Migrate from gsl::span to std::span (C++20) (#758)

Summary:
Pull Request resolved: #758

Now that C++20 is the default standard, migrate from `gsl::span` to `std::span` throughout the codebase to use the standard library implementation instead of the Guidelines Support Library (GSL).

Changes:
- Replaced `gsl::span<T>` with `std::span<T>` in headers and source files
- Replaced `gsl::make_span()` with direct `std::span` construction
- Replaced `gsl::as_bytes()` with `std::as_bytes()`
- Updated `#include <gsl/gsl>` to `#include <span>`
- Updated code style documentation to reference `std::span`
- Kept `DeduceSpanType<T>` utility (required for template argument deduction with nested templates)
- Kept `gsl::narrow` and `gsl::narrow_cast` utilities (no C++20 standard replacement available)

Reviewed By: cstollmeta

Differential Revision: D85808123

fbshipit-source-id: 1bf09f19e30eb6ed448a4df7a65cfcae15d1f217

Author: jeongseok-meta

momentum/character/blend_shape.cpp

@@ -9,6 +9,8 @@
 
 #include "momentum/common/checks.h"
 
+#include <gsl/narrow>
+
 namespace momentum {
 
 template <typename T>
@@ -37,7 +39,7 @@ void BlendShape::computeShape(
 }
 
 VectorXf BlendShape::estimateCoefficients(
-    gsl::span<const Vector3f> vertices,
+    std::span<const Vector3f> vertices,
     const float regularization,
     const VectorXf& weights) const {
   MT_CHECK(
@@ -86,12 +88,12 @@ VectorXf BlendShape::estimateCoefficients(
   }
 }
 
-BlendShape::BlendShape(gsl::span<const Vector3f> baseShape, const size_t numShapes)
+BlendShape::BlendShape(std::span<const Vector3f> baseShape, const size_t numShapes)
     : BlendShapeBase(baseShape.size(), numShapes),
       baseShape_(baseShape.begin(), baseShape.end()),
       factorizationValid_(false) {}
 
-void BlendShape::setShapeVector(const size_t index, gsl::span<const Vector3f> shape) {
+void BlendShape::setShapeVector(const size_t index, std::span<const Vector3f> shape) {
   BlendShapeBase::setShapeVector(index, shape);
 
   // mark as not up to date

momentum/character/blend_shape.h

@@ -22,9 +22,9 @@ struct BlendShape : public BlendShapeBase {
 
   /// @param baseShape Base shape vertices
   /// @param numShapes Number of blend shapes
-  BlendShape(gsl::span<const Vector3f> baseShape, size_t numShapes);
+  BlendShape(std::span<const Vector3f> baseShape, size_t numShapes);
 
-  void setBaseShape(gsl::span<const Vector3f> baseShape) {
+  void setBaseShape(std::span<const Vector3f> baseShape) {
     baseShape_.assign(baseShape.begin(), baseShape.end());
   }
 
@@ -67,15 +67,15 @@ struct BlendShape : public BlendShapeBase {
   /// @param weights Optional per-vertex importance weights
   /// @return Estimated blend shape coefficients
   [[nodiscard]] VectorXf estimateCoefficients(
-      gsl::span<const Vector3f> vertices,
+      std::span<const Vector3f> vertices,
       float regularization = 1.0f,
       const VectorXf& weights = VectorXf()) const;
 
   /// Overrides base method to also invalidate factorization
   ///
   /// @param index Index of the shape vector to set
   /// @param shapeVector Vector of vertex offsets
-  void setShapeVector(size_t index, gsl::span<const Vector3f> shapeVector);
+  void setShapeVector(size_t index, std::span<const Vector3f> shapeVector);
 
   /// Compares all components of two blend shapes
   ///

momentum/character/blend_shape_base.cpp

@@ -9,6 +9,8 @@
 
 #include "momentum/common/checks.h"
 
+#include <gsl/narrow>
+
 namespace momentum {
 
 template <typename T>
@@ -33,7 +35,7 @@ void BlendShapeBase::applyDeltas(
 BlendShapeBase::BlendShapeBase(const size_t modelSize, const size_t numShapes)
     : shapeVectors_(MatrixXf::Zero(modelSize * 3, numShapes)) {}
 
-void BlendShapeBase::setShapeVector(const size_t index, gsl::span<const Vector3f> shape) {
+void BlendShapeBase::setShapeVector(const size_t index, std::span<const Vector3f> shape) {
   MT_CHECK(modelSize() == shape.size(), "{} is not {}", modelSize(), shape.size());
   MT_CHECK(
       gsl::narrow<Eigen::Index>(shape.size()) * 3 == shapeVectors_.rows(),

momentum/character/blend_shape_base.h

@@ -52,7 +52,7 @@ struct BlendShapeBase {
 
   /// @param index Index of the shape vector to set
   /// @param shapeVector Vector of vertex offsets
-  void setShapeVector(size_t index, gsl::span<const Vector3f> shapeVector);
+  void setShapeVector(size_t index, std::span<const Vector3f> shapeVector);
 
   [[nodiscard]] Eigen::Index shapeSize() const {
     return shapeVectors_.cols();

momentum/character/character_utility.cpp

@@ -598,7 +598,7 @@ Character replaceSkeletonHierarchy(
       tgtCharacter.blendShape};
 }
 
-Character removeJoints(const Character& character, gsl::span<const size_t> jointsToRemove) {
+Character removeJoints(const Character& character, std::span<const size_t> jointsToRemove) {
   std::vector<bool> toRemove(character.skeleton.joints.size(), false);
   for (const auto& j : jointsToRemove) {
     MT_THROW_IF(j >= character.skeleton.joints.size(), "Invalid joint found in removeJoints.");

momentum/character/character_utility.h

@@ -62,7 +62,7 @@ namespace momentum {
 /// specified joints.
 [[nodiscard]] Character removeJoints(
     const Character& character,
-    gsl::span<const size_t> jointsToRemove);
+    std::span<const size_t> jointsToRemove);
 
 /// Maps the input ModelParameter motion to a target character by matching model parameter names.
 /// Mismatched names will be discarded (source) or set to zero (target).

momentum/character/linear_skinning.cpp

@@ -14,6 +14,7 @@
 #include "momentum/math/mesh.h"
 
 #include <dispenso/parallel_for.h>
+#include <gsl/narrow>
 
 namespace momentum {
 
@@ -246,7 +247,7 @@ Affine3f getInverseSSDTransformation(
 void applyInverseSSD(
     const TransformationList& inverseBindPose,
     const SkinWeights& skin,
-    gsl::span<const Vector3f> points,
+    std::span<const Vector3f> points,
     const SkeletonState& state,
     Mesh& mesh) {
   // some sanity checks
@@ -258,7 +259,7 @@ void applyInverseSSD(
 std::vector<Vector3f> applyInverseSSD(
     const TransformationList& inverseBindPose,
     const SkinWeights& skin,
-    gsl::span<const Vector3f> points,
+    std::span<const Vector3f> points,
     const SkeletonState& state) {
   MT_CHECK(
       state.jointState.size() == inverseBindPose.size(),

momentum/character/linear_skinning.h

@@ -116,7 +116,7 @@ Affine3f getInverseSSDTransformation(
 std::vector<Vector3f> applyInverseSSD(
     const TransformationList& inverseBindPose,
     const SkinWeights& skin,
-    gsl::span<const Vector3f> points,
+    std::span<const Vector3f> points,
     const SkeletonState& state);
 
 /// Applies inverse SSD to points, storing results in a mesh
@@ -132,7 +132,7 @@ std::vector<Vector3f> applyInverseSSD(
 void applyInverseSSD(
     const TransformationList& inverseBindPose,
     const SkinWeights& skin,
-    gsl::span<const Vector3f> points,
+    std::span<const Vector3f> points,
     const SkeletonState& state,
     Mesh& mesh);
 

momentum/character/parameter_transform.cpp

@@ -49,7 +49,7 @@ ParameterTransformT<T> ParameterTransformT<T>::empty(size_t nJointParameters) {
 }
 
 template <typename T>
-ParameterTransformT<T> ParameterTransformT<T>::identity(gsl::span<const std::string> jointNames) {
+ParameterTransformT<T> ParameterTransformT<T>::identity(std::span<const std::string> jointNames) {
   ParameterTransformT<T> result;
   const size_t nJoints = jointNames.size();
   const size_t nJointParameters = nJoints * kParametersPerJoint;

momentum/character/parameter_transform.h

@@ -11,7 +11,7 @@
 #include <momentum/character/types.h>
 #include <momentum/math/utility.h>
 
-#include <gsl/span>
+#include <span>
 
 #include <string>
 #include <unordered_map>
@@ -102,7 +102,7 @@ struct ParameterTransformT {
 
   /// Return a ParameterTransform object where the model parameters are identical to the joint
   /// parameters.
-  [[nodiscard]] static ParameterTransformT<T> identity(gsl::span<const std::string> jointNames);
+  [[nodiscard]] static ParameterTransformT<T> identity(std::span<const std::string> jointNames);
 
   /// Compute activeJointParams based on the transform and the input ParameterSet.
   [[nodiscard]] VectorX<bool> computeActiveJointParams(const ParameterSet& ps = allParams()) const;