distances.hpp

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#ifndef KNNCOLLE_DISTANCES_HPP
#define KNNCOLLE_DISTANCES_HPP
 
#include <cmath>
#include <cstddef>
 
namespace knncolle {
 
template<typename Data_, typename Distance_>
class DistanceMetric {
public:
    DistanceMetric() = default;
    DistanceMetric(const DistanceMetric&) = default;
    DistanceMetric(DistanceMetric&&) = default;
    DistanceMetric& operator=(const DistanceMetric&) = default;
    DistanceMetric& operator=(DistanceMetric&&) = default;
    virtual ~DistanceMetric() = default;
public:
    virtual Distance_ raw(std::size_t num_dimensions, const Data_* x, const Data_* y) const = 0;
 
    virtual Distance_ normalize(Distance_ raw) const = 0;
 
    virtual Distance_ denormalize(Distance_ norm) const = 0;
};
 
template<typename Data_, typename Distance_>
class EuclideanDistance final : public DistanceMetric<Data_, Distance_> {
public:
    Distance_ raw(std::size_t num_dimensions, const Data_* x, const Data_* y) const {
        Distance_ output = 0;
        for (std::size_t d = 0; d < num_dimensions; ++d) {
            auto delta = static_cast<Distance_>(x[d]) - static_cast<Distance_>(y[d]); // casting to ensure consistent precision/signedness regardless of Data_.
            output += delta * delta;
        }
        return output;
    }
 
    Distance_ normalize(Distance_ raw) const {
        return std::sqrt(raw);
    }
 
    Distance_ denormalize(Distance_ norm) const {
        return norm * norm;
    }
};
 
 
template<typename Data_, typename Distance_>
class ManhattanDistance final : public DistanceMetric<Data_, Distance_> {
public:
    Distance_ raw(std::size_t num_dimensions, const Data_* x, const Data_* y) const {
        Distance_ output = 0;
        for (std::size_t d = 0; d < num_dimensions; ++d) {
            auto delta = static_cast<Distance_>(x[d]) - static_cast<Distance_>(y[d]); // casting to ensure consistent precision/signedness regardless of Data_.
            output += std::abs(delta);
        }
        return output;
    }
 
    Distance_ normalize(Distance_ raw) const {
        return raw;
    }
 
    Distance_ denormalize(Distance_ norm) const {
        return norm;
    }
};
 
}
 
#endif