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+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2016 David Craven <david@craven.ch>
+;;; Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
+;;; Copyright © 2019 Ivan Petkov <ivanppetkov@gmail.com>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU 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.
+;;;
+;;; GNU 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 GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (guix build cargo-utils)
+  #:use-module (guix build utils)
+  #:use-module (ice-9 popen)
+  #:use-module (ice-9 rdelim)
+  #:export (generate-checksums))
+
+;; Commentary:
+;;
+;; Stand alone utilities for building Rust crates or the compiler itself,
+;; without depending on the entire cargo build-system itself.
+;;
+;; Code:
+
+(define (file-sha256 file-name)
+  "Calculate the hexdigest of the sha256 checksum of FILE-NAME and return it."
+  (let ((port (open-pipe* OPEN_READ
+                          "sha256sum"
+                          "--"
+                          file-name)))
+    (let ((result (read-delimited " " port)))
+      (close-pipe port)
+      result)))
+
+(define (generate-checksums dir-name src-name)
+  "Given DIR-NAME, a store directory, checksum all the files in it one
+by one and put the result into the file \".cargo-checksum.json\" in
+the same directory.  Also includes the checksum of an extra file
+SRC-NAME as if it was part of the directory DIR-NAME with name
+\"package\"."
+  (let* ((file-names (find-files dir-name "."))
+         (dir-prefix-name (string-append dir-name "/"))
+         (dir-prefix-name-len (string-length dir-prefix-name))
+         (checksums-file-name (string-append dir-name "/.cargo-checksum.json")))
+    (call-with-output-file checksums-file-name
+      (lambda (port)
+        (display "{\"files\":{" port)
+        (let ((sep ""))
+          (for-each (lambda (file-name)
+            (let ((file-relative-name (string-drop file-name dir-prefix-name-len)))
+                  (display sep port)
+                  (set! sep ",")
+                  (write file-relative-name port)
+                  (display ":" port)
+                  (write (file-sha256 file-name) port))) file-names))
+        (display "},\"package\":" port)
+        (write (file-sha256 src-name) port)
+        (display "}" port)))))