From bf8996c9530a6a1857c35f226f6ba2041f1ba1cc Mon Sep 17 00:00:00 2001
From: Samrat Man Singh <samratmansingh@gmail.com>
Date: Fri, 11 Mar 2016 12:06:01 +0545
Subject: [PATCH] Use `option` to signal when no object is hit

---
 src/raytracer.ml | 74 +++++++++++++++++++++++++-----------------------
 1 file changed, 38 insertions(+), 36 deletions(-)

diff --git a/src/raytracer.ml b/src/raytracer.ml
index ca4d2013..80da2b25 100644
--- a/src/raytracer.ml
+++ b/src/raytracer.ml
@@ -9,10 +9,12 @@ type sphere = { center: Vec3.vec3;
 type hitable = Sphere of sphere
              | World of hitable list
 
-type hit = { t : float;
-             p: Vec3.vec3;
-             normal: Vec3.vec3;
-             hit: bool }
+type hit_rec = { t : float;
+                 p: Vec3.vec3;
+                 normal: Vec3.vec3; }
+
+type hit = hit_rec option
+
 
 let hit_sphere sphere ray (tmin, tmax) =
   let oc = sub ray.origin sphere.center in
@@ -21,11 +23,6 @@ let hit_sphere sphere ray (tmin, tmax) =
   let c = (dot oc oc) -. (sphere.radius *. sphere.radius) in
   let discriminant = b*.b -. a*.c in
 
-  let no_hit = { t = -1.0;
-                 p = Vec3.of_floats (Float.infinity, Float.infinity, Float.infinity);
-                 normal = Vec3.of_floats (0., 0., 0.);
-                 hit = false } in
-
   if (discriminant > 0.0)
   then
     let t = (-.b +. (sqrt discriminant)) /. a in
@@ -33,34 +30,33 @@ let hit_sphere sphere ray (tmin, tmax) =
     if (t < tmax && t > tmin)
     then 
       let p = Ray.point_at_parameter ray t in
-      { t = t;
-        p = p;
-        normal = unit_vector (sub p sphere.center);
-        hit = true }
+      Some { t = t;
+             p = p;
+             normal = unit_vector (sub p sphere.center); }
     else
       let t = (-.b -. (sqrt discriminant)) /. a in
       if (t < tmax && t > tmin)
       then 
         let p = Ray.point_at_parameter ray t in
-        { t = t;
-          p = p;
-          normal = unit_vector (sub p sphere.center);
-          hit = true }
-      else no_hit
-  else no_hit
+        Some { t = t;
+               p = p;
+               normal = unit_vector (sub p sphere.center); }
+      else None
+  else None
 
 let rec hit_world world ray (tmin, tmax) =
-  let init_rec = { t = tmax;
-                   p = Vec3.of_floats (-1., -1., -1.);
-                   normal = Vec3.of_floats (-1., -1., -1.);
-                   hit = false; } in
   List.fold world
-    ~init: init_rec
-    ~f: (fun acc h -> 
-        let hit_rec = (hit h ray (tmin, acc.t)) in
-        if (hit_rec.hit) 
-        then hit_rec
-        else acc)
+    ~init: None
+    ~f: (fun acc h ->
+        let prev_rec = match acc with
+            None -> { t = tmax;
+                      p = Vec3.of_floats (-1., -1., -1.);
+                      normal = Vec3.of_floats (-1., -1., -1.); }
+          | Some r -> r in
+        match (hit h ray (tmin, prev_rec.t)) with
+          Some r -> Some r
+        | None -> acc)
+
 and hit h ray (tmin, tmax) =
   match h with
     Sphere s -> hit_sphere s ray (tmin, tmax)
@@ -72,13 +68,19 @@ let get_color ray =
                         radius = 0.5 } in
   let sphere2 = Sphere {center = Vec3.of_floats (0., -100.5, -1.);
                         radius = 100.0 } in
-  let world = World [sphere1; sphere2] in
-  let hit_result = (hit world ray (0., Float.infinity)) in
-  let t = hit_result.t in
-  if (hit_result.hit && (t > 0.0))
-  then let n = hit_result.normal in
-    mul 0.5 (Vec3.of_floats (n.x +. 1., n.y +. 1., n.z +. 1.))
-  else let unit_direction = unit_vector ray.dir in
+  let world = World [sphere2; sphere1] in
+
+  match (hit world ray (0., Float.infinity)) with
+  Some hit_result ->
+    let t = hit_result.t in
+    if (t > 0.0)
+    then let n = hit_result.normal in
+      mul 0.5 (Vec3.of_floats (n.x +. 1., n.y +. 1., n.z +. 1.))
+    else let unit_direction = unit_vector ray.dir in
+      let t = 0.5 *. (unit_direction.y +. 1.0) in
+      add (mul (1.0 -. t) {x= 1.0; y=1.0; z= 1.0}) (mul t {x= 0.5; y= 0.7; z= 1.0})
+  | None ->
+    let unit_direction = unit_vector ray.dir in
     let t = 0.5 *. (unit_direction.y +. 1.0) in
     add (mul (1.0 -. t) {x= 1.0; y=1.0; z= 1.0}) (mul t {x= 0.5; y= 0.7; z= 1.0})