1 files changed, 118 insertions, 0 deletions
diff --git a/day4/main.go b/day4/main.go
new file mode 100644
index 0000000..e278541
--- /dev/null
+++ b/day4/main.go
@@ -0,0 +1,118 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+)
+
+func main() {
+	fmt.Println(part1(readData("example.txt")))
+	fmt.Println(part1(readData("data.txt")))
+	fmt.Println(part2(readData("example2.txt")))
+	fmt.Println(part2(readData("data.txt")))
+}
+
+func readData(fileName string) (output []string) {
+	fp, err := os.Open(fileName)
+	if err != nil {
+		panic(err)
+	}
+
+	scanner := bufio.NewScanner(fp)
+	output = make([]string, 0)
+
+	for scanner.Scan() {
+		output = append(output, strings.TrimSpace(scanner.Text()))
+	}
+
+	return
+}
+
+func part1(s []string) (count int) {
+	width := len(s[0])
+	height := len(s)
+
+	for y := 0; y < height; y++ {
+		for x := 0; x < width; x++ {
+			if s[y][x] == 'X' {
+				count += checkXmas0(s, x, y)
+			}
+		}
+	}
+	return
+}
+
+func checkXmas0(s []string, x, y int) (count int) {
+	count += checkXmas1(s, x, y, 0)
+	if x >= 3 {
+		count += checkXmas1(s, x, y, -1)
+	}
+	if x < len(s[0])-3 {
+		count += checkXmas1(s, x, y, 1)
+	}
+	return
+}
+
+func checkXmas1(s []string, x, y, dx int) (count int) {
+	count += checkXmas2(s, x, y, dx, 0)
+	if y >= 3 {
+		count += checkXmas2(s, x, y, dx, -1)
+	}
+	if y < len(s)-3 {
+		count += checkXmas2(s, x, y, dx, 1)
+	}
+	return
+}
+
+const WORD string = "XMAS"
+
+func checkXmas2(s []string, x, y, dx, dy int) int {
+	if dx == 0 && dy == 0 {
+		return 0
+	}
+	for i := 0; i < 4; i++ {
+		if s[y][x] != WORD[i] {
+			return 0
+		}
+		x += dx
+		y += dy
+	}
+	return 1
+}
+
+func part2(s []string) (count int) {
+	width := len(s[0])
+	height := len(s)
+	str := []byte{0, 0, 0, 0}
+
+	for y := 1; y < height-1; y++ {
+		for x := 1; x < width-1; x++ {
+			if s[y][x] == 'A' {
+				str[0] = s[y-1][x-1]
+				str[1] = s[y-1][x+1]
+				str[2] = s[y+1][x+1]
+				str[3] = s[y+1][x-1]
+				count += checkXMas(str)
+			}
+		}
+	}
+	return
+}
+
+func checkXMas(b []byte) int {
+	i := 3
+	j := 0
+	for n := 0; n < 4; n++ {
+		if b[i] == 'S' && b[j] == 'M' {
+			break
+		}
+		i = j
+		j += 1
+	}
+	if b[j%4] == 'M' && b[(j+1)%4] == 'M' && b[(j+2)%4] == 'S' && b[(j+3)%4] == 'S' {
+		return 1
+	}
+	return 0
+}