feat: implement new finder using io/fs

Signed-off-by: Mark Sagi-Kazar <mark.sagikazar@gmail.com>

fs.go

@@ -0,0 +1,65 @@
+//go:build go1.16
+// +build go1.16
+
+package viper
+
+import (
+	"errors"
+	"io/fs"
+	"path/filepath"
+)
+
+type finder struct {
+	paths      []string
+	fileNames  []string
+	extensions []string
+
+	withoutExtension bool
+}
+
+func (f finder) Find(fsys fs.FS) (string, error) {
+	for _, path := range f.paths {
+		for _, fileName := range f.fileNames {
+			for _, extension := range f.extensions {
+				filePath := filepath.Join(path, fileName+"."+extension)
+
+				ok, err := fileExists(fsys, filePath)
+				if err != nil {
+					return "", err
+				}
+
+				if ok {
+					return filePath, nil
+				}
+			}
+
+			if f.withoutExtension {
+				filePath := filepath.Join(path, fileName)
+
+				ok, err := fileExists(fsys, filePath)
+				if err != nil {
+					return "", err
+				}
+
+				if ok {
+					return filePath, nil
+				}
+			}
+		}
+	}
+
+	return "", nil
+}
+
+func fileExists(fsys fs.FS, filePath string) (bool, error) {
+	fileInfo, err := fs.Stat(fsys, filePath)
+	if err == nil {
+		return !fileInfo.IsDir(), nil
+	}
+
+	if errors.Is(err, fs.ErrNotExist) {
+		return false, nil
+	}
+
+	return false, err
+}

fs_test.go

@@ -0,0 +1,100 @@
+//go:build go1.16
+// +build go1.16
+
+package viper
+
+import (
+	"io/fs"
+	"testing"
+	"testing/fstest"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestFinder(t *testing.T) {
+	t.Parallel()
+
+	fsys := fstest.MapFS{
+		"home/user/.config":            &fstest.MapFile{},
+		"home/user/config.json":        &fstest.MapFile{},
+		"home/user/config.yaml":        &fstest.MapFile{},
+		"home/user/data.json":          &fstest.MapFile{},
+		"etc/config/.config":           &fstest.MapFile{},
+		"etc/config/a_random_file.txt": &fstest.MapFile{},
+		"etc/config/config.json":       &fstest.MapFile{},
+		"etc/config/config.yaml":       &fstest.MapFile{},
+		"etc/config/config.xml":        &fstest.MapFile{},
+	}
+
+	testCases := []struct {
+		name   string
+		fsys   func() fs.FS
+		finder finder
+		result string
+	}{
+		{
+			name: "find file",
+			fsys: func() fs.FS { return fsys },
+			finder: finder{
+				paths:      []string{"etc/config"},
+				fileNames:  []string{"config"},
+				extensions: []string{"json"},
+			},
+			result: "etc/config/config.json",
+		},
+		{
+			name: "file not found",
+			fsys: func() fs.FS { return fsys },
+			finder: finder{
+				paths:      []string{"var/config"},
+				fileNames:  []string{"config"},
+				extensions: []string{"json"},
+			},
+			result: "",
+		},
+		{
+			name:   "empty search params",
+			fsys:   func() fs.FS { return fsys },
+			finder: finder{},
+			result: "",
+		},
+		{
+			name: "precedence",
+			fsys: func() fs.FS { return fsys },
+			finder: finder{
+				paths:      []string{"var/config", "home/user", "etc/config"},
+				fileNames:  []string{"aconfig", "config"},
+				extensions: []string{"zml", "xml", "json"},
+			},
+			result: "home/user/config.json",
+		},
+		{
+			name: "without extension",
+			fsys: func() fs.FS { return fsys },
+			finder: finder{
+				paths:      []string{"var/config", "home/user", "etc/config"},
+				fileNames:  []string{".config"},
+				extensions: []string{"zml", "xml", "json"},
+
+				withoutExtension: true,
+			},
+			result: "home/user/.config",
+		},
+	}
+
+	for _, testCase := range testCases {
+		testCase := testCase
+
+		t.Run(testCase.name, func(t *testing.T) {
+			t.Parallel()
+
+			fsys := testCase.fsys()
+
+			result, err := testCase.finder.Find(fsys)
+			require.NoError(t, err)
+
+			assert.Equal(t, testCase.result, result)
+		})
+	}
+}