diff --git a/blocks.go b/blocks.go
new file mode 100644
index 0000000..8b3ba09
--- /dev/null
+++ b/blocks.go
@@ -0,0 +1,5 @@
+package defrag
+
+func Blocks(diskMap []byte) ([]byte, error) {
+	return nil, nil
+}
diff --git a/blocks_test.go b/blocks_test.go
new file mode 100644
index 0000000..1e1831c
--- /dev/null
+++ b/blocks_test.go
@@ -0,0 +1,31 @@
+package defrag
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+type tt struct {
+	name      string
+	diskMap   []byte
+	blocks    []byte
+	defragged []byte
+}
+
+func sampleTests() []tt {
+	return []tt{
+		{"short", []byte("12345"), []byte("0..111....22222"), []byte("022111222......")},
+		{"longer", []byte("2333133121414131402"), []byte("00...111...2...333.44.5555.6666.777.888899"), []byte("0099811188827773336446555566..............")},
+	}
+}
+
+func TestBlocks(t *testing.T) {
+	for _, testcase := range sampleTests() {
+		t.Run(testcase.name, func(sub *testing.T) {
+			blocks, err := Blocks(testcase.diskMap)
+			require.NoError(sub, err)
+			require.Equal(sub, testcase.blocks, blocks)
+		})
+	}
+}
diff --git a/cmd/defrag/main.go b/cmd/defrag/main.go
new file mode 100644
index 0000000..29ac7ed
--- /dev/null
+++ b/cmd/defrag/main.go
@@ -0,0 +1,36 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"bytes"
+
+	"codeberg.org/danjones000/advent-of-code/2024-09/defrag"
+)
+
+func handleErr(err error) {
+	if err == nil {
+		return
+	}
+
+	fmt.Fprintln(os.Stderr, err)
+	os.Exit(1)
+}
+
+func main() {
+	name := os.Args[0]
+
+	input, err := io.ReadAll(os.Stdin)
+	handleErr(err)
+
+	fmt.Fprintln(os.Stderr, "Welcome to ", name)
+	input = bytes.TrimSpace(input)
+
+	blocks, err := defrag.Blocks(input)
+	handleErr(err)
+
+	fmt.Println("@TODO")
+	fmt.Println("Got this")
+	fmt.Printf("%s\n", blocks)
+}
diff --git a/defrag.go b/defrag.go
new file mode 100644
index 0000000..82204d2
--- /dev/null
+++ b/defrag.go
@@ -0,0 +1,26 @@
+package defrag
+
+// Defrag performs an in-place sort of blocks.
+// If an error occurs during processing, it will be returned. The slice may have been partially sorted.
+func Defrag(blocks []byte) error {
+	lastPulled := len(blocks)
+	lastPushed := -1
+	for block := 0; block < len(blocks); block++ {
+		if blocks[block] != '.' {
+			continue
+		}
+		if lastPulled <= block {
+			break
+		}
+		for swap := lastPulled - 1; swap > lastPushed; swap-- {
+			if blocks[swap] == '.' {
+				continue
+			}
+			blocks[block], blocks[swap] = blocks[swap], blocks[block]
+			lastPulled = swap
+			lastPushed = block
+			break
+		}
+	}
+	return nil
+}
diff --git a/defrag_test.go b/defrag_test.go
new file mode 100644
index 0000000..62f625b
--- /dev/null
+++ b/defrag_test.go
@@ -0,0 +1,19 @@
+package defrag
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestDefrag(t *testing.T) {
+	for _, testcase := range sampleTests() {
+		t.Run(testcase.name, func(sub *testing.T) {
+			blocks := make([]byte, len(testcase.blocks))
+			copy(blocks, testcase.blocks)
+			err := Defrag(blocks)
+			require.NoError(sub, err)
+			require.Equal(sub, testcase.defragged, blocks)
+		})
+	}
+}
diff --git a/go.mod b/go.mod
index 8879fe6..610e26e 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,11 @@
-module codeberg.org/danjones000/advent-of-code-2024-09
+module codeberg.org/danjones000/advent-of-code/2024-09/defrag
 
 go 1.23.1
+
+require github.com/stretchr/testify v1.10.0
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..713a0b4
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,10 @@
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/testify v1.10.0 h1:Xv5erBjTwe/5IxqUQTdXv5kgmIvbHo3QQyRwhJsOfJA=
+github.com/stretchr/testify v1.10.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=