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Simplify checksum code

This commit is contained in:
Lassi Kortela 2019-12-28 02:12:24 +02:00
parent 553ddc6eb1
commit 4b9b1eaf06
1 changed files with 9 additions and 11 deletions
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20
lassik/trivial-tar-writer.scm
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@ -9,6 +9,10 @@
(define nulls (make-bytevector 512 0))
(define blank-checksum (make-bytevector 7 (char->integer #\space)))
(define (bytevector-sum bv)
(let loop ((i (- (bytevector-length bv) 1)) (sum 0))
(if (< i 0) sum (loop (- i 1) (+ sum (bytevector-u8-ref bv i))))))
(define (tar-string nbyte string)
(let* ((bytes (string->utf8 string))
(nnull (- nbyte (bytevector-length bytes))))
@ -24,14 +28,6 @@
(bytevector-append (make-bytevector nzero (char->integer #\0))
bytes (bytevector 0))))
(define (tar-checksum . vectors)
(let loop-vectors ((vectors vectors) (sum 0))
(if (null? vectors) (truncate-remainder sum (expt 8 6))
(let* ((v (car vectors)) (n (bytevector-length v)))
(let loop-bytes ((i 0) (sum sum))
(if (= i n) (loop-vectors (cdr vectors) sum)
(loop-bytes (+ i 1) (+ sum (bytevector-u8-ref v i)))))))))
(define (tar-write-file fake-path bytes)
(let* ((unix-time-now 0)
(nbyte (bytevector-length bytes))
@ -53,9 +49,11 @@
(tar-string 32 (cdr (tar-owner)))
(tar-string 32 (cdr (tar-group)))
(make-bytevector 183 0)))
(checksum (tar-octal 7 (tar-checksum header-before-checksum
blank-checksum
header-after-checksum))))
(checksum
(let ((sum (+ (bytevector-sum header-before-checksum)
(bytevector-sum blank-checksum)
(bytevector-sum header-after-checksum))))
(tar-octal 7 (truncate-remainder sum (expt 8 6))))))
(write-bytevector header-before-checksum)
(write-bytevector checksum)
(write-bytevector header-after-checksum)