WIP: Use a goal column when moving down

Still needs to be cleared when moving horizontally.

spec/atom/editor-spec.coffee

@@ -48,15 +48,35 @@ fdescribe "Editor", ->
         editor.moveUp()
         expect(editor.getPosition()).toEqual(row: 0, col: 0)
 
-    describe "when down is pressed on the last line", ->
-      it "moves the cursor to the end of line", ->
-        lastLineIndex = buffer.getLines().length - 1
-        lastLine = buffer.getLine(lastLineIndex)
-        expect(lastLine.length).toBeGreaterThan(0)
+    describe "down", ->
+      describe "when moving between lines of varying length", ->
+        fit "retains a goal column, keeping the cursor as close to the original x position as possible", ->
+          lineLengths = buffer.getLines().map (line) -> line.length
+          expect(lineLengths[3]).toBeGreaterThan(lineLengths[4])
+          expect(lineLengths[5]).toBeGreaterThan(lineLengths[4])
+          expect(lineLengths[6]).toBeGreaterThan(lineLengths[3])
 
-        editor.setPosition(row: lastLineIndex, col: 0)
-        editor.moveDown()
-        expect(editor.getPosition()).toEqual(row: lastLineIndex, col: lastLine.length)
+          editor.setPosition(row: 3, col: lineLengths[3])
+
+          editor.moveDown()
+          expect(editor.getPosition().col).toBe lineLengths[4]
+
+          editor.moveDown()
+          expect(editor.getPosition().col).toBe lineLengths[5]
+
+          editor.moveDown()
+          expect(editor.getPosition().col).toBe lineLengths[3]
+
+
+      describe "when on the last line", ->
+        it "moves the cursor to the end of line", ->
+          lastLineIndex = buffer.getLines().length - 1
+          lastLine = buffer.getLine(lastLineIndex)
+          expect(lastLine.length).toBeGreaterThan(0)
+
+          editor.setPosition(row: lastLineIndex, col: 0)
+          editor.moveDown()
+          expect(editor.getPosition()).toEqual(row: lastLineIndex, col: lastLine.length)
 
     describe "when left is pressed on the first column", ->
       describe "when there is a previous line", ->

src/atom/cursor.coffee

@@ -6,7 +6,8 @@ class Cursor extends Template
     @pre class: 'cursor', style: 'position: absolute;', => @raw ' '
 
   viewProperties:
-    setPosition: (@point) ->
+    setPosition: (point) ->
+      @point = @parentView.clipPosition(point)
       @updateAbsolutePosition()
 
     getPosition: -> @point
@@ -21,11 +22,14 @@ class Cursor extends Template
 
     moveDown: ->
       { row, col } = @getPosition()
+
       if row < @parentView.buffer.numLines() - 1
         row++
       else
         col = @parentView.buffer.getLine(row).length
-      @setPosition({row, col})
+
+      @goalColumn ?= col
+      @setPosition({row, col: @goalColumn || col})
 
     moveRight: ->
       { row, col } = @getPosition()
@@ -46,7 +50,6 @@ class Cursor extends Template
 
       @setPosition({row, col})
 
-
     updateAbsolutePosition: ->
       position = @parentView.pixelPositionFromPoint(@point)
       @css(position)

src/atom/editor.coffee

@@ -42,6 +42,10 @@ class Editor extends Template
           @lines.append $$.pre(line)
       @setPosition(row: 0, col: 0)
 
+    clipPosition: ({row, col}) ->
+      line = @buffer.getLine(row)
+      { row: row, col: Math.min(line.length, col) }
+
     pixelPositionFromPoint: ({row, col}) ->
       { top: row * @lineHeight, left: col * @charWidth }