Update "flatten" to be more versatile

CODEOWNERS

@@ -0,0 +1 @@
+* @theMagicalKarp

README.md

@@ -1,6 +1,8 @@
 # Iter
 
-<img align="right" height="125" src="./docs/assets/iter.png" />
+<p align="center">
+  <img height="125" src="./docs/assets/iter.png" />
+</p>
 
 [![Go Reference](https://pkg.go.dev/badge/github.com/theMagicalKarp/iter.svg)](https://pkg.go.dev/github.com/theMagicalKarp/iter)
 [![Go Report Card](https://goreportcard.com/badge/github.com/theMagicalKarp/iter)](https://goreportcard.com/report/github.com/theMagicalKarp/iter)

pkg/itertools/flatten.go

@@ -34,9 +34,12 @@ func (f *flattenIter[T]) Next() (T, bool) {
 	return empty, false
 }
 
-func Flatten[T any](chain iter.Iterable[iter.Iterable[T]]) iter.Iterable[T] {
+// Flatten returns an iterable that flattens a chain of iterables into a single iterable.
+// It takes a variadic argument `chain` of type `iter.Iterable[iter.Iterable[T]]`, where `T` can be any type.
+// The function returns an iterable of type `iter.Iterable[T]`.
+func Flatten[T any](chain ...iter.Iterable[iter.Iterable[T]]) iter.Iterable[T] {
 	return &flattenIter[T]{
-		chain:   chain,
+		chain:   Chain(chain...),
 		current: nil,
 	}
 }

pkg/itertools/flatten_test.go

@@ -13,6 +13,8 @@ func ExampleFlatten() {
 		iter.New(
 			iter.New(1, 2, 3),
 			iter.New(4, 5, 6),
+		),
+		iter.New(
 			iter.New(7, 8, 9),
 		),
 	)
@@ -45,6 +47,32 @@ func TestFlattenBasic(t *testing.T) {
 	assert.Equal(t, 0, v)
 }
 
+func TestFlattenBasicMulti(t *testing.T) {
+	t.Parallel()
+
+	items := itertools.Flatten(
+		iter.New(
+			iter.New(1, 2, 3),
+			iter.New(4, 5, 6),
+		),
+		iter.New(
+			iter.New(7, 8, 9),
+		),
+	)
+
+	for i := 1; i < 10; i++ {
+		v, more := items.Next()
+		assert.True(t, more)
+		assert.Equal(t, i, v)
+	}
+	v, more := items.Next()
+	assert.False(t, more)
+	assert.Equal(t, 0, v)
+	v, more = items.Next()
+	assert.False(t, more)
+	assert.Equal(t, 0, v)
+}
+
 func TestFlattenWithEmpties(t *testing.T) {
 	t.Parallel()
 
@@ -72,6 +100,49 @@ func TestFlattenWithEmpties(t *testing.T) {
 	assert.Equal(t, 0, v)
 }
 
+func TestFlattenWithEmptiesMulti(t *testing.T) {
+	t.Parallel()
+
+	items := itertools.Flatten(
+		iter.New(
+			iter.New[int](),
+			iter.New(1, 2, 3),
+			iter.New(4, 5, 6),
+			iter.New[int](),
+			iter.New(7, 8, 9),
+			iter.New[int](),
+		),
+		iter.New[iter.Iterable[int]](),
+		iter.New(
+			iter.New[int](),
+			iter.New(1, 2, 3),
+			iter.New(4, 5, 6),
+			iter.New[int](),
+			iter.New(7, 8, 9),
+			iter.New[int](),
+		),
+	)
+
+	for i := 1; i < 10; i++ {
+		v, more := items.Next()
+		assert.True(t, more)
+		assert.Equal(t, i, v)
+	}
+
+	for i := 1; i < 10; i++ {
+		v, more := items.Next()
+		assert.True(t, more)
+		assert.Equal(t, i, v)
+	}
+
+	v, more := items.Next()
+	assert.False(t, more)
+	assert.Equal(t, 0, v)
+	v, more = items.Next()
+	assert.False(t, more)
+	assert.Equal(t, 0, v)
+}
+
 func TestFlattenEmpty(t *testing.T) {
 	t.Parallel()
 
@@ -84,3 +155,20 @@ func TestFlattenEmpty(t *testing.T) {
 	assert.False(t, more)
 	assert.Equal(t, 0, v)
 }
+
+func TestFlattenEmptyMulti(t *testing.T) {
+	t.Parallel()
+
+	items := itertools.Flatten(
+		iter.New[iter.Iterable[int]](),
+		iter.New[iter.Iterable[int]](),
+		iter.New[iter.Iterable[int]](),
+	)
+
+	v, more := items.Next()
+	assert.False(t, more)
+	assert.Equal(t, 0, v)
+	v, more = items.Next()
+	assert.False(t, more)
+	assert.Equal(t, 0, v)
+}