Add option for very accurate dirty flag

Set the `fastdirty` option flag to off if you really want accurate
reporting on whether the buffer is modified. This is more resource
intensive but it can be useful for people who don't mind.

Closes #787
Closes #467

cmd/micro/buffer.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bytes"
+	"crypto/md5"
 	"encoding/gob"
 	"io"
 	"io/ioutil"
@@ -57,6 +58,9 @@ type Buffer struct {
 	syntaxDef   *highlight.Def
 	highlighter *highlight.Highlighter
 
+	// Hash of the original buffer -- empty if fastdirty is on
+	origHash [16]byte
+
 	// Buffer local settings
 	Settings map[string]interface{}
 }
@@ -174,7 +178,7 @@ func NewBuffer(reader io.Reader, size int64, path string) *Buffer {
 			}
 
 			if b.Settings["saveundo"].(bool) {
-				// We should only use last time's eventhandler if the file wasn't by someone else in the meantime
+				// We should only use last time's eventhandler if the file wasn't modified by someone else in the meantime
 				if b.ModTime == buffer.ModTime {
 					b.EventHandler = buffer.EventHandler
 					b.EventHandler.buf = b
@@ -188,6 +192,10 @@ func NewBuffer(reader io.Reader, size int64, path string) *Buffer {
 		screen.EnableMouse()
 	}
 
+	if !b.Settings["fastdirty"].(bool) {
+		b.origHash = md5.Sum([]byte(b.String()))
+	}
+
 	b.cursors = []*Cursor{&b.Cursor}
 
 	return b
@@ -451,6 +459,13 @@ func (b *Buffer) SaveAsWithSudo(filename string) error {
 	return err
 }
 
+func (b *Buffer) Modified() bool {
+	if b.Settings["fastdirty"].(bool) {
+		return b.IsModified
+	}
+	return b.origHash != md5.Sum([]byte(b.String()))
+}
+
 func (b *Buffer) insert(pos Loc, value []byte) {
 	b.IsModified = true
 	b.LineArray.insert(pos, value)