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+;;; Guix --- Nix package management from Guile.         -*- coding: utf-8 -*-
+;;; Copyright (C) 2012 Ludovic Courtès <ludo@gnu.org>
+;;;
+;;; This file is part of Guix.
+;;;
+;;; Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+;;;
+;;; Download a binary file from an external source.
+;;;
+
+(use-modules (ice-9 match)
+             (web uri)
+             (web client)
+             (rnrs io ports)
+             (srfi srfi-11)
+             (guix utils))
+
+(define %url-base
+  "http://www.fdn.fr/~lcourtes/software/guix/packages")
+
+(define (file-name->uri file)
+  "Return the URI for FILE."
+  (match (string-tokenize file (char-set-complement (char-set #\/)))
+    ((_ ... system basename)
+     (string->uri (string-append %url-base "/" system "/" basename)))))
+
+(match (command-line)
+  ((_ file expected-hash)
+   (let ((uri (file-name->uri file)))
+     (format #t "downloading file `~a' from `~a'...~%"
+             file (uri->string uri))
+     (let*-values (((resp data) (http-get uri #:decode-body? #f))
+                   ((hash)      (bytevector->base16-string (sha256 data)))
+                   ((part)      (string-append file ".part")))
+       (if (string=? expected-hash hash)
+           (begin
+             (call-with-output-file part
+               (lambda (port)
+                 (put-bytevector port data)))
+             (rename-file part file))
+           (begin
+             (format (current-error-port)
+                     "file at `~a' has SHA256 ~a; expected ~a~%"
+                     (uri->string uri) hash expected-hash)
+             (exit 1)))))))