From f2e0698608bbbf84a8a9bfd69ffae54c5128d860 Mon Sep 17 00:00:00 2001
From: Joseph Montanaro <jfmontanaro@modg.org>
Date: Mon, 19 Jul 2021 11:45:51 -0700
Subject: [PATCH] switch to fastrand (not actually any faster)

---
 combinators.nim |  4 ++--
 fastrand.nim    | 46 ++++++++++++++++++++++++++++++++++------------
 2 files changed, 36 insertions(+), 14 deletions(-)

diff --git a/combinators.nim b/combinators.nim
index 903d1a7..01b4c0d 100644
--- a/combinators.nim
+++ b/combinators.nim
@@ -1,5 +1,5 @@
 import algorithm, random, sugar
-import fixedseq, game
+import fastrand, fixedseq, game
 
 
 proc nextPermutation(x: var FixedSeq): bool =
@@ -93,4 +93,4 @@ proc randomFuture*(dice: FixedSeq, r: var Rand): FixedSeq[5, Die, int8] =
   result.initFixedSeq
   let order = dice.dup(shuffle(r))
   for i, color in order:
-    result.add((color, r.rand(1..3)))
+    result.add((color, r.fastRand(1..3)))
diff --git a/fastrand.nim b/fastrand.nim
index 9480411..93f02e1 100644
--- a/fastrand.nim
+++ b/fastrand.nim
@@ -17,13 +17,27 @@ proc formatRate(n: Natural, d: Duration): string =
   result = formatNum(1_000_000'f64 * n.float64 / d.inMicroseconds.float64)
 
 
-proc fastRand(r: var Rand, x: Natural): uint64 =
-  # return ((r.next shr 32) * x.uint64) shr 32
-  let x = x.uint64
-  if x <= (uint64.high shl 32):
-    return ((r.next shr 32) * x.uint64) shr 32
+const upperBound = uint64(uint32.high)
+
+
+proc fastRand*[T: Natural](r: var Rand, x: T): T =
+  # Nim ranges are usually inclusive, but this algorithm is exclusive
+  let x = x.uint64 + 1
+  let num = if x <= upperBound:
+    ((r.next shr 32) * x.uint64) shr 32
   else:
-    return r.next mod x.uint64
+    r.next mod x.uint64
+  result = T(num)
+
+
+proc fastRand*(r: var Rand; x, y: Natural): Natural =
+  let lim = (y - x)
+  result = fastRand(r, lim) + x
+
+
+proc fastRand*[T](r: var Rand, slice: HSlice[T, T]): T =
+  let n = fastRand(r, slice.a.Natural, slice.b.Natural)
+  result = T(n)
 
 
 proc testFastRand(num = 1_000_000_000): Duration =
@@ -44,9 +58,17 @@ proc testStdRand(num = 1_000_000_000): Duration =
   # echo "std rand execution rate: ", 1000 * num / dur.inMilliseconds.int, " generated per second."
 
 
-randomize()
-let runs = 100_000_000
-let fr = testFastRand(runs)
-echo "fastrand execution rate: ", formatNum(1_000_000 * runs / fr.inMicroseconds.int)
-let sr = testStdRand(runs)
-echo "standard execution rate: ", formatNum(1_000_000 * runs / sr.inMicroseconds.int)
+when isMainModule:
+  randomize()
+  var r = initRand(rand(int64))
+  let runs = 100_000_000
+  var totals: array[5..9, int]
+  for i in 1 .. runs:
+    let n = r.fastRand(5..9)
+    totals[n] += 1
+  echo totals
+
+  # let fr = testFastRand(runs)
+  # echo "fastrand execution rate: ", formatNum(1_000_000 * runs / fr.inMicroseconds.int)
+  # let sr = testStdRand(runs)
+  # echo "standard execution rate: ", formatNum(1_000_000 * runs / sr.inMicroseconds.int)