inverse_vector.h

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#ifndef ROUTING_KIT_INVERSE_VECTOR_H
#define ROUTING_KIT_INVERSE_VECTOR_H
 
#include <routingkit/sort.h>
#include <routingkit/min_max.h>
 
#include <assert.h>
#include <algorithm>
#include <vector>
 
namespace RoutingKit{
 
//
// The inverse vector p of a vector v is a vector such that the elements
// v[p[i]], v[p[i]+1], v[p[i]+2], ..., v[p[i+1]-1] are exactly the elements
// with value i in v. If i does not occur in v, then p[i] == p[i+1]. v must be
// a sorted vector of unsigned integers.
//
 
inline
std::vector<unsigned>invert_vector(const std::vector<unsigned>&v, unsigned element_count){
    std::vector<unsigned>index(element_count+1);
    if(v.empty()){
        std::fill(index.begin(), index.end(), 0);
    }else{
        assert(is_sorted_using_less(v));
        assert(max_element_of(v) < element_count);
 
        index[0] = 0;
 
        unsigned pos = 0;
        for(unsigned i=0; i<element_count; ++i){
            while(pos < v.size() && v[pos] < i)
                ++pos;
            index[i] = pos;
        }
        index[element_count] = v.size();
    }
    return index;
}
 
inline
std::vector<unsigned>invert_inverse_vector(const std::vector<unsigned>&sorted_index){
    assert(!sorted_index.empty());
 
    std::vector<unsigned>v(sorted_index.back());
 
    for(unsigned i=0; i<sorted_index.size()-1; ++i)
        for(unsigned j=sorted_index[i]; j<sorted_index[i+1]; ++j)
            v[j] = i;
 
    return v;
}
 
} // RoutingKit
 
#endif