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+;;; loadhist.el --- lisp functions for working with feature groups
+
+;; Author: Eric S. Raymond <esr@snark.thyrsus.com>
+;; Version: 1.0
+;; Keywords: internal
+
+;;; Commentary:
+
+;; These functions exploit the load-history system variable.
+;; Entry points include `unload-feature', `symbol-file', and `feature-file'.
+
+;;; Code:
+
+(defun symbol-file (sym)
+  "Return the input source from which SYM was loaded.
+This is a file name, or nil if the source was a buffer with no associated file."
+  (catch 'foundit
+    (mapcar
+     (function (lambda (x) (if (memq sym (cdr x)) (throw 'foundit (car x)))))
+     load-history)
+    nil))
+
+(defun feature-symbols (feature)
+  "Return the file and list of symbols associated with a given FEATURE."
+   (catch 'foundit
+     (mapcar
+      (function (lambda (x) 
+		  (if (member (cons 'provide feature) (cdr x))
+		      (throw 'foundit x))))
+      load-history)
+     nil))
+
+(defun feature-file (feature)
+  "Return the file name from which a given FEATURE was loaded.
+Actually, return the load argument, if any; this is sometimes the name of a
+Lisp file without an extension.  If the feature came from an eval-buffer on
+a buffer with no associated file, or an eval-region, return nil."
+  (if (not (featurep feature))
+      (error "%s is not a currently loaded feature." (symbol-name feature))
+    (car (feature-symbols feature))))
+
+(defun file-provides (file)
+  "Return the list of features provided by FILE."
+  (let ((symbols (cdr (assoc file load-history))) (provides nil))
+    (mapcar
+     (function (lambda (x)
+		 (if (and (consp x) (eq (car x) 'provide))
+		     (setq provides (cons (cdr x) provides)))))
+     symbols)
+    provides
+    ))
+
+(defun file-requires (file)
+  "Return the list of features required by FILE."
+  (let ((symbols (cdr (assoc file load-history))) (requires nil))
+    (mapcar
+     (function (lambda (x)
+		 (if (and (consp x) (eq (car x) 'require))
+		     (setq requires (cons (cdr x) requires)))))
+     symbols)
+    requires
+    ))
+
+(defun set-intersect (p q)
+  ;; Return the set intersection of two lists
+  (let ((ret nil))
+    (mapcar
+     (function (lambda (x) (if (memq x q) (setq ret (cons x ret)))))
+     p)
+    ret
+    ))
+
+(defun file-dependents (file)
+  ;; Return the list of loaded libraries that depend on FILE.
+  (let ((provides (file-provides file)) (dependents nil))
+    (mapcar
+     (function (lambda (x) 
+		 (if (set-intersect provides (file-requires (car x)))
+		     (setq dependents (cons (car x) dependents)))))
+     load-history)
+    dependents
+    ))
+
+;;;###autoload
+(defun unload-feature (feature &optional force)
+  "Unload the library that provided FEATURE, restoring all its autoloads.
+If the feature is required by any other loaded code, and optional FORCE
+is nil, raise an error."
+  (interactive "SFeature: ")
+  (if (not (featurep feature))
+      (error "%s is not a currently loaded feature." (symbol-name feature)))
+  (if (not force)
+      (let* ((file (feature-file feature)) (dependents (file-dependents file)))
+	(if dependents
+	    (error "Loaded libraries %s depend on %s."
+		   (prin1-to-string dependents) file)
+	    )))
+  (let* ((flist (feature-symbols feature)) (file (car flist)))
+    (mapcar
+     (function (lambda (x) 
+		 (cond ((stringp x) nil)
+		       ((boundp x) (makunbound x))
+		       ((fboundp x)
+			(fmakunbound x)
+			(let ((aload (get x 'autoload)))
+			  (if aload (fset x aload))))))
+	       )
+     (cdr flist))))
+
+(provide 'loadhist)
+
+;;; loadhist.el ends here