Fetch gem changelogs based on Gemfile.lock diff, you can update gems in Gemfile.lock, then run `bundle exec ruby fetch_gem_changelogs.rb | more`

fetch_gem_changelogs.rb

require 'bundler'
require 'net/http'
require 'json'
require 'uri'

# Function to fetch gem information from RubyGems.org
def fetch_gem_info(gem_name)
  url = URI("https://rubygems.org/api/v1/gems/#{gem_name}.json")
  response = Net::HTTP.get_response(url)

  if response.is_a?(Net::HTTPSuccess)
    JSON.parse(response.body)
  else
    nil
  end
end

# Function to extract updated gems from `git diff`
def updated_gems
  diff_output = `git diff Gemfile.lock`
  gems = {}

  diff_output.each_line do |line|
    if line.match(/^\-\s+(\S+)\s+\((\d+\.\d+\.\d+)\)/)
      gem_name, old_version = line.match(/^\-\s+(\S+)\s+\((\d+\.\d+\.\d+)\)/).captures
      gems[gem_name] ||= {}
      gems[gem_name][:old] = old_version
    elsif line.match(/^\+\s+(\S+)\s+\((\d+\.\d+\.\d+)\)/)
      gem_name, new_version = line.match(/^\+\s+(\S+)\s+\((\d+\.\d+\.\d+)\)/).captures
      gems[gem_name] ||= {}
      gems[gem_name][:new] = new_version
    end
  end

  gems.select { |_, versions| versions[:old] && versions[:new] }
end

# Function to fetch and display relevant CHANGELOG sections
def fetch_and_display_changelog(gem_name, old_version, new_version, changelog_url, source_code_url)
  # Check if the changelog_url is not on GitHub
  unless changelog_url.include?('github.com')
    # If source_code_url is on GitHub, construct the GitHub CHANGELOG URL
    if source_code_url && source_code_url.include?('github.com')
      changelog_url = "#{source_code_url}/blob/main/CHANGELOG.md"
    else
      puts "#{gem_name}: The changelog is not available on GitHub and no fallback could be constructed."
      return
    end
  end

  # Convert GitHub URL to raw URL if needed
  if changelog_url.include?('github.com') && !changelog_url.include?('/raw/')
    changelog_url = changelog_url.gsub('github.com', 'raw.githubusercontent.com').gsub('/blob/', '/')
  end

  uri = URI(changelog_url)
  response = Net::HTTP.get_response(uri)

  if response.is_a?(Net::HTTPSuccess)
    changelog_content = response.body
    extract_relevant_changelog(gem_name, old_version, new_version, changelog_content)
  else
    puts "#{gem_name}: Failed to fetch CHANGELOG from #{changelog_url}"
  end
end

# Function to extract relevant section from the CHANGELOG content
def extract_relevant_changelog(gem_name, old_version, new_version, changelog_content)
  puts "\n====================================================\n"
  puts "#{gem_name} - Changes from version #{old_version} to #{new_version}:\n\n"

  # Refined regex to match version headers more accurately
  version_section_regex = /^##?.*\s*v?(\d+\.\d+\.\d+)/i
  lines = changelog_content.lines
  relevant_lines = []
  capturing = false

  lines.each do |line|
    if line.match(version_section_regex)
      version = line.match(version_section_regex)[1]
      if Gem::Version.new(version) <= Gem::Version.new(new_version) && Gem::Version.new(version) > Gem::Version.new(old_version)
        capturing = true
      elsif Gem::Version.new(version) <= Gem::Version.new(old_version)
        capturing = false
      end
    end
    relevant_lines << line if capturing
  end

  if relevant_lines.empty?
    puts "No specific changelog entries found between versions #{old_version} and #{new_version}."
  else
    puts relevant_lines.join
  end
end

# Fetch updated gems list
gems_to_check = updated_gems

if gems_to_check.empty?
  puts "No updated gems found in git diff."
else
  gems_to_check.each do |gem_name, versions|
    gem_info = fetch_gem_info(gem_name)

    if gem_info
      changelog_url = gem_info['changelog_uri']
      source_code_url = gem_info['source_code_uri']

      if changelog_url
        fetch_and_display_changelog(gem_name, versions[:old], versions[:new], changelog_url, source_code_url)
      elsif source_code_url && source_code_url.include?('github.com')
        fetch_and_display_changelog(gem_name, versions[:old], versions[:new], "#{source_code_url}/blob/main/CHANGELOG.md", source_code_url)
      else
        puts "#{gem_name}: No changelog or source code URL found on RubyGems.org"
      end
    else
      puts "#{gem_name}: Failed to fetch gem information from RubyGems.org"
    end
  end
end