blah

mini-odeint/mini-odeint.cpp

@@ -5,6 +5,7 @@
 #include <cassert>
 #include <cmath>
 #include <iterator>
+#include <ranges>
 #include <span>
 
 namespace mini_odeint {
@@ -63,96 +64,47 @@ template <typename T> struct DormandPrince {
         90730570.0 / 29380423.0, -8293050.0 / 29380423.0}}};
 };
 
-template <typename E> struct OdeVector {
+template <typename E> struct Vec3 {
   using value_type = E;
 
-  E value;
+  E x, y, z;
 
-  constexpr OdeVector() = default;
-  constexpr explicit OdeVector(E value) : value(std::move(value)) {}
+  constexpr Vec3() = default;
+  constexpr Vec3(E x, E y, E z) : x(x), y(y), z(z) {}
 
-  constexpr operator E() const { return value; }
+  constexpr explicit Vec3(std::array<E, 3> v) : x(v[0]), y(v[1]), z(v[2]) {}
 
-  friend constexpr OdeVector operator+(const OdeVector &lhs,
-                                       const OdeVector &rhs) {
-    return OdeVector{lhs.value + rhs.value};
+  friend constexpr Vec3 operator+(const Vec3 &lhs, const Vec3 &rhs) {
+    return Vec3{lhs.x + rhs.x, lhs.y + rhs.y, lhs.z + rhs.z};
   }
-  friend constexpr OdeVector operator*(const OdeVector &lhs,
-                                       const value_type &rhs) {
-    return OdeVector{lhs.value * rhs};
+  friend constexpr Vec3 operator*(const Vec3 &lhs, const value_type &rhs) {
+    return Vec3{lhs.x * rhs, lhs.y * rhs, lhs.z * rhs};
   }
-  friend constexpr OdeVector operator*(const value_type &lhs,
-                                       const OdeVector &rhs) {
-    return OdeVector{lhs * rhs.value};
+  friend constexpr Vec3 operator*(const value_type &lhs, const Vec3 &rhs) {
+    return Vec3{lhs * rhs.x, lhs * rhs.y, lhs * rhs.z};
   }
-  constexpr OdeVector &operator+=(const OdeVector &rhs) {
-    value += rhs.value;
+  constexpr Vec3 &operator+=(const Vec3 &rhs) {
+    x += rhs.x;
+    y += rhs.y;
+    z += rhs.z;
     return *this;
   }
-  constexpr OdeVector &operator+=(const value_type &rhs) {
-    value += rhs;
-    return *this;
-  }
-  value_type inf_norm() const { return std::abs(value); }
 };
 
-template <typename T, std::size_t N> struct OdeVector<std::array<T, N>> {
-  using array_type = std::array<T, N>;
-  using value_type = T;
-  std::array<T, N> value;
-
-  constexpr OdeVector() = default;
-  constexpr explicit OdeVector(std::array<T, N> value)
-      : value(std::move(value)) {}
-
-  operator std::array<T, N>() const { return value; }
-
-  friend constexpr OdeVector operator+(const OdeVector &lhs,
-                                       const OdeVector &rhs) {
-    OdeVector result{lhs};
-    for (std::size_t i = 0; i < N; ++i) {
-      result.value[i] += rhs.value[i];
-    }
-    return result;
-  }
-  friend constexpr OdeVector operator*(const OdeVector &lhs,
-                                       const value_type &rhs) {
-    OdeVector result{lhs};
-    for (std::size_t i = 0; i < N; ++i) {
-      result.value[i] *= rhs;
-    }
-    return result;
-  }
-  friend constexpr OdeVector operator*(const value_type &lhs,
-                                       const OdeVector &rhs) {
-    OdeVector result{rhs};
-    for (std::size_t i = 0; i < N; ++i) {
-      result.value[i] *= lhs;
-    }
-    return result;
-  }
-  constexpr OdeVector &operator+=(const OdeVector &rhs) {
-    for (std::size_t i = 0; i < N; ++i) {
-      value[i] += rhs.value[i];
-    }
-    return *this;
+namespace alg {
+template <typename T> inline auto inf_norm(const T &v) {
+  std::ranges::max_element(v, {}, [](auto n) { return std::abs(n); });
 }
 
-  constexpr OdeVector &operator+=(const value_type &rhs) {
-    for (std::size_t i = 0; i < N; ++i) {
-      value[i] += rhs;
-    }
-    return *this;
+inline float inf_norm(float v) { return std::abs(v); }
+
+inline double inf_norm(double v) { return std::abs(v); }
+
+template <typename E> inline E inf_norm(const Vec3<E> &v) {
+  return std::max({std::abs(v.x), std::abs(v.y), std::abs(v.z)});
 }
 
-  value_type inf_norm() const {
-    auto max = std::abs(value[0]);
-    for (std::size_t i = 1; i < N; ++i) {
-      max = std::max(max, std::abs(value[i]));
-    }
-    return max;
-  }
-};
+} // namespace alg
 
 template <typename Vector, typename Scalar = std::iter_value_t<Vector>,
           typename Tableau = DormandPrince<Scalar>>
@@ -202,7 +154,7 @@ inline std::size_t explicitRungeKutta(std::span<Vector> ys,
       const auto last_k_store = k[stages - 1];
       k[0] = k[stages - 1];
       for (std::size_t i = 1; i < stages; ++i) {
-        OdeVector<Vector> sum_ak{};
+        Vector sum_ak{};
         for (std::size_t j = 0; j < i; ++j) {
           sum_ak += a[i][j] * k[j];
         }
@@ -210,8 +162,8 @@ inline std::size_t explicitRungeKutta(std::span<Vector> ys,
       }
 
       // calculate final value and error
-      OdeVector<Vector> error{};
-      OdeVector<Vector> sum_bk{};
+      Vector error{};
+      Vector sum_bk{};
       for (std::size_t i = 0; i < stages; ++i) {
         sum_bk += b_hat[i] * k[i];
         error += (b_hat[i] - b[i]) * k[i];
@@ -219,13 +171,13 @@ inline std::size_t explicitRungeKutta(std::span<Vector> ys,
       const auto y_hat_np1 = y_hat_n + h_n * sum_bk;
 
       // check if step is successful, ie error is within tolerance
-      const auto E_hp1 = (h_n * error).inf_norm();
+      const auto E_hp1 = alg::inf_norm(h_n * error);
       if (E_hp1 < tol) {
         // if moved over any requested times then interpolate their values
         const auto t_np1 = t_n + h_n;
         while (it < N && t_np1 >= ts[it]) {
           const auto sigma = (ts[it] - t_n) / h_n;
-          OdeVector<Vector> Phi{};
+          Vector Phi{};
           for (std::size_t i = 0; i < stages; ++i) {
             auto term = sigma;
             auto b_i = term * p[i][0];
@@ -266,22 +218,21 @@ inline std::size_t explicitRungeKutta(std::span<Vector> ys,
 
 int main() {
   using namespace mini_odeint;
-  // make vector of doubles from 0.0 to 1.0 by 0.001
+  // make vector of floats from 0.0 to 1.0 by 0.001
   std::vector<float> times;
   times.reserve(1001);
   for (int i = 0; i < 1000; ++i) {
     times.push_back(i / 1000.0);
   }
 
-  std::vector<std::array<float, 2>> ys(times.size());
+  std::vector<Vec3<float>> ys(times.size());
 
   explicitRungeKutta(
-      std::span(ys), std::span<const float>(times),
-      std::array<float, 2>{1.0, 2.0}, float(1e-6),
-      [](auto y, auto t) { return std::array<float, 2>{-y[0], -y[1]}; });
+      std::span(ys), std::span<const float>(times), Vec3<float>{1.0, 1.0, 1.0},
+      float(1e-6), [](auto y, auto t) { return Vec3<float>{-y.x, 0.0, 0.0}; });
 
   for (const auto &v : ys) {
-    std::cout << v[0] << v[1] << '\n';
+    std::cout << v.x << '\n';
   }
 }