Trade: add MaterialData::attributeDataOffset().

src/Magnum/Trade/MaterialData.cpp

@@ -367,6 +367,14 @@ MaterialData::~MaterialData() = default;
 
 MaterialData& MaterialData::operator=(MaterialData&&) noexcept = default;
 
+UnsignedInt MaterialData::attributeDataOffset(const UnsignedInt layer) const {
+    CORRADE_ASSERT(layer <= layerCount(),
+        "Trade::MaterialData::attributeDataOffset(): index" << layer << "out of range for" << layerCount() << "layers", {});
+    if(!_layerOffsets)
+        return layer == 0 ? 0 : _data.size();
+    return layer == 0 ? 0 : _layerOffsets[layer - 1];
+}
+
 UnsignedInt MaterialData::findLayerIdInternal(const Containers::StringView layer) const {
     for(std::size_t i = 1; i < _layerOffsets.size(); ++i) {
         if(_layerOffsets[i] > _layerOffsets[i - 1] &&

src/Magnum/Trade/MaterialData.h

@@ -2238,6 +2238,17 @@ class MAGNUM_TRADE_EXPORT MaterialData {
             return _layerOffsets.isEmpty() ? 1 : _layerOffsets.size();
         }
 
+        /**
+         * @brief Offset of a layer inside attribute data
+         *
+         * Returns the offset where attributes for @p layer start in the array
+         * returned by @ref atttributeData() const. The @p layer is expected
+         * to be less *or equal to* @ref layerCount(), i.e. it's always
+         * possible to call this function with @cpp layer @ce and
+         * @cpp layer + 1 @ce to get the attribute range for given layer.
+         */
+        UnsignedInt attributeDataOffset(UnsignedInt layer) const;
+
         /**
          * @brief Whether a material has given named layer
          *
@@ -2579,6 +2590,7 @@ class MAGNUM_TRADE_EXPORT MaterialData {
          *
          * The @p layer is expected to be smaller than @ref layerCount() const,
          * @p id is expected to be smaller than @ref attributeCount(UnsignedInt) const.
+         * @see @ref attributeDataOffset()
          */
         const MaterialAttributeData& attributeData(UnsignedInt layer, UnsignedInt id) const;
 
@@ -2586,6 +2598,7 @@ class MAGNUM_TRADE_EXPORT MaterialData {
          * @brief Raw attribute data in the base material
          *
          * The @p @p id is expected to be smaller than @ref attributeCount() const.
+         * @see @ref attributeDataOffset()
          */
         const MaterialAttributeData& attributeData(UnsignedInt id) const {
             return attributeData(0, id);

src/Magnum/Trade/Test/MaterialDataTest.cpp

@@ -991,6 +991,9 @@ void MaterialDataTest::construct() {
     CORRADE_COMPARE(data.layerName(0), "");
     CORRADE_VERIFY(!data.hasLayer(""));
 
+    CORRADE_COMPARE(data.attributeDataOffset(0), 0);
+    CORRADE_COMPARE(data.attributeDataOffset(1), 4);
+
     /* Verify sorting */
     CORRADE_COMPARE(data.attributeName(0), "AmbientTextureMatrix");
     CORRADE_COMPARE(data.attributeName(1), "DiffuseTextureCoordinates");
@@ -1184,6 +1187,12 @@ void MaterialDataTest::constructLayers() {
     CORRADE_COMPARE(data.layerCount(), 4);
     CORRADE_COMPARE(data.layerData().size(), 4);
 
+    CORRADE_COMPARE(data.attributeDataOffset(0), 0);
+    CORRADE_COMPARE(data.attributeDataOffset(1), 2);
+    CORRADE_COMPARE(data.attributeDataOffset(2), 5);
+    CORRADE_COMPARE(data.attributeDataOffset(3), 5);
+    CORRADE_COMPARE(data.attributeDataOffset(4), 7);
+
     CORRADE_COMPARE(data.attributeData().size(), 7);
     CORRADE_COMPARE(data.attributeCount(0), 2);
     CORRADE_COMPARE(data.attributeCount(1), 3);
@@ -2580,8 +2589,12 @@ void MaterialDataTest::accessLayerOutOfBounds() {
         {MaterialAttribute::SpecularTexture, 3u}
     }, {0, 2}};
 
+    /* This is fine */
+    data.attributeDataOffset(2);
+
     std::ostringstream out;
     Error redirectError{&out};
+    data.attributeDataOffset(3);
     data.layerName(2);
     data.layerFactor(2);
     data.layerFactorTexture(2);
@@ -2622,6 +2635,7 @@ void MaterialDataTest::accessLayerOutOfBounds() {
     data.attributeOr(2, "AlphaMask", false);
     data.attributeOr(2, MaterialAttribute::AlphaMask, false);
     CORRADE_COMPARE(out.str(),
+        "Trade::MaterialData::attributeDataOffset(): index 3 out of range for 2 layers\n"
         "Trade::MaterialData::layerName(): index 2 out of range for 2 layers\n"
         "Trade::MaterialData::layerFactor(): index 2 out of range for 2 layers\n"
         "Trade::MaterialData::layerFactorTexture(): index 2 out of range for 2 layers\n"