packages: Add 'package-unique-version-prefix'.

* gnu/packages.scm (package-unique-version-prefix): New procedure.
* guix/scripts/package.scm (manifest-entry-version-prefix): Use it.
* tests/packages.scm ("package-unique-version-prefix, gcc@8")
("package-unique-version-prefix, grep"): New tests.

gnu/packages.scm

@@ -66,6 +66,8 @@
             specification->location
             specifications->manifest
 
+            package-unique-version-prefix
+
             generate-package-cache))
 
 ;;; Commentary:
@@ -559,3 +561,22 @@ output."
   ;; fiddle with multiple-value returns.
   (packages->manifest
    (map (compose list specification->package+output) specs)))
+
+(define (package-unique-version-prefix name version)
+  "Search among all the versions of package NAME that are available, and
+return the shortest unambiguous version prefix to designate VERSION.  If only
+one version of the package is available, return the empty string."
+  (match (map package-version (find-packages-by-name name))
+    ((_)
+     ;; A single version of NAME is available, so do not specify the version
+     ;; number, even if the available version doesn't match VERSION.
+     "")
+    (versions
+     ;; If VERSION is the latest version, don't specify any version.
+     ;; Otherwise return the shortest unique version prefix.  Note that this
+     ;; is based on the currently available packages so the result may vary
+     ;; over time.
+     (if (every (cut version>? version <>)
+                (delete version versions))
+         ""
+         (version-unique-prefix version versions)))))

guix/scripts/package.scm

@@ -334,24 +334,8 @@ Alternately, see @command{guix package --search-paths -p ~s}.")
   "Search among all the versions of ENTRY's package that are available, and
 return the shortest unambiguous version prefix for this package.  If only one
 version of ENTRY's package is available, return the empty string."
-  (let ((name (manifest-entry-name entry)))
-    (match (map package-version (find-packages-by-name name))
-      ((_)
-       ;; A single version of NAME is available, so do not specify the
-       ;; version number, even if the available version doesn't match ENTRY.
-       "")
-      (versions
-       ;; If ENTRY uses the latest version, don't specify any version.
-       ;; Otherwise return the shortest unique version prefix.  Note that
-       ;; this is based on the currently available packages, which could
-       ;; differ from the packages available in the revision that was used
-       ;; to build MANIFEST.
-       (let ((current (manifest-entry-version entry)))
-         (if (every (cut version>? current <>)
-                    (delete current versions))
-             ""
-             (version-unique-prefix (manifest-entry-version entry)
-                                    versions)))))))
+  (package-unique-version-prefix (manifest-entry-name entry)
+                                 (manifest-entry-version entry)))
 
 (define* (export-manifest manifest
                           #:optional (port (current-output-port)))

tests/packages.scm

@@ -1923,6 +1923,19 @@
   (package-location (specification->package "guile@2"))
   (specification->location "guile@2"))
 
+(test-equal "package-unique-version-prefix, gcc@8"
+  "8"
+  (let ((gcc (specification->package "gcc-toolchain@8")))
+    (package-unique-version-prefix (package-name gcc)
+                                   (package-version gcc))))
+
+(test-equal "package-unique-version-prefix, grep"
+  ""
+  (let ((grep (specification->package "grep")))
+    (package-unique-version-prefix (package-name grep)
+                                   (package-version grep))))
+
+
 (test-eq "this-package-input, exists"
   hello
   (package-arguments