bug: Fix day13 solution

Author: truggeri

src/day13/day13.go

@@ -70,24 +70,11 @@ func parse(input string) []machine {
 }
 
 func solveMachine(m machine) (bool, uint) {
-	if outOfRange(m) {
-		return false, 0
-	}
-
 	a, b := cramersRule([2][2]int{{m.buttonA.x, m.buttonB.x}, {m.buttonA.y, m.buttonB.y}}, [2][1]int{{m.prize.x}, {m.prize.y}})
 	if a == 0 && b == 0 {
 		return manualSolution(m)
 	}
-	positiveOnly := a > 0 && b >= 0 || a >= 0 && b > 0
-	return positiveOnly, uint(a*BUTTON_A_COST + b*BUTTON_B_COST)
-}
-
-func outOfRange(m machine) bool {
-	maxX := max(m.buttonA.x, m.buttonB.x)
-	cantReachX := maxX != 0 && m.prize.x/maxX > BUTTON_PUSH_LIMIT+1
-	maxY := max(m.buttonA.y, m.buttonB.y)
-	cantReachY := maxY != 0 && m.prize.y/maxY > BUTTON_PUSH_LIMIT+1
-	return cantReachX || cantReachY
+	return validSolution(m, a, b), uint(a*BUTTON_A_COST + b*BUTTON_B_COST)
 }
 
 // @see{https://en.wikipedia.org/wiki/Cramer's_rule#Explicit_formulas_for_small_systems}
@@ -108,6 +95,14 @@ func determinate(input [2][2]int) int {
 	return input[0][0]*input[1][1] - input[0][1]*input[1][0]
 }
 
+func validSolution(m machine, a, b int) bool {
+	positiveOnly := (a > 0 && b >= 0) || (a >= 0 && b > 0)
+	withinLimits := a <= 100 && b <= 100
+	validAnswer := (a*m.buttonA.x+b*m.buttonB.x == m.prize.x) && (a*m.buttonA.y+b*m.buttonB.y == m.prize.y)
+
+	return positiveOnly && withinLimits && validAnswer
+}
+
 func manualSolution(m machine) (bool, uint) {
 	a, costa := linearSolution(m.buttonA, m.prize, BUTTON_A_COST)
 	b, costb := linearSolution(m.buttonB, m.prize, BUTTON_B_COST)

src/day13/day13_test.go

@@ -95,3 +95,23 @@ Prize: X=0, Y=9`
 		t.Errorf("Calculated solution was not expected")
 	}
 }
+
+func TestStretchesTowardsMax(t *testing.T) {
+	input := `Button A: X+98, Y+61
+Button B: X+23, Y+50
+Prize: X=10248, Y=7378`
+	prizes, tokens := Solve(input)
+	if prizes != 1 || tokens != 322 {
+		t.Errorf("Calculated solution was not expected")
+	}
+}
+
+func TestDetNotWholeNumber(t *testing.T) {
+	input := `Button A: X+31, Y+54
+Button B: X+30, Y+14
+Prize: X=1879, Y=1440`
+	prizes, tokens := Solve(input)
+	if prizes != 0 || tokens != 0 {
+		t.Errorf("Calculated solution was not expected")
+	}
+}