boot.rb

$__load_path_size = nil
$__cached_load_paths = {}
$__cache_scanned = {}

PATH_START_SLASH = "/"
PATH_START_PERIOD = "."
PATH_START_TILDE = "~"

::Kernel.class_eval do
  alias __gem_original_require gem_original_require

  def bundler_cached_require(requirement)
    r0 = requirement[0]
    # This seems to be the fastest construct I've found for testing if the path starts with any of those three characters.
    r = if r0 == PATH_START_SLASH or r0 == PATH_START_PERIOD or r0 == PATH_START_TILDE
      requirement
    else
      # If we have a cached lookup, use it. Otherwise, let Kernel#require do its thing.
      (build_require_paths[requirement] || requirement)
    end
    __gem_original_require r
  end

  def build_require_paths
    h = $:.length
    if $__load_path_size != h
      # If we've removed paths, we'll have to wipe and rebuild the cache
      # This isn't foolproof; you could remove paths but then add more between requires to get you back
      # into a positive difference. It's much faster than array diffs, though.
      if $__load_path_size and h < $__load_path_size
        $__cached_load_paths = {}
        $__cache_scanned = {}
      end
      $__load_path_size = h
      $:.each do |path|
        next if $__cache_scanned[path]   # No need to reglob a path we've already scanned.
        $__cache_scanned[path] = true
        cache_load_paths Dir.glob(File.join(path, '**', '*.{rb,so}')), path   # require 'foo' can load foo.so or foo.rb
      end
    end
    $__cached_load_paths
  end

  def cache_load_paths(files, chomp)
    files.each do |file|
      # for /path/to/foo.rb with a load prefix of /path, cache to/foo and to/foo.rb
      key = file[chomp.length + 1, file.length - chomp.length - 4]       
      key_ext = file[chomp.length + 1, file.length - chomp.length - 1]
      $__cached_load_paths[key] ||= file
      $__cached_load_paths[key_ext] ||= file
    end
  end

  alias require bundler_cached_require

  # Undef gem_original_require as bundler tries to restore it if it finds it.
  undef gem_original_require
end