From 25cc02431b61097b2adfc2fbc493b648b0300c3b Mon Sep 17 00:00:00 2001
From: Mason Reed <mason@vector35.com>
Date: Mon, 10 Mar 2025 11:05:40 -0400
Subject: [SharedCache] Refactor Shared Cache

In absence of a better name, this commit refactors the shared cache code.
---
 view/sharedcache/core/FileAccessorCache.cpp | 112 ++++++++++++++++++++++++++++
 1 file changed, 112 insertions(+)
 create mode 100644 view/sharedcache/core/FileAccessorCache.cpp

(limited to 'view/sharedcache/core/FileAccessorCache.cpp')

diff --git a/view/sharedcache/core/FileAccessorCache.cpp b/view/sharedcache/core/FileAccessorCache.cpp
new file mode 100644
index 00000000..047bbe19
--- /dev/null
+++ b/view/sharedcache/core/FileAccessorCache.cpp
@@ -0,0 +1,112 @@
+#include "FileAccessorCache.h"
+
+#include <cassert>
+
+CacheAccessorID GetCacheAccessorID(const std::string& filePath)
+{
+	constexpr std::hash<std::string> hasher;
+	return static_cast<CacheAccessorID>(hasher(filePath));
+}
+
+FileAccessorCache::FileAccessorCache(size_t cacheSize)
+{
+	m_cacheSize = cacheSize;
+	m_accessors = {};
+}
+
+void FileAccessorCache::EvictLastUsed()
+{
+	if (m_cache.empty())
+		return;
+	// Evict the least recently used element.
+	const auto lruID = m_cache.front();
+	m_cache.pop_front();
+	// Ensure the least recently used ID actually exists in the accessors map.
+	assert(m_accessors.find(lruID) != m_accessors.end() && "Evicting non-existent ID from accessors map");
+	m_accessors.erase(lruID);
+}
+
+FileAccessorCache& FileAccessorCache::Global()
+{
+	static FileAccessorCache cache {};
+	return cache;
+}
+
+
+WeakFileAccessor FileAccessorCache::Open(const std::string& filePath)
+{
+	const auto id = GetCacheAccessorID(filePath);
+	std::unique_lock lock(m_mutex);
+
+	// Check if the file is already in the cache.
+	if (const auto it = m_accessors.find(id); it != m_accessors.end())
+	{
+		// Move the accessed ID to the back so we keep it in the cache.
+		auto pos = std::find(m_cache.begin(), m_cache.end(), id);
+		if (pos != m_cache.end())
+			m_cache.erase(pos);
+		m_cache.push_back(id);
+
+		return WeakFileAccessor(it->second, filePath);
+	}
+
+	// Evict if we are going to go above the limit.
+	if (m_cache.size() >= m_cacheSize)
+		EvictLastUsed();
+
+	// Create a new file accessor and add it to the cache.
+	auto accessor = MappedFileAccessor::Open(filePath);
+	if (accessor == nullptr)
+	{
+		// We failed to open the file, we must throw hard!
+		// TODO: Make this mechanism more thought out...
+		throw std::runtime_error("Failed to open file: " + filePath);
+	}
+	auto sharedAccessor = std::make_shared<MappedFileAccessor>(std::move(*accessor));
+	m_accessors.insert_or_assign(id, sharedAccessor);
+	m_cache.push_back(id);
+
+	return WeakFileAccessor(sharedAccessor, filePath);
+}
+
+void FileAccessorWriteLog::AddPointer(const size_t address, const size_t pointer)
+{
+	// TODO: A sharded map here would be better. see: rust dashmap
+	std::unique_lock<std::shared_mutex> lock(m_persistedMutex);
+	m_persistedPointers[address] = pointer;
+}
+
+void FileAccessorWriteLog::ApplyWrites(MappedFileAccessor& accessor)
+{
+	std::shared_lock<std::shared_mutex> lock(m_persistedMutex);
+	for (const auto& [address, pointer] : m_persistedPointers)
+		accessor.WritePointer(address, pointer);
+}
+
+std::shared_ptr<MappedFileAccessor> WeakFileAccessor::lock()
+{
+	auto sharedPtr = m_weakPtr.lock();
+	if (!sharedPtr)
+	{
+		// This will revive other weak pointers to the same shared ptr.
+		// Update the weak pointer to the newly created shared instance
+		m_weakPtr = FileAccessorCache::Global().Open(m_filePath).m_weakPtr;
+		sharedPtr = m_weakPtr.lock();
+
+		// Apply any previously written pointers back.
+		if (sharedPtr)
+			m_writeLog->ApplyWrites(*sharedPtr);
+	}
+
+	return sharedPtr;
+}
+
+void WeakFileAccessor::WritePointer(const size_t address, const size_t pointer)
+{
+	// Persist the pointer after the file accessor is revived.
+	m_writeLog->AddPointer(address, pointer);
+
+	// And then actually apply the written pointer...
+	if (auto sharedPtr = m_weakPtr.lock())
+		sharedPtr->WritePointer(address, pointer);
+}
-- 
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