;MAKE-PIECES - Make the pieces part of a BitTorrent info-hash via data-flow by CONCATENATED-STREAMs.
;Written in 2026 by Prince Trippy <programmer@verisimilitudes.net>.

;To the extent possible under law, the author(s) have dedicated all copyright
;and related and neighboring rights to this software to the public domain worldwide.
;This software is distributed without any warranty.

;You should have received a copy of the CC0 Public Domain Dedication along with this software.
;If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.

(defun make-pieces (dolist block &aux list)
  "Generate the pieces part of a BitTorrent info-hash.
DOLIST is a list of pathnames and BLOCK the block size in octets.
A function HASH corresponding to SHA1 operating on VECTORs must be present.
The function HASH must have only one required parameter, the VECTOR to digest.
This function exists as I noticed how easy a CONCATENATED-STREAM makes the task."
  (check-type block (integer 1))
  (unwind-protect
       (progn (dolist (pathname dolist)
                (loop (restart-case (progn (push (open pathname :element-type 'octet) list)
                                           (return))
                        (abort ()
                          :report "Give up and return from MAKE-PIECES with NIL."
                          (return-from make-pieces))
                        (continue ()
                          :report "Skip only the file in question."
                          (return))
                        (use-value (filename)
                          :report "Provide an alternative pathname to be tried."
                          :interactive (lambda ()
                                         (with-standard-io-syntax
                                           (let (*read-eval*)
                                             (list (read *query-io*)))))
                          (setq pathname filename))
                        (store-value (use-value)
                          :report "Provide an expression for an open stream to be used instead."
                          :interactive (lambda ()
                                         (with-standard-io-syntax
                                           (list (eval (read *query-io*)))))
                          (push use-value list)
                          (return)))))
              (setq list (nreverse list))
              (with-open-stream (stream (apply 'make-concatenated-stream list))
                (loop :with vector := (make-array block :element-type 'octet)
                      :for count := (read-sequence vector stream)
                      :until (zerop count)
                      :if (= count block)
                        :collect (hash vector)
                      :else
                        :collect (hash (make-array count
                                                   :element-type 'octet
                                                   :displaced-to vector
                                                   :displaced-index-offset 0)))))
    (mapc 'close list)))
.