permutation.h

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#ifndef GEOGRAM_BASIC_PERMUTATION
#define GEOGRAM_BASIC_PERMUTATION
 
#include <geogram/basic/common.h>
#include <geogram/basic/numeric.h>
#include <geogram/basic/memory.h>
 
namespace GEO {
 
    namespace Permutation {
 
        inline bool is_valid(const vector<index_t>& permutation) {
            std::vector<bool> visited(permutation.size(), false);
            for(index_t i = 0; i < permutation.size(); i++) {
                if(permutation[i] >= permutation.size()) {
                    return false;
                }
                if(visited[permutation[i]]) {
                    return false;
                }
                visited[permutation[i]] = true;
            }
            return true;
        }
 
        inline void mark(
            vector<index_t>& permutation,
            index_t i
        ) {
            geo_debug_assert(i < permutation.size());
            geo_debug_assert(signed_index_t(permutation[i]) >= 0);
            permutation[i] = index_t(-signed_index_t(permutation[i]) - 1);
        }
 
        inline bool is_marked(
            const vector<index_t>& permutation,
            index_t i
        ) {
            geo_debug_assert(i < permutation.size());
            return signed_index_t(permutation[i]) < 0;
        }
 
        inline void unmark(
            vector<index_t>& permutation,
            index_t i
        ) {
            geo_debug_assert(is_marked(permutation, i));
            permutation[i] = index_t(-signed_index_t(permutation[i]) - 1);
        }
 
        inline void apply(
            void* data, const vector<index_t>& permutation_in,
            size_t elemsize
        ) {
            Memory::pointer pdata = (Memory::pointer) (data);
            vector<index_t>& permutation =
                const_cast<vector<index_t>&>(permutation_in);
            geo_debug_assert(is_valid(permutation));
            Memory::byte* temp = static_cast<Memory::byte*>(alloca(elemsize));
            for(index_t k = 0; k < permutation.size(); k++) {
                if(is_marked(permutation, k)) {
                    continue;
                }
                index_t i = k;
                index_t j = permutation[k];
                Memory::copy(temp, pdata + i * elemsize, elemsize);
                mark(permutation, k);
                while(j != k) {
                    Memory::copy(
                        pdata + i * elemsize, pdata + j * elemsize, elemsize
                    );
                    index_t nj = permutation[j];
                    mark(permutation, j);
                    i = j;
                    j = nj;
                }
                Memory::copy(pdata + i * elemsize, temp, elemsize);
            }
            for(index_t k = 0; k < permutation.size(); k++) {
                unmark(permutation, k);
            }
        }
 
        template <class T>
        inline void apply(
            vector<T>& data, const vector<index_t>& permutation_in
        ) {
            vector<index_t>& permutation =
                const_cast<vector<index_t>&>(permutation_in);
            geo_debug_assert(is_valid(permutation));
            T temp;
            for(index_t k = 0; k < permutation.size(); k++) {
                if(is_marked(permutation, k)) {
                    continue;
                }
                index_t i = k;
                temp = data[i];
                index_t j = permutation[k];
                mark(permutation, k);
                while(j != k) {
                    data[i] = data[j];
                    index_t nj = permutation[j];
                    mark(permutation, j);
                    i = j;
                    j = nj;
                }
                data[i] = temp;
            }
            for(index_t k = 0; k < permutation.size(); k++) {
                unmark(permutation, k);
            }
        }
 
        inline void invert(vector<index_t>& permutation) {
            geo_debug_assert(is_valid(permutation));
            for(index_t k = 0; k < permutation.size(); k++) {
                if(is_marked(permutation, k)) {
                    continue;
                }
                index_t i = k;
                index_t j = permutation[i];
                while(j != k) {
                    index_t temp = permutation[j];
                    permutation[j] = i;
                    mark(permutation, j);
                    i = j;
                    j = temp;
                }
                permutation[j] = i;
                mark(permutation, j);
            }
            for(index_t k = 0; k < permutation.size(); k++) {
                unmark(permutation, k);
            }
        }
 
        inline void invert(
            const vector<index_t>& permutation, vector<index_t>& invert
        ) {
            geo_debug_assert(is_valid(permutation));
            invert.resize(permutation.size());
            for(index_t i=0; i<permutation.size(); ++i) {
                invert[permutation[i]] = i;
            }
        }
 
    }
}
 
#endif