89 lines
1.9 KiB
C++
89 lines
1.9 KiB
C++
#ifndef DISTINCT_H_
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#define DISTINCT_H_
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#include <boost/type_traits/decay.hpp>
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#include <boost/utility/result_of.hpp>
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template<typename Source, typename KeyExtract>
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class Distinct
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{
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private:
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typedef typename boost::decay<
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typename boost::result_of<KeyExtract(typename Source::ElementType&)>::type
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>::type KeyType;
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public:
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typedef typename Source::ElementType ElementType;
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typedef typename Source::MarkerType MarkerType;
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public:
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Distinct(Source source, KeyExtract keyExtract)
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: source(source), keyExtract(keyExtract), hasKey(false)
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{}
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bool step()
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{
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if(!this->source.step() )
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return false;
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while(this->hasKey && this->key == this->keyExtract(*this->source.get() ) )
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{
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if(!this->source.step() )
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return false;
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}
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this->hasKey = true;
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this->key = this->keyExtract(*this->source.get() );
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return true;
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}
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ElementType* get()
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{
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return this->source.get();
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}
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MarkerType mark() const
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{
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return this->source.mark();
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}
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void restore(MarkerType mark)
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{
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this->source.restore(mark);
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this->hasKey = true;
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this->key = this->keyExtract(*this->source.get() );
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}
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private:
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Source source;
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KeyType key;
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KeyExtract keyExtract;
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bool hasKey;
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};
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namespace db {
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template<typename KeyExtract>
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struct DistinctOp
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{
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KeyExtract keyExtract;
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template<typename Source>
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friend Distinct<Source, KeyExtract> operator|(Source source, const DistinctOp& op)
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{
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return Distinct<Source, KeyExtract>(source, op.keyExtract);
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}
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};
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template<typename KeyExtract>
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inline DistinctOp<KeyExtract> distinctBy(KeyExtract keyExtract)
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{
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DistinctOp<KeyExtract> result = {keyExtract};
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return result;
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}
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}
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#endif
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