package mesh

import "math"

// BBox returns the axis-aligned bounding box of the mesh.
func (m *Mesh) BBox() (min, max Vec3) {
	if len(m.Verts) == 0 {
		return Vec3{}, Vec3{}
	}
	min, max = m.Verts[0], m.Verts[0]
	for _, v := range m.Verts[1:] {
		min = min.Min(v)
		max = max.Max(v)
	}
	return
}

// SurfaceArea sums the area of all triangles.
func (m *Mesh) SurfaceArea() float64 {
	sum := 0.0
	for _, t := range m.Tris {
		a, b, c := m.Verts[t[0]], m.Verts[t[1]], m.Verts[t[2]]
		sum += b.Sub(a).Cross(c.Sub(a)).Len() / 2
	}
	return sum
}

// SignedVolume computes the enclosed volume via the divergence theorem.
// Only meaningful for closed meshes; positive when windings face outward.
func (m *Mesh) SignedVolume() float64 {
	sum := 0.0
	for _, t := range m.Tris {
		a, b, c := m.Verts[t[0]], m.Verts[t[1]], m.Verts[t[2]]
		sum += a.Dot(b.Cross(c))
	}
	return sum / 6
}

// Volume is the absolute enclosed volume.
func (m *Mesh) Volume() float64 { return math.Abs(m.SignedVolume()) }

// SceneBBox returns the bounding box over the given meshes.
func SceneBBox(meshes []*Mesh) (min, max Vec3) {
	first := true
	for _, m := range meshes {
		if len(m.Verts) == 0 {
			continue
		}
		mn, mx := m.BBox()
		if first {
			min, max = mn, mx
			first = false
		} else {
			min = min.Min(mn)
			max = max.Max(mx)
		}
	}
	return
}