From 411a588850604d67e8a606c0496999f5065d35d5 Mon Sep 17 00:00:00 2001
From: Bertrand Augereau
Date: Sun, 30 Mar 2008 05:42:39 +0000
Subject: Introduction of a fixed size memory pool with a typical free list
 implementation + : amortized O(1) allocation, O(1) deallocation, less
 overhead per allocation - : unused memory is not reclaimed until death or
 manual invocation of a function

svn-id: r31320
---
 common/memorypool.cpp | 94 +++++++++++++++++++++++++++++++++++++++++++++++++++
 common/memorypool.h   | 34 +++++++++++++++++++
 common/module.mk      |  1 +
 3 files changed, 129 insertions(+)
 create mode 100644 common/memorypool.cpp
 create mode 100644 common/memorypool.h

diff --git a/common/memorypool.cpp b/common/memorypool.cpp
new file mode 100644
index 0000000000..8bd7f802ca
--- /dev/null
+++ b/common/memorypool.cpp
@@ -0,0 +1,94 @@
+#include "common/memorypool.h"
+#include <algorithm>
+
+namespace Common
+{
+
+static const size_t CHUNK_PAGE_SIZE = 32;
+
+void* MemoryPool::allocPage() {
+  void* result = ::malloc(CHUNK_PAGE_SIZE * _chunkSize);
+  _pages.push_back(result);
+  void* current = result;
+  for(size_t i=1; i<CHUNK_PAGE_SIZE; ++i)
+  {
+    void* next    = ((char*)current + _chunkSize);
+    *(void**)current = next;
+    
+    current = next;
+  }
+  *(void**)current = NULL;
+  return result;
+}
+
+MemoryPool::MemoryPool(size_t chunkSize) {
+   // You must at least fit the pointer in the node (technically unneeded considering the next rounding statement)
+  _chunkSize = std::max(chunkSize, sizeof(void*));
+  // There might be an alignment problem on some platforms when trying to load a void* on a non natural boundary
+  // so we round to the next sizeof(void*)
+  _chunkSize = (_chunkSize + sizeof(void*) - 1) & (~(sizeof(void*) - 1));
+
+  _next = NULL;
+}
+
+MemoryPool::~MemoryPool() {
+  for(size_t i=0; i<_pages.size(); ++i)
+    free(_pages[i]);
+}
+
+void* MemoryPool::malloc() {
+#if 1
+  if(!_next)
+    _next = allocPage();
+
+  void* result = _next;
+  _next = *(void**)result;
+  return result;
+#else
+  return ::malloc(_chunkSize);
+#endif
+}
+
+void MemoryPool::free(void* ptr) {
+#if 1
+  *(void**)ptr = _next;
+  _next = ptr;
+#else
+  ::free(ptr);
+#endif
+}
+
+// Technically not compliant C++ to compare unrelated pointers. In practice...
+bool MemoryPool::isPointerInPage(void* ptr, void* page) {
+    return (ptr >= page) && (ptr < (char*)page + CHUNK_PAGE_SIZE * _chunkSize);
+}
+
+void MemoryPool::freeUnusedPages() {
+  //std::sort(_pages.begin(), _pages.end());
+  Array<int> numberOfFreeChunksPerPage;
+  numberOfFreeChunksPerPage.resize(_pages.size());
+  for(size_t i=0; i<numberOfFreeChunksPerPage.size(); ++i) {
+      numberOfFreeChunksPerPage[i] = 0;
+  }
+  
+  void* iterator = _next;
+  while(iterator) {
+    // This should be a binary search
+    for(size_t i=0; i<_pages.size(); ++i) {
+      if(isPointerInPage(iterator, _pages[i])) {
+	++numberOfFreeChunksPerPage[i];
+	break;
+      }
+    }
+    iterator = *(void**)iterator;
+  }
+
+  for(size_t i=0; i<_pages.size(); ++i) {
+    if(numberOfFreeChunksPerPage[i] == CHUNK_PAGE_SIZE) {
+      free(_pages[i]);
+      _pages[i] = NULL; // TODO : Remove NULL values
+    }
+  }
+}
+
+}
diff --git a/common/memorypool.h b/common/memorypool.h
new file mode 100644
index 0000000000..05a58e430c
--- /dev/null
+++ b/common/memorypool.h
@@ -0,0 +1,34 @@
+#ifndef COMMON_MEMORYPOOL_H
+#define COMMON_MEMORYPOOL_H
+
+#include <cstring>
+#include "common/array.h"
+
+namespace Common
+{
+
+class MemoryPool
+{
+ private:
+  MemoryPool(const MemoryPool&);
+  MemoryPool& operator=(const MemoryPool&);
+
+  size_t       _chunkSize;
+  Array<void*> _pages;
+  void*        _next;
+
+  void* allocPage();
+  bool  isPointerInPage(void* ptr, void* page);
+ public:
+  MemoryPool(size_t chunkSize);
+  ~MemoryPool();
+
+  void* malloc();
+  void  free(void* ptr);
+
+  void freeUnusedPages();
+};
+
+}
+
+#endif
diff --git a/common/module.mk b/common/module.mk
index bd3e9f21a0..a7c0f4eb90 100644
--- a/common/module.mk
+++ b/common/module.mk
@@ -7,6 +7,7 @@ MODULE_OBJS := \
 	file.o \
 	fs.o \
 	hashmap.o \
+	memorypool.o \
 	md5.o \
 	mutex.o \
 	str.o \
-- 
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