Day 8

4 files changed, 191 insertions, 0 deletions

diff --git a/day8/data.txt b/day8/data.txt
new file mode 100644
index 0000000..3cefaeb
--- /dev/null
+++ b/day8/data.txt
@@ -0,0 +1,50 @@
+.....wV....q.....................................n

+.......w......q.h.....Vn.........................D

+............w.S..G.....................DT.........

+......S........h......e..T.....y......D...........

+......m.......Ae.......T........o.................

+....m....S........................................

+...m..........................n........8..........

+.........2...G......................n.............

+..2........V.......h................Q.............

+............................o.....................

+.Z......I..U....e...u.....G....o..................

+...N..G.........................................y.

+.....I............q.......h...................s...

+......U........qI....o.V..Rz........8........k....

+......d.Z.........................R.......8y......

+.........e..............T.....l...................

+.......2.........................u...R............

+.....d.............................Q..............

+...................v.....................s.Q....M.

+........2..........4.....................8..7.k...

+...........x..N..................A..........k.....

+...........ZN...........v...............K.........

+...d.......N.....................Ky.6.............

+...........................l6.....................

+....L....g.................4.......k..K.......0...

+..............L...........4R................s.....

+U......r..............H.4.........................

+.......U.............a.......H.............u......

+......xY...............l..........................

+...................................6..u...........

+........Y......L......l............0..............

+......9..L...........A.....v..HEa........K........

+..................v........6.EX.............z.....

+d..Y.............m......A.........................

+......................a.i......M...........z......

+...................g.......................0......

+...............................H.........i........

+..........3................W........E...i...0.....

+.................t.a....g.................5.......

+.r...t...........................7.....5..........

+....................................7....5........

+....................g.Y...wMz.....................

+9..........O....3................W.7..E..XD...1...

+t..............3.x.....9..........W.M.............

+...9............W.................................

+Z.............x................X.i......5.........

+...........3.....................................1

+...................O.......s....X.................

+..............r...................................

+..........................O.................1.....

diff --git a/day8/example.txt b/day8/example.txt
new file mode 100644
index 0000000..79e3286
--- /dev/null
+++ b/day8/example.txt
@@ -0,0 +1,12 @@
+............

+........0...

+.....0......

+.......0....

+....0.......

+......A.....

+............

+............

+........A...

+.........A..

+............

+............

diff --git a/day8/go.mod b/day8/go.mod
new file mode 100644
index 0000000..5baa592
--- /dev/null
+++ b/day8/go.mod
@@ -0,0 +1,3 @@
+module stevenlr.com/aoc2024/day8
+
+go 1.22.2
diff --git a/day8/main.go b/day8/main.go
new file mode 100644
index 0000000..11b0d09
--- /dev/null
+++ b/day8/main.go
@@ -0,0 +1,126 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+)
+
+type Position struct {
+	X, Y int
+}
+
+func ByteToAntenna(b byte) int {
+	if b >= '0' && b <= '9' {
+		return int(b - '0')
+	} else if b >= 'a' && b <= 'z' {
+		return int(b - 'a' + 10)
+	} else if b >= 'A' && b <= 'Z' {
+		return int(b - 'A' + 36)
+	} else {
+		return -1
+	}
+}
+
+func IsInMap(p Position, w, h int) bool {
+	if p.X < 0 || p.X >= w {
+		return false
+	}
+	if p.Y < 0 || p.Y >= h {
+		return false
+	}
+	return true
+}
+
+func ComputeAntinodes(a, b Position, w, h int, first, repeats int) (antinodes []Position) {
+	dx := b.X - a.X
+	dy := b.Y - a.Y
+
+	for i := first; i <= repeats; i++ {
+		n1, n2 := Position{a.X - dx*i, a.Y - dy*i}, Position{b.X + dx*i, b.Y + dy*i}
+		n1InMap := IsInMap(n1, w, h)
+		n2InMap := IsInMap(n2, w, h)
+
+		if n1InMap {
+			antinodes = append(antinodes, n1)
+		}
+
+		if n2InMap {
+			antinodes = append(antinodes, n2)
+		}
+
+		if !n1InMap && !n2InMap {
+			break
+		}
+	}
+
+	return
+}
+
+func main() {
+	{
+		width, height, antinodes := readData("example.txt")
+		fmt.Println(countAntinodes(width, height, antinodes, 1, 1))
+		fmt.Println(countAntinodes(width, height, antinodes, 0, 999))
+	}
+	{
+		width, height, antinodes := readData("data.txt")
+		fmt.Println(countAntinodes(width, height, antinodes, 1, 1))
+		fmt.Println(countAntinodes(width, height, antinodes, 0, 999))
+	}
+}
+
+func countAntinodes(width, height int, antennas map[int][]Position, first, repeats int) (count int) {
+	marked := make([][]bool, height)
+	for y := 0; y < height; y++ {
+		marked[y] = make([]bool, width)
+	}
+
+	for _, positions := range antennas {
+		for i := 0; i < len(positions)-1; i++ {
+			for j := i + 1; j < len(positions); j++ {
+				for _, p := range ComputeAntinodes(positions[i], positions[j], width, height, first, repeats) {
+					marked[p.Y][p.X] = true
+				}
+			}
+		}
+	}
+
+	for y := 0; y < height; y++ {
+		for x := 0; x < len(marked[y]); x++ {
+			if marked[y][x] {
+				count++
+			}
+		}
+	}
+
+	return
+}
+
+func readData(fileName string) (width, height int, antennas map[int][]Position) {
+	fp, err := os.Open(fileName)
+	if err != nil {
+		panic(err)
+	}
+
+	scanner := bufio.NewScanner(fp)
+	antennas = make(map[int][]Position)
+
+	y := 0
+	for ; scanner.Scan(); y++ {
+		line := strings.TrimSpace(scanner.Text())
+		width = len(line)
+
+		for x := 0; x < width; x++ {
+			antenna := ByteToAntenna(line[x])
+			if antenna >= 0 {
+				antennas[antenna] = append(antennas[antenna], Position{x, y})
+			}
+		}
+	}
+
+	height = y
+
+	return
+}