much improved heatmap code

Merging buckets is now done in the initial pass. The entire heatmap
is calculated in two passes. The first creates the buckets while the
second counts the replies.

h/js/plugin/heatmap.coffee

@@ -59,7 +59,7 @@ class Annotator.Plugin.Heatmap extends Annotator.Plugin
     below = []
 
     # Construct control points for the heatmap highlights
-    points = $.map highlights, (hl, i) =>
+    points = highlights.reduce (points, hl, i) =>
       x = hl.offset.top - wrapper.offset().top - offset
       h = hl.height
       d = hl.data
@@ -69,81 +69,73 @@ class Annotator.Plugin.Heatmap extends Annotator.Plugin
       else if x + h >= $(window).height() - @BUCKET_SIZE
         if d not in below then below.push d
       else
-        return [
-          [x, 1, d]
-          [x + h, -1, d]
-        ]
-      return []
-
-    # Accumulate the overlapping annotations into buckets
-    {@buckets, @index} = points.sort(this._collate)
-      .reduce ({annotations, buckets, index}, [x, d, a], i, points) =>
-
-        # remove all instances of this annotation from the accumulator
-        annotations = annotations.reduce (acc, value) ->
-          {values, arrays} = acc
-          if value is a
-            arrays.push values
-            acc.values = []
+        points.push [x, 1, d]
+        points.push [x + h, -1, d]
+      points
+    , []
+
+    # Accumulate the overlapping annotations into buckets.
+    # The algorithm goes like this:
+    # - Collate the points by sorting on position then delta (+1 or -1)
+    # - Reduce over the sorted points
+    #   - For +1 points, add the annotation at this point to an array of
+    #     "carried" annotations. If it already exists, increase the
+    #     corresponding value in an array of counts which maintains the
+    #     number of points that include this annotation.
+    #   - For -1 points, decrement the value for the annotation at this point
+    #     in the carried array of counts. If the count is now zero, remove the
+    #     annotation from the carried array of annotations.
+    #   - If this point is the first, last, sufficiently far from the previous,
+    #     or there are no more carried annotations, add a bucket marker at this
+    #     point.
+    #   - Otherwise, if the last bucket was not isolated (the one before it
+    #     has at least one annotation) then remove it and ensure that its
+    #     annotations and the carried annotations are merged into the previous
+    #     bucket.
+    {@buckets, @index} = points
+    .sort(this._collate)
+    .reduce ({buckets, index, carry}, [x, d, a], i, points) =>
+      if d > 0                                            # Add annotation
+        if (j = carry.annotations.indexOf a) < 0
+          carry.annotations.unshift a
+          carry.counts.unshift 1
         else
-            values.push value
-          acc
-        ,
-          values: []
-          arrays: []
-        annotations = d3.merge annotations.arrays
-
-        if d > 0
-          # if this is a +1 control point, (re-)include the current annotation
-          # by removing and then adding, duplicates are easily avoided
-          annotations.push a
-          buckets.push annotations
+          carry.counts[j]++
+      else                                                # Remove annotation
+        j = carry.annotations.indexOf a                   # XXX: assert(i >= 0)
+        if --carry.counts[j] is 0
+          carry.annotations.splice j, 1
+          carry.counts.splice j, 1
+
+      if (
+        (index.length is 0 or i is points.length - 1) or  # First or last?
+        carry.annotations.length is 0 or                  # A zero marker?
+        x - index[index.length-1] > 180                   # A large gap?
+      )                                                   # Mark a new bucket.
+        buckets.push carry.annotations.slice()
         index.push x
       else
-          # if this is a -1 control point, exclude the current annotation
-          buckets.push annotations
-          index.push x
+        # Merge the previous bucket, making sure its predecessor contains
+        # all the carried annotations and the annotations in the previous
+        # bucket.
+        if buckets[buckets.length-2]?.length
+          last = buckets[buckets.length-2]
+          toMerge = buckets.pop()
+          index.pop()
+        else
+          last = buckets[buckets.length-1]
+          toMerge = []
+        last.push a0 for a0 in carry.annotations when a0 not in last
+        last.push a0 for a0 in toMerge when a0 not in last
 
-        {annotations, buckets, index}
+      {buckets, index, carry}
     ,
-      annotations: []
       buckets: []
       index: []
-
-    # Remove redundant points and merge close buckets until done
-    while @buckets.length > 2
-      # Find the two closest points
-      small = 0
-      threshold = min = 60
-      for i in [0..@index.length-2]
-        # Don't merge empty with non-empty buckets
-        if @buckets[i].length and not @buckets[i+1].length
-          continue
-
-        # Maintain the index of the smallest delta
-        if (w = @index[i+1] - @index[i]) < min
-          small = i
-          min = w
-          break if min == 0 # short-circuit optimization
-
-      # Merge them if they are close enough
-      if min < threshold
-        # Prefer merging the successor bucket backward but not if it's last
-        # since the gradient must always return to 0 at the end
-        if @buckets[small+2]?
-          from = small + 1
-          to = small
-
-          for b in @buckets[from]
-            @buckets[to].push b if b not in @buckets[to]
-        else
-          from = small
-
-        # Drop the merged bucket and index
-        @buckets.splice(from, 1)
-        @index.splice(from, 1)
-      else
-        break
+      carry:
+        annotations: []
+        counts: []
+        latest: 0
 
     # Add the scroll buckets
     @buckets.unshift above, []