From 418918920e3f804c5a7bb38e8d506bf7105a14b9 Mon Sep 17 00:00:00 2001
From: Wojtek Figat <wojtek@figat.pl>
Date: Mon, 13 Nov 2023 11:38:37 +0100
Subject: [PATCH] Add `GetSplineSegmentLength` to get spline segment length

#1879
---
 Source/Engine/Level/Actors/Spline.cpp | 35 +++++++++++++++++++++++++--
 Source/Engine/Level/Actors/Spline.h   |  7 ++++++
 2 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/Source/Engine/Level/Actors/Spline.cpp b/Source/Engine/Level/Actors/Spline.cpp
index df93ea840..dfe7c2be8 100644
--- a/Source/Engine/Level/Actors/Spline.cpp
+++ b/Source/Engine/Level/Actors/Spline.cpp
@@ -149,8 +149,8 @@ float Spline::GetSplineDuration() const
 float Spline::GetSplineLength() const
 {
     float sum = 0.0f;
-    const int32 slices = 20;
-    const float step = 1.0f / (float)slices;
+    constexpr int32 slices = 20;
+    constexpr float step = 1.0f / (float)slices;
     Vector3 prevPoint = Vector3::Zero;
     if (Curve.GetKeyframes().Count() != 0)
     {
@@ -181,6 +181,37 @@ float Spline::GetSplineLength() const
     return Math::Sqrt(sum);
 }
 
+float Spline::GetSplineSegmentLength(int32 index) const
+{
+    if (index == 0)
+        return 0.0f;
+    CHECK_RETURN(index > 0 && index < GetSplinePointsCount(), 0.0f);
+    float sum = 0.0f;
+    constexpr int32 slices = 20;
+    constexpr float step = 1.0f / (float)slices;
+    const auto& a = Curve[index - 1];
+    const auto& b = Curve[index];
+    Vector3 startPoint = a.Value.Translation * _transform.Scale;
+    {
+        const float length = Math::Abs(b.Time - a.Time);
+        Vector3 leftTangent, rightTangent;
+        AnimationUtils::GetTangent(a.Value.Translation, a.TangentOut.Translation, length, leftTangent);
+        AnimationUtils::GetTangent(b.Value.Translation, b.TangentIn.Translation, length, rightTangent);
+
+        // TODO: implement sth more analytical than brute-force solution
+        for (int32 slice = 0; slice < slices; slice++)
+        {
+            const float t = (float)slice * step;
+            Vector3 pos;
+            AnimationUtils::Bezier(a.Value.Translation, leftTangent, rightTangent, b.Value.Translation, t, pos);
+            pos *= _transform.Scale;
+            sum += (float)Vector3::DistanceSquared(pos, startPoint);
+            startPoint = pos;
+        }
+    }
+    return Math::Sqrt(sum);
+}
+
 float Spline::GetSplineTime(int32 index) const
 {
     CHECK_RETURN(index >= 0 && index < GetSplinePointsCount(), 0.0f)
diff --git a/Source/Engine/Level/Actors/Spline.h b/Source/Engine/Level/Actors/Spline.h
index cd022890b..4a81ee186 100644
--- a/Source/Engine/Level/Actors/Spline.h
+++ b/Source/Engine/Level/Actors/Spline.h
@@ -167,6 +167,13 @@ public:
     /// </summary>
     API_PROPERTY() float GetSplineLength() const;
 
+    /// <summary>
+    /// Gets the length of the spline segment (distance between pair of two points).
+    /// </summary>
+    /// <param name="index">The index of the segment end index. Zero-based, smaller than GetSplinePointsCount().</param>
+    /// <returns>The spline segment length.</returns>
+    API_FUNCTION() float GetSplineSegmentLength(int32 index) const;
+
     /// <summary>
     /// Gets the time of the spline keyframe.
     /// </summary>