Only consider BindingSets with a binding for first keystroke of event

The keymap was slow because it would find the binding sets with matching selectors for every ancestor of every keydown event. The problem was, it was considering a lot of binding sets that didn't even have a binding for the event in question. Now we index all binding sets on what bindings they contain, and filter them by that first. Rejecting an event that has no bindings is almost instantaneous now.

src/app/keymap.coffee

@@ -8,10 +8,12 @@ Specificity = require 'specificity'
 module.exports =
 class Keymap
   bindingSets: null
+  bindingSetsByFirstKeystroke: null
   queuedKeystrokes: null
 
   constructor: ->
     @bindingSets = []
+    @bindingSetsByFirstKeystroke = {}
 
   bindDefaultKeys: ->
     @bindKeys "*",
@@ -25,8 +27,12 @@ class Keymap
     $(document).on 'open', => atom.open()
 
   bindKeys: (selector, bindings) ->
-    index = @bindingSets.length
-    @bindingSets.unshift(new BindingSet(selector, bindings, index))
+    bindingSet = new BindingSet(selector, bindings, @bindingSets.length)
+    @bindingSets.unshift(bindingSet)
+    for keystrokes of bindingSet.commandsByKeystrokes
+      keystroke = keystrokes.split(' ')[0] # only index by first keystroke
+      @bindingSetsByFirstKeystroke[keystroke] ?= []
+      @bindingSetsByFirstKeystroke[keystroke].push(bindingSet)
 
   bindingsForElement: (element) ->
     keystrokeMap = {}
@@ -43,9 +49,14 @@ class Keymap
     event.keystrokes = @multiKeystrokeStringForEvent(event)
     isMultiKeystroke = @queuedKeystrokes?
     @queuedKeystrokes = null
+
+    firstKeystroke = event.keystrokes.split(' ')[0]
+    bindingSetsForFirstKeystroke = @bindingSetsByFirstKeystroke[firstKeystroke]
+    return true unless bindingSetsForFirstKeystroke?
+
     currentNode = $(event.target)
     while currentNode.length
-      candidateBindingSets = @bindingSetsForNode(currentNode)
+      candidateBindingSets = @bindingSetsForNode(currentNode, bindingSetsForFirstKeystroke)
       for bindingSet in candidateBindingSets
         command = bindingSet.commandForEvent(event)
         if command
@@ -61,8 +72,8 @@ class Keymap
 
     !isMultiKeystroke
 
-  bindingSetsForNode: (node) ->
-    bindingSets = @bindingSets.filter (set) -> node.is(set.selector)
+  bindingSetsForNode: (node, candidateBindingSets = @bindingSets) ->
+    bindingSets = candidateBindingSets.filter (set) -> node.is(set.selector)
     bindingSets.sort (a, b) ->
       if b.specificity == a.specificity
         b.index - a.index