Add a linear algebra library dependency

Add `mdspan` and `stdBLAS` as dependencies in order to allow use of a
linear algebra library.

Change-Id: I9d2e1f43f6df315a922753d1d88e5904888b3d16

WORKSPACE.bazel

@@ -124,6 +124,61 @@ http_archive(
     url = "https://github.com/fmtlib/fmt/archive/%s.tar.gz" % FMT_VERSION,
 )
 
+MDSPAN_VERSION = "9d0a451e11177cbdeaef035c7914b0aa73ddd1e2"
+
+http_archive(
+    name = "mdspan",
+    build_file_content = """
+load("@rules_cc//cc:defs.bzl", "cc_library")
+
+cc_library(
+    name = "mdspan",
+    hdrs = glob(["include/**"]),
+    includes = ["include"],
+    defines = [
+        "MDSPAN_USE_BRACKET_OPERATOR=1",
+        "MDSPAN_USE_PAREN_OPERATOR=1", # needed by stdBLAS
+    ],
+    visibility = ["//visibility:public"],
+)
+""",
+    sha256 = "d7751653cd93f3e73118796fd5f42f0424ef20ffeaa224005b4008b8987e6d81",
+    strip_prefix = "mdspan-%s" % MDSPAN_VERSION,
+    url = "https://github.com/kokkos/mdspan/archive/%s.tar.gz" % MDSPAN_VERSION,
+)
+
+LINALG_VERSION = "d1a1a116a1a62a03d726d70220e573aa7c3dba68"
+
+http_archive(
+    name = "linalg",
+    build_file_content = """
+load("@rules_cc//cc:defs.bzl", "cc_library")
+
+# It appears this can be empty if we don't use atomic ref, blas, or kokkos
+# https://github.com/kokkos/stdBLAS/blob/main/include/experimental/__p1673_bits/linalg_config.h.in
+genrule(
+    name = "linalg_config",
+    outs = ["include/experimental/__p1673_bits/linalg_config.h"],
+    cmd = "touch $@",
+)
+
+cc_library(
+    name = "linalg",
+    srcs = ["linalg_config"],
+    hdrs = glob(["include/**"]),
+    includes = [
+        "include",
+        "include/experimental",
+    ],
+    deps = ["@mdspan"],
+    visibility = ["//visibility:public"],
+)
+""",
+    sha256 = "2cff70d080186949dcfa6da509eab41c3ba25df23c15bcbca8d810d0a1370bdb",
+    strip_prefix = "stdBLAS-%s" % LINALG_VERSION,
+    url = "https://github.com/kokkos/stdblas/archive/%s.tar.gz" % LINALG_VERSION,
+)
+
 BOOST_UT_VERSION = "e53a47d37bc594e80bd5f1b8dc1ade8dce4429d3"
 
 http_archive(

test/BUILD.bazel

@@ -7,5 +7,7 @@ cc_test(
     deps = [
         "@boost_ut",
         "@fmt",
+        "@linalg",
+        "@mdspan",
     ],
 )

test/dummy_test.cpp

@@ -1,15 +1,87 @@
 #include <boost/ut.hpp>
 
+#include <array>
+#include <cassert>
+#include <cstddef>
+#include <experimental/linalg>
+#include <experimental/mdspan>
 #include <fmt/core.h>
 
+namespace stx = std::experimental;
+
+template <class T, std::size_t R, std::size_t C>
+struct matrix
+{
+  static_assert(R != std::dynamic_extent);
+  static_assert(C != std::dynamic_extent);
+
+  using extents_type = stx::extents<std::size_t, R, C>;
+  using mdspan_type = stx::mdspan<T, extents_type>;
+  using const_mdspan_type = stx::mdspan<const T, extents_type>;
+
+  std::array<T, R * C> data{};
+
+  matrix() = default;
+
+  matrix(std::initializer_list<std::initializer_list<T>> init)
+  {
+    assert(R == init.size());
+
+    auto i = std::size_t{};
+    for (const auto& row : init) {
+      assert(C == row.size());
+
+      auto j = std::size_t{};
+      for (const auto& elem : row) {
+        span()[i, j] = elem;
+
+        ++j;
+      }
+
+      ++i;
+    }
+  }
+
+  [[nodiscard]]
+  constexpr auto span() -> mdspan_type { return mdspan_type{data.data()}; }
+  [[nodiscard]]
+  constexpr auto span() const -> const_mdspan_type
+  {
+    return const_mdspan_type{data.data()};
+  }
+
+  [[nodiscard]]
+  constexpr auto
+  operator[](std::size_t i, std::size_t j) -> mdspan_type::reference
+  {
+    return span()[i, j];
+  }
+  [[nodiscard]]
+  constexpr auto
+  operator[](std::size_t i, std::size_t j) const -> const_mdspan_type::reference
+  {
+    return span()[i, j];
+  }
+};
+
 auto main() -> int
 {
   using ::boost::ut::expect;
   using ::boost::ut::test;
 
+  // NOLINTBEGIN(readability-magic-numbers)
   test("true is true") = [] {
-    const auto s = fmt::format("The answer is {}.", 42);
+    using Mat = matrix<int, 2, 2>;
+
+    auto A = Mat{{1, 2}, {3, 4}};
+    auto B = Mat{{5, 6}, {7, 8}};
+    auto C = Mat{};
+
+    stx::linalg::matrix_product(A.span(), B.span(), C.span());
+
+    const auto s = fmt::format("The answer is {}.", C[0, 0]);
 
-    expect("The answer is 42." == s);
+    expect("The answer is 19." == s);
   };
+  // NOLINTEND(readability-magic-numbers)
 }