Use cached disk contents instead of signature for Buffer.isModified

Computing the signature was really slow for large files. Keeping a cached copy of the disk contents and comparing directly costs less in the benchmark. We'll have to turn on the status bar in benchmarks to see if the comparison is too expensive.

src/app/buffer.coffee

@@ -8,14 +8,12 @@ UndoManager = require 'undo-manager'
 BufferChangeOperation = require 'buffer-change-operation'
 Anchor = require 'anchor'
 AnchorRange = require 'anchor-range'
-{hex_md5}  = require 'md5'
 
 module.exports =
 class Buffer
   @idCounter = 1
   undoManager: null
-  diskContentSignature: null
-  memoryContentSignature: null
+  cachedDiskContents: null
   conflict: false
   lines: null
   file: null
@@ -32,20 +30,12 @@ class Buffer
     if path
       throw "Path '#{path}' does not exist" unless fs.exists(path)
       @setPath(path)
-      @setText(fs.read(@getPath()))
-      @computeDiskContentSignature()
+      @reload()
     else
       @setText('')
 
-    @computeMemoryContentSignature()
     @undoManager = new UndoManager(this)
 
-  computeDiskContentSignature: ->
-    @diskContentSignature = fs.md5ForPath(@getPath())
-
-  computeMemoryContentSignature: ->
-    @memoryContentSignature = hex_md5(@getText())
-
   destroy: ->
     throw new Error("Destroying buffer twice with path '#{@getPath()}'") if @destroyed
     @file?.off()
@@ -65,11 +55,10 @@ class Buffer
     @file.on "contents-change", =>
       if @isModified()
         @conflict = true
-        @computeDiskContentSignature()
+        @updateCachedDiskContents()
         @trigger "contents-change-on-disk"
       else
-        @setText(fs.read(@file.getPath()))
-        @computeDiskContentSignature()
+        @reload()
 
     @file.on "remove", =>
       @file = null
@@ -79,7 +68,11 @@ class Buffer
       @trigger "path-change", this
 
   reload: ->
-    @setText(fs.read(@getPath()))
+    @updateCachedDiskContents()
+    @setText(@cachedDiskContents)
+
+  updateCachedDiskContents: ->
+    @cachedDiskContents = fs.read(@getPath())
 
   getBaseName: ->
     @file?.getBaseName()
@@ -212,8 +205,7 @@ class Buffer
 
   replaceLines: (startRow, endRow, newLines) ->
     @lines[startRow..endRow] = newLines
-    @computeMemoryContentSignature()
-    @conflict = false unless @isModified()
+    @conflict = false if @conflict and !@isModified()
 
   pushOperation: (operation, editSession) ->
     if @undoManager
@@ -237,15 +229,16 @@ class Buffer
     if not path then throw new Error("Can't save buffer with no file path")
 
     @trigger 'before-save'
-    fs.write path, @getText()
+    @cachedDiskContents = @getText()
+    fs.write path, @cachedDiskContents
     @file?.updateMd5()
     @setPath(path)
-    @computeDiskContentSignature()
+
     @trigger 'after-save'
 
   isModified: ->
     if @file
-      @memoryContentSignature != @diskContentSignature
+      @getText() != @cachedDiskContents
     else
       not @isEmpty()