add slice assignment for openarray

stew/byteutils.nim

@@ -22,6 +22,20 @@ func `&`*[N1, N2: static[int], T](
   result[0 ..< N1] = a
   result[N1 ..< result.len] = b
 
+template `^^`(s, i: untyped): untyped =
+  (when i is BackwardsIndex: s.len - int(i) else: int(i))
+
+func `[]=`*[T, U, V](r: var openArray[T], s: HSlice[U, V], v: openArray[T]) =
+  ## openArray slice assignment:
+  ## v[0..<2] = [0, 1]
+  let a = r ^^ s.a
+  let b = r ^^ s.b
+  let L = b - a + 1
+  if L == v.len:
+    for i in 0..<L: r[i + a] = v[i]
+  else:
+    raise newException(RangeError, "different lengths for slice assignment")
+
 ########################################################################################################
 #####################################   Hex utilities   ################################################
 

tests/test_byteutils.nim

@@ -87,3 +87,13 @@ suite "Byte utils":
       "a".toBytes() == @[byte(ord('a'))]
       string.fromBytes([byte(ord('a'))]) == "a"
       cast[ptr UncheckedArray[byte]](cstring(string.fromBytes([byte(ord('a'))])))[1] == byte(0)
+
+  test "slices":
+    var a: array[4, byte]
+    a[0..<2] = [2'u8, 3]
+    check:
+      a[1] == 3
+
+    a.toOpenArray(0, 3)[0..<2] = [4'u8, 5]
+    check:
+      a[1] == 5