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+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2015 David Thompson <davet@gnu.org>
+;;;
+;;; 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 import gem)
+  #:use-module (ice-9 match)
+  #:use-module (ice-9 pretty-print)
+  #:use-module (rnrs bytevectors)
+  #:use-module (json)
+  #:use-module (web uri)
+  #:use-module (guix import utils)
+  #:use-module (guix import json)
+  #:use-module (guix packages)
+  #:use-module (guix licenses)
+  #:use-module (guix base32)
+  #:export (gem->guix-package))
+
+(define (rubygems-fetch name)
+  "Return an alist representation of the RubyGems metadata for the package NAME,
+or #f on failure."
+  (json-fetch
+   (string-append "https://rubygems.org/api/v1/gems/" name ".json")))
+
+(define (ruby-package-name name)
+  "Given the NAME of a package on RubyGems, return a Guix-compliant name for
+the package."
+  (if (string-prefix? "ruby-" name)
+      (snake-case name)
+      (string-append "ruby-" (snake-case name))))
+
+(define (hex-string->bytevector str)
+  "Convert the hexadecimal encoded string STR to a bytevector."
+  (define hex-char->int
+    (match-lambda
+     (#\0 0)
+     (#\1 1)
+     (#\2 2)
+     (#\3 3)
+     (#\4 4)
+     (#\5 5)
+     (#\6 6)
+     (#\7 7)
+     (#\8 8)
+     (#\9 9)
+     (#\a 10)
+     (#\b 11)
+     (#\c 12)
+     (#\d 13)
+     (#\e 14)
+     (#\f 15)))
+
+  (define (read-byte i)
+    (let ((j (* 2 i)))
+      (+ (hex-char->int (string-ref str (1+ j)))
+         (* (hex-char->int (string-ref str j)) 16))))
+
+  (let* ((len (/ (string-length str) 2))
+         (bv  (make-bytevector len)))
+    (let loop ((i 0))
+      (if (= i len)
+          bv
+          (begin
+            (bytevector-u8-set! bv i (read-byte i))
+            (loop (1+ i)))))))
+
+(define (make-gem-sexp name version hash home-page description
+                       dependencies licenses)
+  "Return the `package' s-expression for a Ruby package with the given NAME,
+VERSION, HASH, HOME-PAGE, DESCRIPTION, DEPENDENCIES, and LICENSES."
+  `(package
+     (name ,(ruby-package-name name))
+     (version ,version)
+     (source (origin
+               (method url-fetch)
+               (uri (rubygems-uri ,name version))
+               (sha256
+                (base32
+                 ,(bytevector->nix-base32-string
+                   (hex-string->bytevector hash))))))
+     (build-system ruby-build-system)
+     ,@(if (null? dependencies)
+           '()
+           `((propagated-inputs
+              (,'quasiquote
+               ,(map (lambda (name)
+                       `(,name
+                         (,'unquote
+                          ,(string->symbol name))))
+                     dependencies)))))
+     (synopsis ,description)    ; nothing better to use
+     (description ,description)
+     (home-page ,home-page)
+     (license ,(match licenses
+                 ((license) (license->symbol license))
+                 (_ (map license->symbol licenses))))))
+
+(define* (gem->guix-package package-name #:optional version)
+  "Fetch the metadata for PACKAGE-NAME from rubygems.org, and return the
+`package' s-expression corresponding to that package, or #f on failure."
+  (let ((package (rubygems-fetch package-name)))
+    (and package
+         (let ((name         (assoc-ref package "name"))
+               (version      (assoc-ref package "version"))
+               (hash         (assoc-ref package "sha"))
+               (description  (assoc-ref package "info"))
+               (home-page    (assoc-ref package "homepage_uri"))
+               (dependencies (map (lambda (dep)
+                                    (let ((name (assoc-ref dep "name")))
+                                      (if (string=? name "bundler")
+                                          "bundler" ; special case, no prefix
+                                          (ruby-package-name name))))
+                                  (assoc-ref* package "dependencies"
+                                              "runtime")))
+               (licenses     (map string->license
+                                  (assoc-ref package "licenses"))))
+           (make-gem-sexp name version hash home-page
+                          description dependencies licenses)))))