From 92c319247fae9d8be3f3291d6fa64266224ef61c Mon Sep 17 00:00:00 2001
From: Jaron Kent-Dobias <jaron@kent-dobias.com>
Date: Tue, 2 Apr 2024 17:24:14 +0200
Subject: Added tensor support.

---
 tensor.hpp | 108 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 108 insertions(+)
 create mode 100644 tensor.hpp

(limited to 'tensor.hpp')

diff --git a/tensor.hpp b/tensor.hpp
new file mode 100644
index 0000000..58c2434
--- /dev/null
+++ b/tensor.hpp
@@ -0,0 +1,108 @@
+#pragma once
+
+#include <array>
+#include <functional>
+
+#include <eigen3/unsupported/Eigen/CXX11/Tensor>
+
+#include "types.hpp"
+#include "factorial.hpp"
+
+using Vector = Eigen::Matrix<Real, Eigen::Dynamic, 1>;
+
+using Matrix = Eigen::Matrix<Real, Eigen::Dynamic, Eigen::Dynamic>;
+
+template <int p>
+using Tensor = Eigen::Tensor<Real, p>;
+
+template <int p>
+using constTensor = Eigen::Tensor<const Real, p>;
+
+template <int p, std::size_t... Indices>
+Tensor<p> initializeJHelper(unsigned N, unsigned M, std::index_sequence<Indices...>) {
+  std::array<unsigned, p> Ns;
+  std::fill_n(Ns.begin(), p, N);
+  Ns[p-1] = M;
+  return Tensor<p>(std::get<Indices>(Ns)...);
+}
+
+template <int p>
+Tensor<p> initializeJ(unsigned N, unsigned M) {
+  return initializeJHelper<p>(N, M, std::make_index_sequence<p>());
+}
+
+template <int p, std::size_t... Indices>
+void setJHelper(Tensor<p>& J, const std::array<unsigned, p>& ind, Real val, std::index_sequence<Indices...>) {
+  J(std::get<Indices>(ind)...) = val;
+}
+
+template <int p>
+void setJ(Tensor<p>& J, std::array<unsigned, p> ind, Real val) {
+  setJHelper<p>(J, ind, val, std::make_index_sequence<p>());
+}
+
+template <int p, std::size_t... Indices>
+Real getJHelper(const Tensor<p>& J, const std::array<unsigned, p>& ind, std::index_sequence<Indices...>) {
+  return J(std::get<Indices>(ind)...);
+}
+
+template <int p>
+Real getJ(const Tensor<p>& J, const std::array<unsigned, p>& ind) {
+  return getJHelper<p>(J, ind, std::make_index_sequence<p>());
+}
+
+template <int p>
+void iterateOverHelper(Tensor<p>& J,
+    std::function<void(Tensor<p>&, std::array<unsigned, p>)>& f,
+    unsigned l, std::array<unsigned, p> is) {
+  if (l == 0) {
+    f(J, is);
+  } else {
+    for (unsigned i = 0; i < J.dimension(p - l); i++) {
+      std::array<unsigned, p> js = is;
+      js[p - l] = i;
+      iterateOverHelper<p>(J, f, l - 1, js);
+    }
+  }
+}
+
+template <int p>
+void iterateOver(Tensor<p>& J, std::function<void(Tensor<p>&, std::array<unsigned, p>)>& f) {
+  std::array<unsigned, p> is;
+  iterateOverHelper<p>(J, f, p, is);
+}
+
+template <int p, class Distribution, class Generator>
+Tensor<p> generateCouplings(unsigned N, unsigned M, Distribution d, Generator& r) {
+  Tensor<p> J = initializeJ<p>(N, M);
+
+  std::function<void(Tensor<p>&, std::array<unsigned, p>)> setRandom =
+    [&d, &r] (Tensor<p>& JJ, std::array<unsigned, p> ind) {
+      setJ<p>(JJ, ind, d(r));
+    };
+
+  iterateOver<p>(J, setRandom);
+
+  return J;
+}
+
+template <int p, class Generator>
+Tensor<p> generateRealPSpinCouplings(unsigned N, unsigned M, Generator& r) {
+  Real σp = sqrt(factorial(p-1) / ((Real)2 * pow(N, p - 1)));
+
+  return generateCouplings<p>(N, M, std::normal_distribution<Real>(0, σp), r);
+}
+
+Tensor<3> contractDown(const Tensor<3>& J, const Vector& z) {
+  return J;
+}
+
+const std::array<Eigen::IndexPair<int>, 1> ip00 = {Eigen::IndexPair<int>(0, 0)};
+const std::array<Eigen::IndexPair<int>, 1> ip20 = {Eigen::IndexPair<int>(2, 0)};
+
+template <int r>
+Tensor<3> contractDown(const Tensor<r>& J, const Vector& z) {
+  Tensor<1> zT = Eigen::TensorMap<constTensor<1>>(z.data(), {z.size()});
+  Tensor<r - 1> Jz = J.contract(zT, ip00);
+  return contractDown(Jz, z);
+}
-- 
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