(defconst elpa-download-log-line-re
  (rx (group (1+ (or digit "." ":")))
      " - - "
      "[" (group (repeat 2 digit) "/" (repeat 3 alpha) "/" (repeat 4 digit)
                 ":" (repeat 2 digit) ":" (repeat 2 digit) ":" (repeat 2 digit)
                 " " (or "+" "-") (repeat 4 digit)) "]"
      ;; ;; HTTP query:
      " \"" (group (1+ alpha))         ; method
      " " (group (1+ (not (any blank)))) ; path
      " " "HTTP/" (1+ (or alnum ".")) "\"" ; protocol
      " " (group (1+ digit))               ; status code
      " " (group (1+ digit))               ; size
      " \"-\" "                            ; ?
      "\"" (group (1+ (not (any "\"")))) "\""))

(defun elpa-log-to-package-stats (log-string)
  "Return alist of (PACKAGE . DOWNLOADS) seen in LOG-STRING.
    LOG-STRING is an HTTP download log."
  (let ((stats (make-hash-table :test #'equal)))
    (with-temp-buffer
      (insert log-string)
      (goto-char (point-min))
      (cl-loop while (re-search-forward elpa-download-log-line-re nil t)
               for file = (match-string 4)
               when (string-match-p (rx bos "/packages/" (1+ anything) (or ".tar" ".el") eos)
                                    file)
               do (if (gethash file stats)
                      (cl-incf (gethash file stats))
                    (setf (gethash file stats) 1))))
    (map-into stats 'alist)))

;; (("/packages/seq-2.23.tar" . 1) ("/packages/let-alist-1.0.6.el" . 1))