1 files changed, 35 insertions, 2 deletions
diff --git a/lib/pure/random.nim b/lib/pure/random.nim
index c458d51eb..d6501c87e 100644
--- a/lib/pure/random.nim
+++ b/lib/pure/random.nim
@@ -14,6 +14,8 @@
 ##
 ## **Do not use this module for cryptographic purposes!**
 
+import algorithm                    #For upperBound
+
 include "system/inclrtl"
 {.push debugger:off.}
 
@@ -155,14 +157,45 @@ proc rand*[T](x: HSlice[T, T]): T =
   ## For a slice `a .. b` returns a value in the range `a .. b`.
   result = rand(state, x)
 
-proc rand*[T](r: var Rand; a: openArray[T]): T =
+proc rand*[T](r: var Rand; a: openArray[T]): T {.deprecated.} =
   ## returns a random element from the openarray `a`.
+  ## **Deprecated since v0.20.0:** use ``sample`` instead.
   result = a[rand(r, a.low..a.high)]
 
-proc rand*[T](a: openArray[T]): T =
+proc rand*[T](a: openArray[T]): T {.deprecated.} =
   ## returns a random element from the openarray `a`.
+  ## **Deprecated since v0.20.0:** use ``sample`` instead.
   result = a[rand(a.low..a.high)]
 
+proc sample*[T](r: var Rand; a: openArray[T]): T =
+  ## returns a random element from openArray ``a`` using state in ``r``.
+  result = a[r.rand(a.low..a.high)]
+
+proc sample*[T](a: openArray[T]): T =
+  ## returns a random element from openArray ``a`` using non-thread-safe state.
+  result = a[rand(a.low..a.high)]
+
+proc sample*[T, U](r: var Rand; a: openArray[T], w: openArray[U], n=1): seq[T] =
+  ## Return a sample (with replacement) of size ``n`` from elements of ``a``
+  ## according to convertible-to-``float``, not necessarily normalized, and
+  ## non-negative weights ``w``.  Uses state in ``r``.  Must have sum ``w > 0.0``.
+  assert(w.len == a.len)
+  var cdf = newSeq[float](a.len)   # The *unnormalized* CDF
+  var tot = 0.0                    # Unnormalized is fine if we sample up to tot
+  for i, w in w:
+    assert(w >= 0)
+    tot += float(w)
+    cdf[i] = tot
+  assert(tot > 0.0)                # Need at least one non-zero weight
+  for i in 0 ..< n:
+    result.add(a[cdf.upperBound(r.rand(tot))])
+
+proc sample*[T, U](a: openArray[T], w: openArray[U], n=1): seq[T] =
+  ## Return a sample (with replacement) of size ``n`` from elements of ``a``
+  ## according to convertible-to-``float``, not necessarily normalized, and
+  ## non-negative weights ``w``.  Uses default non-thread-safe state.
+  state.sample(a, w, n)
+
 
 proc initRand*(seed: int64): Rand =
   ## Creates a new ``Rand`` state from ``seed``.