package main

import (
	"encoding/binary"
	"fmt"
	"math"
)

// ReadSTL parses a binary STL byte slice into triangles.
// Format: 80-byte header, uint32 count, N × 50-byte records
// (12 bytes normal, 36 bytes vertices, 2 bytes attribute).
func ReadSTL(data []byte) ([][3]Point, error) {
	if len(data) < 84 {
		return nil, fmt.Errorf("STL data too short (%d bytes, need at least 84)", len(data))
	}

	count := binary.LittleEndian.Uint32(data[80:84])
	expected := 84 + int(count)*50
	debugLog("ReadSTL: %d bytes, header count=%d, expected=%d", len(data), count, expected)

	if len(data) < expected {
		return nil, fmt.Errorf("STL truncated: have %d bytes, need %d for %d triangles", len(data), expected, count)
	}

	triangles := make([][3]Point, 0, count)
	off := 84
	for i := uint32(0); i < count; i++ {
		// Skip 12 bytes of face normal
		off += 12

		var tri [3]Point
		for v := 0; v < 3; v++ {
			tri[v].X = float64(math.Float32frombits(binary.LittleEndian.Uint32(data[off:])))
			tri[v].Y = float64(math.Float32frombits(binary.LittleEndian.Uint32(data[off+4:])))
			tri[v].Z = float64(math.Float32frombits(binary.LittleEndian.Uint32(data[off+8:])))
			off += 12
		}

		// Skip 2-byte attribute
		off += 2
		triangles = append(triangles, tri)
	}

	debugLog("  ReadSTL: parsed %d triangles", len(triangles))
	return triangles, nil
}