From 9014d77ab028b70d80dd6e93ff60b9744bd1780c Mon Sep 17 00:00:00 2001
From: Nathan Sashihara <21227491+n8sh@users.noreply.github.com>
Date: Thu, 30 Jan 2020 22:07:37 -0800
Subject: [PATCH] Fix Issue 20548 - Use bit vector instead of bool[] in
 RandomCover when choices cannot be packed in a single word

---
 std/random.d | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/std/random.d b/std/random.d
index 84aaa3d17..58fe5c5a0 100644
--- a/std/random.d
+++ b/std/random.d
@@ -2950,9 +2950,10 @@ do
 - bigger length means non-GC heap allocation(s) and dealloc. +/
 private struct RandomCoverChoices
 {
-    private void* buffer;
+    private size_t* buffer;
     private immutable size_t _length;
     private immutable bool hasPackedBits;
+    private enum BITS_PER_WORD = typeof(buffer[0]).sizeof * 8;
 
     void opAssign(T)(T) @disable;
 
@@ -2963,8 +2964,9 @@ private struct RandomCoverChoices
 
         if (!hasPackedBits && buffer !is null)
         {
-            void* nbuffer = enforceMalloc(_length);
-            buffer = memcpy(nbuffer, buffer, _length);
+            const nBytesToAlloc = 8 * (_length / BITS_PER_WORD + int(_length % BITS_PER_WORD != 0));
+            void* nbuffer = enforceMalloc(nBytesToAlloc);
+            buffer = cast(size_t*) memcpy(nbuffer, buffer, nBytesToAlloc);
         }
     }
 
@@ -2976,7 +2978,8 @@ private struct RandomCoverChoices
         hasPackedBits = _length <= size_t.sizeof * 8;
         if (!hasPackedBits)
         {
-            buffer = enforceCalloc(numChoices, 1);
+            const nWordsToAlloc = _length / BITS_PER_WORD + int(_length % BITS_PER_WORD != 0);
+            buffer = cast(size_t*) enforceCalloc(nWordsToAlloc, BITS_PER_WORD / 8);
         }
     }
 
@@ -2993,8 +2996,9 @@ private struct RandomCoverChoices
     bool opIndex(size_t index) const pure nothrow @nogc @trusted
     {
         assert(index < _length);
+        import core.bitop : bt;
         if (!hasPackedBits)
-            return *((cast(bool*) buffer) + index);
+            return cast(bool) bt(buffer, index);
         else
             return ((cast(size_t) buffer) >> index) & size_t(1);
     }
@@ -3004,7 +3008,11 @@ private struct RandomCoverChoices
         assert(index < _length);
         if (!hasPackedBits)
         {
-            *((cast(bool*) buffer) + index) = value;
+            import core.bitop : btr, bts;
+            if (value)
+                bts(buffer, index);
+            else
+                btr(buffer, index);
         }
         else
         {