5 files changed, 192 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
+}
diff --git a/go.work b/go.work
index 6c04533..ebf24bc 100644
--- a/go.work
+++ b/go.work
@@ -8,4 +8,5 @@ use (
 	./day5
 	./day6
 	./day7
+	./day8
 )