题记：写这篇博客要主是加深自己对元素缓冲区的认识和总结实现算法时的一些验经和训教，如果有错误请指出，万分感谢。
关于形环缓冲区的识知，请看这里
http://en.wikipedia.org/wiki/Circular_buffer
面上这个网址已绍介得非常详细了。
面下这个网址有 RingBuffer的C代码实现， 实际上是一个C的源开库 liblcthw 里实现的。
http://c.learncodethehardway.org/book/ex44.html
源开库 liblcthw的网址为 https://github.com/zedshaw/liblcthw， 用C代码实现了一些用常的数据结构，list，map，tree，字符串函数，ring buffer等，习学C语言的人值得看看。
boost 库里也有形环缓冲区的实现， 详细用使的例子如下：
#include <boost/circular_buffer.hpp>
int main(int /*argc*/, char* /*argv*/[]) {
// 创立一个形环缓冲区来放存三个int类型的数据
boost::circular_buffer<int> cb(3);
//插入元素
cb.push_back(1);
cb.push_back(2);
cb.push_back(3);
int a = cb[0]; // a == 1
int b = cb[1]; // b == 2
int c = cb[2]; // c == 3
//形环缓冲区在现已满了，继承插入元素将会覆盖掉最面前的元素
cb.push_back(4); // 用4覆盖了1
cb.push_back(5); // 用5覆盖了2
//形环缓冲区在现含包元素 3, 4 和 5.
a = cb[0]; // a == 3
b = cb[1]; // b == 4
c = cb[2]; // c == 5
//元素可以被从面后掏出也可从面前掏出
cb.pop_back(); // 5 被掏出
cb.pop_front(); // 3 被掏出
int d = cb[0]; // d == 4
return 0;
}
面上的例子很单简， 可以让我们对用使方法做一个单简的解理。
有兴致的同窗还可以看看面下的这个例子。
#include <boost/circular_buffer.hpp>
#include <numeric>
#include <assert.h>
int main(int /*argc*/, char* /*argv*/[])
{
//创立一个容量为3的形环缓冲区
boost::circular_buffer<int> cb(3);
//插入2个元素进入形环缓冲区
cb.push_back(1);
cb.push_back(2);
// assertions
assert(cb[0] == 1);
assert(cb[1] == 2);
assert(!cb.full());
assert(cb.size() == 2);
assert(cb.capacity() == 3);
//再插入2个元素
cb.push_back(3);
cb.push_back(4);
//算计容器里全部元素之和
int sum = std::accumulate(cb.begin(), cb.end(), 0);
//断言
assert(cb[0] == 2);
assert(cb[1] == 3);
assert(cb[2] == 4);
assert(*cb.begin() == 2);
assert(cb.front() == 2);
assert(cb.back() == 4);
assert(sum == 9);
assert(cb.full());
assert(cb.size() == 3);
assert(cb.capacity() == 3);
return 0;
}
还有一种特别的形环缓冲区叫 界边缓冲区，界边缓冲区是一个型典的产生者消费者模式，产生者产生和存储元素，消费者掏出元素，然后行进处置。界边缓冲区有个别特的地方，就是缓冲区满了的时候， 产生者证保不会行进插入元素的任务， 直一要到有闲暇空间的时候，才会插入新元素。
界边缓冲区的实现如下所示 ：
#include <boost/circular_buffer.hpp>
#include <boost/thread/mutex.hpp>
#include <boost/thread/condition.hpp>
#include <boost/thread/thread.hpp>
#include <boost/call_traits.hpp>
#include <boost/progress.hpp>
#include <boost/bind.hpp>
template <class T>
class bounded_buffer {
public:
typedef boost::circular_buffer<T> container_type;
typedef typename container_type::size_type size_type;
typedef typename container_type::value_type value_type;
typedef typename boost::call_traits<value_type>::param_type param_type;
explicit bounded_buffer(size_type capacity) : m_unread(0), m_container(capacity) {}
void push_front(boost::call_traits<value_type>::param_type item) {
// param_type represents the "best" way to pass a parameter of type value_type to a method
boost::mutex::scoped_lock lock(m_mutex);
m_not_full.wait(lock, boost::bind(&bounded_buffer<value_type>::is_not_full, this));
m_container.push_front(item);
++m_unread;
lock.unlock();
m_not_empty.notify_one();
}
void pop_back(value_type* pItem) {
boost::mutex::scoped_lock lock(m_mutex);
m_not_empty.wait(lock, boost::bind(&bounded_buffer<value_type>::is_not_empty, this));
*pItem = m_container[--m_unread];
lock.unlock();
m_not_full.notify_one();
}
private:
bounded_buffer(const bounded_buffer&); // Disabled copy constructor
bounded_buffer& operator = (const bounded_buffer&); // Disabled assign operator
bool is_not_empty() const { return m_unread > 0; }
bool is_not_full() const { return m_unread < m_container.capacity(); }
size_type m_unread;
container_type m_container;
boost::mutex m_mutex;
boost::condition m_not_empty;
boost::condition m_not_full;
};
1. push_front() 方法被产生者程线用调，的目是插入新元素到buffer中。这个方法会锁住mutex，直一等到有新空间可以插入新元素。 (Mutex锁在待等间期是没有锁住的，只有条件满意的时候才会把锁锁上) 假如在buffer中有一个可用的空间，执行就会继承，该方法就会插入元素进入到形环缓冲区的尾末。 然后未读元素的数量就会增长，然后主动解锁。 (在例子中，Mutex锁解锁是会抛出一个异常，锁会在scoped_lock象对的析构函数中主动被打开). 最后，这个方法会通知其中的一个消费者程线，告知他们，有一个新的元素插入了缓冲区。
2. pop_back() 方法被消费者程线用调，的目是为了从buffer中读取下一个元素。这个方法会锁住Mutex然后待等，直到有一个未读的元素进入缓冲区。 假如至少有一个未读元素的时候，这个方法就会增长未读元素的数量，然后从circular_buffer中读取一个未读元素。然后就解锁Mutex，并通知待等中的一个产生者程线，告知它又新的空间可以插入新元素了。
3. 这个 pop_back() 方法移除元素，元素仍然留在 circular_buffer 里，这样的话，当这个技巧比移除一个元素更有率效。circular_buffer满的时候它就会被产生者程线用一个新的元素代替。
面下的网址是一个形环缓冲区的 C++ 实现，可以用来处置二进制数据，改天有空了翻译一下，便利大家浏览。
http://www.asawicki.info/news_1468_circular_buffer_of_raw_binary_data_in_c.html
Circular Buffer of Raw Binary Data in C++
Circular Buffer, Cyclic Buffer or Ring Buffer is a data structure that effectively manages a queue of some items. Items can be added at the back and removed from the front. It has limited capacity because it is based on preallocated array. Functionality is implemented using two pointers or indices - pointing to the first and past the last valid element. The Begin pointer is incremented whenever an item is popped from the front so that it "chases" the End pointer, which is incremented whenever a new item is pushed to the back. They can both wrap around the size of the array. Both operations are done very effectively - in constant time O(1) and no reallocations are needed. This makes circular buffers perfect solution for queues of some data streams, like video or audio.
It's not very sophisticated data structure, but there is one problem. Sample codes of circular buffers you can find on the Internet, just like for many other data structures, operate usually on a single object of some user-defined type. What if we need a buffer for raw binary data, stored as array of bytes? We can treat single bytes as data items, but enqueueing and dequeueing single bytes with separate function calls would not be efficient. We can, on the other hand, define some block of data (like 4096 bytes) as the type of item, but this limits us to operating on on such block at a time.
Best solution would be to write an implementation that operates on binary data in form of (const char *bytes, size_t byte_count) and allows writing and reading arbitrary amount of data in a single call, just like functions for writing and reading files do. The only problem that arises in such code is that sometimes the block of data you want to write to or read from the buffer is not in a continuous region of memory, but wraps around to the beginning of the array so we have to process it on two parts - first at the end of the array and the second at the beginning.
Here is my C++ implementation of a circular buffer for raw binary data:
#include <algorithm> // for std::min
class CircularBuffer
{
public:
CircularBuffer(size_t capacity);
~CircularBuffer();
size_t size() const { return size_; }
size_t capacity() const { return capacity_; }
// Return number of bytes written.
size_t write(const char *data, size_t bytes);
// Return number of bytes read.
size_t read(char *data, size_t bytes);
private:
size_t beg_index_, end_index_, size_, capacity_;
char *data_;
};
CircularBuffer::CircularBuffer(size_t capacity)
: beg_index_(0)
, end_index_(0)
, size_(0)
, capacity_(capacity)
{
data_ = new char[capacity];
}
CircularBuffer::~CircularBuffer()
{
delete [] data_;
}
size_t CircularBuffer::write(const char *data, size_t bytes)
{
if (bytes == 0) return 0;
size_t capacity = capacity_;
size_t bytes_to_write = std::min(bytes, capacity - size_);
// Write in a single step
if (bytes_to_write <= capacity - end_index_)
{
memcpy(data_ + end_index_, data, bytes_to_write);
end_index_ += bytes_to_write;
if (end_index_ == capacity) end_index_ = 0;
}
// Write in two steps
else
{
size_t size_1 = capacity - end_index_;
memcpy(data_ + end_index_, data, size_1);
size_t size_2 = bytes_to_write - size_1;
memcpy(data_, data + size_1, size_2);
end_index_ = size_2;
}
size_ += bytes_to_write;
return bytes_to_write;
}
size_t CircularBuffer::read(char *data, size_t bytes)
{
if (bytes == 0) return 0;
size_t capacity = capacity_;
size_t bytes_to_read = std::min(bytes, size_);
// Read in a single step
if (bytes_to_read <= capacity - beg_index_)
{
memcpy(data, data_ + beg_index_, bytes_to_read);
beg_index_ += bytes_to_read;
if (beg_index_ == capacity) beg_index_ = 0;
}
// Read in two steps
else
{
size_t size_1 = capacity - beg_index_;
memcpy(data, data_ + beg_index_, size_1);
size_t size_2 = bytes_to_read - size_1;
memcpy(data + size_1, data_, size_2);
beg_index_ = size_2;
}
size_ -= bytes_to_read;
return bytes_to_read;
}
Similar phenomenon can be observed in API of the FMOD sound library. Just like graphical textures in DirectX, sound samples in FMOD can also be "locked" to get pointer to a raw memory we can read or fill. But DirectX textures lie in the continuous memory region, so we get a single pointer. The only difficult thing in understanding locking textures is the concept of "stride", which can be greater than the width of a single row. Here in FMOD the Sound::lock() method returns two pointers and two lengths, probably because the locked region can wrap over end of internally used circular buffer like the one shown above.
文章结束给大家分享下程序员的一些笑话语录：
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