From f6dae4edbcd541d06b204dd85f76694256f8f70d Mon Sep 17 00:00:00 2001
From: Zahary Karadjov <zahary@gmail.com>
Date: Sat, 23 Jun 2018 16:25:33 +0300
Subject: [PATCH] bugfix: allow the slice operators to return empty ranges

---
 ranges/bitranges.nim   | 5 +++--
 ranges/typedranges.nim | 7 ++++++-
 tests/ttypedranges.nim | 5 ++++-
 3 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/ranges/bitranges.nim b/ranges/bitranges.nim
index d796d14..05c6fde 100644
--- a/ranges/bitranges.nim
+++ b/ranges/bitranges.nim
@@ -137,8 +137,9 @@ proc `[]`*(x: BitRange, idx: int): bool {.inline.} =
 proc sliceNormalized(x: BitRange, ibegin, iend: int): BitRange =
   assert ibegin >= 0 and
          ibegin < x.len and
-         iend >= ibegin and
-         iend < x.len
+         iend < x.len and
+         iend + 1 >= ibegin # the +1 here allows the result to be
+                            # an empty range
 
   result.data  = x.data
   result.start = x.start + ibegin
diff --git a/ranges/typedranges.nim b/ranges/typedranges.nim
index 8cb7a4b..daa4519 100644
--- a/ranges/typedranges.nim
+++ b/ranges/typedranges.nim
@@ -113,7 +113,12 @@ proc `$`*(r: Range): string =
   result &= "]"
 
 proc sliceNormalized[T](r: Range[T], ibegin, iend: int): Range[T] =
-  assert(ibegin >= 0 and ibegin < r.len and iend >= ibegin and iend < r.len)
+  assert ibegin >= 0 and
+         ibegin < r.len and
+         iend < r.len and
+         iend + 1 >= ibegin # the +1 here allows the result to be
+                            # an empty range
+
   when rangesGCHoldEnabled:
     result.gcHold = r.gcHold
   result.start = r.start.shift(ibegin)
diff --git a/tests/ttypedranges.nim b/tests/ttypedranges.nim
index b2ea9df..27bec80 100644
--- a/tests/ttypedranges.nim
+++ b/tests/ttypedranges.nim
@@ -19,6 +19,9 @@ suite "Typed ranges":
     let b = toRange(@[1, 2, 3])
     a[1 .. 3] = b
     check a.toSeq == @[0, 1, 2, 3, 0]
+    check:
+      a[2 .. 2].len == 1
+      a[1 ..< 1].len == 0
 
   test "equality operator":
     var x = toRange(@[0, 1, 2, 3, 4, 5])
@@ -95,4 +98,4 @@ suite "Typed ranges":
     check hash(uu) == hash(u)
     check hash(v) == hash(vv)
     check hash(uu) != hash(vv)
-     
\ No newline at end of file
+