Add solution day 18 part 1

terminalnode · terminalnode · commit ca5d2a8d93c5 · 2024-12-18T15:49:04.000+01:00
diff --git a/README.md b/README.md
@@ -50,7 +50,7 @@ automatically rebuilt and redeployed every time the `common` module or their own
 | 02  | ⭐ ⭐      | 15  | ⭐ ⭐      |
 | 03  | ⭐ ⭐      | 16  | ⭐ 🥸     |
 | 04  | ⭐ ⭐      | 17  | ⭐        |
-| 05  | ⭐ ⭐      | 18  |          |
+| 05  | ⭐ ⭐      | 18  | ⭐        |
 | 06  | ⭐ ⭐      | 19  |          |
 | 07  | ⭐ ⭐      | 20  |          |
 | 08  | ⭐ ⭐      | 21  |          |
diff --git a/solutions/day18/main.go b/solutions/day18/main.go
@@ -1,9 +1,108 @@
 package main
 
 import (
+	"fmt"
 	"github.com/terminalnode/adventofcode2024/common"
+	"github.com/terminalnode/adventofcode2024/common/util"
 )
 
 func main() {
-	common.Setup(18, nil, nil)
+	common.Setup(18, part1, nil)
+}
+
+func part1(
+	input string,
+) string {
+	out, err := run(input, 70, 70, 1024)
+	if err != nil {
+		return err.Error()
+	}
+	return fmt.Sprintf("Shortest path is %d steps", out)
+}
+
+func run(
+	input string,
+	maxX int,
+	maxY int,
+	fallenBytes int,
+) (int, error) {
+	m := buildMatrix(maxX, maxY)
+	cs, err := parse(input)
+	if err != nil {
+		return -1, err
+	}
+
+	if len(cs) > fallenBytes {
+		cs = cs[:fallenBytes]
+	}
+
+	for _, c := range cs {
+		m[c.Y][c.X] = true
+	}
+
+	visitedSet := buildVisitedSet(maxY)
+	shortestPath(
+		m,
+		util.Coordinate{X: 0, Y: 0},
+		util.Coordinate{X: maxX, Y: maxY},
+		0,
+		visitedSet,
+	)
+
+	return visitedSet[maxY][maxX], nil
+}
+
+func buildMatrix(
+	maxX int,
+	maxY int,
+) [][]bool {
+	out := make([][]bool, maxY+1)
+	for y := 0; y <= maxY; y++ {
+		out[y] = make([]bool, maxX+1)
+	}
+	return out
+}
+
+func buildVisitedSet(
+	maxY int,
+) map[int]map[int]int {
+	visitedSet := make(map[int]map[int]int)
+	for y := 0; y <= maxY; y++ {
+		visitedSet[y] = make(map[int]int)
+	}
+	return visitedSet
+}
+
+func shortestPath(
+	m [][]bool,
+	c util.Coordinate,
+	goal util.Coordinate,
+	steps int,
+	visitedSet map[int]map[int]int,
+) {
+	visit := visitedSet[c.Y][c.X]
+	if visit > 0 && visit <= steps {
+		// Someone beat us to this position
+		return
+	}
+
+	endVisit := visitedSet[goal.Y][goal.X]
+	if endVisit > 0 && endVisit <= steps {
+		// Someone beat us to the end goal
+		return
+	}
+
+	if !util.In2DArray(c, m) || m[c.Y][c.X] {
+		// The current position is impassable or outside the matrix
+		return
+	}
+
+	// Register our place on the map
+	visitedSet[c.Y][c.X] = steps
+
+	// Travel new places
+	shortestPath(m, c.North(), goal, steps+1, visitedSet)
+	shortestPath(m, c.East(), goal, steps+1, visitedSet)
+	shortestPath(m, c.South(), goal, steps+1, visitedSet)
+	shortestPath(m, c.West(), goal, steps+1, visitedSet)
 }
diff --git a/solutions/day18/parse.go b/solutions/day18/parse.go
@@ -0,0 +1,35 @@
+package main
+
+import (
+	"fmt"
+	"github.com/terminalnode/adventofcode2024/common/util"
+	"strconv"
+	"strings"
+)
+
+func parse(
+	input string,
+) ([]util.Coordinate, error) {
+	split := strings.Split(input, "\n")
+	out := make([]util.Coordinate, len(split))
+	for i, s := range split {
+		xy := strings.Split(s, ",")
+		if len(xy) != 2 {
+			return out, fmt.Errorf("expected '%s' split by ',' to become a size 2 list", s)
+		}
+
+		x, err := strconv.ParseInt(xy[0], 10, 0)
+		if err != nil {
+			return out, err
+		}
+
+		y, err := strconv.ParseInt(xy[1], 10, 0)
+		if err != nil {
+			return out, err
+		}
+
+		out[i] = util.Coordinate{X: int(x), Y: int(y)}
+	}
+
+	return out, nil
+}
