From 9717ae49a479e3266aed834dc4b1f12239dd83b8 Mon Sep 17 00:00:00 2001
From: ADAM David Alan Martin <adam@recursive.engineer>
Date: Thu, 5 Sep 2024 18:48:07 -0400
Subject: [PATCH] Use fast random to decide when to split Blobs.

---
 Memory/Blob.h | 41 +++++++++++++++++++++++++++++------------
 1 file changed, 29 insertions(+), 12 deletions(-)

diff --git a/Memory/Blob.h b/Memory/Blob.h
index 5123be8..1481725 100644
--- a/Memory/Blob.h
+++ b/Memory/Blob.h
@@ -5,10 +5,12 @@ static_assert( __cplusplus > 2020'99 );
 #include <Alepha/Alepha.h>
 
 #include <cassert>
+#include <cstdlib>
 
 #include <memory>
 
 #include <Alepha/swappable.h>
+#include <Alepha/fastRandom.h>
 #include <Alepha/Exception.h>
 #include <Alepha/error.h>
 
@@ -30,6 +32,9 @@ namespace Alepha::Hydrogen  ::detail::  Blob_m
 
 	namespace C
 	{
+		const auto doubleBlobPointerOption= "ALEPHA_USE_DOUBLE_BLOB_POINTERS";
+		const int storageSplitRandomBitDepth= 12;		// TODO: Environment tunable?  Global tunable?
+
 		const bool debug= false;
 		const bool debugLifecycle= false or C::debug;
 		const bool debugCtors= false or C::debugLifecycle or C::debug;
@@ -77,20 +82,29 @@ namespace Alepha::Hydrogen  ::detail::  Blob_m
 			Buffer< Mutable > buffer;
 			std::size_t viewLimit= 0; // TODO: Consider allowing for unrooted sub-buffer views?
 
-			// TODO: Take the `storage` parameter and make it not increment when this ctor is called -- only when the dice roll passes.
+			// Potentially rollover the internal storage, during a blob sharing operation.
+			void
+			maybeRollover()
+			{
+				if( not ::getenv( C::doubleBlobPointerOption ) ) return;
+
+				if( Alepha::fastRandomBits( C::storageSplitRandomBitDepth ) ) return;
+
+				// The inner pointer gets incremented.
+				std::shared_ptr< Blob > inner= *storage;
+
+				// Capture the new inner into the outer.
+				storage= std::make_shared< std::shared_ptr< Blob > >( std::move( inner ) );
+
+				// Post rollover, there should only be one reference to start.
+				assert( storage.use_count() == 1 );
+			}
+
+
+			// Takeover a portion of a shared block
 			explicit
 			Blob( IndirectStorage storage, Buffer< Mutable > buffer ) noexcept
-				: storage( Utility::evaluate <=[storage= std::move( storage )] () -> IndirectStorage
-				{
-					//if( fastRandomBits( C::storageSplitRandomBitDepth ) )
-					return std::move( storage );
-					//if( C::debugSplitSharing ) error() << "Observed a use count of " << storage.use_count() << " when we failed the dice roll." << std::endl;
-					//auto split= std::make_shared< std::shared_ptr< Blob > >( *storage );
-					//if( C::
-					//return split;
-				}),
-				buffer( buffer ),
-				viewLimit( buffer.size() )
+				: storage( std::move( storage ) ), buffer( buffer ), viewLimit( buffer.size() )
 			{}
 
 		public:
@@ -242,7 +256,10 @@ namespace Alepha::Hydrogen  ::detail::  Blob_m
 					viewLimit= (*storage)->viewLimit;
 				}
 
+				assert( storage );
+
 				// Now we assume that there's a two-layer scheme, so we operate based upon that.
+				maybeRollover();
 
 				Blob rv{ storage, Buffer< Mutable >{ buffer, amount } };
 				buffer= buffer + amount;