cache.hxx

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#pragma once
 
#include <sal/config.h>
 
#include <cassert>
#include <cstddef>
#include <map>
#include <list>
 
#include <sal/types.h>
 
namespace binaryurp {
 
namespace cache {
 
enum { size = 256, ignore = 0xFFFF };
 
}
 
template< typename T > class Cache {
public:
    typedef sal_uInt16 IdxType;
 
    explicit Cache(std::size_t size):
        size_(size)
    {
        assert(size < cache::ignore);
    }
 
    IdxType add( const T& rContent, bool* pbFound) {
        assert( pbFound != nullptr);
        if( !size_) {
            *pbFound = false;
            return cache::ignore;
        }
        // try to insert into the map
        list_.push_front( rContent); // create a temp entry
        auto const [it, inserted] = map_.emplace( list_.begin(), 0 );
        *pbFound = !inserted;
 
        if( !inserted) { // insertion not needed => found the entry
            list_.pop_front(); // remove the temp entry
            list_.splice( list_.begin(), list_, it->first); // the found entry is moved to front
            return it->second;
        }
 
        // test insertion successful => it was new so we keep it
        IdxType n = static_cast<IdxType>( map_.size() - 1);
        if( n >= size_) { // cache full => replace the LRU entry
            // find the least recently used element in the map
            typename LruItMap::iterator lru = map_.find( --list_.end());
            n = lru->second;
            map_.erase( lru); // remove it from the map
            list_.pop_back(); // remove from the list
        }
        it->second = n;
        return n;
    }
 
private:
    Cache(const Cache&) = delete;
    Cache& operator=(const Cache&) = delete;
 
    typedef std::list<T> LruList; // last recently used list
    typedef typename LruList::iterator LruListIt;
    struct CmpT{ bool operator()( const LruListIt& rA, const LruListIt& rB) const { return (*rA<*rB);}};
    typedef std::map< LruListIt, IdxType, CmpT > LruItMap; // a map into a LruList
 
    std::size_t size_;
    LruItMap map_;
    LruList list_;
};
 
}
 
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