(in-package :fwoar.cl-git)
 
 (defun edges-in-fanout (toc s sha)
   (let* ((fanout-offset (getf toc :fanout)))
     (file-position s (+ fanout-offset (* 4 (1- (elt sha 0)))))
     (destructuring-bind ((_ . cur) (__ . next))
         (fwoar.bin-parser:extract '((cur 4 fwoar.bin-parser:be->int)
                                     (next 4 fwoar.bin-parser:be->int))
                                   s)
       (declare (ignore _ __))
       (values cur next))))
 
 (defun find-sha-between-terms (toc s start end sha)
   (unless (>= start end)
     (let* ((sha-offset (getf toc :shas))
            (mid (floor (+ start end)
                        2)))
       (file-position s (+ sha-offset (* 20 mid)))
       (let ((sha-at-mid (read-bytes 20 'fwoar.bin-parser:byte-array-to-hex-string s)))

         (cond ((serapeum:string-prefix-p sha sha-at-mid)
                (values mid sha-at-mid))
               ((string< sha sha-at-mid)

                (find-sha-between-terms toc s start mid sha))
               ((string> sha sha-at-mid)
                (find-sha-between-terms toc s (1+ mid) end sha))

               (t (values mid sha-at-mid)))))))

 (defun find-sha-in-pack (pack-file id)

   (with-open-file (s (index-file pack-file)
                      :element-type '(unsigned-byte 8))
     (let ((binary-sha (ironclad:hex-string-to-byte-array id))

           (toc (idx-toc pack-file)))

       (multiple-value-bind (_ end) (edges-in-fanout toc s binary-sha)
         (declare (ignore _))

         (multiple-value-bind (midpoint sha)
             (find-sha-between-terms toc s 0 end id)

           (and midpoint
                (values pack-file

                        midpoint
                        sha)))))))

 
 (defun find-object-in-pack-files (repo id)
   (dolist (pack-file (pack-files repo))

     (multiple-value-bind (pack mid sha) (find-sha-in-pack pack-file id)

       (when pack
         (return-from find-object-in-pack-files

           (values pack mid sha))))))

 (defun raw-object-for-ref (packed-ref)
   (let ((pack (packed-ref-pack packed-ref)))
     (with-pack-streams (i p) pack
       (file-position p (read-4-byte-offset pack (packed-ref-offset packed-ref)))
       (get-object-from-pack p))))
 
 (defun get-object-from-pack (s)
   (let* ((metadata (fwoar.bin-parser:extract-high s))
          (type (object-type->sym (get-object-type metadata)))
          (size (get-object-size metadata)))
     (case type
       (:ref-delta (error ":ref-delta not implemented yet"))
       (:ofs-delta (get-ofs-delta-offset-streaming s)))
     (let ((decompressed (chipz:decompress nil (chipz:make-dstate 'chipz:zlib) s)))
       (values (concatenate
                '(vector fwoar.cl-git.types:octet)
                (ecase type
                  (:commit #.(babel:string-to-octets "commit" :encoding :ascii))
                  (:blob #.(babel:string-to-octets "blob" :encoding :ascii))
                  (:tree #.(babel:string-to-octets "tree" :encoding :ascii)))
                #(32)
                (babel:string-to-octets (prin1-to-string size ):encoding :ascii)
                #(0)
                decompressed)
               size
               (length decompressed)))))
 
 (defun get-ofs-delta-offset-streaming (buf)
   (let* ((idx 0))
     (flet ((advance ()
              (read-byte buf)))
       (loop
         for c = (advance)
         for ofs = (logand c 127) then (+ (ash (1+ ofs)
                                               7)
                                          (logand c 127))
         while (> (logand c 128) 0)
         finally
            (return (values (- ofs) idx))))))
 

 (defun read-object-from-pack (s repository ref)

   (let* ((pos (file-position s))
          (metadata (fwoar.bin-parser:extract-high s))

          (type (object-type->sym (get-object-type metadata)))

          (size (get-object-size metadata))

          (delta-base (case type
                        (:ref-delta (error ":ref-delta not implemented yet"))
                        (:ofs-delta (get-ofs-delta-offset-streaming s))))
          (decompressed (chipz:decompress nil (chipz:make-dstate 'chipz:zlib) s))
          (object-data (extract-object-of-type type decompressed repository pos (pathname s) ref delta-base)))

     (list (cons :type (object-type->sym type))
           (cons :decompressed-size size)
           (cons :object-data object-data)

           (cons :raw-data decompressed))))

 (defun extract-object-of-type (type s repository pos packfile ref delta-base)

   (with-simple-restart (continue "Skip object of type ~s at position ~d"
                                  type
                                  pos)

     (-extract-object-of-type (object-type->sym type)
                              s
                              repository

                              :offset-from pos

                              :packfile packfile

                              :hash (ref-hash ref)
                              :base delta-base)))

 (defun pack-offset-for-object (index-file obj-number)
   (let ((offset-offset (getf index-file
                              :4-byte-offsets)))
     (+ offset-offset
        (* 4 obj-number))))
 

 (defun extract-object-at-pos (pack pos ref)
   (with-open-file (p (pack-file pack) :element-type '(unsigned-byte 8))
     (file-position p pos)
     (read-object-from-pack p
                            (repository pack)
                            ref)))
 

 (defun read-4-byte-offset (pack obj-number)
   (with-pack-streams (s _) pack
     (file-position s
                    (pack-offset-for-object (idx-toc pack)
                                            obj-number))
     (read-bytes 4 'fwoar.bin-parser:be->int s)))
 

 (defun extract-object-from-pack (pack obj-number ref)

   (let ((object-offset-in-pack (read-4-byte-offset pack obj-number)))
     (extract-object-at-pos pack
                            object-offset-in-pack
                            ref)))

 (defun extract-loose-object (repo file ref)

   (with-open-file (s file :element-type '(unsigned-byte 8))

     (alexandria:when-let ((result (chipz:decompress nil (chipz:make-dstate 'chipz:zlib)
                                                     s)))

       (destructuring-bind (type rest)
           (partition (char-code #\space) result)
         (extract-object-of-type (object-type->sym (babel:octets-to-string type))
                                 (elt (partition 0 rest)
                                      1)

                                 repo

                                 0

                                 nil

                                 ref
                                 nil)))))

 (defparameter *want-delta* nil)

 (defgeneric extract-object (object)
   (:method ((object loose-ref))
     (extract-loose-object (ref-repo object)

                           (loose-ref-file object)
                           object))

   (:method ((object packed-ref))

     (let ((maybe-delta (data-lens.lenses:view *object-data-lens*
                                               (extract-object-from-pack (packed-ref-pack object)
                                                                         (packed-ref-offset object)
                                                                         object))))
       (if *want-delta*
           maybe-delta
           (resolve-delta object
                          maybe-delta)))))