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package main
import (
"encoding/binary"
"fmt"
"math"
"strings"
"testing"
)
// A cube, written in both STL flavours, is the fixture: it exercises every
// axis, has known dimensions, and lets the two parsers be checked against each
// other rather than against hand-written expectations.
// cubeTriangles returns the 12 triangles of an axis-aligned cube spanning
// [0,size]^3, as flat [9]float32 vertex triples.
func cubeTriangles(size float32) [][9]float32 {
s := size
// Eight corners.
c := [8][3]float32{
{0, 0, 0}, {s, 0, 0}, {s, s, 0}, {0, s, 0},
{0, 0, s}, {s, 0, s}, {s, s, s}, {0, s, s},
}
quads := [6][4]int{
{0, 3, 2, 1}, // bottom
{4, 5, 6, 7}, // top
{0, 1, 5, 4}, // front
{2, 3, 7, 6}, // back
{1, 2, 6, 5}, // right
{0, 4, 7, 3}, // left
}
var out [][9]float32
for _, q := range quads {
for _, tri := range [2][3]int{{0, 1, 2}, {0, 2, 3}} {
var t [9]float32
for i, vi := range tri {
v := c[q[vi]]
t[i*3], t[i*3+1], t[i*3+2] = v[0], v[1], v[2]
}
out = append(out, t)
}
}
return out
}
func binaryCubeSTL(size float32, withNormals bool) []byte {
tris := cubeTriangles(size)
buf := make([]byte, 84+len(tris)*50)
copy(buf[:80], "binary cube fixture")
binary.LittleEndian.PutUint32(buf[80:84], uint32(len(tris)))
for i, t := range tris {
off := 84 + i*50
put := func(n int, v float32) {
binary.LittleEndian.PutUint32(buf[off+n*4:], math.Float32bits(v))
}
if withNormals {
nx, ny, nz := faceNormal(t)
put(0, nx)
put(1, ny)
put(2, nz)
}
for v := 0; v < 9; v++ {
put(3+v, t[v])
}
}
return buf
}
func asciiCubeSTL(size float32) []byte {
var b strings.Builder
b.WriteString("solid cube\n")
for _, t := range cubeTriangles(size) {
nx, ny, nz := faceNormal(t)
fmt.Fprintf(&b, " facet normal %e %e %e\n outer loop\n", nx, ny, nz)
for v := 0; v < 3; v++ {
fmt.Fprintf(&b, " vertex %e %e %e\n", t[v*3], t[v*3+1], t[v*3+2])
}
b.WriteString(" endloop\n endfacet\n")
}
b.WriteString("endsolid cube\n")
return []byte(b.String())
}
func TestParseBinarySTL(t *testing.T) {
m, err := parseSTL(binaryCubeSTL(10, true))
if err != nil {
t.Fatalf("parseSTL: %v", err)
}
if m.Triangles != 12 {
t.Errorf("triangles = %d, want 12", m.Triangles)
}
if got, want := len(m.Verts), 12*3*vertexStride; got != want {
t.Errorf("len(Verts) = %d, want %d", got, want)
}
}
func TestParseASCIISTL(t *testing.T) {
m, err := parseSTL(asciiCubeSTL(10))
if err != nil {
t.Fatalf("parseSTL: %v", err)
}
if m.Triangles != 12 {
t.Errorf("triangles = %d, want 12", m.Triangles)
}
}
// The two flavours describe the same solid, so after parsing they must yield
// the same buffer. This is the check that keeps the parsers from drifting.
func TestBinaryAndASCIIAgree(t *testing.T) {
bin, err := parseSTL(binaryCubeSTL(10, true))
if err != nil {
t.Fatalf("binary: %v", err)
}
asc, err := parseSTL(asciiCubeSTL(10))
if err != nil {
t.Fatalf("ascii: %v", err)
}
if len(bin.Verts) != len(asc.Verts) {
t.Fatalf("vertex count differs: binary %d, ascii %d", len(bin.Verts), len(asc.Verts))
}
for i := range bin.Verts {
// The ASCII fixture round-trips through %e, so exact equality is not
// available; a loose epsilon still catches any structural mismatch.
if math.Abs(float64(bin.Verts[i]-asc.Verts[i])) > 1e-5 {
t.Fatalf("vertex %d differs: binary %v, ascii %v", i, bin.Verts[i], asc.Verts[i])
}
}
}
// A binary STL whose 80-byte header starts with "solid" is a real-world
// export quirk that a naive prefix check would misparse as ASCII.
func TestBinarySTLWithSolidHeader(t *testing.T) {
data := binaryCubeSTL(10, true)
copy(data[:80], "solid created by some exporter")
if !isBinarySTL(data) {
t.Fatal("binary STL with 'solid' header detected as ASCII")
}
m, err := parseSTL(data)
if err != nil {
t.Fatalf("parseSTL: %v", err)
}
if m.Triangles != 12 {
t.Errorf("triangles = %d, want 12", m.Triangles)
}
}
// Zeroed normals are common in exported files; the parser must derive them
// rather than storing a zero vector, which would render the face unlit.
func TestZeroNormalsAreRecomputed(t *testing.T) {
m, err := parseSTL(binaryCubeSTL(10, false))
if err != nil {
t.Fatalf("parseSTL: %v", err)
}
for i := 0; i < len(m.Verts); i += vertexStride {
nx, ny, nz := m.Verts[i+3], m.Verts[i+4], m.Verts[i+5]
l := math.Sqrt(float64(nx*nx + ny*ny + nz*nz))
if math.Abs(l-1) > 1e-4 {
t.Fatalf("vertex %d has non-unit normal (%v, %v, %v), length %v", i/vertexStride, nx, ny, nz, l)
}
}
}
// Normalisation is what lets the viewer use one fixed camera for every model,
// so it must hold regardless of the model's original scale or position.
func TestNormalizeFitsUnitBox(t *testing.T) {
for _, size := range []float32{0.5, 10, 250} {
m, err := parseSTL(binaryCubeSTL(size, true))
if err != nil {
t.Fatalf("size %v: %v", size, err)
}
var minv, maxv [3]float32
for a := 0; a < 3; a++ {
minv[a], maxv[a] = math.MaxFloat32, -math.MaxFloat32
}
for i := 0; i < len(m.Verts); i += vertexStride {
for a := 0; a < 3; a++ {
v := m.Verts[i+a]
if v < minv[a] {
minv[a] = v
}
if v > maxv[a] {
maxv[a] = v
}
}
}
for a := 0; a < 3; a++ {
if d := maxv[a] - minv[a]; d > 1.0001 {
t.Errorf("size %v: axis %d extent %v exceeds unit box", size, a, d)
}
if c := (minv[a] + maxv[a]) / 2; math.Abs(float64(c)) > 1e-5 {
t.Errorf("size %v: axis %d not centred (centre %v)", size, a, c)
}
}
}
}
// Malformed input arrives from the network, so the parser must reject it
// rather than panic or return a nonsensical mesh.
func TestParseSTLRejectsGarbage(t *testing.T) {
cases := map[string][]byte{
"empty": {},
"short": []byte("solid"),
"truncated binary": binaryCubeSTL(10, true)[:100],
"text": []byte("this is not an STL file at all\njust some prose\n"),
}
for name, data := range cases {
t.Run(name, func(t *testing.T) {
if _, err := parseSTL(data); err == nil {
t.Error("expected an error, got nil")
}
})
}
}
func TestPackMeshRoundTrip(t *testing.T) {
m, err := parseSTL(binaryCubeSTL(10, true))
if err != nil {
t.Fatalf("parseSTL: %v", err)
}
packed := packMesh(m)
if got, want := len(packed), len(m.Verts)*4; got != want {
t.Fatalf("packed length = %d, want %d", got, want)
}
for i, want := range m.Verts {
got := math.Float32frombits(binary.LittleEndian.Uint32(packed[i*4:]))
if got != want {
t.Fatalf("float %d: got %v, want %v", i, got, want)
}
}
}
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