override `Kernel.require` to track loaded files, and allow reloading

reloader.rb

require 'pp'

class Reloader

  def initialize
    # normalized path => mtime
    @mtimecache = Hash.new

    # normalized path => load path
    # this will only lookup once, storing _nil_ in the cache if not found
    @pathcache = Hash.new do |cache, path|
      cache[path] = search_load_path(path)
    end
  end

  def inspect
    "<Reloader: #{@mtimecache.count} entries>"
  end

  # @see Kernel.require
  def track(path)
    if (path = @pathcache[path])
      @mtimecache[path] = File.mtime(path)
    end
  end

  def reload(*paths, force: false)
    # with no given paths, just use everything we've tracked
    paths = @mtimecache.keys if paths.empty?
    paths.each do |path|
      # for each path, normalize from the load path
      if (path = @pathcache[path])
        # and if we're either forcing the reload or it's been changed
        if force or (File.mtime(path) > @mtimecache[path])
          # delete from the list of loaded modules, and rerequire
          $".delete path
          require path
        end
      end
    end
  end

  # dump the current autoreload state
  def dump
    puts "path => mtime:"
    pp @mtimecache

    puts "require => path:"
    pp @pathcache

    return nil
  end

  # reset the path cache, in the case where we grabbed the wrong thing
  def reset!
    @pathcache.clear
  end

  private

  # search the load path for the given requireable path
  def search_load_path(path)
    # if the path is absolute, it's already found, just look for extensions
    if path =~ /^\//
      options = Dir.glob("#{path}{.rb,.so,}")

    # if the path is relative, we're globbing for from the cwd
    elsif path =~ /^\.\.?\// # relative path
      dirpath = File.expand_path(path)
      options = Dir.glob("#{dirpath}{.rb,.so,}")

    # otherwise it's a bare word, so it must be on the load path
    else
      options = $LOAD_PATH.lazy.flat_map do |dir|
        dirpath = File.join(dir, path)
        Dir.glob("#{dirpath}{.rb,.so,}")
      end
    end

    # return the first thing we found that's actually a file
    options.select { |o| File.file?(o) }.first
  end

end

# monkeypatch the kernel
module Kernel
  alias original_require require

  def reloader
    @reloader ||= Reloader.new
  end

  def require(path)
    reloader.track(path)
    original_require(path)
  end

  def reload!
    reloader.reload
    return
  end
end