Day 3

day3/main.go

@@ -0,0 +1,97 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+func main() {
+	content, err := os.ReadFile("data.txt")
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Println(part1("xmul(2,4)%&mul[3,7]!@^do_not_mul(5,5)+mul(32,64]then(mul(11,8)mul(8,5))"))
+	fmt.Println(part1(string(content)))
+
+	fmt.Println(part2("xmul(2,4)&mul[3,7]!^don't()_mul(5,5)+mul(32,64](mul(11,8)undo()?mul(8,5))"))
+	fmt.Println(part2(string(content)))
+}
+
+func part1(s string) (result int) {
+	for {
+		index := strings.Index(s, "mul(")
+		if index < 0 {
+			return
+		}
+
+		s = s[index+4:]
+		op1, opLength := parseOperand(&s)
+		if opLength < 1 || opLength > 3 {
+			continue
+		}
+
+		if !expectChar(&s, ',') {
+			continue
+		}
+
+		op2, opLength := parseOperand(&s)
+		if opLength < 1 || opLength > 3 {
+			continue
+		}
+
+		if !expectChar(&s, ')') {
+			continue
+		}
+
+		result += op1 * op2
+	}
+}
+
+func part2(s string) (result int) {
+	for {
+		dont_index := strings.Index(s, "don't()")
+		if dont_index < 0 {
+			result += part1(s)
+			return
+		}
+
+		result += part1(s[:dont_index])
+		s = s[dont_index+7:]
+
+		do_index := strings.Index(s, "do()")
+		if do_index < 0 {
+			return
+		}
+
+		s = s[do_index+4:]
+	}
+
+	return
+}
+
+func expectChar(s *string, r byte) bool {
+	if len(*s) == 0 || (*s)[0] != r {
+		return false
+	} else {
+		*s = (*s)[1:]
+		return true
+	}
+}
+
+func parseOperand(s *string) (operand int, length int) {
+	for len(*s) > 0 {
+		c := (*s)[0]
+
+		if c < '0' || c > '9' {
+			return
+		}
+
+		operand = operand*10 + int(c-'0')
+		*s = (*s)[1:]
+		length++
+	}
+
+	return
+}