From e69e1967bb377e943f53636b984ac59bc006b734 Mon Sep 17 00:00:00 2001
From: Doc <Doctor_Luigi@aol.com>
Date: Sun, 7 Sep 2025 15:34:33 +0200
Subject: [PATCH] added side and split geom logic

---
 internal/geom/side.go  |  5 +++++
 internal/geom/split.go | 29 +++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)
 create mode 100644 internal/geom/side.go
 create mode 100644 internal/geom/split.go

diff --git a/internal/geom/side.go b/internal/geom/side.go
new file mode 100644
index 0000000..9b5e6e9
--- /dev/null
+++ b/internal/geom/side.go
@@ -0,0 +1,5 @@
+package geom
+
+func Side(P, O, D Vec) float64 {
+	return Cross(D, Sub(P, O))
+}
diff --git a/internal/geom/split.go b/internal/geom/split.go
new file mode 100644
index 0000000..7e3e4e8
--- /dev/null
+++ b/internal/geom/split.go
@@ -0,0 +1,29 @@
+package geom
+
+func SplitSeg(s Seg, O, D Vec) (front, back []Seg) {
+	sa := Side(s.A, O, D)
+	sb := Side(s.B, O, D)
+
+	if sa >= -EPS && sb >= -EPS {
+		return []Seg{s}, nil
+	}
+	if sa <= EPS && sb <= EPS {
+		return nil, []Seg{s}
+	}
+
+	ok, t := SegLineIntersect(s.A, s.B, O, D)
+	if !ok {
+
+		if sa >= 0 {
+			return []Seg{s}, nil
+		}
+		return nil, []Seg{s}
+	}
+	M := Vec{s.A.X + t*(s.B.X-s.A.X), s.A.Y + t*(s.B.Y-s.A.Y)}
+	a := Seg{s.A, M}
+	b := Seg{M, s.B}
+	if sa > 0 {
+		return []Seg{a}, []Seg{b}
+	}
+	return []Seg{b}, []Seg{a}
+}