circular fifo from the web added to the svn

git-svn-id: file:///afs/psi.ch/project/sls_det_software/svn/slsDetectorSoftware@331 951219d9-93cf-4727-9268-0efd64621fa3

slsDetectorSoftware/slsReceiver/circularFifo.h

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+/* CircularFifo.h 
+* Not any company's property but Public-Domain
+* Do with source-code as you will. No requirement to keep this
+* header if need to use it/change it/ or do whatever with it
+*
+* Note that there is No guarantee that this code will work 
+* and I take no responsibility for this code and any problems you
+* might get if using it. The code is highly platform dependent!
+*
+* Code & platform dependent issues with it was originally 
+* published at http://www.kjellkod.cc/threadsafecircularqueue
+* 2009-11-02
+* @author Kjell Hedström, hedstrom@kjellkod.cc */
+
+#ifndef CIRCULARFIFO_H_
+#define CIRCULARFIFO_H_
+
+/** Circular Fifo (a.k.a. Circular Buffer) 
+* Thread safe for one reader, and one writer */
+template<typename Element, unsigned int Size>
+class CircularFifo {
+public:
+   enum {Capacity = Size+1};
+
+   CircularFifo() : tail(0), head(0){}
+   virtual ~CircularFifo() {}
+
+   bool push(Element& item_);
+   bool pop(Element& item_);
+   
+   bool isEmpty() const;
+   bool isFull() const;
+   
+private:
+   volatile unsigned int tail; // input index
+   Element array[Capacity];
+   volatile unsigned int head; // output index
+
+   unsigned int increment(unsigned int idx_) const;
+};
+
+
+
+
+/** Producer only: Adds item to the circular queue. 
+* If queue is full at 'push' operation no update/overwrite
+* will happen, it is up to the caller to handle this case
+*
+* \param item_ copy by reference the input item
+* \return whether operation was successful or not */
+template<typename Element, unsigned int Size>
+bool CircularFifo<Element, Size>::push(Element& item_)
+{
+   int nextTail = increment(tail);
+   if(nextTail != head)
+   {
+      array[tail] = item_;
+      tail = nextTail;
+      return true;
+   }
+
+   // queue was full
+   return false;
+}
+
+/** Consumer only: Removes and returns item from the queue
+* If queue is empty at 'pop' operation no retrieve will happen
+* It is up to the caller to handle this case
+*
+* \param item_ return by reference the wanted item
+* \return whether operation was successful or not */
+template<typename Element, unsigned int Size>
+bool CircularFifo<Element, Size>::pop(Element& item_)
+{
+   if(head == tail)
+      return false;  // empty queue
+
+   item_ = array[head];
+   head = increment(head);
+   return true;
+}
+
+/** Useful for testinng and Consumer check of status
+  * Remember that the 'empty' status can change quickly
+  * as the Procuder adds more items.
+  *
+  * \return true if circular buffer is empty */
+template<typename Element, unsigned int Size>
+bool CircularFifo<Element, Size>::isEmpty() const
+{
+   return (head == tail);
+}
+
+/** Useful for testing and Producer check of status
+  * Remember that the 'full' status can change quickly
+  * as the Consumer catches up.
+  *
+  * \return true if circular buffer is full.  */
+template<typename Element, unsigned int Size>
+bool CircularFifo<Element, Size>::isFull() const
+{
+   int tailCheck = (tail+1) % Capacity;
+   return (tailCheck == head);
+}
+
+/** Increment helper function for index of the circular queue
+* index is inremented or wrapped
+*
+*  \param idx_ the index to the incremented/wrapped
+*  \return new value for the index */
+template<typename Element, unsigned int Size>
+unsigned int CircularFifo<Element, Size>::increment(unsigned int idx_) const
+{
+   // increment or wrap
+   // =================
+   //    index++;
+   //    if(index == array.lenght) -> index = 0;
+   //
+   //or as written below:   
+   //    index = (index+1) % array.length
+   idx_ = (idx_+1) % Capacity;
+   return idx_;
+}
+
+#endif /* CIRCULARFIFO_H_ */