...----------------)
     #     result:          [---------)
     #
-    return Segm[P,S].new(p.right, iv.right)
+    return Segm[P,S].new(p.right - extend, iv.right)
 
   # now: iv.right <= p.right and p.left <= iv.left:
   if p.left <= iv.left:

tests/test_interval_set.nim

@@ -239,6 +239,18 @@ suite "IntervalSet: Intervals of FancyPoint entries over FancyScalar":
     check br.ge(uHigh) == iv(uHigh,uHigh)
     check br.envelope(uHigh) == iv(uHigh,uHigh)
 
+  test "Merge overlapping intervals":
+    br.clear()
+    check br.merge(100, 199) == 100
+    check br.merge(150, 200) == 1
+    check br.total == 101
+    check br.chunks == 1
+    check br.verify.isOk
+    check br.merge( 99, 150) == 1
+    check br.total == 102
+    check br.chunks == 1
+    check br.verify.isOk
+
   test "Merge disjunct intervals on 1st set":
     br.clear()
     check br.merge(  0,  99) == 100