;;; fetch_page.el --- Web fetch tool for gptel (HTML/text/links) -*- lexical-binding: t; -*-

;; Copyright (C) 2025  cjennings
;; Author: cjennings and AI assistant

;;; Commentary:
;;
;; fetch_page: In-memory web fetch-and-parse tool for gptel tools.
;;
;; Features
;; - Fetch a URL (http/https), follow redirects.
;; - Return one of: rendered text (DOM+shr), raw HTML, or extracted links.
;; - Size/time guards: max_bytes, max_chars, timeout.
;; - User-Agent presets; Accept/Accept-Language overrides.
;; - Predictable, LLM-friendly outputs and explicit error suggestions.
;;
;; Notes
;; - This tool uses url.el (asynchronous url-retrieve) and libxml/shr for HTML.
;; - For non-text content (e.g., PDFs), it returns an explicit error and suggests
;;   using a download tool (e.g., download_url) instead.
;;
;; Installation
;; - Load after gptel and registers a gptel tool named "fetch_page" (category "web").
;;
;;; Code:

(require 'url)
(require 'url-http) ;; for url-http-end-of-headers, response status, etc.
(require 'subr-x)
(eval-when-compile (require 'cl-lib))
(require 'dom)
(require 'shr)

;; Internal helpers are prefixed gptel-fetch-page--

(defconst gptel-fetch-page--default-max-bytes (* 16 1024 1024))
(defconst gptel-fetch-page--max-allowed-bytes (* 32 1024 1024))
(defconst gptel-fetch-page--default-max-chars 300000)
(defconst gptel-fetch-page--default-timeout 30)

(defconst gptel-fetch-page--ua-presets
  '(("desktop-chrome"  . "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36")
	("desktop-firefox" . "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:121.0) Gecko/20100101 Firefox/121.0")
	("desktop-safari"  . "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.0 Safari/605.1.15")
	("edge-windows"    . "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36 Edg/120.0.0.0")
	("mobile-iphone"   . "Mozilla/5.0 (iPhone; CPU iPhone OS 17_0 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.0 Mobile/15E148 Safari/604.1")
	("mobile-android"  . "Mozilla/5.0 (Linux; Android 13; Pixel 6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Mobile Safari/537.36")
	("emacs-default"   . "Emacs gptel/fetch_page (+https://github.com/karthink/gptel)") )
  "Preset User-Agent strings.")

(defun gptel-fetch-page--coerce-bool (v)
  (cond
   ((eq v :json-false) nil)
   ((stringp v) (not (string-blank-p v)))
   (t (and v t))))

(defun gptel-fetch-page--coerce-int (v &optional default min max)
  (let ((n (cond
			((numberp v) v)
			((stringp v) (string-to-number v))
			(t nil))))
	(setq n (or n default))
	(when (and min n (< n min)) (setq n min))
	(when (and max n (> n max)) (setq n max))
	n))

(defun gptel-fetch-page--ua (ua)
  "Return a User-Agent string from UA preset or use UA directly."
  (cond
   ((null ua) (cdr (assoc "desktop-chrome" gptel-fetch-page--ua-presets)))
   ((stringp ua)
	(or (cdr (assoc ua gptel-fetch-page--ua-presets)) ua))
   (t (cdr (assoc "desktop-chrome" gptel-fetch-page--ua-presets)))))

(defun gptel-fetch-page--ok-scheme-p (url)
  (let ((u (ignore-errors (url-generic-parse-url url))))
	(and u (member (url-type u) '("http" "https")))))

(defun gptel-fetch-page--parse-headers ()
  "Parse headers in current buffer up to `url-http-end-of-headers'."
  (let (hdrs)
	(save-excursion
	  (goto-char (point-min))
	  (while (re-search-forward "^\\([^:]+\\):[ \t]*\\(.*\\)$" url-http-end-of-headers t)
		(push (cons (downcase (match-string 1)) (string-trim (match-string 2))) hdrs)))
	(nreverse hdrs)))

(defun gptel-fetch-page--content-type (headers)
  "Return (mime charset) from HEADERS alist."
  (let* ((ct (cdr (assoc "content-type" headers)))
		 (mime (and ct (car (split-string ct ";" t "[ \t]*"))))
		 (charset (and ct (when (string-match "charset=\\([^;]+\\)" ct)
							(downcase (match-string 1 ct))))))
	(list (or mime "unknown") charset)))

(defun gptel-fetch-page--header-bytes (headers)
  "Extract and return the Content-Length from HEADERS as a number.

HEADERS is an alist representing HTTP headers. This function looks
up the 'Content-Length' header, safely converting its value to a number,
or returning nil if the header is absent or conversion fails."
  (let ((cl (cdr (assoc "content-length" headers))))
	(and cl (ignore-errors (string-to-number cl)))))

(defun gptel-fetch-page--region-string (beg end)
  (save-excursion
	(goto-char beg)
	(buffer-substring-no-properties beg end)))

(defun gptel-fetch-page--html-to-dom (html)
  (with-temp-buffer
	(insert html)
	(condition-case err
		(if (fboundp 'libxml-parse-html-region)
			(libxml-parse-html-region (point-min) (point-max))
		  (signal 'error '(libxml-missing)))
	  (error (signal (car err) (cdr err))))))

(defun gptel-fetch-page--dom-to-text (dom)
  (with-temp-buffer
	;; Use shr to render; keep output lean and deterministic.
	(let ((shr-use-fonts nil)
		  (shr-inhibit-images t)
		  (shr-width (min 120 (window-body-width))))
	  (shr-insert-document dom))
	(buffer-string)))

(defun gptel-fetch-page--dom-base (dom final-url)
  (or (when-let* ((node (car (dom-by-tag dom 'base)))
				  (href (dom-attr node 'href))) href)
	  final-url))

(defun gptel-fetch-page--resolve (href base)
  (condition-case _
	  (if (and href (not (string-empty-p href)))
		  (url-expand-file-name href base)
		nil)
	(error nil)))

(defun gptel-fetch-page--collect-links (dom final-url include-titles)
  (let* ((base (gptel-fetch-page--dom-base dom final-url))
		 (seen (make-hash-table :test 'equal))
		 (out '()))
	(cl-labels ((push-link (u ttitle)
				  (when (and u (not (gethash u seen)))
					(puthash u t seen)
					(push (if include-titles
							  (format "%s\t%s" u (or (and ttitle (string-trim ttitle)) ""))
							u)
						  out))))
	  ;; <a> and <area>
	  (dolist (tag '(a area))
		(dolist (n (dom-by-tag dom tag))
		  (let ((href (dom-attr n 'href)))
			(push-link (gptel-fetch-page--resolve href base)
					   (ignore-errors (string-trim (dom-texts n)))))))
	  ;; <link rel="stylesheet"> included by request
	  (dolist (n (dom-by-tag dom 'link))
		(let* ((rel (downcase (or (dom-attr n 'rel) "")))
			   (is-style (string-match-p "stylesheet" rel))
			   (href (dom-attr n 'href)))
		  (when is-style
			(push-link (gptel-fetch-page--resolve href base)
					   (or (dom-attr n 'title) rel))))))
	(mapconcat #'identity (nreverse out) "\n")))

(defun gptel-fetch-page--truncate (s max-chars)
  (if (and max-chars (> (length s) max-chars))
	  (concat (substring s 0 max-chars)
			  "\nnote: output truncated at " (number-to-string max-chars) " chars")
	s))

(defun gptel-fetch-page--format-error (&rest kv)
  (let ((summary (plist-get kv :summary))
		(status  (or (plist-get kv :status) "unknown"))
		(url     (or (plist-get kv :url) "unknown"))
		(type    (or (plist-get kv :type) "unknown"))
		(limit   (plist-get kv :limit))
		(suggest (plist-get kv :suggest)))
	(string-join
	 (delq nil
		   (list (format "ERROR: %s" summary)
				 (format "details: status=%s url=%s type=%s" status url type)
				 (and limit (format "limit: %s" limit))
				 (and suggest (format "suggest: %s" suggest))))
	 "\n")))

(defun gptel-fetch-page--format-header (status final-url type size)
  (format "[%s] final-url: %s | type: %s | size: %s B"
		  status final-url type (or size "unknown")))

;; ----------------------------
;; Refactored orchestrator bits
;; ----------------------------

(defun gptel-fetch-page--normalize-args (url out-format timeout max-bytes max-chars include-titles
											 user-agent accept accept-language debug)
  "Coerce and validate arguments into a request plist."
  (let* ((out-format (or out-format "text"))
		 (timeout (gptel-fetch-page--coerce-int timeout gptel-fetch-page--default-timeout 1 nil))
		 (max-bytes (gptel-fetch-page--coerce-int max-bytes gptel-fetch-page--default-max-bytes
												  1 gptel-fetch-page--max-allowed-bytes))
		 (max-chars (gptel-fetch-page--coerce-int max-chars gptel-fetch-page--default-max-chars 1000 nil))
		 (include-titles (gptel-fetch-page--coerce-bool include-titles))
		 (debug (gptel-fetch-page--coerce-bool debug))
		 (ua (gptel-fetch-page--ua user-agent))
		 (accept (or accept "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8"))
		 (accept-language (or accept-language "en-US,en;q=0.9")))
	(list :url url
		  :out-format out-format
		  :timeout timeout
		  :max-bytes max-bytes
		  :max-chars max-chars
		  :include-titles include-titles
		  :debug debug
		  :ua ua
		  :accept accept
		  :accept-language accept-language)))

(defun gptel-fetch-page--build-request-headers (req)
  "Build url.el headers alist from REQ."
  (let ((max-bytes (plist-get req :max-bytes)))
	`(("User-Agent" . ,(plist-get req :ua))
	  ("Accept" . ,(plist-get req :accept))
	  ("Accept-Language" . ,(plist-get req :accept-language))
	  ,@(and max-bytes (list (cons "Range" (format "bytes=0-%d" (1- max-bytes))))))))

(defun gptel-fetch-page--start-timeout (req callback done-cell)
  "Start a timer based on REQ's :timeout that calls CALLBACK with an error.
DONE-CELL is a cons cell used as shared mutable state; its car is set non-nil once done."
  (let ((timeout (plist-get req :timeout))
		(orig-url (plist-get req :url)))
	(run-at-time timeout nil
				 (lambda ()
				   (unless (car done-cell)
					 (setcar done-cell t)
					 (funcall callback
							  (gptel-fetch-page--format-error
							   :summary (format "Request timed out after %ss" timeout)
							   :url orig-url
							   :suggest "retry with a larger timeout or probe connectivity")))))))

(defun gptel-fetch-page--parse-response-buffer ()
  "Parse the current url-retrieve buffer into a response plist."
  (save-excursion
	(goto-char (point-min))
	(let* ((status-line (buffer-substring (line-beginning-position) (line-end-position)))
		   (http-status
			(or (and (boundp 'url-http-response-status) url-http-response-status)
				(and (string-match "HTTP/[0-9.]+ +\\([0-9]+\\)" status-line)
					 (string-to-number (match-string 1 status-line)))))
		   (headers (gptel-fetch-page--parse-headers))
		   (final-url (or (and (boundp 'url-current-object)
							   (ignore-errors (url-recreate-url url-current-object)))
						  "unknown"))
		   (ctype (gptel-fetch-page--content-type headers))
		   (mime (car ctype))
		   (charset (cadr ctype))
		   (declared-bytes (gptel-fetch-page--header-bytes headers))
		   (body (gptel-fetch-page--region-string url-http-end-of-headers (point-max)))
		   (actual-bytes (string-bytes body)))
	  (list :status http-status
			:status-line status-line
			:headers headers
			:final-url final-url
			:mime mime
			:charset charset
			:declared-bytes declared-bytes
			:body body
			:actual-bytes actual-bytes))))

(defun gptel-fetch-page--enforce-limits (resp req)
  "Check response sizes and return either (:ok t RESP) or (:error STRING)."
  (let* ((http-status (plist-get resp :status))
		 (final-url (plist-get resp :final-url))
		 (mime (plist-get resp :mime))
		 (declared (plist-get resp :declared-bytes))
		 (actual (plist-get resp :actual-bytes))
		 (max-bytes (plist-get req :max-bytes))
		 (max-chars (plist-get req :max-chars))
		 (timeout (plist-get req :timeout)))
	(cond
	 ((and declared max-bytes (> declared max-bytes))
	  (list :error (gptel-fetch-page--format-error
					:summary (format "Response exceeds max_bytes (%d bytes) for in-memory fetch" max-bytes)
					:status http-status
					:url final-url
					:type mime
					:limit (format "max_bytes=%d max_chars=%d timeout=%ss" max-bytes max-chars timeout)
					:suggest (format "call download_url to save the file, or retry fetch_page with a larger max_bytes (up to %d)"
									 gptel-fetch-page--max-allowed-bytes)))))
	 ((and max-bytes (> actual max-bytes))
	  (list :error (gptel-fetch-page--format-error
					:summary (format "Response exceeds max_bytes (%d bytes) during retrieval" max-bytes)
					:status http-status
					:url final-url
					:type mime
					:limit (format "max_bytes=%d max_chars=%d timeout=%ss" max-bytes max-chars timeout)
					:suggest (format "call download_url to save the file, or retry fetch_page with a larger max_bytes (up to %d)"
									 gptel-fetch-page--max-allowed-bytes)))))
	 (t (list :ok t resp)))))

(defun gptel-fetch-page--maybe-decode-body (body charset)
  "Decode BODY using CHARSET if recognized; otherwise return BODY unchanged."
  (let ((cs (and charset (ignore-errors (coding-system-from-name charset)))))
	(if (and cs (coding-system-p cs))
		(condition-case _
			(decode-coding-string body cs)
		  (error body))
	  body)))

(defun gptel-fetch-page--render (resp req)
  "Render the response according to request preferences.
Returns either (:ok t STRING) or (:error STRING)."
  (let* ((http-status (plist-get resp :status))
		 (final-url (plist-get resp :final-url))
		 (mime (plist-get resp :mime))
		 (charset (plist-get resp :charset))
		 (raw-body (plist-get resp :body))
		 (body (gptel-fetch-page--maybe-decode-body raw-body charset))
		 (out-format (plist-get req :out-format))
		 (max-chars (plist-get req :max-chars))
		 (debug (plist-get req :debug))
		 (include-titles (plist-get req :include-titles))
		 (actual-bytes (plist-get resp :actual-bytes)))
	(pcase mime
	  ((or "text/html" "application/xhtml+xml")
	   (pcase out-format
		 ("html"
		  (let* ((content (gptel-fetch-page--truncate body max-chars))
				 (out (if debug
						  (concat (gptel-fetch-page--format-header http-status final-url mime actual-bytes)
								  "\n\n" content)
						content)))
			(list :ok t out)))
		 ((or "text" "links")
		  (if (fboundp 'libxml-parse-html-region)
			  (condition-case _
				  (let* ((dom (gptel-fetch-page--html-to-dom body))
						 (payload (if (string= out-format "text")
									  (gptel-fetch-page--dom-to-text dom)
									(gptel-fetch-page--collect-links dom final-url include-titles)))
						 (content (gptel-fetch-page--truncate payload max-chars))
						 (out (if debug
								  (concat (gptel-fetch-page--format-header http-status final-url mime actual-bytes)
										  "\n\n" content)
								content)))
					(list :ok t out))
				(error
				 (list :error (gptel-fetch-page--format-error
							   :summary "HTML parsing failed"
							   :status http-status
							   :url final-url
							   :type mime
							   :suggest "return html format (raw), use download_url, or install Emacs with libxml2"))))
			(list :error (gptel-fetch-page--format-error
						  :summary "HTML parsing requires Emacs built with libxml2"
						  :status http-status
						  :url final-url
						  :type mime
						  :suggest "return html format (raw), use download_url, or install Emacs with libxml2"))))
		 (_
		  (list :error (gptel-fetch-page--format-error
						:summary (format "Invalid format '%s' for HTML content" out-format)
						:status http-status
						:url final-url
						:type mime
						:suggest "use format=text, format=html, or format=links")))))
	  ("text/plain"
	   (let* ((content (gptel-fetch-page--truncate body max-chars))
			  (out (if debug
					   (concat (gptel-fetch-page--format-header http-status final-url mime actual-bytes)
							   "\n\n" content)
					 content)))
		 (list :ok t out)))
	  (_
	   (list :error (gptel-fetch-page--format-error
					 :summary (format "Non-text content not supported for parsing (%s)" mime)
					 :status http-status
					 :url final-url
					 :type mime
					 :suggest "use download_url to save the file locally, then parse with a file tool"))))))

;; ----------------------------
;; Orchestrator
;; ----------------------------

(defun gptel-fetch-page--run (callback url out-format timeout max-bytes max-chars include-titles
									   user-agent accept accept-language debug)
  "Orchestrate async fetch and rendering for gptel fetch_page."
  (let ((req (gptel-fetch-page--normalize-args url out-format timeout max-bytes max-chars
											   include-titles user-agent accept accept-language debug)))
	(unless (gptel-fetch-page--ok-scheme-p (plist-get req :url))
	  (funcall callback (gptel-fetch-page--format-error
						 :summary "Only http/https URLs are supported"
						 :url (plist-get req :url)
						 :suggest "use an http/https URL"))
	  (cl-return-from gptel-fetch-page--run))
	(let* ((url-request-method "GET")
		   (url-request-extra-headers (gptel-fetch-page--build-request-headers req))
		   (done (list nil)) ;; cons cell used as shared mutable flag
		   (orig-url (plist-get req :url))
		   timer)
	  (setq timer (gptel-fetch-page--start-timeout req callback done))
	  (url-retrieve
	   orig-url
	   (lambda (status)
		 (let ((inhibit-read-only t))
		   (unwind-protect
			   (progn
				 (unless (car done)
				   (setcar done t)
				   (when (timerp timer) (cancel-timer timer))
				   (if (plist-get status :error)
					   (let ((err (plist-get status :error)))
						 (funcall callback (gptel-fetch-page--format-error
											:summary (error-message-string err)
											:url orig-url
											:suggest "check URL or try again")))
					 (let* ((resp (gptel-fetch-page--parse-response-buffer))
							(limit-check (gptel-fetch-page--enforce-limits resp req)))
					   (if (plist-get limit-check :error)
						   (funcall callback (plist-get limit-check :error))
						 (let ((rendered (gptel-fetch-page--render resp req)))
						   (funcall callback
									(or (plist-get rendered :error)
										(caddr rendered))))))))
				 (kill-buffer (current-buffer)))
			 (ignore-errors (kill-buffer (current-buffer))))))
	   nil t nil))))


;; Public entry for the gptel tool function.
(defun gptel-fetch-page--tool (callback url &optional format timeout max_bytes max_chars include_titles user_agent accept accept_language debug)
  (gptel-fetch-page--run callback url format timeout max_bytes max_chars include_titles user_agent accept accept_language debug))

(with-eval-after-load 'gptel
  (gptel-make-tool
   :name "fetch_page"
   :function #'gptel-fetch-page--tool
   :description (concat
				 "Fetch a web page and return rendered text, raw HTML, or links.\n"
				 "- format=text|html|links (default text)\n"
				 "- Guards: timeout (30s), max_bytes (default 16MiB, up to 32MiB), max_chars (300k)\n"
				 "- Includes stylesheets in links mode; relative URLs resolved with <base>/final URL\n"
				 "- Non-text types error with explicit suggestion to use a downloader.")
   :category "web"
   :async t
   :include t
   :args (list
		  '(:name "url" :type string
				  :description "The http/https URL to fetch")
		  '(:name "format" :type string
				  :enum ["text" "html" "links"]
				  :optional t
				  :description "Output format: text (rendered), html (raw), or links (absolute URLs)")
		  '(:name "timeout" :type integer :optional t
				  :description "Timeout in seconds (default 30)")
		  '(:name "max_bytes" :type integer :optional t
				  :description "Max response bytes (default 16MiB, up to 32MiB)")
		  '(:name "max_chars" :type integer :optional t
				  :description "Max output chars (default 300k)")
		  '(:name "include_titles" :type boolean :optional t
				  :description "In links mode, include titles as a tab-separated second column")
		  '(:name "user_agent" :type string :optional t
				  :description "User-Agent preset or full string. Presets: desktop-chrome, desktop-firefox, desktop-safari, edge-windows, mobile-iphone, mobile-android, emacs-default")
		  '(:name "accept" :type string :optional t
				  :description "Accept header override")
		  '(:name "accept_language" :type string :optional t
				  :description "Accept-Language header override (e.g., en-US,en;q=0.9)")
		  '(:name "debug" :type boolean :optional t
				  :description "If true, prepend a compact metadata header"))))

(provide 'fetch_page)
;;; fetch_page.el ends here