Add too slow solution for day 17 part 2

Naive solution that works for the example, but is too slow for regular inputs.

Author: terminalnode

solutions/day17/machine.go

@@ -6,19 +6,17 @@ import (
 )
 
 type machine struct {
-	a     int64
-	b     int64
-	c     int64
-	seq   []int64
-	out   []int64
-	pos   int
-	count int
+	a   int64
+	b   int64
+	c   int64
+	seq []int64
+	out []int64
+	pos int
 }
 
-func (m *machine) run(maxItt int) {
-	for m.pos < len(m.seq) && (maxItt <= 0 || m.count < maxItt) {
+func (m *machine) run(maxOut int) {
+	for m.pos < len(m.seq) && (maxOut < 1 || len(m.out) < maxOut) {
 		m.next()
-		m.count++
 	}
 }
 

solutions/day17/main.go

@@ -6,7 +6,7 @@ import (
 )
 
 func main() {
-	common.Setup(17, part1, nil)
+	common.Setup(17, part1, part2)
 }
 
 func part1(
@@ -19,3 +19,47 @@ func part1(
 	m.run(-1)
 	return m.strOut()
 }
+
+func part2(
+	input string,
+) string {
+	m, err := parseMachine(input)
+	if err != nil {
+		return fmt.Sprintf("Failed to parse machine: %v", err)
+	}
+
+	lenSeq := len(m.seq)
+	initA := int64(0)
+	initB := m.b
+	initC := m.c
+	for initA = 0; true; initA++ {
+		m.run(lenSeq)
+		if compareSeqOut(m, lenSeq) {
+			break
+		}
+		m.a = initA
+		m.b = initB
+		m.c = initC
+		m.out = m.out[:0]
+		m.pos = 0
+	}
+
+	return fmt.Sprintf("Registry A should be %d", initA-1)
+}
+
+func compareSeqOut(
+	m machine,
+	lenSeq int,
+) bool {
+	if len(m.out) != lenSeq {
+		return false
+	}
+
+	for i, seq := range m.seq {
+		if m.out[i] != seq {
+			return false
+		}
+	}
+
+	return true
+}

solutions/day17/main_test.go

@@ -4,13 +4,6 @@ import (
 	"testing"
 )
 
-const ex1Out = "4,6,3,5,6,3,5,2,1,0"
-const ex1 = `Register A: 729
-Register B: 0
-Register C: 0
-
-Program: 0,1,5,4,3,0`
-
 func TestParseMachine(t *testing.T) {
 	testInput := `Register A: -1337
 Register B: 0
@@ -49,9 +42,30 @@ Program: 0,-1,5,-4,3,0`
 	}
 }
 
-func TestPart1_Example1(t *testing.T) {
-	out := part1(ex1)
-	if out != ex1Out {
-		t.Errorf("expected '%s' but got '%s'", ex1Out, out)
+func TestPart1_Example(t *testing.T) {
+	const ex = `Register A: 729
+Register B: 0
+Register C: 0
+
+Program: 0,1,5,4,3,0`
+	const exOut = "4,6,3,5,6,3,5,2,1,0"
+
+	out := part1(ex)
+	if out != exOut {
+		t.Errorf("expected '%s' but got '%s'", exOut, out)
+	}
+}
+
+func TestPart2_Example(t *testing.T) {
+	ex := `Register A: 2024
+Register B: 0
+Register C: 0
+
+Program: 0,3,5,4,3,0`
+	exOut := "Registry A should be 117440"
+
+	out := part2(ex)
+	if out != exOut {
+		t.Errorf("expected '%s' but got '%s'", exOut, out)
 	}
 }