3 files changed, 67 insertions, 3 deletions
diff --git a/entity/err.go b/entity/err.go
new file mode 100644
index 00000000..9356433d
--- /dev/null
+++ b/entity/err.go
@@ -0,0 +1 @@
+package entity
diff --git a/entity/id.go b/entity/id.go
new file mode 100644
index 00000000..7fa1785a
--- /dev/null
+++ b/entity/id.go
@@ -0,0 +1,65 @@
+package entity
+
+import (
+	"fmt"
+	"io"
+	"strings"
+
+	"github.com/pkg/errors"
+)
+
+const IdLengthSHA1 = 40
+const IdLengthSHA256 = 64
+const humanIdLength = 7
+
+const UnsetId = Id("unset")
+
+// Id is an identifier for an entity or part of an entity
+type Id string
+
+func (i Id) String() string {
+	return string(i)
+}
+
+func (i Id) Human() string {
+	format := fmt.Sprintf("%%.%ds", humanIdLength)
+	return fmt.Sprintf(format, i)
+}
+
+func (i Id) HasPrefix(prefix string) bool {
+	return strings.HasPrefix(string(i), prefix)
+}
+
+// UnmarshalGQL implement the Unmarshaler interface for gqlgen
+func (i *Id) UnmarshalGQL(v interface{}) error {
+	_, ok := v.(string)
+	if !ok {
+		return fmt.Errorf("IDs must be strings")
+	}
+
+	*i = v.(Id)
+
+	if err := i.Validate(); err != nil {
+		return errors.Wrap(err, "invalid ID")
+	}
+
+	return nil
+}
+
+// MarshalGQL implement the Marshaler interface for gqlgen
+func (i Id) MarshalGQL(w io.Writer) {
+	_, _ = w.Write([]byte(`"` + i.String() + `"`))
+}
+
+// IsValid tell if the Id is valid
+func (i Id) Validate() error {
+	if len(i) != IdLengthSHA1 && len(i) != IdLengthSHA256 {
+		return fmt.Errorf("invalid length")
+	}
+	for _, r := range i {
+		if (r < 'a' || r > 'z') && (r < '0' || r > '9') {
+			return fmt.Errorf("invalid character")
+		}
+	}
+	return nil
+}
diff --git a/entity/interface.go b/entity/interface.go
index 62b92a58..dd5d69b1 100644
--- a/entity/interface.go
+++ b/entity/interface.go
@@ -2,7 +2,5 @@ package entity
 
 type Interface interface {
 	// Id return the Entity identifier
-	Id() string
-	// HumanId return the Entity identifier truncated for human consumption
-	HumanId() string
+	Id() Id
 }