diff --git a/externalsort/io_test.go b/externalsort/io_test.go
index 7bcb91d..ceb6ed7 100644
--- a/externalsort/io_test.go
+++ b/externalsort/io_test.go
@@ -41,13 +41,19 @@ func TestLineReader(t *testing.T) {
 		{
 			name:     "empty",
 			in:       "",
-			expected: []string{""},
+			expected: nil,
 		},
 		{
 			name:     "one-row",
 			in:       "abc",
 			expected: []string{"abc"},
 		},
+		{
+			name: "linebreak",
+			in: `abc
+`,
+			expected: []string{"abc\n"},
+		},
 		{
 			name: "multiple-rows",
 			in: `a
@@ -55,7 +61,7 @@ func TestLineReader(t *testing.T) {
 b
 b
 `,
-			expected: []string{"a\n", "\n", "b\n", "b\n", ""},
+			expected: []string{"a\n", "\n", "b\n", "b\n"},
 		},
 		{
 			name:     "large-row",
@@ -112,9 +118,6 @@ func TestLineReader_error(t *testing.T) {
 	require.False(t, errors.Is(err, io.EOF))
 
 	r := newStringReader("")
-	_, err = r.ReadLine()
-	require.NoError(t, err)
-
 	_, err = r.ReadLine()
 	require.True(t, errors.Is(err, io.EOF))
 }
diff --git a/externalsort/sort_test.go b/externalsort/sort_test.go
index 6fac5b1..63535ad 100644
--- a/externalsort/sort_test.go
+++ b/externalsort/sort_test.go
@@ -3,7 +3,9 @@ package externalsort
 import (
 	"bufio"
 	"bytes"
+	"fmt"
 	"io/ioutil"
+	"os"
 	"path"
 	"strings"
 	"testing"
@@ -85,12 +87,16 @@ func TestSort(t *testing.T) {
 		testCaseDir := path.Join(testDir, d)
 
 		t.Run(d, func(t *testing.T) {
+			tmpDir, err := ioutil.TempDir("", fmt.Sprintf("sort%s-", d))
+			require.NoError(t, err)
+			defer func() { _ = os.RemoveAll(tmpDir) }()
+
 			in, out := readTestCase(testCaseDir)
+			in = copyFiles(t, in, tmpDir)
 
 			var buf bytes.Buffer
 			w := bufio.NewWriter(&buf)
-			err := Sort(w, in...)
-			require.NoError(t, err)
+			require.NoError(t, Sort(w, in...))
 
 			expected, err := ioutil.ReadFile(out)
 			require.NoError(t, err)
@@ -116,3 +122,27 @@ func listDirs(t *testing.T, dir string) []string {
 
 	return dirs
 }
+
+func copyFiles(t *testing.T, in []string, dir string) []string {
+	t.Helper()
+
+	var ret []string
+	for _, f := range in {
+		ret = append(ret, copyFile(t, f, dir))
+	}
+
+	return ret
+}
+
+func copyFile(t *testing.T, f, dir string) string {
+	t.Helper()
+
+	data, err := ioutil.ReadFile(f)
+	require.NoError(t, err)
+
+	dst := path.Join(dir, path.Base(f))
+	err = ioutil.WriteFile(dst, data, 0644)
+	require.NoError(t, err)
+
+	return dst
+}
diff --git a/externalsort/testdata/sort/2/in1.txt b/externalsort/testdata/sort/2/in1.txt
index 31b838d..8baef1b 100644
Binary files a/externalsort/testdata/sort/2/in1.txt and b/externalsort/testdata/sort/2/in1.txt differ
diff --git a/externalsort/testdata/sort/2/out.txt b/externalsort/testdata/sort/2/out.txt
index 31b838d..8baef1b 100644
Binary files a/externalsort/testdata/sort/2/out.txt and b/externalsort/testdata/sort/2/out.txt differ
diff --git a/externalsort/testdata/sort/3/in3.txt b/externalsort/testdata/sort/3/in3.txt
index 6d9973c..2ceb12b 100644
Binary files a/externalsort/testdata/sort/3/in3.txt and b/externalsort/testdata/sort/3/in3.txt differ
diff --git a/externalsort/testdata/sort/4/in1.txt b/externalsort/testdata/sort/4/in1.txt
new file mode 100644
index 0000000..e69de29
diff --git a/externalsort/testdata/sort/4/in2.txt b/externalsort/testdata/sort/4/in2.txt
new file mode 100644
index 0000000..e69de29
diff --git a/externalsort/testdata/sort/4/out.txt b/externalsort/testdata/sort/4/out.txt
new file mode 100644
index 0000000..e69de29
diff --git a/externalsort/testdata/sort/5/in1.txt b/externalsort/testdata/sort/5/in1.txt
new file mode 100644
index 0000000..8baef1b
Binary files /dev/null and b/externalsort/testdata/sort/5/in1.txt differ
diff --git a/externalsort/testdata/sort/5/in2.txt b/externalsort/testdata/sort/5/in2.txt
new file mode 100644
index 0000000..e69de29
diff --git a/externalsort/testdata/sort/5/in3.txt b/externalsort/testdata/sort/5/in3.txt
new file mode 100644
index 0000000..8baef1b
Binary files /dev/null and b/externalsort/testdata/sort/5/in3.txt differ
diff --git a/externalsort/testdata/sort/5/out.txt b/externalsort/testdata/sort/5/out.txt
new file mode 100644
index 0000000..5d8a556
Binary files /dev/null and b/externalsort/testdata/sort/5/out.txt differ
diff --git a/externalsort/testdata/sort/6/in1.txt b/externalsort/testdata/sort/6/in1.txt
new file mode 100644
index 0000000..88ae695
Binary files /dev/null and b/externalsort/testdata/sort/6/in1.txt differ
diff --git a/externalsort/testdata/sort/6/in2.txt b/externalsort/testdata/sort/6/in2.txt
new file mode 100644
index 0000000..74c9df3
Binary files /dev/null and b/externalsort/testdata/sort/6/in2.txt differ
diff --git a/externalsort/testdata/sort/6/in3.txt b/externalsort/testdata/sort/6/in3.txt
new file mode 100644
index 0000000..34aaebe
Binary files /dev/null and b/externalsort/testdata/sort/6/in3.txt differ
diff --git a/externalsort/testdata/sort/6/out.txt b/externalsort/testdata/sort/6/out.txt
new file mode 100644
index 0000000..b2cf317
Binary files /dev/null and b/externalsort/testdata/sort/6/out.txt differ