blob.rb

Blob = Struct.new(:oid, :content) do
  def type
    Database::BLOB
  end

  def to_s
    content
  end
end

commit.rb

Commit = Struct.new(:oid, :tree, :message) do
  def type
    Database::COMMIT
  end

  def to_s
    ["tree #{ tree.oid }", "", message].join("\n")
  end
end

database.rb

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

require_relative "./blob"
require_relative "./tree"
require_relative "./commit"

class Database
  BLOB   = "blob"
  TREE   = "tree"
  COMMIT = "commit"

  def initialize(pathname)
    @pathname = pathname
  end

  def store(object)
    string  = object.to_s.force_encoding(Encoding::ASCII_8BIT)
    content = "#{ object.type } #{ string.bytesize }\0#{ string }"

    object.oid = Digest::SHA1.hexdigest(content)
    write_file(object.oid, content)
  end

  def write_file(oid, content)
    path = @pathname.join(oid[0..1], oid[2..-1])

    FileUtils.mkdir_p(path.dirname)
    File.write(path, Zlib::Deflate.deflate(content))
  end
end

git.rb

require "fileutils"
require "pathname"

require_relative "./database"
require_relative "./workspace"

work_path = Pathname.new(Dir.getwd)
workspace = Workspace.new(work_path)

git_path = work_path.join(".git")
database = Database.new(git_path.join("objects"))

%w[objects refs].each do |dir|
  FileUtils.mkdir_p(git_path.join(dir))
end

source_files = workspace.list_files

entries = source_files.map do |path|
  data = workspace.read_file(path)
  stat = workspace.stat_file(path)

  blob = Blob.new(nil, data)
  database.store(blob)

  [stat, blob, path]
end

tree = Tree.new(nil, entries)
database.store(tree)

message = "Just enough code to write a commit."
commit = Commit.new(nil, tree, message)
database.store(commit)

File.open(git_path.join("HEAD"), "w") { |file| file.puts(commit.oid) }

tree.rb

Tree = Struct.new(:oid, :entries) do
  def type
    Database::TREE
  end

  def to_s
    entries.sort_by!(&:last)

    lines = entries.map do |stat, blob, path|
      ["100644", path, blob.oid].pack("A7Z*H40")
    end

    lines.join("")
  end
end

workspace.rb

class Workspace
  IGNORE = %w[. .. .git]

  def initialize(pathname)
    @pathname = pathname
  end

  def list_files
    Dir.entries(@pathname) - IGNORE
  end

  def read_file(path)
    File.read(@pathname.join(path))
  end

  def stat_file(path)
    File.stat(@pathname.join(path))
  end
end