Add solution day 16 part 1

Author: terminalnode

README.md

@@ -36,7 +36,7 @@ deployments).
 ### Running with `docker-compose`
 Just run `docker-compose up` and it will run all services as well as the `traefik`
 gateway. If the watch feature of `docker-compose` is enabled, the services will be
-automatically rebuilt and redeployed ever time the `common` module or their own
+automatically rebuilt and redeployed every time the `common` module or their own
 `solutions/dayX` module is updated.
 
 ## Progress
@@ -47,7 +47,7 @@ automatically rebuilt and redeployed ever time the `common` module or their own
 |-----|----------|-----|----------|
 | 01  | ⭐ ⭐      | 14  | ⭐ ⭐      |
 | 02  | ⭐ ⭐      | 15  | ⭐ ⭐      |
-| 03  | ⭐ ⭐      | 16  |          |
+| 03  | ⭐ ⭐      | 16  | ⭐        |
 | 04  | ⭐ ⭐      | 17  |          |
 | 05  | ⭐ ⭐      | 18  |          |
 | 06  | ⭐ ⭐      | 19  |          |

common/util/coordinate.go

@@ -19,6 +19,10 @@ func In2DArray[T any](
 	return !invalid
 }
 
+func (c Coordinate) IsOrigin() bool {
+	return c.X == 0 && c.Y == 0
+}
+
 func (c Coordinate) Add(x int, y int) Coordinate {
 	return Coordinate{X: c.X + x, Y: c.Y + y}
 }

solutions/day16/main.go

@@ -1,9 +1,84 @@
 package main
 
 import (
+	"fmt"
 	"github.com/terminalnode/adventofcode2024/common"
+	"math"
 )
 
 func main() {
-	common.Setup(16, nil, nil)
+	common.Setup(16, part1, nil)
+}
+
+func part1(
+	input string,
+) string {
+	i, err := parse(input)
+	if err != nil {
+		return fmt.Sprintf("Failed to parse input: %v", err)
+	}
+
+	// Initialize visited set with end point
+	set := initializeVisitedSet(i)
+	loop(initialReindeer(i.s), i, set)
+
+	return fmt.Sprintf("Cheapest path: %v", lowestSoFar(i, set))
+}
+
+func loop(
+	r reindeer,
+	i parsedInput,
+	set visitedSet,
+) {
+	if r.visitAndCheckIfDead(set) {
+		return
+	}
+
+	lowest := lowestSoFar(i, set)
+	if lowest != 0 && r.score > lowest {
+		return
+	}
+
+	if r.p.Equals(i.e) {
+		return
+	}
+
+	loop(r.turnClockwise(), i, set)
+	loop(r.turnCounterClockwise(), i, set)
+	fwd, err := r.forward(i.m)
+	if err == nil {
+		loop(fwd, i, set)
+	}
+
+	return
+}
+
+func lowestSoFar(
+	i parsedInput,
+	set visitedSet,
+) int {
+	lowest := 0
+	for _, n := range set[i.e.Y][i.e.X] {
+		if lowest == 0 || n < lowest {
+			lowest = n
+		}
+	}
+	return lowest
+}
+
+func initializeVisitedSet(
+	i parsedInput,
+) visitedSet {
+	set := make(visitedSet)
+	for y, line := range i.m {
+		set[y] = make(map[intX]map[intDirection]intScore)
+		for x, _ := range line {
+			set[y][x] = make(map[intDirection]intScore)
+			set[y][x][North] = math.MaxInt
+			set[y][x][South] = math.MaxInt
+			set[y][x][West] = math.MaxInt
+			set[y][x][East] = math.MaxInt
+		}
+	}
+	return set
 }

solutions/day16/parse.go

@@ -0,0 +1,56 @@
+package main
+
+import (
+	"errors"
+	"github.com/terminalnode/adventofcode2024/common/util"
+	"strings"
+)
+
+type raceMap = [][]int32
+type parsedInput struct {
+	m raceMap
+	s util.Coordinate
+	e util.Coordinate
+}
+
+const (
+	Start  = 'S'
+	End    = 'E'
+	Ground = '.'
+	Wall   = '#'
+)
+
+func parse(
+	input string,
+) (parsedInput, error) {
+	lines := strings.Split(input, "\n")
+	m := make(raceMap, len(lines))
+	s := util.Coordinate{}
+	e := util.Coordinate{}
+
+	for y, line := range lines {
+		m[y] = []int32(line)
+		if s.IsOrigin() || e.IsOrigin() {
+			for x, ch := range line {
+				if ch == Start {
+					s = util.Coordinate{X: x, Y: y}
+					m[y][x] = Ground
+				} else if ch == End {
+					e = util.Coordinate{X: x, Y: y}
+					m[y][x] = Ground
+				}
+			}
+		}
+	}
+
+	var err error
+	switch {
+	case s.IsOrigin() && e.IsOrigin():
+		err = errors.New("neither start nor end was found")
+	case s.IsOrigin():
+		err = errors.New("start not found")
+	case e.IsOrigin():
+		err = errors.New("end not found")
+	}
+	return parsedInput{m: m, s: s, e: e}, err
+}

solutions/day16/reindeer.go

@@ -0,0 +1,92 @@
+package main
+
+import (
+	"fmt"
+	"github.com/terminalnode/adventofcode2024/common/util"
+)
+
+type intX = int
+type intY = int
+type intDirection = int
+type intScore = int
+type visitedSet = map[intY]map[intX]map[intDirection]intScore
+
+const (
+	North = iota
+	East
+	South
+	West
+)
+
+type reindeer struct {
+	p         util.Coordinate
+	score     int
+	direction int
+}
+
+func initialReindeer(
+	start util.Coordinate,
+) reindeer {
+	return reindeer{
+		p:         start,
+		score:     0,
+		direction: East,
+	}
+}
+
+func (r reindeer) forward(m raceMap) (reindeer, error) {
+	var newP util.Coordinate
+	switch r.direction {
+	case North:
+		newP = r.p.North()
+	case East:
+		newP = r.p.East()
+	case South:
+		newP = r.p.South()
+	case West:
+		newP = r.p.West()
+	default:
+		return r, fmt.Errorf("illegal direction %d", r.direction)
+	}
+
+	if m[r.p.Y][r.p.X] == '#' {
+		return r, fmt.Errorf("wall hit")
+	}
+
+	return reindeer{
+		p:         newP,
+		score:     1 + r.score,
+		direction: r.direction,
+	}, nil
+}
+
+func (r reindeer) turnClockwise() reindeer {
+	return reindeer{
+		p:         r.p,
+		score:     1000 + r.score,
+		direction: (r.direction + 1) % 4,
+	}
+}
+
+func (r reindeer) turnCounterClockwise() reindeer {
+	return reindeer{
+		p:         r.p,
+		score:     1000 + r.score,
+		direction: (r.direction - 1 + 4) % 4,
+	}
+}
+
+func (r reindeer) visitAndCheckIfDead(
+	set visitedSet,
+) bool {
+	v := set[r.p.Y][r.p.X][r.direction]
+	if v == 0 || r.score < v {
+		set[r.p.Y][r.p.X][r.direction] = r.score
+		return false
+	}
+	return true
+}
+
+func (r reindeer) String() string {
+	return fmt.Sprintf("Reindeer{p:%s, score:%d, direction:%d}", r.p, r.score, r.direction)
+}