edit_message.rb

#!/usr/bin/env ruby

require "digest/sha1"
require "fileutils"
require "zlib"

GIT_DIR = ".git"

Commit = Struct.new(:id, :data) do
  def self.path(id)
    File.join(GIT_DIR, "objects", id[0..1], id[2..-1])
  end

  def self.load(id)
    path = Commit.path(id)
    blob = Zlib::Inflate.inflate(File.read(path))
    data = blob.split("\0", 2)[1]

    new(id, data)
  end

  def self.store(data)
    blob = "commit #{ data.bytesize }\0#{ data }"
    id   = Digest::SHA1.hexdigest(blob)
    path = Commit.path(id)

    unless File.readable?(path)
      FileUtils.mkdir_p(File.dirname(path))
      File.open(path, "w", 0444) do |f|
        f.write Zlib::Deflate.deflate(blob, Zlib::BEST_SPEED)
      end
    end

    new(id, data)
  end

  def parent
    parent_id = data.lines.grep(/^parent /).first.split(/\s+/)[1]
    Commit.load(parent_id)
  end
end

target_id = ARGV.first

head_name = "HEAD"
head_id   = nil

until head_id
  head_data = File.read(File.join(GIT_DIR, head_name)).strip
  case head_data
  when /^ref: (.*)$/
    head_name = $1
  when /^[0-9a-f]{40}$/
    head_id = head_data
  end
end

commit = Commit.load(head_id)
chain  = []

until commit.id == target_id
  chain.unshift(commit)
  commit = commit.parent
end

msg_path = File.join(GIT_DIR, "COMMIT_EDITMSG")
File.open(msg_path, "w") { |f| f.write(commit.data) }
exit unless system("vim", msg_path)

new_data   = File.read(msg_path)
new_commit = Commit.store(new_data)

exit if new_commit.id == commit.id

new_head = chain.inject(new_commit) do |parent, cmt|
  data = cmt.data.gsub(/^parent.*$/, "parent #{ parent.id }")
  Commit.store(data)
end

File.open(File.join(GIT_DIR, head_name), "w") do |f|
  f.puts new_head.id
end

old_ids = (chain + [commit]).map(&:id)

old_ids.each do |id|
  path = Commit.path(id)
  dir  = File.dirname(path)

  File.unlink(path)
  begin
    Dir.rmdir(dir)
  rescue Errno::ENOTEMPTY
  end

  puts "dropped: #{ path }"
end

puts "HEAD is now: #{ new_head.id }"