Replace anchor point/range with a single concept: markers

A "marker" is basically like a persistent selection/cursor composite,
having a head and a tail. The "head" is like the cursor in a selection,
and the "tail" is like the part of the selection that doesn't move. My
goal is for markers to be the only construct used to track regions
in the buffer. I want to replace anchors with them.

src/app/buffer-marker.coffee

@@ -0,0 +1,81 @@
+_ = require 'underscore'
+Point = require 'point'
+Range = require 'range'
+
+module.exports =
+class BufferMarker
+  headPosition: null
+  tailPosition: null
+  stayValid: false
+
+  constructor: ({@id, @buffer, range, @stayValid}) ->
+    @setRange(range)
+
+  setRange: (range) ->
+    range = @buffer.clipRange(range)
+    @tailPosition = range.start
+    @headPosition = range.end
+
+  getRange: ->
+    new Range(@tailPosition, @headPosition)
+
+  getStartPosition: ->
+    @getRange().start
+
+  getEndPosition: ->
+    @getRange().end
+
+  tryToInvalidate: (oldRange) ->
+    containsStart = oldRange.containsPoint(@getStartPosition(), exclusive: true)
+    containsEnd = oldRange.containsPoint(@getEndPosition(), exclusive: true)
+    return unless containsEnd or containsStart
+
+    if @stayValid
+      previousRange = @getRange()
+      if containsStart and containsEnd
+        @setRange([oldRange.end, oldRange.end])
+      else if containsStart
+        @setRange([oldRange.end, @getEndPosition()])
+      else
+        @setRange([@getStartPosition(), oldRange.start])
+      [@id, previousRange]
+    else
+      @invalidate()
+      [@id]
+
+  handleBufferChange: (bufferChange) ->
+    @setTailPosition(@updatePosition(@tailPosition, bufferChange, true), clip: false)
+    @setHeadPosition(@updatePosition(@headPosition, bufferChange, false), clip: false)
+
+  updatePosition: (position, bufferChange, isFirstPoint) ->
+    { oldRange, newRange } = bufferChange
+
+    return position if oldRange.containsPoint(position, exclusive: true)
+    return position if isFirstPoint and oldRange.start.isEqual(position)
+    return position if position.isLessThan(oldRange.end)
+
+    newRow = newRange.end.row
+    newColumn = newRange.end.column
+
+    if position.row == oldRange.end.row
+      newColumn += position.column - oldRange.end.column
+    else
+      newColumn = position.column
+      newRow += position.row - oldRange.end.row
+
+    [newRow, newColumn]
+
+  setTailPosition: (tailPosition, options={}) ->
+    @tailPosition = Point.fromObject(tailPosition)
+    @tailPosition = @buffer.clipPosition(@tailPosition) if options.clip ? true
+
+  setHeadPosition: (headPosition, options={}) ->
+    @headPosition = Point.fromObject(headPosition)
+    @headPosition = @buffer.clipPosition(@headPosition) if options.clip ? true
+
+  getPosition: ->
+    @position
+
+  invalidate: (preserve) ->
+    delete @buffer.validMarkers[@id]
+    @buffer.invalidMarkers[@id] = this