map-file-lines

[Ted Zlatanov]

Emacs Lisp lacks a good way to iterate over all the lines of a file,
especially for a large file.  The following code tries to provide a
solution, concentrating on reading a block of data in one shot and then
processing it line by line.  It may be more efficient to write this in
C.  Also, it does not deal with cases where the first line read is
bigger than the buffer size, and may have other bugs, but it works for
me so I thought I'd post it for comments and criticism.

Thanks
Ted

(defun map-file-lines (file func &optional bufsize)
  (let ((filepos 0)
        (linenum 0)
        (bufsize (or bufsize 4096)))
    (with-temp-buffer
      (while
          (let*
              ((inserted (insert-file-contents
                          file nil
                          filepos (+ filepos bufsize) 
                          t))
               (numlines (count-lines (point-min) (point-max)))
               (read (nth 1 inserted))
               (done (< 1 read)))
            (dotimes (n (count-lines (point-min) (point-max)))
                (goto-char (point-min))
                (funcall func 
                         (buffer-substring 
                          (line-beginning-position) 
                          (line-end-position)) 
                         (incf linenum))
                (incf filepos (line-end-position))
                (forward-line))
            done)))
    linenum))

;;(map-file-lines "/tmp/test" (lambda (line num) (message "%d: %s" line)))