From df01f2eed7532e266851c8cab89df44f8149e673 Mon Sep 17 00:00:00 2001
From: Gregory Eremin <g@erem.in>
Date: Sun, 17 Jun 2018 13:34:01 +0200
Subject: [PATCH] Add file cache implementation

---
 filecache/filecache.go      | 30 +++++++++++++++++++++++++
 filecache/filecache_test.go | 45 +++++++++++++++++++++++++++++++++++++
 2 files changed, 75 insertions(+)
 create mode 100644 filecache/filecache.go
 create mode 100644 filecache/filecache_test.go

diff --git a/filecache/filecache.go b/filecache/filecache.go
new file mode 100644
index 0000000..cefe692
--- /dev/null
+++ b/filecache/filecache.go
@@ -0,0 +1,30 @@
+package filecache
+
+import (
+	"encoding/json"
+	"io/ioutil"
+	"os"
+	"reflect"
+)
+
+// Load will look for a given file. If it exists, it would unmarshal its
+// contents into the given value. If the file does not exist, a supplied
+// function would be called and its output would be written to a file.
+func Load(val interface{}, filename string, fn func() interface{}) error {
+	_, err := os.Stat(filename)
+	if os.IsNotExist(err) {
+		out := fn()
+		reflect.ValueOf(val).Elem().Set(reflect.ValueOf(out))
+
+		body, err := json.Marshal(val)
+		if err != nil {
+			return err
+		}
+		return ioutil.WriteFile(filename, body, 0766)
+	}
+	body, err := ioutil.ReadFile(filename)
+	if err != nil {
+		return err
+	}
+	return json.Unmarshal(body, val)
+}
diff --git a/filecache/filecache_test.go b/filecache/filecache_test.go
new file mode 100644
index 0000000..29eaeeb
--- /dev/null
+++ b/filecache/filecache_test.go
@@ -0,0 +1,45 @@
+package filecache
+
+import (
+	"io/ioutil"
+	"os"
+	"testing"
+)
+
+func TestFileCache(t *testing.T) {
+	filename := getTempFileName(t)
+	const exp = 100
+	var res int
+	var nCalled int
+	for i := 0; i < 10; i++ {
+		err := Load(&res, filename, func() interface{} {
+			nCalled++
+			return exp
+		})
+		if err != nil {
+			t.Fatalf("Error occurred while loading cache: %v", err)
+		}
+
+		if res != exp {
+			t.Errorf("Expected %d, got %d", exp, res)
+		}
+	}
+	if nCalled != 1 {
+		t.Errorf("Epected a function to be called once, was called %d times", nCalled)
+	}
+}
+
+func getTempFileName(t *testing.T) string {
+	t.Helper()
+	fd, err := ioutil.TempFile("", "filecache")
+	if err != nil {
+		t.Fatalf("Failed to create a temporary file: %v", err)
+	}
+	if err := fd.Close(); err != nil {
+		t.Fatalf("Failed to close temporary file: %v", err)
+	}
+	if err := os.Remove(fd.Name()); err != nil {
+		t.Fatalf("Failed to delete temporary file: %v", err)
+	}
+	return fd.Name()
+}