70 lines
2.9 KiB
Racket
70 lines
2.9 KiB
Racket
#lang racket/base
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;; "varints" from Google Protocol Buffers,
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;; https://developers.google.com/protocol-buffers/docs/encoding#varints
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;;
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;; "Each byte in a varint, except the last byte, has the most
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;; significant bit (msb) set – this indicates that there are further
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;; bytes to come. The lower 7 bits of each byte are used to store the
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;; two's complement representation of the number in groups of 7 bits,
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;; least significant group first."
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(provide write-varint
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read-varint
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encode-varint
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decode-varint)
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(require racket/port)
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(define (write-varint v out-port)
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(if (< v 128)
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(write-byte v out-port)
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(begin (write-byte (+ 128 (modulo v 128)) out-port)
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(write-varint (quotient v 128) out-port))))
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(define (read-varint in-port)
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(let/ec return
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(let loop ()
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(define b (read-byte in-port))
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(cond [(eof-object? b) (return b)]
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[(< b 128) b]
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[else (+ (* (loop) 128) (- b 128))]))))
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(define (encode-varint v)
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(call-with-output-bytes (lambda (p) (write-varint v p))))
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(define (decode-varint bs ks kf)
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((call-with-input-bytes bs (lambda (p)
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(define v (read-varint p))
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(cond [(eof-object? v) (lambda () (kf #t))]
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[else (define rest (port->bytes p))
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(lambda () (ks v rest))])))))
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(module+ test
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(require rackunit)
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(check-equal? (encode-varint 0) (bytes 0))
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(check-equal? (encode-varint 1) (bytes 1))
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(check-equal? (encode-varint 127) (bytes 127))
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(check-equal? (encode-varint 128) (bytes 128 1))
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(check-equal? (encode-varint 255) (bytes 255 1))
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(check-equal? (encode-varint 256) (bytes 128 2))
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(check-equal? (encode-varint 300) (bytes #b10101100 #b00000010))
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(check-equal? (encode-varint 1000000000) (bytes 128 148 235 220 3))
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(define (ks* v rest) (list v rest))
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(define (kf* [short? #f]) (if short? 'short (void)))
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(check-equal? (decode-varint (bytes) ks* kf*) 'short)
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(check-equal? (decode-varint (bytes 0) ks* kf*) (list 0 (bytes)))
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(check-equal? (decode-varint (bytes 0 99) ks* kf*) (list 0 (bytes 99)))
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(check-equal? (decode-varint (bytes 1) ks* kf*) (list 1 (bytes)))
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(check-equal? (decode-varint (bytes 127) ks* kf*) (list 127 (bytes)))
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(check-equal? (decode-varint (bytes 128) ks* kf*) 'short)
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(check-equal? (decode-varint (bytes 128 1) ks* kf*) (list 128 (bytes)))
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(check-equal? (decode-varint (bytes 128 1 99) ks* kf*) (list 128 (bytes 99)))
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(check-equal? (decode-varint (bytes 255 1) ks* kf*) (list 255 (bytes)))
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(check-equal? (decode-varint (bytes 128 2) ks* kf*) (list 256 (bytes)))
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(check-equal? (decode-varint (bytes #b10101100 #b00000010) ks* kf*) (list 300 (bytes)))
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(check-equal? (decode-varint (bytes 128 148 235 220 3) ks* kf*) (list 1000000000 (bytes)))
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(check-equal? (decode-varint (bytes 128 148 235 220 3 99) ks* kf*) (list 1000000000 (bytes 99))))
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