diff --git a/externalsort/README.md b/externalsort/README.md
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+++ b/externalsort/README.md
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+## externalsort
+
+В этой задаче нужно написать однопроходную внешнюю сортировку слиянием.
+Моделируется ситуация, в которой, данные расположены на внешних устройствах и суммарно не вмещаются в оперативную память,
+но каждый кусочек по-отдельности вмещается.
+
+Задача разбита на 3 составные части.
+
+#### Reader & writer
+
+Реализовать интерфейсы для построчного чтения/записи строк:
+```
+type LineReader interface {
+	ReadLine() (string, error)
+}
+
+type LineWriterFlusher interface {
+	Write(l string) error
+	Flush() error
+}
+```
+и два конструктора:
+```
+func NewReader(r io.Reader) LineReader
+func NewWriterFlusher(w io.Writer) LineWriterFlusher
+```
+
+`NewLineReader` оборачивает переданный `io.Reader` в `LineReader`.
+
+Вызов `ReadLine` должен читать одну строку.
+Строка имеет произвольную длину.
+Конец строки определяется переводом строки ('\n').
+Непустая последовательность символов после последнего перевода строки также считается строкой.
+
+`ReadLine` должен возращать `io.EOF` при достижении конца файла.
+
+#### Merge
+
+Функция слияния произвольного количества отсортированных групп строк:
+```
+func Merge(w LineWriterFlusher, readers ...LineReader) error
+```
+
+`Merge` по необходимости читает из reader'ов и пишет во writer.
+
+#### Sort
+
+```
+Sort(w io.Writer, in ...string) error
+```
+
+Функция принимает на вход произвольное количество файлов, каждый их которых помещается в оперативную память,
+а также writer для записи результата.
+
+Результаты сортировки отдельных файлов можно записывать поверх входных данных.
+
+### Ссылки
+
+* container/heap: https://golang.org/pkg/container/heap/
diff --git a/externalsort/io.go b/externalsort/io.go
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--- /dev/null
+++ b/externalsort/io.go
@@ -0,0 +1,12 @@
+// +build !change
+
+package externalsort
+
+type LineReader interface {
+	ReadLine() (string, error)
+}
+
+type LineWriterFlusher interface {
+	Write(l string) error
+	Flush() error
+}
diff --git a/externalsort/io_test.go b/externalsort/io_test.go
new file mode 100644
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--- /dev/null
+++ b/externalsort/io_test.go
@@ -0,0 +1,147 @@
+package externalsort
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"strings"
+	"testing"
+	"testing/iotest"
+
+	"github.com/stretchr/testify/require"
+)
+
+func newStringReader(s string) LineReader {
+	return NewReader(strings.NewReader(s))
+}
+
+func readAll(r LineReader) (lines []string, err error) {
+	for {
+		l, err := r.ReadLine()
+		if err != nil {
+			if errors.Is(err, io.EOF) {
+				return lines, nil
+			}
+			return nil, err
+		}
+		lines = append(lines, l)
+	}
+}
+
+func TestLineReader(t *testing.T) {
+	type Wrapper func(r io.Reader) io.Reader
+
+	for _, tc := range []struct {
+		name     string
+		in       string
+		wrappers []Wrapper
+		expected []string
+	}{
+		{
+			name:     "empty",
+			in:       "",
+			expected: []string{""},
+		},
+		{
+			name:     "one-row",
+			in:       "abc",
+			expected: []string{"abc"},
+		},
+		{
+			name: "multiple-rows",
+			in: `a
+
+b
+b
+`,
+			expected: []string{"a\n", "\n", "b\n", "b\n", ""},
+		},
+		{
+			name:     "large-row",
+			in:       strings.Repeat("a", 4097),
+			expected: []string{strings.Repeat("a", 4097)},
+		},
+		{
+			name:     "huge-row",
+			in:       strings.Repeat("a", 65537),
+			expected: []string{strings.Repeat("a", 65537)},
+		},
+		{
+			name:     "half-reader",
+			in:       strings.Repeat("a", 1025),
+			wrappers: []Wrapper{iotest.HalfReader},
+			expected: []string{strings.Repeat("a", 1025)},
+		},
+		{
+			name:     "eof",
+			in:       strings.Repeat("a", 1025),
+			wrappers: []Wrapper{iotest.DataErrReader},
+			expected: []string{strings.Repeat("a", 1025)},
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			var r io.Reader
+			r = strings.NewReader(tc.in)
+			for _, w := range tc.wrappers {
+				r = w(r)
+			}
+
+			lineReader := NewReader(r)
+
+			lines, err := readAll(lineReader)
+			require.NoError(t, err)
+
+			require.Equal(t, strings.Join(lines, ""), tc.in)
+			require.Len(t, lines, len(tc.expected),
+				"expected: %+v, got: %+v", tc.expected, lines)
+			require.Equal(t, tc.expected, lines)
+		})
+	}
+}
+
+type brokenReader int
+
+func (b brokenReader) Read(data []byte) (n int, err error) {
+	return 0, errors.New("read is broken")
+}
+
+func TestLineReader_error(t *testing.T) {
+	_, err := NewReader(new(brokenReader)).ReadLine()
+	require.Error(t, err)
+	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))
+}
+
+func TestLineWriterFlusher(t *testing.T) {
+	for _, tc := range []struct {
+		name  string
+		lines []string
+	}{
+		{
+			name: "empty",
+		},
+		{
+			name:  "simple",
+			lines: []string{"a\n", "b\n", "c\n"},
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			var buf bytes.Buffer
+			w := NewWriterFlusher(&buf)
+
+			for _, l := range tc.lines {
+				require.NoError(t, w.Write(l))
+			}
+
+			require.NoError(t, w.Flush())
+
+			require.Equal(t, strings.Join(tc.lines, ""), buf.String())
+		})
+	}
+}
diff --git a/externalsort/sort.go b/externalsort/sort.go
new file mode 100644
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--- /dev/null
+++ b/externalsort/sort.go
@@ -0,0 +1,23 @@
+// +build !solution
+
+package externalsort
+
+import (
+	"io"
+)
+
+func NewReader(r io.Reader) LineReader {
+	panic("implement me")
+}
+
+func NewWriterFlusher(w io.Writer) LineWriterFlusher {
+	panic("implement me")
+}
+
+func Merge(w LineWriterFlusher, readers ...LineReader) error {
+	panic("implement me")
+}
+
+func Sort(w io.Writer, in ...string) error {
+	panic("implement me")
+}
diff --git a/externalsort/sort_test.go b/externalsort/sort_test.go
new file mode 100644
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--- /dev/null
+++ b/externalsort/sort_test.go
@@ -0,0 +1,113 @@
+package externalsort
+
+import (
+	"bytes"
+	"io/ioutil"
+	"path"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"gitlab.com/slon/shad-go/tools/testtool"
+)
+
+func TestMerge(t *testing.T) {
+	for _, tc := range []struct {
+		name string
+		in   []string
+		out  string
+	}{
+		{
+			name: "simple",
+			in: []string{`0`, `1
+1
+1`},
+			out: `0
+1
+1
+1`,
+		},
+		{
+			// Merge believes lines are read in sorted order.
+			name: "single-unsorted-file",
+			in: []string{`1
+0`},
+			out: `1
+0`,
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			out := &bytes.Buffer{}
+			w := NewWriterFlusher(out)
+
+			var readers []LineReader
+			for _, s := range tc.in {
+				readers = append(readers, newStringReader(s))
+			}
+
+			err := Merge(w, readers...)
+			require.NoError(t, err)
+
+			require.Equal(t, tc.out, out.String())
+		})
+	}
+}
+
+func TestSort_fileNotFound(t *testing.T) {
+	var buf bytes.Buffer
+	err := Sort(&buf, testtool.RandomName())
+	require.Error(t, err)
+}
+
+func TestSort(t *testing.T) {
+	testDir := path.Join("./testdata", "sort")
+
+	readTestCase := func(dir string) (in []string, out string) {
+		files, err := ioutil.ReadDir(dir)
+		require.NoError(t, err)
+
+		for _, f := range files {
+			if strings.HasPrefix(f.Name(), "in") {
+				in = append(in, path.Join(dir, f.Name()))
+			}
+			if f.Name() == "out.txt" {
+				out = path.Join(dir, f.Name())
+			}
+		}
+
+		return
+	}
+
+	for _, d := range listDirs(t, testDir) {
+		testCaseDir := path.Join(testDir, d)
+
+		t.Run(d, func(t *testing.T) {
+			in, out := readTestCase(testCaseDir)
+
+			var buf bytes.Buffer
+			err := Sort(&buf, in...)
+			require.NoError(t, err)
+
+			expected, err := ioutil.ReadFile(out)
+			require.NoError(t, err)
+
+			require.Equal(t, string(expected), buf.String())
+		})
+	}
+}
+
+func listDirs(t *testing.T, dir string) []string {
+	t.Helper()
+
+	files, err := ioutil.ReadDir(dir)
+	require.NoError(t, err)
+
+	var dirs []string
+	for _, f := range files {
+		if f.IsDir() {
+			dirs = append(dirs, f.Name())
+		}
+	}
+
+	return dirs
+}
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