fix `baseAddr`, move to `ptrops` (#156)

* return `nil` for empty openArrays / avoid returning `16`
* move to `ptrops`, deprecate `ptr_arith` fully

stew/ptrops.nim

@@ -48,3 +48,13 @@ template distance*[T](a, b: ptr T): int =
   # Number of elements between a and b - undefined behavior when difference
   # exceeds what can be represented in an int
   distance(cast[pointer](a), cast[pointer](b)) div sizeof(T)
+
+proc baseAddr*[T](x: openArray[T]): ptr T =
+  # Return the address of the zero:th element of x or `nil` if x is empty
+  if x.len == 0: nil else: cast[ptr T](x)
+
+template makeOpenArray*[T](p: ptr T, len: Natural): openArray[T] =
+  toOpenArray(cast[ptr UncheckedArray[T]](p), 0, len - 1)
+
+template makeOpenArray*(p: pointer, T: type, len: Natural): openArray[T] =
+  toOpenArray(cast[ptr UncheckedArray[T]](p), 0, len - 1)

stew/ranges/ptr_arith.nim

@@ -1,8 +1,8 @@
+{.deprecated: "use stew/ptrops".}
+
 import ../ptrops
 export ptrops
 
-proc baseAddr*[T](x: openArray[T]): pointer = cast[pointer](x)
-
 # Please note that we use templates here on purpose.
 # As much as I believe in the power of optimizing compilers, it turned
 # out that the use of forced inlining with templates still creates a
@@ -13,10 +13,3 @@ template shift*(p: pointer, delta: int): pointer {.deprecated: "use ptrops".} =
 
 template shift*[T](p: ptr T, delta: int): ptr T {.deprecated: "use ptrops".} =
   p.offset(delta)
-
-template makeOpenArray*[T](p: ptr T, len: Natural): auto =
-  toOpenArray(cast[ptr UncheckedArray[T]](p), 0, len - 1)
-
-template makeOpenArray*(p: pointer, T: type, len: Natural): auto =
-  toOpenArray(cast[ptr UncheckedArray[T]](p), 0, len - 1)
-

tests/test_ptrops.nim

@@ -59,3 +59,30 @@ suite "ptrops":
       p0.distance(p0) == 0
       p0.distance(p1) == 2
       p1.distance(p0) == -2
+
+  test "baseAddr":
+    block arrays:
+      var
+        v0: array[0, int] = []
+        v1 = [22]
+
+      check:
+        baseAddr(v0) == nil
+        baseAddr(v1) == addr v1[0]
+        baseAddr(v1)[] == v1[0]
+
+    block seqs:
+      var
+        v0: seq[int]
+        v1 = @[22]
+
+      check:
+        baseAddr(v0) == nil
+        baseAddr(v1) == addr v1[0]
+        baseAddr(v1)[] == v1[0]
+
+    block oas:
+      var v = 56
+      check:
+        baseAddr(makeOpenArray(nil, int, 0)) == nil
+        baseAddr(makeOpenArray(addr v, 1)) == addr v