3 files changed, 70 insertions, 15 deletions
diff --git a/cmd/linked_token_slice.go b/cmd/linked_token_slice.go
new file mode 100644
index 0000000..c8c876f
--- /dev/null
+++ b/cmd/linked_token_slice.go
@@ -0,0 +1,28 @@
+package cmd
+
+type LinkedTokenList struct {
+	head, tail *Token
+}
+
+func (lts *LinkedTokenList) AddToken(content string) {
+	newToken := &Token{
+		content: content,
+		prev:    nil,
+		next:    nil,
+	}
+
+	if lts.head == nil {
+		lts.head = newToken
+		lts.tail = newToken
+	} else {
+		newToken.SetPreviousToken(lts.tail)
+		lts.tail.SetNextToken(newToken)
+		lts.tail = newToken
+	}
+}
+
+func SliceToLinkedTokenSlice(slice []string, tokenSlice *LinkedTokenList) {
+	for _, item := range slice {
+		tokenSlice.AddToken(item)
+	}
+}
diff --git a/cmd/token.go b/cmd/token.go
new file mode 100644
index 0000000..7652be7
--- /dev/null
+++ b/cmd/token.go
@@ -0,0 +1,30 @@
+package cmd
+
+type Token struct {
+	content    string
+	prev, next *Token
+}
+
+func (t *Token) GetContent() string {
+	return t.content
+}
+
+func (t *Token) SetContent(content string) {
+	t.content = content
+}
+
+func (t *Token) GetPreviousToken() *Token {
+	return t.prev
+}
+
+func (t *Token) SetPreviousToken(newToken *Token) {
+	t.prev = newToken
+}
+
+func (t *Token) GetNextToken() *Token {
+	return t.next
+}
+
+func (t *Token) SetNextToken(newToken *Token) {
+	t.next = newToken
+}
diff --git a/cmd/uptfs.go b/cmd/uptfs.go
index 1bba4c4..531b102 100644
--- a/cmd/uptfs.go
+++ b/cmd/uptfs.go
@@ -24,28 +24,25 @@ func Run() {
 	tokens := strings.Split(inputString, " ")
 	tokens = formatInput(tokens, additionalDelimeters)
 
-	lowercaseFilter := NewLowercaseFilterWithExtraSteps()
-	lowercaseFilter.Greet()
-
-	var newTokens []string
-	for _, token := range tokens {
-		newTokens = append(newTokens, lowercaseFilter.Filter(token))
+	if len(tokens) == 0 {
+		err := errors.New("the slice is empty")
+		fmt.Printf("%v\n", err)
+		os.Exit(1)
 	}
 
-	tokens = newTokens
+	var linkedTokens LinkedTokenList
+	SliceToLinkedTokenSlice(tokens, &linkedTokens)
 
+	lowercaseFilter := NewLowercaseFilterWithExtraSteps()
 	uppercaseFilter := NewUppercaseFilter()
-	uppercaseFilter.Greet()
 
-	newTokens = nil
-	for _, token := range tokens {
-		newTokens = append(newTokens, uppercaseFilter.Filter(token))
+	for current := linkedTokens.head; current != nil; current = current.GetNextToken() {
+		current.content = lowercaseFilter.Filter(current.content)
+		current.content = uppercaseFilter.Filter(current.content)
 	}
 
-	result := strings.Join(newTokens, " ")
-
-	fmt.Println("")
-	fmt.Println(result)
+	fmt.Println("Head: ", linkedTokens.head.content)
+	fmt.Println("Tail: ", linkedTokens.tail.content)
 }
 
 func formatInput(tokenArray []string, delimeterArray []string) []string {