diff --git a/day3.go b/day3.go
new file mode 100644
index 0000000..25da426
--- /dev/null
+++ b/day3.go
@@ -0,0 +1,142 @@
+package main
+
+import (
+	"strconv"
+	"strings"
+)
+
+type Day3Mul struct {
+	size int
+	number1 int
+	number2 int
+}
+
+func Day3ParseMulInstruction(text string) *Day3Mul {
+	max_length := 0
+	max_length += 4 // mul(
+	max_length += 3 // 999
+	max_length += 1 // ,
+	max_length += 3 // 999
+	max_length += 1 // )
+
+	if len(text) < 4 {
+		return nil
+	}
+
+	if len(text) > max_length {
+		text = text[:max_length]
+	}
+
+	if text[0:4] != "mul(" {
+		return nil
+	}
+
+	comma_index := strings.IndexRune(text, ',')
+	if (comma_index == -1) {
+		return nil
+	}
+
+	closing_index := strings.IndexRune(text, ')')
+	if (closing_index == -1) {
+		return nil
+	}
+
+	number1_text := text[4:comma_index]
+	number2_text := text[(comma_index+1):closing_index]
+
+	number1, err := strconv.ParseUint(number1_text, 10, 0)
+	if err != nil {
+		return nil
+	}
+	if number1 > 999 {
+		return nil
+	}
+
+	number2, err := strconv.ParseUint(number2_text, 10, 0)
+	if err != nil {
+		return nil
+	}
+	if number2 > 999 {
+		return nil
+	}
+
+	inst := Day3Mul{
+		size: closing_index + 1,
+		number1: int(number1),
+		number2: int(number2),
+	}
+
+	return &inst
+}
+
+func Day3Part1(lines []string) error {
+	answer := 0
+
+	for _, line := range lines {
+		cursor := 0
+		for cursor < len(line) {
+			inst := Day3ParseMulInstruction(line[cursor:])
+			if inst != nil {
+				cursor += inst.size
+				answer += inst.number1 * inst.number2
+			} else {
+				cursor += 1
+			}
+		}
+	}
+
+
+	println(answer)
+
+	return nil
+}
+
+func Day3StartsWith(haystack string, needle string) bool {
+	if len(haystack) < len(needle) {
+		return false
+	}
+
+	return haystack[:len(needle)] == needle
+}
+
+func Day3Part2(lines []string) error {
+	answer := 0
+
+	do_inst := "do()"
+	dont_inst := "don't()"
+
+	enabled := true
+	for _, line := range lines {
+		cursor := 0
+		for cursor < len(line) {
+			rest := line[cursor:]
+			if Day3StartsWith(rest, do_inst) {
+				enabled = true
+				cursor += len(do_inst)
+				continue
+			}
+
+			if Day3StartsWith(rest, dont_inst) {
+				enabled = false
+				cursor += len(dont_inst)
+				continue
+			}
+
+			if enabled {
+				inst := Day3ParseMulInstruction(rest)
+				if inst != nil {
+					cursor += inst.size
+					answer += inst.number1 * inst.number2
+					continue
+				}
+			}
+
+			cursor += 1
+		}
+	}
+
+
+	println(answer)
+
+	return nil
+}
diff --git a/main.go b/main.go
index 4f7ec9d..ca2d31b 100644
--- a/main.go
+++ b/main.go
@@ -47,5 +47,11 @@ func main() {
 		} else if part == "2" {
 			Day2Part2(lines)
 		}
+	} else if day == "3" {
+		if part == "1" {
+			Day3Part1(lines)
+		} else if part == "2" {
+			Day3Part2(lines)
+		}
 	}
 }