From 1008f463687a373f5c5d4024d25a09125b6c49ec Mon Sep 17 00:00:00 2001
From: Martin Lehoux <martin@lehoux.net>
Date: Sat, 4 Apr 2020 15:40:11 -0700
Subject: feat - Use HitRecord

---
 color.go   |  3 ++-
 hitable.go | 24 +++++++++++++++++++++---
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/color.go b/color.go
index 3514be6..dbcb47a 100644
--- a/color.go
+++ b/color.go
@@ -17,7 +17,8 @@ func LinearBlend(color1 Color, color2 Color, param float64) Color {
 }
 
 func RayColor(ray Ray, hitable Hitable) Color {
-	if hitable.Hit(ray) {
+	hit := hitable.Hit(ray)
+	if hit.time > 0 {
 		return Color{1.0, 0.0, 0.0}
 	}
 	param := 0.5 * (ray.direction.Unit().y + 1)
diff --git a/hitable.go b/hitable.go
index a44026d..1443110 100644
--- a/hitable.go
+++ b/hitable.go
@@ -1,7 +1,16 @@
 package main
 
+import "math"
+
+type HitRecord struct {
+	time   float64
+	point  Vec3D
+	normal Vec3D
+}
+
 type Hitable interface {
-	Hit(ray Ray) bool
+	Hit(ray Ray) HitRecord
+	Normal(point Vec3D) Vec3D
 }
 
 type Sphere struct {
@@ -9,11 +18,20 @@ type Sphere struct {
 	radius float64
 }
 
-func (sphere Sphere) Hit(ray Ray) bool {
+func (sphere Sphere) Normal(point Vec3D) Vec3D {
+	return (SubVec3D(point, sphere.center)).Unit()
+}
+
+func (sphere Sphere) Hit(ray Ray) HitRecord {
 	oc := SubVec3D(ray.origin, sphere.center)
 	a := DotProduct(ray.direction, ray.direction)
 	b := 2.0 * DotProduct(oc, ray.direction)
 	c := DotProduct(oc, oc) - sphere.radius*sphere.radius
 	discriminant := b*b - 4*a*c
-	return discriminant > 0
+	if discriminant < 0 {
+		return HitRecord{}
+	}
+	time := (-b - math.Sqrt(discriminant)) / 2.0 / a
+	point := ray.PointAt(time)
+	return HitRecord{time, point, SubVec3D(point, sphere.Normal(point))}
 }
-- 
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