Add a helper for creating open arrays from pointers (#36)

* Add a helper for creating open arrays from pointers

* doh, toOpenArray is broken in Nim 0.19.0

* Make openarrays from typed pointers

* remove the ptr.toOpenArray seq work-around

ranges/ptr_arith.nim

@@ -8,3 +8,19 @@ proc distance*(a, b: pointer): int {.inline.} =
 
 proc shift*[T](p: ptr T, delta: int): ptr T {.inline.} =
   cast[ptr T](shift(cast[pointer](p), delta * sizeof(T)))
+
+when (NimMajor,NimMinor,NimPatch) >= (0,19,9):
+  template makeOpenArray*[T](p: ptr T, len: int): auto =
+    toOpenArray(cast[ptr UncheckedArray[T]](p), 0, len - 1)
+
+  template makeOpenArray*(p: pointer, T: type, len: int): auto =
+    toOpenArray(cast[ptr UncheckedArray[T]](p), 0, len - 1)
+
+else:
+  # TODO: These are fallbacks until we upgrade to 0.19.9
+  template makeOpenArray*(p: pointer, T: type, len: int): auto =
+    toOpenArray(cast[ptr array[0, T]](p)[], 0, len - 1)
+
+  template makeOpenArray*[T](p: ptr T, len: int): auto =
+    toOpenArray(cast[ptr array[0, T]](p)[], 0, len - 1)
+

tests/tbitranges.nim

@@ -45,7 +45,7 @@ suite "bit ranges":
     check $asciiBits == "0100000101010011010000110100100101001001"
 
   test "concat operator":
-    randomize(0)
+    randomize(5000)
 
     for i in 0..<256:
       var xSeq = randomBytes(rand(i))

tests/ttypedranges.nim

@@ -1,6 +1,6 @@
 import
   unittest, sets,
-  ../ranges/typedranges
+  ../ranges/[typedranges, ptr_arith]
 
 suite "Typed ranges":
   test "basic stuff":
@@ -237,3 +237,18 @@ suite "Typed ranges":
       $b.toOpenArray == "[]"
       b.aecheck(1) == 2
       $b.toOpenArray == "[]"
+
+  test "make openarrays from pointers":
+    var str = "test 1,2,3"
+    var charPtr: ptr char = addr str[7]
+    var regularPtr: pointer = addr str[5]
+
+    check:
+      # (regularPtr.makeOpenArray(char, 4).len == 4)
+      (regularPtr.makeOpenArray(char, 5) == "1,2,3")
+      (regularPtr.makeOpenArray(char, 5) == str[5..9])
+
+      # (charPtr.makeOpenArray(3).len == 3)
+      (charPtr.makeOpenArray(3) == "2,3")
+      (charPtr.makeOpenArray(1) == str[7..7])
+