diff --git a/2024/day01.go b/2024/day01.go
index 7a24926..90913fb 100644
--- a/2024/day01.go
+++ b/2024/day01.go
@@ -2,14 +2,14 @@ package adventoc2024
 
 import (
 	"bufio"
-	"bytes"
 	"fmt"
 	"io"
-	"os"
 	"sort"
 )
 
-func part1(r io.Reader) string {
+type Day1 struct{}
+
+func (d Day1) Part1(r io.Reader) string {
 	s := bufio.NewScanner(r)
 	var sum int
 	var left, right []int
@@ -30,7 +30,7 @@ func part1(r io.Reader) string {
 	return printi(sum)
 }
 
-func part2(r io.Reader) string {
+func (d Day1) Part2(r io.Reader) string {
 	s := bufio.NewScanner(r)
 	var sum int
 	var left []int
@@ -51,11 +51,3 @@ func part2(r io.Reader) string {
 
 	return printi(sum)
 }
-
-func main() {
-	buf := new(bytes.Buffer)
-	tee := io.TeeReader(os.Stdin, buf)
-
-	fmt.Println(part1(tee))
-	fmt.Println(part2(buf))
-}
diff --git a/2024/puzzle_test.go b/2024/puzzle_test.go
index 9379376..5081671 100644
--- a/2024/puzzle_test.go
+++ b/2024/puzzle_test.go
@@ -19,12 +19,13 @@ func TestDay01(t *testing.T) {
 3   3`
 
 	p1, p2 := dup(input)
+	solver := Day1{}
 
-	if got, expected := part1(p1), "11"; got != expected {
+	if got, expected := solver.Part1(p1), "11"; got != expected {
 		t.Errorf("day01 part 1 = %s; expected %s", got, expected)
 	}
 
-	if got, expected := part2(p2), "31"; got != expected {
+	if got, expected := solver.Part2(p2), "31"; got != expected {
 		t.Errorf("day01 part 2 = %s; expected %s", got, expected)
 	}
 }
@@ -38,7 +39,6 @@ func TestDay02(t *testing.T) {
 1 3 6 7 9`
 
 	p1, p2 := dup(input)
-
 	solver := Day2{}
 
 	if got, expected := solver.Part1(p1), "2"; got != expected {
diff --git a/main.go b/main.go
index 4b01527..7e9875f 100644
--- a/main.go
+++ b/main.go
@@ -1,18 +1,46 @@
 package main
 
 import (
-	"adventoc/2024"
+	adventoc2024 "adventoc/2024"
 	"bytes"
+	"flag"
 	"fmt"
 	"io"
+	"log"
 	"os"
+	"strconv"
 )
 
+type puzzle interface {
+	Part1(io.Reader) string
+	Part2(io.Reader) string
+}
+
+var puzzles = []puzzle{
+	adventoc2024.Day1{},
+	adventoc2024.Day2{},
+}
+
 func main() {
+	flag.Parse()
+	pi := 0
+	if i := flag.Arg(0); i != "" {
+		var err error
+		pi, err = strconv.Atoi(i)
+		if err != nil {
+			log.Fatalf("invalid puzzle day: %s", i)
+		}
+		pi--
+
+		if pi < 0 || pi > len(puzzles) {
+			log.Fatalf("puzzle index %d is out of range", pi)
+		}
+	}
+
 	buf := new(bytes.Buffer)
 	tee := io.TeeReader(os.Stdin, buf)
 
-	puzzle := adventoc2024.Day2{}
+	puzzle := puzzles[pi]
 
 	fmt.Println("Part 1:", puzzle.Part1(tee))
 	fmt.Println("Part 2:", puzzle.Part2(buf))