arraybuf: new constructor (#236)

* arraybuf: new constructor

also add a block to avoid symbol leaks

* speed up for same length

stew/arraybuf.nim

@@ -48,19 +48,22 @@ template len*(b: ArrayBuf): int =
   int(b.n)
 
 template setLen*(b: var ArrayBuf, newLenParam: int) =
-  newLenParam.evalOnceAs(newLen)
-  let nl = typeof(b.n)(newLen)
-  for i in newLen ..< b.len():
-    reset(b.buf[i]) # reset cleared items when shrinking
-  b.n = nl
+  block:
+    newLenParam.evalOnceAs(newLen)
+    let nl = typeof(b.n)(newLen)
+    for i in newLen ..< b.len():
+      reset(b.buf[i]) # reset cleared items when shrinking
+    b.n = nl
 
 template data*(bParam: ArrayBuf): openArray =
-  bParam.evalOnceAs(b)
-  b.buf.toOpenArray(0, b.len() - 1)
+  block:
+    bParam.evalOnceAs(b)
+    b.buf.toOpenArray(0, b.len() - 1)
 
 template data*(bParam: var ArrayBuf): var openArray =
-  bParam.evalOnceAs(b)
-  b.buf.toOpenArray(0, b.len() - 1)
+  block:
+    bParam.evalOnceAs(b)
+    b.buf.toOpenArray(0, b.len() - 1)
 
 iterator items*[N, T](b: ArrayBuf[N, T]): lent T =
   for i in 0 ..< b.len:
@@ -92,6 +95,22 @@ template `==`*(a, b: ArrayBuf): bool =
 template `<`*(a, b: ArrayBuf): bool =
   a.data() < b.data()
 
+template initCopyFrom*[N, T](
+    _: type ArrayBuf[N, T], data: openArray[T]
+): ArrayBuf[N, T] =
+  var v: ArrayBuf[N, T]
+  v.n = typeof(v.n)(v.buf.copyFrom(data))
+  v
+
+template initCopyFrom*[N, T](
+    _: type ArrayBuf[N, T], data: array[N, T]
+): ArrayBuf[N, T] =
+  # Shortcut version that avoids zeroMem on matching lengths
+  ArrayBuf[N, T](
+    buf: data,
+    n: N
+  )
+
 template add*[N, T](b: var ArrayBuf[N, T], v: T) =
   ## Adds items up to capacity then drops the rest
   # TODO `b` is evaluated multiple times but since it's a `var` this should

tests/test_arraybuf.nim

@@ -56,3 +56,16 @@ suite "ArrayBuf":
       v.len == 1
     v.setLen(2)
     doAssert v.data() == [byte 1, 0]
+
+  test "construction":
+    let
+      a0 = ArrayBuf[4, byte].initCopyFrom([])
+      a2 = ArrayBuf[2, byte].initCopyFrom([byte 2, 3, 4, 5])
+      a3 = ArrayBuf[5, byte].initCopyFrom([byte 2, 3, 4])
+      a5 = ArrayBuf[5, byte].initCopyFrom([byte 2, 3])
+
+    check:
+      a0.len == 0
+      a2.data() == [byte 2, 3]
+      a3.data() == [byte 2, 3, 4]
+      a5.data() == [byte 2, 3]