challenge: Display an overall summary.

* guix/scripts/challenge.scm (summarize-report-list): New procedure.
(guix-challenge): Use it.
* doc/guix.texi (Invoking guix challenge): Adjust command output in
example.

doc/guix.texi

@@ -7276,6 +7276,13 @@ updating list of substitutes from 'https://guix.example.org'... 100.0%
   local hash: 0k4v3m9z1zp8xzzizb7d8kjj72f9172xv078sq4wl73vnq9ig3ax
   https://hydra.gnu.org/nar/@dots{}-pius-2.1.1: 0k4v3m9z1zp8xzzizb7d8kjj72f9172xv078sq4wl73vnq9ig3ax
   https://guix.example.org/nar/@dots{}-pius-2.1.1: 1cy25x1a4fzq5rk0pmvc8xhwyffnqz95h2bpvqsz2mpvlbccy0gs
+
+@dots{}
+
+6,406 store items were analyzed:
+  - 4,749 (74.1%) were identical
+  - 525 (8.2%) differed
+  - 1,132 (17.7%) were inconclusive
 @end smallexample
 
 @noindent

guix/scripts/challenge.scm

@@ -210,6 +210,20 @@ inconclusive reports."
        (report (G_ "~a contents match:~%") item)
        (report-hashes item local narinfos)))))
 
+(define (summarize-report-list reports)
+  "Display the overall summary of REPORTS."
+  (let ((total         (length reports))
+        (inconclusive  (count comparison-report-inconclusive? reports))
+        (matches       (count comparison-report-match? reports))
+        (discrepancies (count comparison-report-mismatch? reports)))
+    (report (G_ "~h store items were analyzed:~%") total)
+    (report (G_ "  - ~h (~,1f%) were identical~%")
+            matches (* 100. (/ matches total)))
+    (report (G_ "  - ~h (~,1f%) differed~%")
+            discrepancies (* 100. (/ discrepancies total)))
+    (report (G_ "  - ~h (~,1f%) were inconclusive~%")
+            inconclusive (* 100. (/ inconclusive total)))))
+
 
 ;;;
 ;;; Command-line options.
@@ -292,6 +306,8 @@ Challenge the substitutes for PACKAGE... provided by one or more servers.\n"))
                                     (reports (compare-contents items urls)))
                  (for-each (cut summarize-report <> #:verbose? verbose?)
                            reports)
+                 (report "\n")
+                 (summarize-report-list reports)
 
                  (exit (cond ((any comparison-report-mismatch? reports) 2)
                              ((every comparison-report-match? reports) 0)