Add cache purge (#7)

Reviewed-on: #7
Co-authored-by: jolheiser <john.olheiser@gmail.com>
Co-committed-by: jolheiser <john.olheiser@gmail.com>

bench.txt

@@ -1,9 +0,0 @@
-go test -benchmem -bench=.
-goos: linux
-goarch: amd64
-pkg: go.jolheiser.com/overlay
-cpu: Intel(R) Core(TM) i7-4700MQ CPU @ 2.40GHz
-BenchmarkCache-8     	134959974	         9.003 ns/op	       0 B/op	       0 allocs/op
-BenchmarkNoCache-8   	  897212	      1369 ns/op	     280 B/op	       4 allocs/op
-PASS
-ok  	go.jolheiser.com/overlay	3.360s

overlay.go

@@ -18,12 +18,17 @@ type FS struct {
 	cache   map[string]bool
 }
 
+// PurgeCache purges the cache
+func (f *FS) PurgeCache() {
+	f.cache = make(map[string]bool)
+}
+
 func (f *FS) apn(name string) string {
 	return path.Join(f.root, name)
 }
 
 func (f *FS) exists(name string) bool {
-	if has, ok := f.cache[name]; ok && f.doCache {
+	if has, ok := f.cache[name]; ok {
 		return has
 	}
 	_, err := os.Stat(f.apn(name))
@@ -63,10 +68,9 @@ type Option func(*FS) error
 // New returns a new FS
 func New(root string, fs fs.FS, opts ...Option) (*FS, error) {
 	x := &FS{
-		fs:      fs,
-		root:    root,
-		doCache: true,
-		cache:   make(map[string]bool),
+		fs:    fs,
+		root:  root,
+		cache: make(map[string]bool),
 	}
 
 	for _, opt := range opts {