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

;; Copyright (C) 2025  cjennings
;; Author: cjennings and AI assistant
;; Keywords: convenience, tools, web
;; Package-Requires: ((emacs "27.1"))
;; SPDX-License-Identifier: GPL-3.0-or-later

;;; 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 '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)
  (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
    (let ((shr-use-fonts nil)
          (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 (or (string-match-p "stylesheet" rel)
                             (string-match-p "\bstyle\b" 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")))

(defun gptel-fetch-page--run (callback url format timeout max-bytes max-chars include-titles
									   user-agent accept accept-language debug)
  (let* ((format (or 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")))
	(unless (gptel-fetch-page--ok-scheme-p url)
	  (funcall callback (gptel-fetch-page--format-error
						 :summary "Only http/https URLs are supported"
						 :url url
						 :suggest "use an http/https URL") )
	  (cl-return-from gptel-fetch-page--run))

	(let* ((url-request-method "GET")
		   (url-request-extra-headers
			`(("User-Agent" . ,ua)
			  ("Accept" . ,accept)
			  ("Accept-Language" . ,accept-language)
			  ;; Byte range to hint the server to cap size
			  ,@(and max-bytes (list (cons "Range" (format "bytes=0-%d" (1- max-bytes)))))))
		   (done nil)
		   (timer nil)
		   (orig-url url))
	  (setq timer (run-at-time timeout nil
							   (lambda ()
								 (unless done
								   (setq done 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")))) )
			(url-retrieve
			 url
			 (lambda (status)
			   (let ((inhibit-read-only t))
				 (unwind-protect
					 (progn
					   (when (and (not done)) (setq done t) (when timer (cancel-timer timer)))
					   (let ((bail nil))
						 (when (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")))
						   (setq bail t))
						 ;; Parse status line & headers
						 (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)))))
								(status-msg (or (and (boundp 'url-http-response-status) url-http-response-status)
												http-status))
								(_ (search-forward "\n\n" nil t))
								(headers (gptel-fetch-page--parse-headers))
								(final-url (or (and (boundp 'url-current-object)
													(ignore-errors (url-recreate-url url-current-object))) orig-url))
								(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)))
						   ;; Enforce size limits
						   (when (and (not bail) declared-bytes max-bytes (> declared-bytes max-bytes))
							 (funcall callback (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)))
							 (setq bail t))
						   (when (and (not bail) max-bytes (> actual-bytes max-bytes))
							 (funcall callback (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)))
							 (setq bail t))

						   (unless bail
							 ;; Route based on content-type & requested format
							 (pcase mime
							   ((or "text/html" "application/xhtml+xml")
								(pcase format
								  ("html"
								   (let* ((content (gptel-fetch-page--truncate body max-chars))
										  (out (if debug
												   (concat (gptel-fetch-page--format-header status-msg final-url mime actual-bytes)
														   "\n\n" content)
												 content)))
									 (funcall callback out)))
								  ("text"
								   (if (fboundp 'libxml-parse-html-region)
									   (condition-case err
										   (let* ((dom (gptel-fetch-page--html-to-dom body))
												  (txt (gptel-fetch-page--truncate (gptel-fetch-page--dom-to-text dom) max-chars))
												  (out (if debug
														   (concat (gptel-fetch-page--format-header status-msg final-url mime actual-bytes)
																   "\n\n" txt)
														 txt)))
											 (funcall callback out))
										 (error
										  (funcall callback (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")))
										 (funcall callback (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"))))
								   ("links"
									(if (fboundp 'libxml-parse-html-region)
										(condition-case err
											(let* ((dom (gptel-fetch-page--html-to-dom body))
												   (links (gptel-fetch-page--collect-links dom final-url include-titles))
												   (content (gptel-fetch-page--truncate links max-chars))
												   (out (if debug
															(concat (gptel-fetch-page--format-header status-msg final-url mime actual-bytes)
																	"\n\n" content)
														  content)))
											  (funcall callback out))
										  (error
										   (funcall callback (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")))
										  (funcall callback (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"))))))
								  ("text/plain"
								   (let* ((content (gptel-fetch-page--truncate body max-chars))
										  (out (if debug
												   (concat (gptel-fetch-page--format-header status-msg final-url mime actual-bytes)
														   "\n\n" content)
												 content)))
									 (funcall callback out)))
								  (_
								   (funcall callback (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")))))))
						   (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"))))

;; Automatically add to gptel-tools on load
(add-to-list 'gptel-tools (gptel-get-tool '("web" "fetch_page")))


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