/**********************************************************************
* path tracing demo
*
* Copyright (c) 2019-2023 Dario Deledda. All rights reserved.
* Use of this source code is governed by an MIT license
* that can be found in the LICENSE file.
*
* This file contains a path tracer example in less of 500 line of codes
* 3 demo scenes included
*
* This code is inspired by:
* - "Realistic Ray Tracing" by Peter Shirley 2000 ISBN-13: 978-1568814612
* - https://www.kevinbeason.com/smallpt/
*
* Known limitations:
* - there are some approximation errors in the calculations
* - to speed-up the code a cos/sin table is used
* - the full precision code is present but commented, can be restored very easily
* - an higher number of samples ( > 60) can block the program on higher resolutions
*   without a stack size increase
* - as a recursive program this code depend on the stack size,
*   for higher number of samples increase the stack size
*   in linux: ulimit -s byte_size_of_the_stack
*   example: ulimit -s 16000000
* - No OpenMP support
**********************************************************************/
import os
import math
import rand
import time
import term

const (
    inf = 1e+10
    eps = 1e-4
    f_0 = 0.0
)

//**************************** 3D Vector utility struct *********************
struct Vec {
mut:
    x f64 = 0.0
    y f64 = 0.0
    z f64 = 0.0
}

[inline]
fn (v Vec) + (b Vec) Vec {
    return Vec{v.x + b.x, v.y + b.y, v.z + b.z}
}

[inline]
fn (v Vec) - (b Vec) Vec {
    return Vec{v.x - b.x, v.y - b.y, v.z - b.z}
}

[inline]
fn (v Vec) * (b Vec) Vec {
    return Vec{v.x * b.x, v.y * b.y, v.z * b.z}
}

[inline]
fn (v Vec) dot(b Vec) f64 {
    return v.x * b.x + v.y * b.y + v.z * b.z
}

[inline]
fn (v Vec) mult_s(b f64) Vec {
    return Vec{v.x * b, v.y * b, v.z * b}
}

[inline]
fn (v Vec) cross(b Vec) Vec {
    return Vec{v.y * b.z - v.z * b.y, v.z * b.x - v.x * b.z, v.x * b.y - v.y * b.x}
}

[inline]
fn (v Vec) norm() Vec {
    tmp_norm := 1.0 / math.sqrt(v.x * v.x + v.y * v.y + v.z * v.z)
    return Vec{v.x * tmp_norm, v.y * tmp_norm, v.z * tmp_norm}
}

//********************************Image**************************************
struct Image {
    width  int
    height int
    data   &Vec = unsafe { nil }
}

fn new_image(w int, h int) Image {
    vecsize := int(sizeof(Vec))
    return Image{
        width: w
        height: h
        data: unsafe { &Vec(vcalloc(vecsize * w * h)) }
    }
}

// write out a .ppm file
fn (image Image) save_as_ppm(file_name string) {
    npixels := image.width * image.height
    mut f_out := os.create(file_name) or { panic(err) }
    f_out.writeln('P3') or { panic(err) }
    f_out.writeln('${image.width} ${image.height}') or { panic(err) }
    f_out.writeln('255') or { panic(err) }
    for i in 0 .. npixels {
        c_r := to_int(unsafe { image.data[i] }.x)
        c_g := to_int(unsafe { image.data[i] }.y)
        c_b := to_int(unsafe { image.data[i] }.z)
        f_out.write_string('${c_r} ${c_g} ${c_b} ') or { panic(err) }
    }
    f_out.close()
}

//********************************** Ray ************************************
struct Ray {
    o Vec
    d Vec
}

// material types, used in radiance()
enum Refl_t {
    diff
    spec
    refr
}

//******************************** Sphere ***********************************
struct Sphere {
    rad  f64 = 0.0 // radius
    p    Vec    // position
    e    Vec    // emission
    c    Vec    // color
    refl Refl_t // reflection type => [diffuse, specular, refractive]
}

fn (sp Sphere) intersect(r Ray) f64 {
    op := sp.p - r.o // Solve t^2*d.d + 2*t*(o-p).d + (o-p).(o-p)-R^2 = 0
    b := op.dot(r.d)
    mut det := b * b - op.dot(op) + sp.rad * sp.rad

    if det < 0 {
        return 0
    }

    det = math.sqrt(det)

    mut t := b - det
    if t > eps {
        return t
    }

    t = b + det
    if t > eps {
        return t
    }
    return 0
}

/*********************************** Scenes **********************************
* 0) Cornell Box with 2 spheres
* 1) Sunset
* 2) Psychedelic
* The sphere fields are: Sphere{radius, position, emission, color, material}
******************************************************************************/
const (
    cen     = Vec{50, 40.8, -860} // used by scene 1
    spheres = [
        [// scene 0 cornnel box
            Sphere{
                rad: 1e+5
                p: Vec{1e+5 + 1, 40.8, 81.6}
                e: Vec{}
                c: Vec{.75, .25, .25}
                refl: .diff
            }, // Left
            Sphere{
                rad: 1e+5
                p: Vec{-1e+5 + 99, 40.8, 81.6}
                e: Vec{}
                c: Vec{.25, .25, .75}
                refl: .diff
            }, // Rght
            Sphere{
                rad: 1e+5
                p: Vec{50, 40.8, 1e+5}
                e: Vec{}
                c: Vec{.75, .75, .75}
                refl: .diff
            }, // Back
            Sphere{
                rad: 1e+5
                p: Vec{50, 40.8, -1e+5 + 170}
                e: Vec{}
                c: Vec{}
                refl: .diff
            }, // Frnt
            Sphere{
                rad: 1e+5
                p: Vec{50, 1e+5, 81.6}
                e: Vec{}
                c: Vec{.75, .75, .75}
                refl: .diff
            }, // Botm
            Sphere{
                rad: 1e+5
                p: Vec{50, -1e+5 + 81.6, 81.6}
                e: Vec{}
                c: Vec{.75, .75, .75}
                refl: .diff
            }, // Top
            Sphere{
                rad: 16.5
                p: Vec{27, 16.5, 47}
                e: Vec{}
                c: Vec{1, 1, 1}.mult_s(.999)
                refl: .spec
            }, // Mirr
            Sphere{
                rad: 16.5
                p: Vec{73, 16.5, 78}
                e: Vec{}
                c: Vec{1, 1, 1}.mult_s(.999)
                refl: .refr
            }, // Glas
            Sphere{
                rad: 600
                p: Vec{50, 681.6 - .27, 81.6}
                e: Vec{12, 12, 12}
                c: Vec{}
                refl: .diff
            }, // Lite
        ],
        [// scene 1 sunset
            Sphere{
                rad: 1600
                p: Vec{1.0, 0.0, 2.0}.mult_s(3000)
                e: Vec{1.0, .9, .8}.mult_s(1.2e+1 * 1.56 * 2)
                c: Vec{}
                refl: .diff
            }, // sun
            Sphere{
                rad: 1560
                p: Vec{1, 0, 2}.mult_s(3500)
                e: Vec{1.0, .5, .05}.mult_s(4.8e+1 * 1.56 * 2)
                c: Vec{}
                refl: .diff
            }, // horizon sun2
            Sphere{
                rad: 10000
                p: cen + Vec{0, 0, -200}
                e: Vec{0.00063842, 0.02001478, 0.28923243}.mult_s(6e-2 * 8)
                c: Vec{.7, .7, 1}.mult_s(.25)
                refl: .diff
            }, // sky
            Sphere{
                rad: 100000
                p: Vec{50, -100000, 0}
                e: Vec{}
                c: Vec{.3, .3, .3}
                refl: .diff
            }, // grnd
            Sphere{
                rad: 110000
                p: Vec{50, -110048.5, 0}
                e: Vec{.9, .5, .05}.mult_s(4)
                c: Vec{}
                refl: .diff
            }, // horizon brightener
            Sphere{
                rad: 4e+4
                p: Vec{50, -4e+4 - 30, -3000}
                e: Vec{}
                c: Vec{.2, .2, .2}
                refl: .diff
            }, // mountains
            Sphere{
                rad: 26.5
                p: Vec{22, 26.5, 42}
                e: Vec{}
                c: Vec{1, 1, 1}.mult_s(.596)
                refl: .spec
            }, // white Mirr
            Sphere{
                rad: 13
                p: Vec{75, 13, 82}
                e: Vec{}
                c: Vec{.96, .96, .96}.mult_s(.96)
                refl: .refr
            }, // Glas
            Sphere{
                rad: 22
                p: Vec{87, 22, 24}
                e: Vec{}
                c: Vec{.6, .6, .6}.mult_s(.696)
                refl: .refr
            }, // Glas2
        ],
        [// scene 3 Psychedelic
            Sphere{
                rad: 150
                p: Vec{50 + 75, 28, 62}
                e: Vec{1, 1, 1}.mult_s(0e-3)
                c: Vec{1, .9, .8}.mult_s(.93)
                refl: .refr
            },
            Sphere{
                rad: 28
                p: Vec{50 + 5, -28, 62}
                e: Vec{1, 1, 1}.mult_s(1e+1)
                c: Vec{1, 1, 1}.mult_s(0)
                refl: .diff
            },
            Sphere{
                rad: 300
                p: Vec{50, 28, 62}
                e: Vec{1, 1, 1}.mult_s(0e-3)
                c: Vec{1, 1, 1}.mult_s(.93)
                refl: .spec
            },
        ],
    ] // end of scene array
)

//********************************** Utilities ******************************
[inline]
fn clamp(x f64) f64 {
    if x < 0 {
        return 0
    }
    if x > 1 {
        return 1
    }
    return x
}

[inline]
fn to_int(x f64) int {
    p := math.pow(clamp(x), 1.0 / 2.2)
    return int(p * 255.0 + 0.5)
}

fn intersect(r Ray, spheres &Sphere, nspheres int) (bool, f64, int) {
    mut d := 0.0
    mut t := inf
    mut id := 0
    for i := nspheres - 1; i >= 0; i-- {
        d = unsafe { spheres[i] }.intersect(r)
        if d > 0 && d < t {
            t = d
            id = i
        }
    }
    return t < inf, t, id
}

// some casual random function, try to avoid the 0
fn rand_f64() f64 {
    x := rand.u32() & 0x3FFF_FFFF
    return f64(x) / f64(0x3FFF_FFFF)
}

const (
    cache_len  = 65536 // the 2*pi angle will be split in 2^16 parts
    cache_mask = cache_len - 1 // mask to speed-up the module process
)

struct Cache {
mut:
    sin_tab [65536]f64
    cos_tab [65536]f64
}

fn new_tabs() Cache {
    mut c := Cache{}
    inv_len := 1.0 / f64(cache_len)
    for i in 0 .. cache_len {
        x := f64(i) * math.pi * 2.0 * inv_len
        c.sin_tab[i] = math.sin(x)
        c.cos_tab[i] = math.cos(x)
    }
    return c
}

//************ Cache for sin/cos speed-up table and scene selector **********
const (
    tabs = new_tabs()
)

//****************** main function for the radiance calculation *************
fn radiance(r Ray, depthi int, scene_id int) Vec {
    if depthi > 1024 {
        eprintln('depthi: ${depthi}')
        eprintln('')
        return Vec{}
    }
    mut depth := depthi // actual depth in the reflection tree
    mut t := 0.0 // distance to intersection
    mut id := 0 // id of intersected object
    mut res := false // result of intersect

    v_1 := 1.0
    // v_2 := f64(2.0)

    scene := spheres[scene_id]
    // res, t, id = intersect(r, id, tb.scene)
    res, t, id = intersect(r, scene.data, scene.len)
    if !res {
        return Vec{}
    }
    // if miss, return black

    obj := scene[id] // the hit object

    x := r.o + r.d.mult_s(t)
    n := (x - obj.p).norm()

    nl := if n.dot(r.d) < 0.0 { n } else { n.mult_s(-1) }

    mut f := obj.c

    // max reflection
    mut p := f.z
    if f.x > f.y && f.x > f.z {
        p = f.x
    } else {
        if f.y > f.z {
            p = f.y
        }
    }

    depth++
    if depth > 5 {
        if rand_f64() < p {
            f = f.mult_s(f64(1.0) / p)
        } else {
            return obj.e // R.R.
        }
    }

    if obj.refl == .diff { // Ideal DIFFUSE reflection
        // **Full Precision**
        // r1  := f64(2.0 * math.pi) * rand_f64()

        // tabbed speed-up
        r1 := rand.u32() & cache_mask

        r2 := rand_f64()
        r2s := math.sqrt(r2)

        w := nl

        mut u := if math.abs(w.x) > f64(0.1) { Vec{0, 1, 0} } else { Vec{1, 0, 0} }
        u = u.cross(w).norm()

        v := w.cross(u)

        // **Full Precision**
        // d := (u.mult_s(math.cos(r1) * r2s) + v.mult_s(math.sin(r1) * r2s) + w.mult_s(1.0 - r2)).norm()

        // tabbed speed-up
        d := (u.mult_s(tabs.cos_tab[r1] * r2s) + v.mult_s(tabs.sin_tab[r1] * r2s) +
            w.mult_s(math.sqrt(f64(1.0) - r2))).norm()

        return obj.e + f * radiance(Ray{x, d}, depth, scene_id)
    } else {
        if obj.refl == .spec { // Ideal SPECULAR reflection
            return obj.e + f * radiance(Ray{x, r.d - n.mult_s(2.0 * n.dot(r.d))}, depth, scene_id)
        }
    }

    refl_ray := Ray{x, r.d - n.mult_s(2.0 * n.dot(r.d))} // Ideal dielectric REFRACTION
    into := n.dot(nl) > 0 // Ray from outside going in?

    nc := f64(1.0)
    nt := f64(1.5)

    nnt := if into { nc / nt } else { nt / nc }

    ddn := r.d.dot(nl)
    cos2t := v_1 - nnt * nnt * (v_1 - ddn * ddn)
    if cos2t < 0.0 { // Total internal reflection
        return obj.e + f * radiance(refl_ray, depth, scene_id)
    }

    dirc := if into { f64(1) } else { f64(-1) }
    tdir := (r.d.mult_s(nnt) - n.mult_s(dirc * (ddn * nnt + math.sqrt(cos2t)))).norm()

    a := nt - nc
    b := nt + nc
    r0 := a * a / (b * b)
    c := if into { v_1 + ddn } else { v_1 - tdir.dot(n) }

    re := r0 + (v_1 - r0) * c * c * c * c * c
    tr := v_1 - re
    pp := f64(.25) + f64(.5) * re
    rp := re / pp
    tp := tr / (v_1 - pp)

    mut tmp := Vec{}
    if depth > 2 {
        // Russian roulette
        tmp = if rand_f64() < pp {
            radiance(refl_ray, depth, scene_id).mult_s(rp)
        } else {
            radiance(Ray{x, tdir}, depth, scene_id).mult_s(tp)
        }
    } else {
        tmp = (radiance(refl_ray, depth, scene_id).mult_s(re)) +
            (radiance(Ray{x, tdir}, depth, scene_id).mult_s(tr))
    }
    return obj.e + (f * tmp)
}

//*********************** beam scan routine *********************************
fn ray_trace(w int, h int, samps int, file_name string, scene_id int) Image {
    image := new_image(w, h)

    // inverse costants
    w1 := f64(1.0 / f64(w))
    h1 := f64(1.0 / f64(h))
    samps1 := f64(1.0 / f64(samps))

    cam := Ray{Vec{50, 52, 295.6}, Vec{0, -0.042612, -1}.norm()} // cam position, direction
    cx := Vec{f64(w) * 0.5135 / f64(h), 0, 0}
    cy := cx.cross(cam.d).norm().mult_s(0.5135)
    mut r := Vec{}

    // speed-up constants
    v_1 := f64(1.0)
    v_2 := f64(2.0)

    // OpenMP injection point! #pragma omp parallel for schedule(dynamic, 1) shared(c)
    for y := 0; y < h; y++ {
        if y & 7 == 0 || y + 1 == h {
            term.cursor_up(1)
            eprintln('Rendering (${samps * 4} spp) ${(100.0 * f64(y)) / (f64(h) - 1.0):5.2f}%')
        }
        for x in 0 .. w {
            i := (h - y - 1) * w + x
            mut ivec := unsafe { &image.data[i] }
            // we use sx and sy to perform a square subsampling of 4 samples
            for sy := 0; sy < 2; sy++ {
                for sx := 0; sx < 2; sx++ {
                    r = Vec{0, 0, 0}
                    for _ in 0 .. samps {
                        r1 := v_2 * rand_f64()
                        dx := if r1 < v_1 { math.sqrt(r1) - v_1 } else { v_1 - math.sqrt(v_2 - r1) }

                        r2 := v_2 * rand_f64()
                        dy := if r2 < v_1 { math.sqrt(r2) - v_1 } else { v_1 - math.sqrt(v_2 - r2) }

                        d := cx.mult_s(((f64(sx) + 0.5 + dx) * 0.5 + f64(x)) * w1 - .5) +
                            cy.mult_s(((f64(sy) + 0.5 + dy) * 0.5 + f64(y)) * h1 - .5) + cam.d
                        r = r + radiance(Ray{cam.o +
                            d.mult_s(140.0), d.norm()}, 0, scene_id).mult_s(samps1)
                    }
                    tmp_vec := Vec{clamp(r.x), clamp(r.y), clamp(r.z)}.mult_s(.25)
                    (*ivec) = *ivec + tmp_vec
                }
            }
        }
    }
    return image
}

fn main() {
    if os.args.len > 6 {
        eprintln('Usage:\n     path_tracing [samples] [image.ppm] [scene_n] [width] [height]')
        exit(1)
    }
    mut width := 320 // width of the rendering in pixels
    mut height := 200 // height of the rendering in pixels
    mut samples := 4 // number of samples per pixel, increase for better quality
    mut scene_id := 0 // scene to render [0 cornell box,1 sunset,2 psyco]
    mut file_name := 'image.ppm' // name of the output file in .ppm format

    if os.args.len >= 2 {
        samples = os.args[1].int() / 4
    }
    if os.args.len >= 3 {
        file_name = os.args[2]
    }
    if os.args.len >= 4 {
        scene_id = os.args[3].int()
    }
    if os.args.len >= 5 {
        width = os.args[4].int()
    }
    if os.args.len == 6 {
        height = os.args[5].int()
    }
    // change the seed for a different result
    rand.seed([u32(2020), 0])

    t1 := time.ticks()

    eprintln('Path tracing samples: ${samples}, file_name: ${file_name}, scene_id: ${scene_id}, width: ${width}, height: ${height}')
    eprintln('')
    image := ray_trace(width, height, samples, file_name, scene_id)
    t2 := time.ticks()

    eprintln('Rendering finished. Took: ${(t2 - t1):5}ms')

    image.save_as_ppm(file_name)
    t3 := time.ticks()

    eprintln('Image saved as [${file_name}]. Took: ${(t3 - t2):5}ms')
}