package main

// This reads obj files?
import (
	"bufio"
	"fmt"
	"io"
	"log"
	"math"
	"strings"
)

// These should probably go somewhere else but they're here because they're tied
// with the object format specification and I don't want a lot of granularity right now

type Vec3f struct {
	X, Y, Z float32
}

// A ROW MAJOR matrix
type Mat44f [16]float32

func (m *Mat44f) Set(x int, y int, val float32) {
	m[x+y*4] = val
}
func (m *Mat44f) Get(x int, y int) float32 {
	return m[x+y*4]
}
func (m *Mat44f) ZeroFill() {
	for i := range m {
		m[i] = 0
	}
}
func (m *Mat44f) SetIdentity() {
	m.ZeroFill()
	for i := range 4 {
		m.Set(i, i, 1)
	}
}

// Compute the projection matrix, filling the given matrix. FOV is in degrees
func (m *Mat44f) SetProjection(fov float32, near float32, far float32) {
	// Projection matrix is
	// S    0    0    0
	// 0    S    0    0
	// 0    0 -f/(f-n) -1
	// 0    0 -fn/(f-n) 0
	// where S (scale) is 1 / tan(fov / 2) (assuming fov is radians)
	m.ZeroFill()
	scale := float32(1 / math.Tan(float64(fov)*0.5*math.Pi/180))
	m.Set(0, 0, scale)
	m.Set(1, 1, scale)
	m.Set(2, 2, -far/(far-near))
	m.Set(3, 2, -1)
	m.Set(2, 3, -far*near/(far-near))
}

// Multiply the given point by our vector. Remember this is row-major order
func (m *Mat44f) MultiplyPoint3(p Vec3f) Vec3f {
	var out Vec3f
	// We hope very much that Go will optimize the function calls for us,
	// along with computing the constants.
	out.X = p.X*m.Get(0, 0) + p.Y*m.Get(0, 1) + p.Z*m.Get(0, 2) + m.Get(0, 3)
	out.Y = p.X*m.Get(1, 0) + p.Y*m.Get(1, 1) + p.Z*m.Get(1, 2) + m.Get(1, 3)
	out.Z = p.X*m.Get(2, 0) + p.Y*m.Get(2, 1) + p.Z*m.Get(2, 2) + m.Get(2, 3)
	w := p.X*m.Get(3, 0) + p.Y*m.Get(3, 1) + p.Z*m.Get(3, 2) + m.Get(3, 3)
	if w != 1 {
		out.X /= w
		out.Y /= w
		out.Z /= w
	}
	return out
}

type Facef [3]Vec3f

type ObjModel struct {
	Vertices []Vec3f
	Faces    []Facef
}

// Parse an obj file at the given reader. Only handles v and f right now
func ParseObj(reader io.Reader) (*ObjModel, error) {
	result := ObjModel{
		Vertices: make([]Vec3f, 0),
		Faces:    make([]Facef, 0),
	}
	breader := bufio.NewReader(reader)
	done := false
	for !done {
		// Scan a line
		line, err := breader.ReadString('\n')
		if err != nil {
			if err == io.EOF {
				done = true
			} else {
				log.Printf("NOT EOF ERR?")
				return nil, err
			}
		}
		line = strings.Trim(line, " \t\n\r")
		if len(line) == 0 {
			continue
		}
		// Find the first "item", whatever that is. This also gets rid of comments
		// since we just don't use lines that start with # (no handler
		var t string
		_, err = fmt.Sscan(line, &t)
		if err != nil {
			log.Printf("SSCANF ERR")
			return nil, err
		}
		line = line[len(t):]
		if t == "v" {
			// Read a vertex, should be just three floats
			var vertex Vec3f
			_, err := fmt.Sscan(line, &vertex.X, &vertex.Y, &vertex.Z)
			if err != nil {
				return nil, err
			}
			result.Vertices = append(result.Vertices, vertex)
		} else if t == "f" {
			// Read a face; in our example, it's always three sets.
			// For THIS example, we throw away those other values
			var face Facef
			var vi [3]int
			var ti int
			_, err := fmt.Sscanf(line, "%d/%d/%d %d/%d/%d %d/%d/%d",
				&vi[0], &ti, &ti, &vi[1], &ti, &ti, &vi[2], &ti, &ti)
			if err != nil {
				return nil, err
			}
			for i := range 3 {
				if vi[i] > len(result.Vertices) || vi[i] < 1 {
					return nil, fmt.Errorf("Face vertex index out of bounds: %d", vi[i])
				}
				face[i] = result.Vertices[vi[i]-1]
			}
			result.Faces = append(result.Faces, face)
		}
	}
	return &result, nil
}