A script to fill the ruby/ruby ChangeLog with the commit information as a Git commit hook.

changelog.rb

#!/usr/bin/ruby
# cd .git/hooks && mv PATH/TO/THIS_FILE.rb post-commit && chmod +x post-commit
# https://gist.github.com/eregon/3f8182f48b2aa667041e

def error msg
  $stderr.puts msg
  exit 0 # We would not want to stop anything
end

error "Need ruby >= 2.0" unless RUBY_VERSION >= "2.0" and RUBY_ENGINE == 'ruby'

commit = `git log --format=medium --date=raw -1`.lines.map(&:chomp)

error commit[2] unless /^Date:   (?<date>\d+)/ =~ commit[2]
date = Time.at(Integer(date))
time = date.localtime("+09:00").ctime

error commit[1] unless /^Author: (?<author>.+) <(?<email>.+)>$/ =~ commit[1]
me = `git config user.name`.chomp

error "Ignoring commit by #{author}" unless me == author

git_padding = "    "
description = commit[4..-1].map { |line|
  error line unless line.start_with? git_padding
  line[git_padding.size..-1]
}.grep(/^(?:\* |  )/).slice_before(/^\*/).map { |entry|
  entry.map { |line| "\t#{line}" }.join("\n")
}.join("\n\n")

entry = <<-EOS
#{time}  #{author}  <#{email}>

#{description}

EOS

# Prevent recursion
previous_entry = open("ChangeLog") { |file|
  file.each_line.take(entry.lines.count)
}.join

if previous_entry == entry
  exit 0
end

puts "Writing ChangeLog"
contents = entry + `git show HEAD^1:ChangeLog`
File.write("ChangeLog", contents)
system 'git add ChangeLog'
system 'git commit -q --amend -C HEAD'