// vim: shiftwidth=4 tabstop=4 noexpandtab

/*
Wikipedia about STL file format: https://en.wikipedia.org/wiki/STL_(file_format)
*/

package main

import (
	"io/ioutil"
	"log"
	"encoding/binary"
	"math"
)

// representation of binary STL file content
type StlModel struct {
	header []byte
	numberOfTriangles uint32
	surface Surface
}

// read and parse a given binary STL file
func ReadBinaryStlFile(filePath string) (StlModel, error) {

	fileContent, err := ioutil.ReadFile(filePath)
	if err != nil {
		return StlModel{}, err
	}

	model := StlModel{}
	model.surface = Surface{}

	model.header = fileContent[0:80]
	log.Printf("STL header: '%s'\n", string(model.header))

	model.numberOfTriangles = binary.LittleEndian.Uint32(fileContent[80:84])
	log.Printf("STL model has %d triangles\n", model.numberOfTriangles)

	var i uint32
	for i = 0; i < model.numberOfTriangles; i++ {  // for each expected triangle
		start := 84 + i*50  // 50 bytes is length of one triangle
		end := 84 + (i+1)*50
		model.surface.triangles = append(model.surface.triangles,
			ParseBinaryStlTriangle(fileContent[start:end]))
	}

	return model,nil
}

// parse the 50 bytes of the STL file representing a triangle (surface normal is ignored)
func ParseBinaryStlTriangle(data []byte) *Triangle {  // FIXME: This function should only accept 50 byte slices/arrays

	// allocate a new triangle and three corner points on the heap
	triangle := new(Triangle)
	triangle.a = new(Point)  // corner point a
	triangle.b = new(Point)  // corner point b
	triangle.c = new(Point)  // corner point c

	// parse x, y and z coordinate for corner point a
	triangle.a.x = math.Float32frombits(binary.LittleEndian.Uint32(data[12:16]))
	triangle.a.y = math.Float32frombits(binary.LittleEndian.Uint32(data[16:20]))
	triangle.a.z = math.Float32frombits(binary.LittleEndian.Uint32(data[20:24]))

	// parse x, y and z coordinate for corner point b
	triangle.b.x = math.Float32frombits(binary.LittleEndian.Uint32(data[24:28]))
	triangle.b.y = math.Float32frombits(binary.LittleEndian.Uint32(data[28:32]))
	triangle.b.z = math.Float32frombits(binary.LittleEndian.Uint32(data[32:36]))

	// parse x, y and z coordinate for corner point c
	triangle.c.x = math.Float32frombits(binary.LittleEndian.Uint32(data[36:40]))
	triangle.c.y = math.Float32frombits(binary.LittleEndian.Uint32(data[40:44]))
	triangle.c.z = math.Float32frombits(binary.LittleEndian.Uint32(data[44:48]))

	return triangle
}

func (stl StlModel) toVertices() []float32 {

	retval := make([]float32, stl.numberOfTriangles * 9)

	for index,triangle := range(stl.surface.triangles) {

		retval[index*9+0] = triangle.a.x
		retval[index*9+1] = triangle.a.y
		retval[index*9+2] = triangle.a.z

		retval[index*9+3] = triangle.b.x
		retval[index*9+4] = triangle.b.y
		retval[index*9+5] = triangle.b.z

		retval[index*9+6] = triangle.c.x
		retval[index*9+7] = triangle.c.y
		retval[index*9+8] = triangle.c.z

	}

	return retval
}