Added some initial code.

entry.go

@@ -0,0 +1,35 @@
+package main
+
+import (
+	"encoding/json"
+	"os"
+)
+
+// Entry represents a file in storage.
+type Entry struct {
+	URL           string `json:"url"`
+	ContentLength int64  `json:"content_length"`
+	ContentType   string `json:"content_type"`
+}
+
+// LoadEntry loads an entry from disk.
+func LoadEntry(filename string) (*Entry, error) {
+	f, err := os.Open(filename)
+	if err != nil {
+		return nil, err
+	}
+	e := &Entry{}
+	if err = json.NewDecoder(f).Decode(e); err != nil {
+		return nil, err
+	}
+	return e, nil
+}
+
+// Save writes the entry to disk.
+func (e *Entry) Save(filename string) error {
+	f, err := os.Create(filename)
+	if err != nil {
+		return err
+	}
+	return json.NewEncoder(f).Encode(e)
+}

storage.go

@@ -0,0 +1,39 @@
+package main
+
+import (
+	"crypto/md5"
+	"fmt"
+	"io"
+	"os"
+	"path"
+	"sync"
+)
+
+// Storage provides read and write access to items in the cache. In order to
+// avoid race conditions, adding and testing for entries in the cache are
+// protected by a mutex.
+type Storage struct {
+	Directory string
+	mutex     sync.Mutex
+}
+
+// GetReader returns an io.Reader for the specified URL. If the file does not
+// exist, both a writer (for downloading the file) and a reader are created.
+func (s *Storage) GetReader(url string) (io.Reader, error) {
+	var (
+		hash         = string(md5.Sum([]byte(url)))
+		jsonFilename = path.Join(s.Directory, fmt.Sprintf("%s.json", hash))
+		dataFilename = path.Join(s.Directory, fmt.Sprintf("%s.data", hash))
+	)
+	s.mutex.Lock()
+	defer s.mutex.Unlock()
+	i, err := os.Stat(jsonFilename)
+	if err != nil {
+		if os.IsNotExist(err) {
+			NewWriter(url, jsonFilename, dataFilename)
+		} else {
+			return nil, err
+		}
+	}
+	return NewReader(url, jsonFilename, dataFilename)
+}