Fix bug in Bounds2i iterators.

For degenerate bounds, they would end up going on forever (or at least a
very long time, visiting all sorts of points that weren't inside the
bounds).

Added unit tests to test both their regular operation as well as degenerate
cases.

src/core/geometry.h

@@ -1299,7 +1299,15 @@ inline Bounds2iIterator begin(const Bounds2i &b) {
 }
 
 inline Bounds2iIterator end(const Bounds2i &b) {
-    return Bounds2iIterator(b, Point2i(b.pMin.x, b.pMax.y));
+    // Normally, the ending point is at the minimum x value and one past
+    // the last valid y value.
+    Point2i pEnd(b.pMin.x, b.pMax.y);
+    // However, if the bounds are degenerate, override the end point to
+    // equal the start point so that any attempt to iterate over the bounds
+    // exits out immediately.
+    if (b.pMin.x >= b.pMax.x || b.pMin.y >= b.pMax.y)
+        pEnd = b.pMin;
+    return Bounds2iIterator(b, pEnd);
 }
 
 template <typename T>

src/tests/bounds.cpp

@@ -0,0 +1,42 @@
+
+#include "tests/gtest/gtest.h"
+#include "pbrt.h"
+#include "geometry.h"
+
+TEST(Bounds2, IteratorBasic) {
+    Bounds2i b{{0, 1}, {2, 3}};
+    Point2i e[] = { { 0, 1 }, { 1, 1 }, { 0, 2 }, { 1, 2 } };
+    int offset = 0;
+    for (auto p : b) {
+        EXPECT_LT(offset, sizeof(e) / sizeof(e[0]));
+        EXPECT_EQ(e[offset], p) << "offset = " << offset;
+        ++offset;
+    }
+}
+
+TEST(Bounds2, IteratorDegenerate) {
+    Bounds2i b{{0, 0}, {0, 10}};
+    for (auto p : b) {
+        // This loop should never run.
+        bool reached = true;
+        EXPECT_FALSE(reached) << "p = " << p;
+        break;
+    }
+
+    Bounds2i b2{{0, 0}, {4, 0}};
+    for (auto p : b2) {
+        // This loop should never run.
+        bool reached = true;
+        EXPECT_FALSE(reached) << "p = " << p;
+        break;
+    }
+
+    Bounds2i b3;
+    for (auto p : b3) {
+        // This loop should never run.
+        bool reached = true;
+        EXPECT_FALSE(reached) << "p = " << p;
+        break;
+    }
+}
+