geom/sample: add subregion sampler for extents

CMakeLists.txt

@@ -360,6 +360,7 @@ list (
     geom/sample/edge.hpp
     geom/sample/surface.hpp
     geom/sample/volume.hpp
+    geom/sample/subregion.hpp
     geom/segment.cpp
     geom/segment.hpp
     geom/sphere.cpp
@@ -697,6 +698,7 @@ if (TESTS)
         geom/plane
         geom/ray
         geom/sample/edge
+        geom/sample/subregion
         geom/segment
         geom/sphere
         hash/buzhash

geom/sample/subregion.hpp

@@ -0,0 +1,63 @@
+/*
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * Copyright 2020, Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+#include <algorithm>
+
+#include "../../extent.hpp"
+#include "../../region.hpp"
+#include "../../random.hpp"
+
+
+namespace cruft::geom::sample {
+    /// Given a desired subregion shape, and a possible field of locations,
+    /// select a random location for surface placement.
+    ///
+    /// Valid placements allow for rotations, but are exclusive of far edges
+    /// for integer types.
+    ///
+    /// The largest axis of the shape must not be larger than the smallest
+    /// axis of the placement field otherwise the chosen region may fall
+    /// outside the field.
+    class subregion {
+    public:
+        subregion (cruft::extent2i _shape, cruft::extent2i _field)
+            : m_shape (_shape)
+            , m_field (_field)
+        {
+            CHECK (all (m_shape <= m_field));
+        }
+
+
+        // Choose placements by looking at each axis in turn.
+        //
+        // We support rotations by randomising the axis ordering for the shape
+        // but not the field.
+        template <typename GeneratorT>
+        cruft::region2i
+        operator() (GeneratorT &&gen)
+        {
+            int axes[] = { 0, 1 };
+            std::shuffle (std::begin (axes), std::end (axes), gen);
+
+            cruft::region2i res;
+
+            for (int i = 0; i < std::ssize (axes); ++i) {
+                res.e[i] = m_shape[axes[i]];
+                res.p[i] = cruft::random::uniform (m_field[i] - res.e[i]);
+            }
+
+            return res;
+        }
+
+    private:
+        cruft::extent2i m_shape;
+        cruft::extent2i m_field;
+    };
+}

test/geom/sample/subregion.cpp

@@ -0,0 +1,47 @@
+
+#include <cruft/util/tap.hpp>
+
+#include <cruft/util/extent.hpp>
+#include <cruft/util/geom/sample/subregion.hpp>
+
+
+int
+main ()
+{
+    cruft::TAP::logger tap;
+
+    static const struct {
+        cruft::extent2i shape;
+        cruft::extent2i field;
+        char const *message;
+    } TESTS[] = {
+        { {  1,  1 }, {   1,   1 }, "Unit shape and unit field" },
+        { {  1,  1 }, { 100, 100 }, "Unit shape and large field" },
+        { { 10, 10 }, { 100, 100 }, "Square shape and large field" },
+        { {  1, 10 }, { 100, 100 }, "Tall shape and large field" },
+        { { 10,  1 }, { 100, 100 }, "" },
+        { {  2,  2 }, { 100, 100 }, "" },
+    };
+
+    static const int ITERATIONS = 1'000;
+
+    for (auto const &[shape, field, msg]: TESTS) {
+        cruft::rand::xoshiro256plusplus gen (42);
+        cruft::geom::sample::subregion sampler {shape, field};
+
+        cruft::region2i const valid (cruft::point2i (0), field);
+
+        bool success = true;
+        for (int i = 0; i < ITERATIONS; ++i) {
+            auto const choice = sampler (gen);
+            if (!valid.covers (choice)) {
+                success = false;
+                break;
+            }
+        }
+
+        tap.expect (success, "subregion coverage validity, %!", msg);
+    }
+
+    return tap.status ();
+}