[core] Avoid NaNs in TransformState unit conversions

Fix cases where e.g. state has either zero width or height, causing the
unit convertion functions would return NaNs.

src/mbgl/map/transform.cpp

@@ -45,24 +45,23 @@ Transform::Transform(std::function<void(MapChange)> callback_,
 #pragma mark - Map View
 
 bool Transform::resize(const std::array<uint16_t, 2> size) {
-    if (state.width != size[0] || state.height != size[1]) {
-
-        if (callback) {
-            callback(MapChangeRegionWillChange);
-        }
-
-        state.width = size[0];
-        state.height = size[1];
-        state.constrain(state.scale, state.x, state.y);
-
-        if (callback) {
-            callback(MapChangeRegionDidChange);
-        }
-
-        return true;
-    } else {
+    if (state.width == size[0] && state.height == size[1]) {
         return false;
     }
+
+    if (callback) {
+        callback(MapChangeRegionWillChange);
+    }
+
+    state.width = size[0];
+    state.height = size[1];
+    state.constrain(state.scale, state.x, state.y);
+
+    if (callback) {
+        callback(MapChangeRegionDidChange);
+    }
+
+    return true;
 }
 
 #pragma mark - Camera

src/mbgl/map/transform_state.cpp

@@ -230,6 +230,10 @@ double TransformState::worldSize() const {
 }
 
 ScreenCoordinate TransformState::latLngToScreenCoordinate(const LatLng& latLng) const {
+    if (width == 0 || height == 0) {
+        return {};
+    }
+
     mat4 mat = coordinatePointMatrix(getZoom());
     vec4 p;
     Point<double> pt = project(latLng) / double(util::tileSize);
@@ -239,6 +243,10 @@ ScreenCoordinate TransformState::latLngToScreenCoordinate(const LatLng& latLng)
 }
 
 LatLng TransformState::screenCoordinateToLatLng(const ScreenCoordinate& point, LatLng::WrapMode wrapMode) const {
+    if (width == 0 || height == 0) {
+        return {};
+    }
+
     float targetZ = 0;
     mat4 mat = coordinatePointMatrix(getZoom());
 

test/map/transform.cpp

@@ -474,3 +474,43 @@ TEST(Transform, Camera) {
     transform.updateTransitions(transform.getTransitionStart() + transform.getTransitionDuration());
     ASSERT_FALSE(transform.inTransition());
 }
+
+TEST(Transform, DefaultTransform) {
+    Transform transform;
+    const TransformState& state = transform.getState();
+
+    LatLng nullIsland, latLng = {};
+    ScreenCoordinate center, point = {};
+    const uint16_t min = std::numeric_limits<uint16_t>::min();
+    const uint16_t max = std::numeric_limits<uint16_t>::max();
+
+    auto testConversions = [&](const LatLng& coord, const ScreenCoordinate& screenCoord) {
+        latLng = state.screenCoordinateToLatLng(center);
+        ASSERT_NEAR(latLng.latitude, coord.latitude, 0.000001);
+        ASSERT_NEAR(latLng.longitude, coord.longitude, 0.000001);
+        point = state.latLngToScreenCoordinate(nullIsland);
+        ASSERT_DOUBLE_EQ(point.x, screenCoord.x);
+        ASSERT_DOUBLE_EQ(point.y, screenCoord.y);
+    };
+
+    testConversions(nullIsland, center);
+
+    // Cannot assign the current size.
+    ASSERT_FALSE(transform.resize({{}}));
+
+    ASSERT_TRUE(transform.resize({{ min, max }}));
+    testConversions(nullIsland, center);
+
+    ASSERT_TRUE(transform.resize({{ max, min }}));
+    testConversions(nullIsland, center);
+
+    ASSERT_TRUE(transform.resize({{ min, min }}));
+    testConversions(nullIsland, center);
+
+    center = { max / 2., max / 2. };
+
+    // -1 evaluates to UINT_MAX.
+    ASSERT_TRUE(transform.resize({{ static_cast<uint16_t>(-1), static_cast<uint16_t>(-1) }}));
+    ASSERT_FALSE(transform.resize({{ max, max }}));
+    testConversions(nullIsland, center);
+}