homebrew-cask-versions/lib/cask/download.rb

require 'digest'

class Cask::Download
  attr_reader :cask

  def initialize(cask)
    @cask = cask
  end

  def perform(force=false)
    if cask.url.using == :svn
      downloader = Cask::SubversionDownloadStrategy.new(cask)
    elsif cask.url.using == :post
      downloader = Cask::CurlPostDownloadStrategy.new(cask)
    else
      downloader = Cask::CurlDownloadStrategy.new(cask)
    end
    downloader.clear_cache if force
    begin
      downloaded_path = downloader.fetch
    rescue StandardError => e
      raise CaskError.new("Download failed on Cask '#{cask}' with message: #{e}")
    end
    begin
      # this symlink helps track which downloads are ours
      File.symlink downloaded_path,
                   HOMEBREW_CACHE_CASKS.join(downloaded_path.basename)
    rescue StandardError => e
    end
    _check_sums(downloaded_path, cask.sums) unless cask.sums === :no_check
    downloaded_path
  end

  private
  def _check_sums(path, sums)
    has_sum = false
    sums.each do |sum|
      unless sum.empty?
        computed = Checksum.new(sum.hash_type, Digest.const_get(sum.hash_type.to_s.upcase).file(path).hexdigest)
        if sum == computed
          odebug "Checksums match"
        else
          ohai 'Note: running "brew update" may fix checksum errors'
          raise ChecksumMismatchError.new(path, sum, computed)
        end
        has_sum = true
      end
    end
    raise ChecksumMissingError.new("Checksum required: sha256 '#{Digest::SHA256.file(path).hexdigest}'") unless has_sum
  end
end