Remove "magical" NewPrefix function, rename Prefix to Encoder.

encoder_test.go

@@ -0,0 +1,33 @@
+package multibase
+
+import (
+	"testing"
+)
+
+func TestInvalidPrefix(t *testing.T) {
+	_, err := NewEncoder('q')
+	if err == nil {
+		t.Error("expected failure")
+	}
+}
+
+func TestPrefix(t *testing.T) {
+	for str, base := range Encodings {
+		prefix, err := NewEncoder(base)
+		if err != nil {
+			t.Fatalf("NewEncoder(%c) failed: %v", base, err)
+		}
+		str1, err := Encode(base, sampleBytes)
+		if err != nil {
+			t.Fatal(err)
+		}
+		str2 := prefix.Encode(sampleBytes)
+		if str1 != str2 {
+			t.Errorf("encoded string mismatch: %s != %s", str1, str2)
+		}
+		_, err = EncoderByName(str)
+		if err != nil {
+			t.Fatalf("NewEncoder(%s) failed: %v", str, err)
+		}
+	}
+}