Add solution day 21 part 1

Author: terminalnode

README.md

@@ -53,7 +53,7 @@ automatically rebuilt and redeployed every time the `common` module or their own
 | 05  | ⭐ ⭐      | 18  | ⭐ ⭐      |
 | 06  | ⭐ ⭐      | 19  | ⭐ ⭐      |
 | 07  | ⭐ ⭐      | 20  | ⭐ ⭐      |
-| 08  | ⭐ ⭐      | 21  |          |
+| 08  | ⭐ ⭐      | 21  | ⭐        |
 | 09  | ⭐ ⭐      | 22  | ⭐ ⭐      |
 | 10  | ⭐ ⭐      | 23  | ⭐ ⭐      |
 | 11  | ⭐ ⭐      | 24  | ⭐ ⭐      |

solutions/day21/keypad.go

@@ -0,0 +1,100 @@
+package main
+
+import (
+	"github.com/terminalnode/adventofcode2024/common/util"
+)
+
+var numStart = util.Coordinate{X: 2, Y: 3}
+var dirStart = util.Coordinate{X: 2, Y: 0}
+
+// Char to Pos on numeric keypad
+func charToPosNum(
+	c int,
+) util.Coordinate {
+	switch c {
+	case '0':
+		return util.Coordinate{X: 1, Y: 3}
+	case 'A':
+		return numStart
+	default:
+		y := 2 - ((c - '0' - 1) / 3)
+		x := (c - '0' - 1) % 3
+		return util.Coordinate{X: int(x), Y: int(y)}
+	}
+}
+
+// Char to Pos on directional keypad
+func charToPosDir(
+	c int,
+) util.Coordinate {
+	switch c {
+	case '^':
+		return util.Coordinate{X: 1, Y: 0}
+	case '<':
+		return util.Coordinate{X: 0, Y: 1}
+	case 'v':
+		return util.Coordinate{X: 1, Y: 1}
+	case '>':
+		return util.Coordinate{X: 2, Y: 1}
+	default:
+		return util.Coordinate{X: 2, Y: 0}
+	}
+}
+
+func shortestPath(
+	start, end util.Coordinate,
+	useNumPad bool,
+) []int {
+	var path []int
+	dx := end.X - start.X
+	dy := end.Y - start.Y
+	pVert, pHor := buildPath(dx, dy)
+
+	left := end.X < start.X
+	var gap bool
+	if useNumPad {
+		gap = (start.Y == 3 && end.X == 0) || (start.X == 0 && end.Y == 3)
+	} else {
+		gap = (start.X == 0 && end.Y == 0) || (start.Y == 0 && end.X == 0)
+	}
+
+	// The gap is on the left side in both pads, so if we're moving left and would cross
+	// the gap we have to avoid it. If not, apparently it's always best to move vertical first.
+	if left != gap {
+		path = append(path, pHor...)
+		path = append(path, pVert...)
+	} else {
+		path = append(path, pVert...)
+		path = append(path, pHor...)
+	}
+	path = append(path, 'A')
+	return path
+}
+
+func buildPath(
+	dx int,
+	dy int,
+) ([]int, []int) {
+	adx := util.AbsInt(dx)
+	ady := util.AbsInt(dy)
+	pVert := make([]int, ady)
+	pHor := make([]int, adx)
+
+	for i := range adx {
+		if dx < 0 {
+			pHor[i] = '<'
+		} else {
+			pHor[i] = '>'
+		}
+	}
+
+	for i := range ady {
+		if dy < 0 {
+			pVert[i] = '^'
+		} else {
+			pVert[i] = 'v'
+		}
+	}
+
+	return pVert, pHor
+}

solutions/day21/main.go

@@ -1,9 +1,53 @@
 package main
 
 import (
+	"fmt"
 	"github.com/terminalnode/adventofcode2024/common"
+	"strconv"
+	"strings"
 )
 
 func main() {
-	common.Setup(21, nil, nil)
+	common.Setup(21, part1, nil)
+}
+
+func part1(
+	input string,
+) string {
+	var out string
+
+	sequences := strings.Split(input, "\n")
+	sum := 0
+	for _, code := range sequences {
+		// Five is the longest path between two keys on numeric pad
+		path := make([]int, 0, 5*len(code))
+		prev := numStart
+		for _, c := range code {
+			curr := charToPosNum(int(c))
+			path = append(path, shortestPath(prev, curr, true)...)
+			prev = curr
+		}
+
+		for range 2 {
+			// Three is the longest path between two keys on directional pad
+			newPath := make([]int, 0, 3*len(path))
+			prev = dirStart
+			for _, c := range path {
+				curr := charToPosDir(c)
+				newPath = append(newPath, shortestPath(prev, curr, false)...)
+				prev = curr
+			}
+			path = newPath
+		}
+
+		codeNumeric, err := strconv.Atoi(code[:len(code)-1])
+		if err != nil {
+			return fmt.Sprintf("Failed to parse code %s as int: %v", code, err)
+		}
+
+		sum += len(path) * codeNumeric
+	}
+
+	out += fmt.Sprintf("Sum: %d", sum)
+	return out
 }