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Re: finding tags files based on location

From: Kevin Rodgers
Subject: Re: finding tags files based on location
Date: Wed, 27 Feb 2008 21:49:05 -0700
User-agent: Thunderbird (Macintosh/20071031)

Mike Mattie wrote:
On Tue, 26 Feb 2008 21:31:28 -0700
Kevin Rodgers <address@hidden> wrote:

gamename wrote:
On Feb 25, 8:08 pm, Kevin Rodgers <address@hidden> wrote:
gamename wrote:
I often have to checkout multiple copies of the same source tree
from cvs.  For example, if I have a source tree 'foo' in cvs, I
may have 2 copies like ~/foo0/bar and ~/foo1/bar in my home
directory at the same time.  Each copy of the foo tree will have
a tags file in 'foo/tags'. Does anyone have code that will enable
emacs to determine which tags file to use based on the tree i'm
currently using?  For example, if I'm in foo1/bar, I would be
using the tags file in foo1/tags, not foo0/ tags.  Is there code
to dynamically determine that?
Is it enough to make tags-file-name a buffer local variable, set
appropriately for each file?  If so:

(defun set-local-tags-file-name ()
   (when (file-exists-p "tags")
     (set (make-local-variable 'tags-file-name)
         (expand-file-name "tags"))))

(add-hook 'find-file-hook 'set-local-tags-file-name)
Thanks Kevin, but I don't think that scales very well.  If I have
thousands of files, that wouldn't be workable.
Actually, it scales perfectly well: No matter how many files you have,
the hook is only run once for each file that you actually visit.

The question is: Does it actually work?  I see now that
vist-tags-table explicitly provides a way to do the same thing, so
perhaps this is better:

(defun set-local-tags-file-name ()
   (when (file-exists-p "tags")
     (visit-tags-table (expand-file-name "tags") t)))

Actually that won't be all that useful as it will scope the tags table to the 
of the file, instead of the entire source tree. If you use version control alot 
can find a good location for the tags table by ascending the FS tree looking 
for the
highest directory that is writable and under version control.

Yes, OP mentioned source trees but since the examples given simply had
the tags file in the same directory as the source file, that's what I

The code below will give you a sketch of the algorithm. It's all recursive so 
it will
hit the stack limit with valid inputs. But it might help you.

If your hook used a global structure that paired the loaded tags tables with 
their directory
you could cheaply determine a best table from the list to associate with a 

Why bother generating a global structure, instead of just searching up
the directory tree from the source file towards the root until you find
a tags file?

(defun set-local-tags-file-name (&optional directory)
"Set the buffer's local `tags-file-name' value to the directory's TAGS file.
If there is no TAGS file in the current directory, search up the
directory tree to the root until a TAGS file is found."
  (let ((file (expand-file-name "TAGS" directory)))
    (cond ((file-exists-p file) (visit-tags-table file t))
          ((equal directory "/") nil)
          (t (set-local-tags-file-name (expand-file-name ".." directory))))))

If you know the root of the tree it's pretty easy to make up the commands to 
generate the table
when it is missing as well.

Now that's an interesting idea!

(defun prefix-strings (prefix list)
  "prefix all the strings of the list concatenating the result."
    (lambda ( string )
      (concat prefix string))

(defun strip-list-last ( list )
  "strip the last element from a list"
  (if (consp (cdr list))
      (car list)
      (strip-list-last (cdr list)))

(defun ascend-to-checkout-root ( dir )
  "The recursive core of find-checkout-root. use that entry point instead."
  (if (and
        (file-accessible-directory-p dir)
        (file-writable-p (concat dir "/_")))

    ;; vc-backend is not robust with inputs. If a directory is given without a 
    ;; slash a nil value will be returned incorrectly for directories under 
    ;; control.

    (if (vc-backend dir)
        ((traverse (strip-list-last (split-string dir "/" t))))

        (if traverse
            ((found (ascend-to-checkout-root (prefix-strings "/" traverse))))
            (if (eq 't found)
          t))   ;; halt when list is exhausted
      t)        ;; halt when the directory is no longer under version control.
    t))         ;; halt when we don't have read and write permission for the 

(defun find-checkout-root ( dir )
  "Find the ancestor directory that is the root of the checkout containing DIR.

   Recursion halts on these conditions:
   * exhausted path.
   * directory is not: accesible,readable, and writable.
   * directory is not under version control according to vc-backend.
    ((found (ascend-to-checkout-root dir)))
    (if (eq 't found)

Kevin Rodgers
Denver, Colorado, USA

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