refactor: add support for go1.16's WalkDir

Add support for using the faster WalkDir method when building against
go1.16.

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go: ['1.16']
+        go: ['1.15', '1.16']
     steps:
     - name: Setup
       uses: actions/setup-go@v2

finder.go

@@ -1,7 +1,6 @@
 package sysfont
 
 import (
-	"os"
 	"path/filepath"
 	"strings"
 
@@ -43,14 +42,7 @@ func NewFinder(opts *FinderOpts) *Finder {
 	}
 
 	var fonts []*Font
-	walker := func(filename string, info os.FileInfo, err error) error {
-		if err != nil {
-			return err
-		}
-		if info.IsDir() {
-			return nil
-		}
-
+	findFonts(opts, func(filename string) error {
 		// Check file extension.
 		if extensions := opts.Extensions; len(extensions) > 0 {
 			extension := filepath.Ext(strings.ToLower(filename))
@@ -67,14 +59,7 @@ func NewFinder(opts *FinderOpts) *Finder {
 
 		fonts = append(fonts, matches...)
 		return nil
-	}
-
-	// Traverse OS font directories.
-	for _, dir := range opts.SearchPaths {
-		if err := filepath.Walk(dir, walker); err != nil {
-			continue
-		}
-	}
+	})
 
 	return &Finder{
 		fonts: fonts,

finder_dirwalk.go

@@ -0,0 +1,29 @@
+//go:build go1.16
+// +build go1.16
+
+package sysfont
+
+import (
+	"io/fs"
+	"path/filepath"
+)
+
+func findFonts(opts *FinderOpts, fn func(filename string) error) {
+	walker := func(filename string, info fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if info.IsDir() {
+			return nil
+		}
+
+		return fn(filename)
+	}
+
+	// Traverse OS font directories.
+	for _, dir := range opts.SearchPaths {
+		if err := filepath.WalkDir(dir, walker); err != nil {
+			continue
+		}
+	}
+}

finder_walk.go

@@ -0,0 +1,29 @@
+//go:build !go1.16
+// +build !go1.16
+
+package sysfont
+
+import (
+	"os"
+	"path/filepath"
+)
+
+func findFonts(opts *FinderOpts, fn func(filename string) error) {
+	walker := func(filename string, info os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+		if info.IsDir() {
+			return nil
+		}
+
+		return fn(filename)
+	}
+
+	// Traverse OS font directories.
+	for _, dir := range opts.SearchPaths {
+		if err := filepath.Walk(dir, walker); err != nil {
+			continue
+		}
+	}
+}