Add feed merging and diffing

[cardmagic]

Summary

Add methods to merge multiple feeds and diff feeds to find new/removed items.

Proposed Features

merge(*feeds)

Combine multiple feeds, deduplicating by guid or link:

def merge(*feeds)
  all_items = [items, *feeds.map(&:items)].flatten
  
  seen = Set.new
  merged_items = all_items.select do |item|
    key = item[:guid] || item[:id] || item[:link]
    next false if key.nil? || seen.include?(key)
    seen.add(key)
    true
  end
  
  # Sort by date, newest first
  merged_items.sort_by! { |i| i[:pubDate] || i[:updated] || Time.at(0) }.reverse!
  
  # Return new SimpleRSS-like object or just the array
  merged_items
end

Usage:

feed1 = SimpleRSS.parse(source1)
feed2 = SimpleRSS.parse(source2)

combined = feed1.merge(feed2)
# or
combined = SimpleRSS.merge(feed1, feed2, feed3)

diff(other)

Find differences between two versions of a feed:

def diff(other)
  my_keys = items.map { |i| i[:guid] || i[:id] || i[:link] }.compact
  other_keys = other.items.map { |i| i[:guid] || i[:id] || i[:link] }.compact
  
  {
    added: other.items.select { |i| !my_keys.include?(i[:guid] || i[:id] || i[:link]) },
    removed: items.select { |i| !other_keys.include?(i[:guid] || i[:id] || i[:link]) }
  }
end

Usage:

old_feed = SimpleRSS.parse(cached_source)
new_feed = SimpleRSS.parse(fresh_source)

changes = old_feed.diff(new_feed)
changes[:added]    # New items
changes[:removed]  # Items no longer in feed

Deduplication helper

def dedupe
  seen = Set.new
  @items = items.select do |item|
    key = item[:guid] || item[:id] || item[:link]
    next true if key.nil?  # Keep items without identifiers
    next false if seen.include?(key)
    seen.add(key)
    true
  end
  self
end

Benefits

• Aggregate multiple feeds into one stream
• Track new items for notifications/alerts
• Handle feeds with duplicate entries
• Useful for feed reader applications