When writing package definition, what if I download sources from git
repository? For example:

(define-public libwebsockets
  (package
    (name "libwebsockets")
    (version "1.3")
    (source (origin
              ;; The project does not publish tarballs, so we have to take
              ;; things from Git.
              (method git-fetch)
              (uri (git-reference
                    (url "git://git.libwebsockets.org/libwebsockets")
                    (commit (string-append "v" version
                                           "-chrome37-firefox30"))))
              (sha256
               (base32
                "12fqh2d2098mgf0ls19p9lzibpsqhv7mc5rn1yvrbfnazmcr40g4"))
              (file-name (string-append name "-" version))))
...

How do I calculate this 12fqh2d...?

Dmitry Nikolaev