(use irregex posix srfi-13 utils)

(define file "your-file")
(define index "index")

; bos = beggining of string
(define byte-num-regexp '(: bos (* num)))

(define (get-byte-offset line)
  (car (irregex-extract byte-num-regexp line)))

(define (grep-output file)
  (call-with-values (lambda () (process (string-append "grep -b '' " file)))
    (lambda (in-port _ _)
      (read-all in-port))))

(define (make-index grep-output)
  (let ((index-fileno (file-open index (+ open/wronly open/append open/creat))))
    (with-input-from-string grep-output
     (lambda ()
       (let loop ((line (read-line)))
	 (unless (eof-object? line)
	   (file-write index-fileno (string-append (string-pad (get-byte-offset line) 16) "\n"))
	   (loop (read-line))))))))

(define (start-index)
  (make-index (grep-output file)))