3 files changed, 124 insertions, 0 deletions
diff --git a/internal/feed/dataversion.go b/internal/feed/dataversion.go
new file mode 100644
index 0000000..c6f1b8d
--- /dev/null
+++ b/internal/feed/dataversion.go
@@ -0,0 +1,36 @@
+package feed
+
+//go:generate stringer -linecomment -type=DataVersion
+
+import (
+  "encoding/json"
+  "fmt"
+)
+
+// Data version for NVD feeds and feed items.
+type DataVersion byte
+
+const (
+  V40 DataVersion = iota // 4.0
+)
+
+// Unmarshal data version from JSON.
+func (me *DataVersion) UnmarshalJSON(b []byte) error {
+  // decode string, check for error
+  var s string
+  if err := json.Unmarshal(b, &s); err != nil {
+    return err
+  }
+
+  // check value
+  switch s {
+  case "4.0":
+    *me = V40
+  default:
+    // return error
+    return fmt.Errorf("unknown data version: %s", s)
+  }
+
+  // return success
+  return nil
+}
diff --git a/internal/feed/dataversion_string.go b/internal/feed/dataversion_string.go
new file mode 100644
index 0000000..26a0fdb
--- /dev/null
+++ b/internal/feed/dataversion_string.go
@@ -0,0 +1,23 @@
+// Code generated by "stringer -linecomment -type=DataVersion"; DO NOT EDIT.
+
+package feed
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[V40-0]
+}
+
+const _DataVersion_name = "4.0"
+
+var _DataVersion_index = [...]uint8{0, 3}
+
+func (i DataVersion) String() string {
+	if i >= DataVersion(len(_DataVersion_index)-1) {
+		return "DataVersion(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _DataVersion_name[_DataVersion_index[i]:_DataVersion_index[i+1]]
+}
diff --git a/internal/feed/dataversion_test.go b/internal/feed/dataversion_test.go
new file mode 100644
index 0000000..6bf683c
--- /dev/null
+++ b/internal/feed/dataversion_test.go
@@ -0,0 +1,65 @@
+package feed
+
+import (
+  "encoding/json"
+  "testing"
+)
+
+func TestDataVersionUnmarshalInvalidData(t *testing.T) {
+  test := []byte(`{}`)
+  var val DataVersion
+
+  if err := json.Unmarshal(test, &val); err == nil {
+    t.Errorf("got \"%s\", exp error", val)
+  }
+}
+
+func TestDataVersionUnmarshalUnknown(t *testing.T) {
+  test := []byte(`"foo"`)
+  exp := "unknown data version: foo"
+  var val DataVersion
+
+  err := json.Unmarshal(test, &val)
+  if err == nil {
+    t.Errorf("got \"%s\", exp error", val)
+    return
+  }
+
+  if err.Error() != exp {
+    t.Errorf("got \"%s\", exp \"%s\"", err.Error(), exp)
+  }
+}
+
+func TestDataVersionUnmarshalValid(t *testing.T) {
+  test := []byte(`"4.0"`)
+  exp := V40
+  var got DataVersion
+
+  if err := json.Unmarshal(test, &got); err != nil {
+    t.Error(err)
+    return
+  }
+
+  if got != exp {
+    t.Errorf("got \"%s\", exp \"%s\"", got, exp)
+  }
+}
+
+func TestDataVersionString(t *testing.T) {
+  tests := []struct {
+    val DataVersion
+    exp string
+  } {
+    { V40, "4.0" },
+    { DataVersion(255), "DataVersion(255)" },
+  }
+
+  for _, test := range(tests) {
+    t.Run(test.val.String(), func(t *testing.T) {
+      got := test.val.String()
+      if got != test.exp {
+        t.Errorf("got \"%s\", exp \"%s\"", got, test.exp)
+      }
+    })
+  }
+}