adding the undo/redo functionality

app/src/main/kotlin/com/simplemobiletools/notes/models/TextHistory.kt

@@ -0,0 +1,35 @@
+package com.simplemobiletools.notes.models
+
+import java.util.*
+
+class TextHistory {
+    var position = 0
+    val history = LinkedList<TextHistoryItem>()
+
+    fun getPrevious(): TextHistoryItem? {
+        if (position == 0) {
+            return null
+        }
+        position--
+        return history[position]
+    }
+
+    fun getNext(): TextHistoryItem? {
+        if (position >= history.size) {
+            return null
+        }
+
+        val item = history[position]
+        position++
+        return item
+    }
+
+    fun add(item: TextHistoryItem) {
+        while (history.size > position) {
+            history.removeLast()
+        }
+
+        history.add(item)
+        position++
+    }
+}

app/src/main/kotlin/com/simplemobiletools/notes/models/TextHistoryItem.kt

@@ -0,0 +1,3 @@
+package com.simplemobiletools.notes.models
+
+data class TextHistoryItem(val start: Int, val before: CharSequence?, val after: CharSequence?)