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#lang racket/base
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(require racket/class
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"base.rkt" "util.rkt" "number.rkt" "array.rkt" racket/stream sugar/unstable/dict)
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(provide (all-defined-out))
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#|
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approximates
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https://github.com/mbutterick/restructure/blob/master/src/LazyArray.coffee
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|#
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(define x:lazy-array%
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(class x:array%
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(super-new)
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(inherit-field [@type type] [@len len])
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(define/override (decode port parent)
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(define starting-pos (pos port)) ; ! placement matters. `resolve-length` will change `pos`
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(define len (resolve-length @len port parent))
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(define new-parent (if (x:int? @len)
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(mhasheq x:parent-key parent
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x:start-offset-key starting-pos
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x:current-offset-key 0
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x:length-key @len)
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parent))
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(define stream-starting-pos (pos port))
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(begin0
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(for/stream ([index (in-range len)])
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(define orig-pos (pos port))
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(pos port (+ stream-starting-pos (* (send @type size #f new-parent) index)))
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(begin0
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(send @type decode port new-parent)
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(pos port orig-pos)))
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(pos port (+ (pos port) (* len (send @type size #f new-parent))))))
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(define/override (encode val port [parent #f])
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(super encode (if (stream? val) (stream->list val) val) port parent))
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(define/override (size [val #f] [parent #f])
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(super size (if (stream? val) (stream->list val) val) parent))))
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(define (x:lazy-array [type-arg #f] [len-arg #f]
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#:type [type-kwarg #f]
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#:length [len-kwarg #f]
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#:pre-encode [pre-proc #f]
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#:post-decode [post-proc #f]
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#:base-class [base-class x:lazy-array%])
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(define type (or type-arg type-kwarg))
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(define len (or len-arg len-kwarg))
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(new (generate-subclass base-class pre-proc post-proc) [type type]
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[len len]
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[count-bytes? #false]))
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(module+ test
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(require rackunit "number.rkt" "base.rkt")
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(define bstr #"ABCD1234")
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(define ds (open-input-bytes bstr))
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(define la (x:lazy-array uint8 4))
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(define ila (decode la ds))
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(check-equal? (pos ds) 4)
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(check-equal? (stream-ref ila 1) 66)
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(check-equal? (stream-ref ila 3) 68)
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(check-equal? (pos ds) 4)
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(check-equal? (stream->list ila) '(65 66 67 68))
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(define la2 (x:lazy-array int16be (λ (t) 4)))
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(check-equal? (encode la2 '(1 2 3 4) #f) #"\0\1\0\2\0\3\0\4")
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(check-equal? (stream->list (decode la2 (open-input-bytes #"\0\1\0\2\0\3\0\4"))) '(1 2 3 4))) |