#!/usr/bin/guile \
--no-auto-compile -e main -s

Broken Link Checker
Copyright (C) 2025 Ray Miller <ray@1729.org.uk>.

This program 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.

This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.
!#
(use-modules
 (ice-9 format)
 (ice-9 match)
 (web client)
 (web response)
 (web uri)
 ((web http) #:select (parse-header))
 ((htmlprag) #:select (html->sxml))
 ((sxml xpath) #:select (sxpath))
 (srfi srfi-1)
 (srfi srfi-9 gnu)
 (srfi srfi-26)
 (srfi srfi-37)
 (srfi srfi-71))

;; Queue of work to do, which is stored as pairs of (referrer . uri) so we
;; can report the referring page if `uri` is invalid
(define queue (list))

;; Hash of URLs already visited
(define seen (make-hash-table))

;; A count of invalid URLs we've seen.
(define error-count 0)

;; Whether or not we should act like a crawler and recurse into all pages
;; reachable from the start page. This will be set by a command-line
;; option.
(define recurse? #f)

;; A list of domains to exclude from checking (for example, sites we know are valid
;; but that return errors to crawlers). Populated by command-line options.
(define excluded-domains (list))

;; Similar to excluded domains, but the entire URL must match, not just the host
;; part.
(define excluded-urls (list))

;; Helper functions

(define (seen? uri)
  "Return #t if this URI has been visited, otherwise #f."
  (hash-ref seen (uri->string uri) #f))

(define (set-seen! uri)
  "Add the given URI to the visited set."
  (hash-set! seen (uri->string uri) #t))

(define (exclude-domain! domain-name)
  "Add a domain to the set of excluded domains."
  (set! excluded-domains (lset-adjoin string=? excluded-domains domain-name)))

(define (exclude-url! url)
  "Add a URL to the set of excluded URLs."
  (set! excluded-urls (lset-adjoin string=? excluded-urls url)))

(define (excluded? uri)
  "Check whether or not the URI is an excluded URL or for an excluded domain."
  (or (member (uri->string uri) excluded-urls string=?)
      (any (cute string-suffix? <> (uri-host uri))
           excluded-domains)))

(define (html-content-type? res)
  "Return #t if the response content type is HTML-like, otherwise #f."
  (let ((ct (first (response-content-type res))))
    (or (equal? ct 'text/html)
        (equal? ct 'application/xhtml+xml)
        (equal? ct 'application/xml))))

(define (ok-response? res)
  "Return #t if the HTTP response code is 200 (OK), otherwise #f."
  (= 200 (response-code res)))


;; List of host names that are forcibly mapped to the base URI.
;; This is a work-around for when running Hugo in development; the
;; site is served on http://127.0.0.1:1313/ but some URLs in the
;; delivered pages are for http://localhost:1313/ and some URLs in the
;; content have hard-coded the live hostname - we want to rewrite
;; both of these to http://127.0.0.1:1313/ to test the development
;; version of the site.
;;
;; The actual overrides are set through command-line arguments rather than
;; being hard-coded here.

(define host-overrides
  '())

(define (add-host-override! host-name)
  (set! host-overrides (lset-adjoin equal? host-overrides host-name)))

(define (override-host-port base-uri uri)
  (if (member (uri-host uri) host-overrides)
      (set-fields uri
                  ((uri-scheme) (uri-scheme base-uri))
                  ((uri-host) (uri-host base-uri))
                  ((uri-port) (uri-port base-uri)))
      uri))

;; List of SXPath expressions to search for links.
(define link-paths (map sxpath
                        '((// a @ href)
                          (// img @ src)
                          (// link @ href)
                          (// video @ src)
                          (// video source @ src)
                          (// audio @ src)
                          (// audio source @ src)
                          (// picture source @ srcset)
                          (// img @ srcset))))

;; Special handling for srcset
(define (parse-srcset s)
  "Parse out the list of URLs from an image srcset, discarding whitespace and the
width/density descriptor."
  (map (compose first (cut string-split <> #\space))
       (map string-trim-both (string-split s #\,))))

(define (extract-links body)
  "Extract links from an HTML document, discarding any tel: or mailto: links."
  (let ((x (html->sxml body)))
    (remove (lambda (s) (or (string-prefix? "tel:" s) (string-prefix? "mailto:" s)))
            (delete-duplicates
             (fold (lambda (f accum)
                     (fold (lambda (x accum)
                             (match-let (((attr val) x))
                               (if (equal? attr 'srcset)
                                   (append (parse-srcset val) accum)
                                   (cons val accum))))
                           accum
                           (f x)))
                   '()
                   link-paths)))))

(define (uri-without-fragment uri)
  "If uri has a fragment, return a new URI without the fragment."
  (if (uri-fragment uri)
      (set-field uri (uri-fragment) #f)
      uri))

(define (resolve-relative-uri base-uri s)
  "If the string s is a relative URI, resolve it relative to base-uri."
  (let ((uri (string->uri-reference s)))
    (if (uri-scheme uri)
        uri
        (build-uri (uri-scheme base-uri)
                   #:userinfo (or (uri-userinfo uri) (uri-userinfo base-uri))
                   #:host (or (uri-host uri) (uri-host base-uri))
                   #:port (or (uri-port uri) (uri-port base-uri))
                   #:path (cond
                           ((string-prefix? "/" (uri-path uri)) (uri-path uri))
                           ((string-suffix? "/" (uri-path base-uri)) (string-append (uri-path base-uri) (uri-path uri)))
                           (else (string-append (uri-path base-uri) "/" (uri-path uri))))
                   #:query (uri-query uri)))))

(define (enqueue-links base-uri)
  (unless (seen? base-uri)
    (hash-set! seen (uri->string base-uri) #t)
    (format #t "[INFO] enqueuing links for ~a~%" (uri->string base-uri))
    (let ((res body (http-get base-uri)))
      (if (ok-response? res)
          (if (html-content-type? res)
              (set! queue (append queue
                                  (map (compose (cut cons base-uri <>)
                                                (cut override-host-port base-uri <>)
                                                uri-without-fragment
                                                (cut resolve-relative-uri base-uri <>))
                                       (extract-links body))))
              (format #t "[ERROR] unexpected content type ~a for ~a~%" (response-content-type res) (uri->string base-uri)))
          (format #t "[ERROR] unexpected status code ~a for ~a~%" (response-code res) (uri->string base-uri))))))

(define (same-host? uri referrer)
  "Return #t if URI has the same host name as REFERRER, otherwise #f."
  (string=? (uri-host uri) (uri-host referrer)))

;; Some sites return 403 errors for bot requests, these headers make
;; us look more like a real browser.
(define request-headers
  (map (match-lambda ((k . v) (cons k (parse-header k v))))
       '((accept . "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8")
         (accept-encoding . "gzip, deflate, br, zstd")
         (accept-language . "en-GB,en;q=0.5")
         (user-agent . "Mozilla/5.0 (X11; Linux x86_64; rv:133.0) Gecko/20100101 Firefox/133.0"))))

;; A memoized version of http-get. This allows us to quickly check a URL we have
;; seen before (when it is linked from multiple different pages) without generating
;; extra load on the target server.
(define http-head-memoized
  (let ((memo (make-hash-table)))
    (lambda (uri)
      (let ((key (uri->string uri)))
        (unless (hash-ref memo key #f)
          (format #t "[DEBUG] checking link ~a~%" key)
          (with-exception-handler
              (lambda (e)
                (hash-set! memo key e))
            (lambda ()
              (let ((res _ (http-head uri #:headers request-headers)))
                (hash-set! memo key res)))
            #:unwind? #t))
        (hash-ref memo key)))))

(define (process-queue)
  (match queue
    (()
     (format #t "[INFO] processing complete, found ~a broken link~p~%" error-count error-count))
    (((referrer . uri) rest ...)
     (set! queue rest)
     (if (excluded? uri)
         (format #t "[INFO] skipping excluded URL ~a~%" (uri->string uri))
         (let ((res (http-head-memoized uri)))
           (cond
            ((exception? res)
             (format #t "[ERROR] HEAD ~a referred to by ~a threw exception ~a~%" (uri->string uri) (uri->string referrer) res)
             (set! error-count (1+ error-count)))
            ((> (response-code res) 399)
             (format #t "[ERROR] HEAD ~a referred to by ~a returned HTTP Status ~a~%" (uri->string uri) (uri->string referrer) (response-code res))
             (set! error-count (1+ error-count)))
            ((and recurse? (ok-response? res) (html-content-type? res) (same-host? uri referrer))
             (enqueue-links uri)))))
     (process-queue))))

(define (show-help)
  (format #t "Usage: broken-link-checker [OPTIONS] URL

Run broken link checker on URL.

 -h, --help                    Show this help message

 -X, --exclude-domain=DOMAIN   Skip checking links to this domain

 -x, --exclude-url=URL         Skip checking this specific link

 -o, --override-host=HOSTNAME  Rewrite links to HOSTNAME to the base URL

 -r, --recurse                 Check all pages on the same host that are reachable
                               from the starting URL.
"))

(define %default-options
  '((exclude-domains . ())
    (exclude-urls . ())
    (host-overrides . ())
    (recurse . #f)
    (urls . ())))

(define %options
  (list
   (option '(#\h "help") #f #f
           (lambda args
             (show-help)
             (exit 0)))
   (option '(#\X "exclude-domain") #t #f
           (lambda (opt name val result)
             (assoc-set! result 'exclude-domains
                         (cons val (assoc-ref result 'exclude-domains)))))
   (option '(#\x "exclude-url") #t #f
           (lambda (opt name val result)
             (assoc-set! result 'exclude-urls
                         (cons val (assoc-ref result 'exclude-urls)))))
   (option '(#\o "override-host") #t #f
           (lambda (opt name val result)
             (assoc-set! result 'host-overrides
                         (cons val (assoc-ref result 'host-overrides)))))
   (option '(#\r "recurse") #f #f
           (lambda (opt name val result)
             (assoc-set! result 'recurse #t)))))

(define (parse-args args)
  (args-fold args %options
             (lambda (opt name arg result)
               (format (current-error-port) "~A: unrecognized option~%" opt)
               (exit 1))
             (lambda (url result)
               (assoc-set! result 'urls (cons url (assoc-ref result 'urls))))
             %default-options))

(define (main args)
  (let ((args (parse-args (cdr args))))
    (set! recurse? (assoc-ref args 'recurse))
    (for-each exclude-domain! (assoc-ref args 'exclude-domains))
    (for-each exclude-url! (assoc-ref args 'exclude-urls))
    (for-each add-host-override! (assoc-ref args 'host-overrides))
    (for-each (compose enqueue-links string->uri) (assoc-ref args 'urls))
    (process-queue)
    (if (> error-count 0)
        (exit 2)
        (exit 0))))