Centralize D&D candidate JSON codecs

internal/modules/dnd/codec/candidatejson/candidate_json_test.go

@@ -0,0 +1,47 @@
+package candidatejson
+
+import (
+	"encoding/json"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+type testCandidate struct {
+	Items []string `json:"items"`
+}
+
+func TestCandidateJSON(t *testing.T) {
+	input := testCandidate{Items: []string{"one", "two"}}
+	content, err := EncodeCandidate("test candidate", input)
+	if err != nil {
+		t.Fatalf("EncodeCandidate() error = %v", err)
+	}
+	if !json.Valid(content) {
+		t.Fatalf("EncodeCandidate() = %q, want JSON", content)
+	}
+	for _, test := range []struct {
+		name    string
+		content []byte
+		want    testCandidate
+		wantErr string
+	}{
+		{name: "typed round trip", content: content, want: input},
+		{name: "unknown field", content: []byte(`{"items":[],"unexpected":true}`), wantErr: "unknown field"},
+		{name: "malformed JSON", content: []byte(`{"items":`), wantErr: "decode test candidate"},
+		{name: "trailing value", content: []byte(`{"items":[]} {}`), wantErr: "multiple JSON values"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			got, err := DecodeCandidate[testCandidate]("test candidate", test.content)
+			if test.wantErr != "" {
+				if err == nil || !strings.Contains(err.Error(), test.wantErr) {
+					t.Fatalf("DecodeCandidate() error = %v, want %q", err, test.wantErr)
+				}
+				return
+			}
+			if err != nil || !reflect.DeepEqual(got, test.want) {
+				t.Fatalf("DecodeCandidate() = %#v, %v; want %#v", got, err, test.want)
+			}
+		})
+	}
+}