Use atan2

Which would've saved me lots of time and headbanging earlier.

10/main.go

@@ -22,25 +22,6 @@ func buildMap(input []string) []*asteroid {
 	return res
 }
 
-func angleBetween(asteroid1, asteroid2 *asteroid) float64 {
-	if asteroid1.y == asteroid2.y {
-		if asteroid2.x > asteroid1.x {
-			return math.Pi / 2
-		} else {
-			return 3 * math.Pi / 2
-		}
-	} else {
-		angle := math.Atan(float64(asteroid2.x-asteroid1.x) / float64(asteroid1.y-asteroid2.y))
-		if asteroid1.y < asteroid2.y {
-			angle += math.Pi
-		}
-		if angle < 0 {
-			angle += math.Pi * 2
-		}
-		return angle
-	}
-}
-
 func checkAngles(asteroid1 *asteroid, others []*asteroid, countVisible bool) map[float64][]*asteroid {
 	angles := make(map[float64][]*asteroid)
 	for _, asteroid2 := range others {
@@ -48,7 +29,10 @@ func checkAngles(asteroid1 *asteroid, others []*asteroid, countVisible bool) map
 			continue
 		}
 
-		angle := angleBetween(asteroid1, asteroid2)
+		angle := math.Atan2(float64(asteroid2.x-asteroid1.x), float64(asteroid1.y-asteroid2.y))
+		if angle < 0 {
+			angle += math.Pi * 2
+		}
 
 		if len(angles[angle]) == 0 && countVisible {
 			asteroid1.visible++

10/main_test.go

@@ -1,34 +0,0 @@
-package main
-
-import (
-	"math"
-	"testing"
-)
-
-func Test_angleBetween(t *testing.T) {
-	type args struct {
-		asteroid1 *asteroid
-		asteroid2 *asteroid
-	}
-	tests := []struct {
-		name string
-		args args
-		want float64
-	}{
-		{"above", args{&asteroid{x: 5, y: 5}, &asteroid{x: 5, y: 0}}, 0},
-		{"below", args{&asteroid{x: 5, y: 5}, &asteroid{x: 5, y: 10}}, math.Pi},
-		{"right", args{&asteroid{x: 5, y: 5}, &asteroid{x: 10, y: 5}}, math.Pi / 2},
-		{"left", args{&asteroid{x: 5, y: 5}, &asteroid{x: 0, y: 5}}, 3 * math.Pi / 2},
-		{"quadrant1", args{&asteroid{x: 5, y: 5}, &asteroid{x: 10, y: 0}}, math.Pi / 4},
-		{"quadrant2", args{&asteroid{x: 5, y: 5}, &asteroid{x: 10, y: 10}}, 3 * math.Pi / 4},
-		{"quadrant3", args{&asteroid{x: 5, y: 5}, &asteroid{x: 0, y: 10}}, 5 * math.Pi / 4},
-		{"quadrant4", args{&asteroid{x: 5, y: 5}, &asteroid{x: 0, y: 0}}, 7 * math.Pi / 4},
-	}
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			if got := angleBetween(tt.args.asteroid1, tt.args.asteroid2); got != tt.want {
-				t.Errorf("angleBetween() = %v, want %v", got, tt.want)
-			}
-		})
-	}
-}