more text edit operations;

--- a/src/Pure/Thy/text_edit.scala	Wed Jan 06 23:18:12 2010 +0100
+++ b/src/Pure/Thy/text_edit.scala	Wed Jan 06 23:18:44 2010 +0100
@@ -11,33 +11,67 @@
 sealed abstract class Text_Edit
 {
   val start: Int
+  def after(offset: Int): Int
   def before(offset: Int): Int
-  def after(offset: Int): Int
+  def edit(string: String, shift: Int): (Option[Text_Edit], String)
 }
 
 
 object Text_Edit
 {
+  /* transform offsets */
+
+  private def after_insert(start: Int, text: String, offset: Int): Int =
+    if (start <= offset) offset + text.length
+    else offset
+
+  private def after_remove(start: Int, text: String, offset: Int): Int =
+    if (start > offset) offset
+    else (offset - text.length) max start
+
+
+  /* edit strings */
+
+  private def insert(index: Int, text: String, string: String): String =
+    string.substring(0, index) + text + string.substring(index)
+
+  private def remove(index: Int, count: Int, string: String): String =
+    string.substring(0, index) + string.substring(index + count)
+
+
+  /* explicit edits */
+
   case class Insert(start: Int, text: String) extends Text_Edit
   {
-    def before(offset: Int): Int =
-      if (start > offset) offset
-      else (offset - text.length) max start
+    def after(offset: Int): Int = after_insert(start, text, offset)
+    def before(offset: Int): Int = after_remove(start, text, offset)
 
-    def after(offset: Int): Int =
-      if (start <= offset) offset + text.length
-      else offset
+    def can_edit(string_length: Int, shift: Int): Boolean =
+      shift <= start && start <= shift + string_length
+
+    def edit(string: String, shift: Int): (Option[Insert], String) =
+      if (can_edit(string.length, shift)) (None, insert(start - shift, text, string))
+      else (Some(this), string)
   }
 
   case class Remove(start: Int, text: String) extends Text_Edit
   {
-    def before(offset: Int): Int =
-      if (start <= offset) offset + text.length
-      else offset
+    def after(offset: Int): Int = after_remove(start, text, offset)
+    def before(offset: Int): Int = after_insert(start, text, offset)
+
+    def can_edit(string_length: Int, shift: Int): Boolean =
+      shift <= start && start < shift + string_length
 
-    def after(offset: Int): Int =
-      if (start > offset) offset
-      else (offset - text.length) max start
+    def edit(string: String, shift: Int): (Option[Remove], String) =
+      if (can_edit(string.length, shift)) {
+        val index = start - shift
+        val count = text.length min (string.length - index)
+        val rest =
+          if (count == text.length) None
+          else Some(Remove(start, text.substring(count)))
+        (rest, remove(index, count, string))
+      }
+      else (Some(this), string)
   }
 }