Paginate samples by date range

In #946, I added a `/pages/:id/versions/sampled` endpoint that returns no more than one page per date. The goal was to reduce the number of queries and the amount of data transferred to load some pages that have a *lot* of snapshots. It turns out this didn't work well because it still paginated by the number of versions it was querying, which means it still required the exact same number of requests (d'oh!). This changes the approach to paginate by date range instead, which should make a big difference. The code is pretty rough, but as long as it works, this needs to deploy so we can try it out with the bigger production data set.

app/controllers/api/v0/versions_controller.rb

@@ -1,6 +1,9 @@
 class Api::V0::VersionsController < Api::V0::ApiController
   include SortingConcern
 
+  SAMPLE_DAYS_DEFAULT = 183
+  SAMPLE_DAYS_MAX = 365
+
   def index
     query = version_collection
     paging = pagination(query)
@@ -22,11 +25,48 @@ def sampled
     # We don't support the complex filters and options #index does; we want to
     # keep this as simple and fast as possible.
     query = page.versions
-    # FIXME: this should probably have special pagination by date, since we
-    # don't want a sample group split across two responses.
-    paging = pagination(query)
 
-    samples = paging[:query].each_with_object({}) do |version, result|
+    now = Time.now
+    # time_range = nil
+    # if params[:from] && params[:to]
+    #   # time_range = parse_unbounded_range!(params[:capture_time], "capture_time") { |d| parse_date!(d) }
+    #   from_time = parse_date!(params[:from]).to_date
+    #   to_time = parse_date!(params[:to]).to_date + 1.day
+    #   if to_time - from_time > SAMPLE_DAYS_MAX.days
+    #     raise Api::InputError, "`from` and `to` parameters must be no more than 365 days apart"
+    #   end
+    #   time_range = [from_time, to_time]
+    # elsif params[:from]
+    #   from_time = parse_date!(params[:from]).to_date
+    #   time_range = [from_time, from_time + SAMPLE_DAYS_DEFAULT.days]
+    # elsif params[:to]
+    #   to_time = parse_date!(params[:to]).to_date + 1.day
+    #   time_range = [to_time - SAMPLE_DAYS_DEFAULT.days, to_time]
+    # else
+    #   to_time = now.to_date + 1.day
+    #   time_range = [to_time - SAMPLE_DAYS_DEFAULT.days, to_time]
+    # end
+
+    time_range = parse_unbounded_range!(params[:capture_time], "capture_time") { |d| parse_date!(d).to_date } || []
+    if time_range[0] && time_range[1]
+      time_range[1] = time_range[1] + 1.day
+      if time_range[1] - time_range[0] > SAMPLE_DAYS_MAX.days
+        raise Api::InputError, "time range must be no more than 365 days"
+      end
+    elsif time_range[1]
+      to_time = now.to_date + 1.day
+      time_range = [to_time - SAMPLE_DAYS_DEFAULT.days, to_time]
+    elsif time_range[0]
+      from_time = time_range[0]
+      time_range = [from_time, from_time + SAMPLE_DAYS_DEFAULT.days]
+    else
+      to_time = now.to_date + 1.day
+      time_range = [to_time - SAMPLE_DAYS_DEFAULT.days, to_time]
+    end
+
+    query = query.where_in_unbounded_range(:capture_time, time_range)
+
+    samples = query.each_with_object({}) do |version, result|
       key = version.capture_time.to_date.iso8601
       if result.key?(key)
         result[key][:version_count] += 1
@@ -46,11 +86,45 @@ def sampled
       sample[:version] = serialize_version(sample[:version])
     end
 
+    next_version = page.versions.where('capture_time < ?', time_range[0]).select(:uuid, :capture_time).first
+    if samples.length.zero? && next_version.nil?
+      raise Api::NotFoundError, "`to` time is older than the oldest version"
+    end
+
+    links = {
+      first: api_v0_page_versions_sampled_url(page),
+      last: nil,
+      next: nil,
+      prev: nil
+    }
+    # Optimize forward pagination by skipping to a time range with data.
+    if next_version
+      next_to = next_version.capture_time.to_date + 1.day
+      # links[:next] = api_v0_page_versions_sampled_url(
+      #   page,
+      #   params: {from: (next_to - SAMPLE_DAYS_DEFAULT.days).iso8601, to: next_to.iso8601}
+      # )
+      links[:next] = api_v0_page_versions_sampled_url(
+        page,
+        params: {capture_time: "#{(next_to - SAMPLE_DAYS_DEFAULT.days).iso8601}..#{next_to.iso8601}"}
+      )
+    end
+    if time_range[1] < Time.now
+      # links[:prev] = api_v0_page_versions_sampled_url(
+      #   page,
+      #   params: {from: time_range[1].iso8601, to: (time_range[1] + SAMPLE_DAYS_DEFAULT.days).iso8601}
+      # )
+      links[:prev] = api_v0_page_versions_sampled_url(
+        page,
+        params: {capture_time: "#{time_range[1].iso8601}..#{(time_range[1] + SAMPLE_DAYS_DEFAULT.days).iso8601}"}
+      )
+    end
+
     render json: {
-      links: paging[:links],
+      links: links,
       meta: {
-        **paging[:meta],
-        sample_period: 'day'
+        sample_period: 'day',
+        warning: 'This API endpoint is experimental and may change'
       },
       data: samples.values
     }