package appview

import (
	"io/fs"
	"testing"
	"testing/fstest"
)

func TestOverlayFS_Open_UpperWins(t *testing.T) {
	lower := fstest.MapFS{
		"file.txt": &fstest.MapFile{Data: []byte("lower")},
	}
	upper := fstest.MapFS{
		"file.txt": &fstest.MapFile{Data: []byte("upper")},
	}

	ofs := newOverlayFS(upper, lower)

	data, err := fs.ReadFile(ofs, "file.txt")
	if err != nil {
		t.Fatal(err)
	}
	if string(data) != "upper" {
		t.Errorf("expected upper content, got %q", data)
	}
}

func TestOverlayFS_Open_FallbackToLower(t *testing.T) {
	lower := fstest.MapFS{
		"only-lower.txt": &fstest.MapFile{Data: []byte("lower")},
	}
	upper := fstest.MapFS{}

	ofs := newOverlayFS(upper, lower)

	data, err := fs.ReadFile(ofs, "only-lower.txt")
	if err != nil {
		t.Fatal(err)
	}
	if string(data) != "lower" {
		t.Errorf("expected lower content, got %q", data)
	}
}

func TestOverlayFS_Open_UpperOnly(t *testing.T) {
	lower := fstest.MapFS{}
	upper := fstest.MapFS{
		"only-upper.txt": &fstest.MapFile{Data: []byte("upper")},
	}

	ofs := newOverlayFS(upper, lower)

	data, err := fs.ReadFile(ofs, "only-upper.txt")
	if err != nil {
		t.Fatal(err)
	}
	if string(data) != "upper" {
		t.Errorf("expected upper content, got %q", data)
	}
}

func TestOverlayFS_Open_NotFound(t *testing.T) {
	lower := fstest.MapFS{}
	upper := fstest.MapFS{}

	ofs := newOverlayFS(upper, lower)

	_, err := ofs.Open("missing.txt")
	if err == nil {
		t.Fatal("expected error for missing file")
	}
}

func TestOverlayFS_ReadDir_Merge(t *testing.T) {
	lower := fstest.MapFS{
		"dir/a.txt": &fstest.MapFile{Data: []byte("a-lower")},
		"dir/b.txt": &fstest.MapFile{Data: []byte("b-lower")},
	}
	upper := fstest.MapFS{
		"dir/b.txt": &fstest.MapFile{Data: []byte("b-upper")},
		"dir/c.txt": &fstest.MapFile{Data: []byte("c-upper")},
	}

	ofs := newOverlayFS(upper, lower)
	rdfs, ok := ofs.(fs.ReadDirFS)
	if !ok {
		t.Fatal("overlayFS does not implement ReadDirFS")
	}

	entries, err := rdfs.ReadDir("dir")
	if err != nil {
		t.Fatal(err)
	}

	// Should have a.txt (lower), b.txt (upper), c.txt (upper) = 3 entries
	if len(entries) != 3 {
		t.Fatalf("expected 3 entries, got %d", len(entries))
	}

	names := make([]string, len(entries))
	for i, e := range entries {
		names[i] = e.Name()
	}

	expected := []string{"a.txt", "b.txt", "c.txt"}
	for i, name := range expected {
		if names[i] != name {
			t.Errorf("entry %d: expected %q, got %q", i, name, names[i])
		}
	}

	// Verify b.txt content comes from upper
	data, err := fs.ReadFile(ofs, "dir/b.txt")
	if err != nil {
		t.Fatal(err)
	}
	if string(data) != "b-upper" {
		t.Errorf("expected b-upper, got %q", data)
	}
}

func TestOverlayFS_ReadFile(t *testing.T) {
	lower := fstest.MapFS{
		"file.txt": &fstest.MapFile{Data: []byte("lower")},
	}
	upper := fstest.MapFS{
		"file.txt": &fstest.MapFile{Data: []byte("upper")},
	}

	ofs := newOverlayFS(upper, lower)
	rffs, ok := ofs.(fs.ReadFileFS)
	if !ok {
		t.Fatal("overlayFS does not implement ReadFileFS")
	}

	data, err := rffs.ReadFile("file.txt")
	if err != nil {
		t.Fatal(err)
	}
	if string(data) != "upper" {
		t.Errorf("expected upper, got %q", data)
	}
}