From ef17306f8b57e2477d9c77d17926b80f48dc53ef Mon Sep 17 00:00:00 2001
From: Zachary Yedidia <zyedidia@gmail.com>
Date: Mon, 30 May 2016 21:01:40 -0400
Subject: [PATCH] Allow changes from external programs to be undone

We use a diff to be able to transform the buffer into an arbitrary
string (usually when we reread the file after an external program such
as gofmt has changed it) and still preserve undo/redo.

Fixes #136
---
 cmd/micro/buffer.go       |  6 +-----
 cmd/micro/eventhandler.go | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/cmd/micro/buffer.go b/cmd/micro/buffer.go
index fbcb602a..0357db52 100644
--- a/cmd/micro/buffer.go
+++ b/cmd/micro/buffer.go
@@ -155,11 +155,7 @@ func (b *Buffer) ReOpen() {
 		messenger.Error(err.Error())
 		return
 	}
-	if txt == "" {
-		b.r = new(rope.Rope)
-	} else {
-		b.r = rope.New(txt)
-	}
+	b.EventHandler.ApplyDiff(txt)
 
 	b.ModTime, _ = GetModTime(b.Path)
 	b.IsModified = false
diff --git a/cmd/micro/eventhandler.go b/cmd/micro/eventhandler.go
index 2c0e6482..84f7465c 100644
--- a/cmd/micro/eventhandler.go
+++ b/cmd/micro/eventhandler.go
@@ -2,6 +2,8 @@ package main
 
 import (
 	"time"
+
+	dmp "github.com/sergi/go-diff/diffmatchpatch"
 )
 
 const (
@@ -55,6 +57,25 @@ func NewEventHandler(buf *Buffer) *EventHandler {
 	return eh
 }
 
+// ApplyDiff takes a string and runs the necessary insertion and deletion events to make
+// the buffer equal to that string
+// This means that we can transform the buffer into any string and still preserve undo/redo
+// through insert and delete events
+func (eh *EventHandler) ApplyDiff(new string) {
+	messenger.Message("Applying diff")
+	differ := dmp.New()
+	diff := differ.DiffMain(eh.buf.String(), new, false)
+	var charNum int
+	for _, d := range diff {
+		if d.Type == dmp.DiffInsert {
+			eh.Insert(charNum, d.Text)
+		} else if d.Type == dmp.DiffDelete {
+			eh.Remove(charNum, charNum+Count(d.Text))
+		}
+		charNum += Count(d.Text)
+	}
+}
+
 // Insert creates an insert text event and executes it
 func (eh *EventHandler) Insert(start int, text string) {
 	e := &TextEvent{