scalable cache friendly spiral sequence generation (SNSystems)

spiral_generator.cpp

#include <cstddef>
#include <algorithm>
#include <array>
#include <iostream>

using std::size_t;

auto constexpr calculate(size_t row, size_t column, size_t width, size_t height, size_t value = 0)
{
    --width;
    --height;

    auto const ring = std::min({ row, column, width - column, height - row });

    auto const doublering = 2 * ring;
    value += doublering * (width + height + 2 - doublering);

    row -= ring;
    column -= ring;
    width -= (2 * ring);
    height -= (2 * ring);

    if (row == 0 || column == width)
    {
        return value + row + column;
    }
    else
    {
        auto const max = value + (width + height) * 2;
        return max - row - column;
    }
}

template <size_t width, size_t height>
constexpr auto generate_spiral(size_t start = 0)
{
    auto data = std::array<std::array<size_t, width>, height>{};

    for (auto row = 0; row < height; ++row)
    {
        for (auto column = size_t{}; column < width; ++column)
        {
            data[row][column] = calculate(row, column, width, height, start);
        }
    }

    return data;
}

int main()
{
    constexpr auto spiral = generate_spiral<9, 12>(100);
    for (auto&& rows : spiral)
    {
        for (auto&& val : rows)
        {
            std::cout << val << "   ";
        }

        std::cout << '\n';
    }
}