Trade: add MeshData::attributeId() for inverse ID mapping.

A helper for knowing e.g. which texture coordinate set is given
texture coordinate attribute.

doc/changelog.dox

@@ -525,6 +525,8 @@ See also:
     lookup compared to @relativeref{Trade::MeshData,hasAttribute()} +
     @relativeref{Trade::MeshData,attributeId()}. This also makes the API
     consistent with @ref Trade::SceneData::findFieldId().
+-   Added @ref Trade::MeshData::attributeId() that returns ID of an attribute
+    in a set of attributes of the same name
 -   Added @ref Trade::TextureType::Texture1DArray,
     @relativeref{Trade::TextureType,Texture2DArray} and
     @relativeref{Trade::TextureType,CubeMapArray} in order to be able to

src/Magnum/Trade/MeshData.cpp

@@ -346,6 +346,16 @@ MeshAttribute MeshData::attributeName(const UnsignedInt id) const {
     return _attributes[id]._name;
 }
 
+UnsignedInt MeshData::attributeId(const UnsignedInt id) const {
+    CORRADE_ASSERT(id < _attributes.size(),
+        "Trade::MeshData::attributeId(): index" << id << "out of range for" << _attributes.size() << "attributes", {});
+    const MeshAttribute name = _attributes[id]._name;
+    UnsignedInt count = 0;
+    for(UnsignedInt i = 0; i != id; ++i)
+        if(_attributes[i]._name == name) ++count;
+    return count;
+}
+
 VertexFormat MeshData::attributeFormat(const UnsignedInt id) const {
     CORRADE_ASSERT(id < _attributes.size(),
         "Trade::MeshData::attributeFormat(): index" << id << "out of range for" << _attributes.size() << "attributes", {});

src/Magnum/Trade/MeshData.h

@@ -1322,6 +1322,18 @@ class MAGNUM_TRADE_EXPORT MeshData {
          */
         MeshAttribute attributeName(UnsignedInt id) const;
 
+        /**
+         * @brief Attribute ID in a set of attributes of the same name
+         * @m_since_latest
+         *
+         * The @p id is expected to be smaller than @ref attributeCount() const.
+         * Returns the number of attributes of the same @ref attributeName()
+         * preceeding @p id, or @cpp 0 @ce if it's the first attribute of
+         * given name.
+         * @see @ref attributeId(MeshAttribute, UnsignedInt) const
+         */
+        UnsignedInt attributeId(UnsignedInt id) const;
+
         /**
          * @brief Attribute format
          *
@@ -1417,6 +1429,7 @@ class MAGNUM_TRADE_EXPORT MeshData {
          *
          * Like @ref findAttributeId(), but the @p id is expected to be smaller
          * than @ref attributeCount(MeshAttribute) const.
+         * @see @ref attributeId(UnsignedInt) const
          */
         UnsignedInt attributeId(MeshAttribute name, UnsignedInt id = 0) const;
 

src/Magnum/Trade/Test/MeshDataTest.cpp

@@ -1295,6 +1295,11 @@ void MeshDataTest::construct() {
     CORRADE_COMPARE(data.attributeName(2), MeshAttribute::Normal);
     CORRADE_COMPARE(data.attributeName(3), MeshAttribute::TextureCoordinates);
     CORRADE_COMPARE(data.attributeName(4), meshAttributeCustom(13));
+    CORRADE_COMPARE(data.attributeId(0), 0);
+    CORRADE_COMPARE(data.attributeId(1), 0);
+    CORRADE_COMPARE(data.attributeId(2), 0);
+    CORRADE_COMPARE(data.attributeId(3), 1);
+    CORRADE_COMPARE(data.attributeId(4), 0);
     CORRADE_COMPARE(data.attributeFormat(0), VertexFormat::Vector3);
     CORRADE_COMPARE(data.attributeFormat(1), VertexFormat::Vector2);
     CORRADE_COMPARE(data.attributeFormat(2), VertexFormat::Vector3);
@@ -3474,6 +3479,7 @@ void MeshDataTest::attributeNotFound() {
     Error redirectError{&out};
     data.attributeData(2);
     data.attributeName(2);
+    data.attributeId(2);
     data.attributeFormat(2);
     data.attributeOffset(2);
     data.attributeStride(2);
@@ -3520,6 +3526,7 @@ void MeshDataTest::attributeNotFound() {
     CORRADE_COMPARE(out.str(),
         "Trade::MeshData::attributeData(): index 2 out of range for 2 attributes\n"
         "Trade::MeshData::attributeName(): index 2 out of range for 2 attributes\n"
+        "Trade::MeshData::attributeId(): index 2 out of range for 2 attributes\n"
         "Trade::MeshData::attributeFormat(): index 2 out of range for 2 attributes\n"
         "Trade::MeshData::attributeOffset(): index 2 out of range for 2 attributes\n"
         "Trade::MeshData::attributeStride(): index 2 out of range for 2 attributes\n"