improved series algorithm efficiency (now up to ~5x faster and ~2.3x …

…lighter than the previous one)

README.md

@@ -18,7 +18,7 @@ Pagy is the ultimate pagination gem that outperforms the others in each and ever
 - __This version requires `ruby 3.0+`. For `ruby <3.0` use `pagy <4.0` (see the [pagy3 branch](https://github.com/ddnexus/pagy/tree/pagy3))__
 - Updating `pagy` from `3.0+` to `4.0+` requires a single renaming in your code, but only if it uses the `searchkick` or the `elasticsearch_rails` extras (see the [Changelog](https://github.com/ddnexus/pagy/blob/master/CHANGELOG.md))
 - Added the docker development environment to ease contributions
-- Big code restyling following ruby 3.0 syntax and cops; tried to make the code simpler, more readable and verbose with almost negligible performance loss.
+- Big code restyling following ruby 3.0 syntax and cops; the code is simpler, more readable and verbose with yet improved performance.
 
 ## Comparison with other gems
 

lib/pagy.rb

@@ -33,10 +33,10 @@ def initialize(vars)
           if @page > @last
 
     @offset = @items * (@page - 1) + @outset
-    @items  = @count - ((@pages-1) * @items) if @page == @last && @count.positive?
+    @items  = @count - ((@pages - 1) * @items) if @page == @last && @count.positive?
     @from   = @count.zero? ? 0 : @offset + 1 - @outset
     @to     = @count.zero? ? 0 : @offset + @items - @outset
-    @prev   = (@page-1 unless @page == 1)
+    @prev   = (@page - 1 unless @page == 1)
     @next   = @page == @last ? (1 if @vars[:cycle]) : @page + 1
   end
 
@@ -45,22 +45,25 @@ def series(size=@vars[:size])
     return [] if size.empty?
     raise VariableError.new(self), "expected 4 items >= 0 in :size; got #{size.inspect}" \
           unless size.size == 4 && size.all?{ |num| num >= 0 rescue false }  # rubocop:disable Style/RescueModifier
-
+    # This algorithm is up to ~5x faster and ~2.3x lighter than the previous one (pagy < 4.3)
+    left_gap_start  =     1 + size[0]
+    left_gap_end    = @page - size[1] - 1
+    right_gap_start = @page + size[2] + 1
+    right_gap_end   = @last - size[3]
+    left_gap_end    = right_gap_end  if left_gap_end   > right_gap_end
+    right_gap_start = left_gap_start if left_gap_start > right_gap_start
     series = []
-    [ *0..size[0],                                   # initial pages from 0
-      *@page-size[1]..@page+size[2],                 # around current page
-      *@last-size[3]+1..@last+1                      # final pages till @last+1
-    ].sort!.each_cons(2) do |left, right|            # sort and loop by 2
-      next if left.negative? || left == right        # skip out of range and duplicates
-      break if left > @last                          # break if out of @last boundary
-      case right
-      when left+1 then series.push(left)             # no gap     -> no additions
-      when left+2 then series.push(left, left+1)     # 1 page gap -> fill with missing page
-      else             series.push(left, :gap)       # n page gap -> add gap
-      end
+    start  = 1
+    if (left_gap_end - left_gap_start).positive?
+      series.push(*start..(left_gap_start - 1), :gap)
+      start = left_gap_end + 1
+    end
+    if (right_gap_end - right_gap_start).positive?
+      series.push(*start..(right_gap_start - 1), :gap)
+      start = right_gap_end + 1
     end
-    series.shift                                     # shift the start boundary (0)
-    series[series.index(@page)] = @page.to_s         # convert the current page to String
+    series.push(*start..@last)
+    series[series.index(@page)] = @page.to_s
     series
   end