Added Insert method

Author: linkdotnet

stringbuilder.go

@@ -85,6 +85,26 @@ func (s *StringBuilder) Remove(start int, length int) error {
 	return nil
 }
 
+func (s *StringBuilder) Insert(index int, text string) error {
+	if index < 0 {
+		return fmt.Errorf("index can't be negative")
+	}
+
+	if index > s.position {
+		return fmt.Errorf("can't write outside the buffer")
+	}
+
+	newLen := s.position + len(text)
+	if newLen >= cap(s.data) {
+		s.grow(newLen)
+	}
+
+	s.data = append(s.data[:index], append([]rune(text), s.data[index:]...)...)
+	s.position = newLen
+
+	return nil
+}
+
 func (s *StringBuilder) grow(lenToAdd int) {
 	// Grow times 2 until lenToAdd fits
 	newLen := cap(s.data)

stringbuilder_test.go

@@ -1,6 +1,8 @@
 package Text
 
-import "testing"
+import (
+	"testing"
+)
 
 func TestAppend(t *testing.T) {
 	const expected string = "Hello World"
@@ -120,3 +122,32 @@ func TestRemoveWhenLengthZero(t *testing.T) {
 		t.Errorf("Actual %q, Expected: %q", result, expected)
 	}
 }
+
+func TestInsertAtIndex(t *testing.T) {
+	const expected string = "Hello my dear and beautiful World"
+	sb := NewFromString("Hello World")
+
+	if err := sb.Insert(5, " my dear and beautiful"); err != nil {
+		t.Errorf("Insert threw an error: %v", err)
+	}
+
+	if result := sb.ToString(); result != expected {
+		t.Errorf("Actual %q, Expected: %q", result, expected)
+	}
+}
+
+func TestInsertShouldThrowIfNegativeIndex(t *testing.T) {
+	sb := StringBuilder{}
+
+	if err := sb.Insert(-1, "Test"); err == nil {
+		t.Error("Should throw error but did not")
+	}
+}
+
+func TestInsertShouldThrowErrirIfOutOfRange(t *testing.T) {
+	sb := StringBuilder{}
+
+	if err := sb.Insert(1, "Test"); err == nil {
+		t.Error("Should throw error but did not")
+	}
+}