Commit 30ca3f0

@@ -1,12 +1,16 @@
 package main
 
-import (
-	"errors"
-)
-
 type Dictionary map[string]string
+type DictionaryError string
+
+var (
+	ErrorNotFound   = DictionaryError("could not find the word you were looking for")
+	ErrorWordExists = DictionaryError("cannot add word because it already exists")
+)
 
-var ErrorNotFound = errors.New("could not find the word you were looking for")
+func (e DictionaryError) Error() string {
+	return string(e)
+}
 
 func (d Dictionary) Search(word string) (string, error) {
 	definition, ok := d[word]
@@ -17,6 +21,17 @@ func (d Dictionary) Search(word string) (string, error) {
 	return definition, nil
 }
 
-func (d Dictionary) Add(word, definition string) {
-	d[word] = definition
+func (d Dictionary) Add(word, definition string) error {
+	_, err := d.Search(word)
+
+	switch err {
+	case ErrorNotFound:
+		d[word] = definition
+	case nil:
+		return ErrorWordExists
+	default:
+		return err
+	}
+
+	return nil
 }

@@ -23,18 +23,25 @@ func TestSearch(t *testing.T) {
 }
 
 func TestAdd(t *testing.T) {
-	dictionary := Dictionary{}
-	dictionary.Add("test", "this is just a test")
+	t.Run("new word", func(t *testing.T) {
+		dictionary := Dictionary{}
+		key := "test"
+		value := "this is just a test"
+		err := dictionary.Add(key, value)
 
-	want := "this is just a test"
-	got, err := dictionary.Search("test")
-	if err != nil {
-		t.Fatal("should find added word:", err)
-	}
+		assertNil(t, err)
+		assertDefinition(t, dictionary, key, value)
+	})
 
-	if want != got {
-		t.Errorf("got %q want %q", got, want)
-	}
+	t.Run("existing word", func(t *testing.T) {
+		key := "test"
+		value := "this is just a test"
+		dictionary := Dictionary{key: value}
+		err := dictionary.Add(key, "new test")
+
+		assertError(t, err, ErrorWordExists)
+		assertDefinition(t, dictionary, key, value)
+	})
 }
 
 func assertStrings(t *testing.T, got, want string) {
@@ -44,3 +51,22 @@ func assertStrings(t *testing.T, got, want string) {
 		t.Errorf("got %q want %q", got, want)
 	}
 }
+
+func assertDefinition(t *testing.T, dictionary Dictionary, key, expected string) {
+	t.Helper()
+
+	actual, err := dictionary.Search("test")
+	if err != nil {
+		t.Fatal("should find added key:", err)
+	}
+
+	if expected != actual {
+		t.Errorf("actual %q want %q", actual, expected)
+	}
+}
+
+func assertNil(t *testing.T, actual error) {
+	if actual != nil {
+		t.Errorf("expected nil got %q", actual)
+	}
+}