Day 13: Part 1 and 2

Signed-off-by: James Griffin <james@unsupervised.ca>

main.go

@@ -3,7 +3,7 @@ package main
 import (
 	"fmt"
 
-	"github.com/thatguygriff/aoc2020/twelve"
+	"github.com/thatguygriff/aoc2020/thirteen"
 )
 
 func main() {
@@ -52,6 +52,10 @@ func main() {
 	// fmt.Println(eleven.PartTwo())
 
 	// Day 12
-	fmt.Println(twelve.PartOne())
-	fmt.Println(twelve.PartTwo())
+	// fmt.Println(twelve.PartOne())
+	// fmt.Println(twelve.PartTwo())
+
+	// Day 13
+	fmt.Println(thirteen.PartOne())
+	fmt.Println(thirteen.PartTwo())
 }

thirteen/day_thirteen.go

@@ -0,0 +1,138 @@
+package thirteen
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+)
+
+type bus struct {
+	id int
+}
+
+type note struct {
+	departure int
+	routes    []bus
+	sequence  []int
+}
+
+func notes(f string) (*note, error) {
+	n := &note{}
+	if err := n.load(f); err != nil {
+		return nil, err
+	}
+
+	return n, nil
+}
+
+func (n *note) load(filename string) error {
+	file, err := os.Open(filename)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+	first := true
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		if first {
+			time, err := strconv.Atoi(scanner.Text())
+			if err != nil {
+				return err
+			}
+			n.departure = time
+			first = false
+			continue
+		}
+
+		buses := strings.Split(scanner.Text(), ",")
+		for _, b := range buses {
+			if b == "x" {
+				n.sequence = append(n.sequence, -1)
+				continue
+			}
+
+			id, err := strconv.Atoi(b)
+			if err != nil {
+				return err
+			}
+			n.routes = append(n.routes, bus{id: id})
+			n.sequence = append(n.sequence, id)
+		}
+	}
+
+	return nil
+}
+
+func (n *note) firstBus() int {
+	wait := 0
+	now := n.departure
+
+	for {
+		for _, bus := range n.routes {
+			if now%bus.id == 0 {
+				fmt.Println("Found", bus.id, "after a wait of", wait)
+				return wait * bus.id
+			}
+		}
+
+		now++
+		wait++
+	}
+}
+
+func (n *note) sequenceStart() int {
+	if len(n.sequence) == 0 {
+		return -1
+	}
+
+	time := 0
+	for {
+		// Naive interval skipping, only check times where the first bus is scheduled
+		interval := n.sequence[0]
+		found := true
+		for i := 1; i < len(n.sequence); i++ {
+			// Skip unconstrained rules
+			if n.sequence[i] == -1 {
+				continue
+			}
+
+			if (time+i)%n.sequence[i] == 0 {
+				// We know that the bus at i is correct, so we can skip interval*bus id ahead for the next time this is valid
+				interval *= n.sequence[i]
+				continue
+			}
+
+			// We reach this point if the sequence is broken. We now advance
+			// to the next time the first bus is valid
+			time += interval
+			found = false
+			break
+		}
+		if found {
+			return time
+		}
+
+	}
+}
+
+// PartOne find the product of the wait * bus id to get to the airport
+func PartOne() string {
+	n, err := notes("thirteen/input.txt")
+	if err != nil {
+		return err.Error()
+	}
+
+	return fmt.Sprintf("Found a product of %d", n.firstBus())
+}
+
+// PartTwo What is the answer to the consequtive departure minutes contest
+func PartTwo() string {
+	n, err := notes("thirteen/input.txt")
+	if err != nil {
+		return err.Error()
+	}
+
+	return fmt.Sprintf("The earliest minute is %d", n.sequenceStart())
+}

thirteen/day_thirteen_test.go

@@ -0,0 +1,77 @@
+package thirteen
+
+import "testing"
+
+func Test_note_load(t *testing.T) {
+	n, err := notes("sample.txt")
+	if err != nil {
+		t.Logf(err.Error())
+		t.Fail()
+	}
+
+	if len(n.routes) != 5 {
+		t.Logf("Expected 5 routes, found %d", n.routes)
+		t.Fail()
+	}
+
+	if n.departure != 939 {
+		t.Logf("Expected departure of 939, found %d", n.departure)
+		t.Fail()
+	}
+}
+
+func Test_find_first(t *testing.T) {
+	n, _ := notes("sample.txt")
+
+	result := n.firstBus()
+
+	if result != 295 {
+		t.Logf("Expected product of 295 for first bus, got %d", result)
+		t.Fail()
+	}
+}
+
+func Test_contest_start(t *testing.T) {
+	n, _ := notes("sample.txt")
+
+	result := n.sequenceStart()
+	if result != 1068781 {
+		t.Logf("Expected time of 1068781 for sequence, got %d", result)
+		t.Fail()
+	}
+
+	n.sequence = []int{17, -1, 13, 19}
+	result = n.sequenceStart()
+	if result != 3417 {
+		t.Logf("Expected time of 3417 for sequence, got %d", result)
+		t.Fail()
+	}
+
+	n.sequence = []int{67, 7, 59, 61}
+	result = n.sequenceStart()
+	if result != 754018 {
+		t.Logf("Expected time of 754018 for sequence, got %d", result)
+		t.Fail()
+	}
+
+	n.sequence = []int{67, -1, 7, 59, 61}
+	result = n.sequenceStart()
+	if result != 779210 {
+		t.Logf("Expected time of 779210 for sequence, got %d", result)
+		t.Fail()
+	}
+
+	n.sequence = []int{67, 7, -1, 59, 61}
+	result = n.sequenceStart()
+	if result != 1261476 {
+		t.Logf("Expected time of 1261476 for sequence, got %d", result)
+		t.Fail()
+	}
+
+	n.sequence = []int{1789, 37, 47, 1889}
+	result = n.sequenceStart()
+	if result != 1202161486 {
+		t.Logf("Expected time of 1202161486 for sequence, got %d", result)
+		t.Fail()
+	}
+}

thirteen/input.txt

@@ -0,0 +1,2 @@
+1002561
+17,x,x,x,x,x,x,x,x,x,x,37,x,x,x,x,x,409,x,29,x,x,x,x,x,x,x,x,x,x,13,x,x,x,x,x,x,x,x,x,23,x,x,x,x,x,x,x,373,x,x,x,x,x,x,x,x,x,41,x,x,x,x,x,x,x,x,19

thirteen/sample.txt

@@ -0,0 +1,2 @@
+939
+7,13,x,x,59,x,31,19