Add solution for day 3 part 1

Author: terminalnode

README.md

@@ -21,7 +21,7 @@ information.
 |-----|----------|-----|----------|
 | 01  | ⭐ ⭐      | 14  |          |
 | 02  | ⭐ ⭐      | 15  |          |
-| 03  |          | 16  |          |
+| 03  | ⭐        | 16  |          |
 | 04  |          | 17  |          |
 | 05  |          | 18  |          |
 | 06  |          | 19  |          |

day03/main.go

@@ -1,21 +1,53 @@
 package main
 
 import (
+	"fmt"
 	"github.com/terminalnode/adventofcode2024/common"
+	"regexp"
+	"strconv"
 )
 
+var findMulRegex = regexp.MustCompile("mul\\((\\d+),(\\d+)\\)")
+
 func main() {
 	common.Setup(2, part1, part2)
 }
 
 func part1(
 	input string,
 ) string {
-	return "Not solved yet"
+	finds := findMulRegex.FindAllSubmatch([]byte(input), -1)
+	sum := 0
+	for _, match := range finds {
+		if len(match) != 3 {
+			return fmt.Sprintf("Expected 3 results from regex, got %q\n", match)
+		}
+
+		multiplied, err := mul(string(match[1]), string(match[2]))
+		if err != nil {
+			return fmt.Sprintf("Failed to parse multiplication of %q:\n%v\n", match, err)
+		}
+		sum += multiplied
+	}
+	return fmt.Sprintf("Sum of all %d multiplications: %d", len(finds), sum)
 }
 
 func part2(
 	input string,
 ) string {
 	return "Not solved yet"
 }
+
+func mul(sub1 string, sub2 string) (int, error) {
+	first, err := strconv.ParseInt(sub1, 10, 0)
+	if err != nil {
+		return 0, err
+	}
+
+	second, err := strconv.ParseInt(sub2, 10, 0)
+	if err != nil {
+		return 0, err
+	}
+
+	return int(first * second), nil
+}