config: Only marshal Slug, UUID, Deprecated of deprecated exercises

We don't need any of the other fields, and it's deceptive to keep them
in the JSON file because a maintainer may mistakenly believe them to
be significant, but in fact they are not; the website does not display
ny of the removed fields:
https://github.com/exercism/website/blob/b3314e84c52b52cf5f253386c3f0bf4127ca09aa/app/views/my/tracks/_show_side_exercises.html.haml#L26-L34

Two new tests were added: TestMarshalActive and TestMarshalDeprecated.
TestMarshalActive passes both before and after the associated code
change.
TestMarshalDeprecated fails before the associated code change, and
passes after.

Closes exercism/configlet#140

track/config.go

@@ -100,6 +100,36 @@ func (cfg *Config) ToJSON() ([]byte, error) {
 	return json.MarshalIndent(&cfg, "", "  ")
 }
 
+// MarshalJSON marshals a exercise metadata to JSON,
+// only marshalling certain fields if the exercise is deprecated.
+func (e *ExerciseMetadata) MarshalJSON() ([]byte, error) {
+	if e.IsDeprecated {
+		// Only marshal Slug, UUID, Deprecated.
+		return json.Marshal(&struct {
+			Slug         string `json:"slug"`
+			UUID         string `json:"uuid"`
+			IsDeprecated bool   `json:"deprecated"`
+		}{
+			Slug:         e.Slug,
+			UUID:         e.UUID,
+			IsDeprecated: true,
+		})
+	} else {
+		// Use the default marshalling.
+		// We can't embed ExerciseMetadata into an anonymous struct,
+		// since that will cause infinite recursion on this MarshalJSON,
+		// But we can embed a new typedef of it,
+		// since the typedef does not have this MarshalJSON function.
+		// Technique discovered from http://choly.ca/post/go-json-marshalling/
+		type ExerciseMetadataJ ExerciseMetadata
+		return json.Marshal(&struct {
+			*ExerciseMetadataJ
+		}{
+			ExerciseMetadataJ: (*ExerciseMetadataJ)(e),
+		})
+	}
+}
+
 func normalizeTopic(t string) string {
 	s := strings.ToLower(t)
 	s = rgxFunkyChars.ReplaceAllString(s, "")

track/config_test.go

@@ -105,6 +105,87 @@ func TestMarshalingNormalizesTopics(t *testing.T) {
 	assert.Equal(t, []string{"apple", "fig", "honeydew_melon"}, dstCfg.Exercises[0].Topics)
 }
 
+var allExercisesKeys = []string{
+	"slug",
+	"uuid",
+}
+
+var activeExercisesKeys = []string{
+	"core",
+	// not auto_approve, since it is omitempty
+	"unlocked_by",
+	"difficulty",
+	"topics",
+}
+
+func TestMarshalActive(t *testing.T) {
+	srcCfg := Config{
+		Exercises: []ExerciseMetadata{
+			ExerciseMetadata{
+				Slug:         "active",
+				Topics:       []string{"topic_one", "topic_two"},
+				IsDeprecated: false,
+			},
+		},
+	}
+
+	dst, err := srcCfg.ToJSON()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// contains all the keys we expect it to contain:
+	for _, key := range append(allExercisesKeys, activeExercisesKeys...) {
+		assert.True(t, strings.Contains(string(dst), key), "expected JSON representation to contain %q, but it didn't: %s", key, string(dst))
+	}
+
+	var dstCfg Config
+	if err := json.NewDecoder(bytes.NewReader(dst)).Decode(&dstCfg); err != nil {
+		t.Fatal(err)
+	}
+
+	// survived an encode -> decode:
+	assert.Equal(t, "active", dstCfg.Exercises[0].Slug)
+	assert.False(t, dstCfg.Exercises[0].IsDeprecated)
+	assert.Equal(t, []string{"topic_one", "topic_two"}, dstCfg.Exercises[0].Topics)
+}
+
+func TestMarshalDeprecated(t *testing.T) {
+	srcCfg := Config{
+		Exercises: []ExerciseMetadata{
+			ExerciseMetadata{
+				Slug:         "deprecated",
+				Topics:       []string{"topic_one", "topic_two"},
+				IsDeprecated: true,
+			},
+		},
+	}
+
+	dst, err := srcCfg.ToJSON()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// contains the keys we want, and not the ones we don't:
+	for _, key := range append(allExercisesKeys, "deprecated") {
+		assert.True(t, strings.Contains(string(dst), key), "expected JSON representation to contain %q, but it didn't: %s", key, string(dst))
+	}
+	for _, key := range activeExercisesKeys {
+		assert.False(t, strings.Contains(string(dst), key), "expected JSON representation NOT to contain %q, but it did: %s", key, string(dst))
+	}
+
+	var dstCfg Config
+	if err := json.NewDecoder(bytes.NewReader(dst)).Decode(&dstCfg); err != nil {
+		t.Fatal(err)
+	}
+
+	// survived an encode -> decode:
+	assert.Equal(t, "deprecated", dstCfg.Exercises[0].Slug)
+	assert.True(t, dstCfg.Exercises[0].IsDeprecated)
+	// Note that since topics was never marshalled, it should be nil.
+	assert.Nil(t, dstCfg.Exercises[0].Topics)
+}
+
 func TestSemanticsOfMissingTopics(t *testing.T) {
 	src := `
 	{