167 lines
6.4 KiB
Scheme
167 lines
6.4 KiB
Scheme
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;;; Copyright (c) 1994 by Olin Shivers
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;;; String collectors
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;;; ===========================================================================
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;;; string-colllector
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;;; (make-string-collector)
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;;; (collect-string! SC S)
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;;; (clear-string-collector! SC)
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;;; (string-collector->string SC)
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;;;
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;;; A string collector is a data structure that reduces the overhead of
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;;; accumulating a large string in bits and pieces. It is basically a
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;;; "chunk list," where a chunk is a string of at least 128 chars. In this
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;;; way, the list overhead is kept under 2% of the whole data structure.
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;;; When a new string is added to the collection, it is added to the current
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;;; chunk. When the chunk reaches 128 chars, it is added to the chunk list,
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;;; and a new chunk is started. If a large string is added to the collection,
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;;; it is added as a chunk itself, so large strings are not split into small
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;;; pieces. (Actually, a *copy* of the original large string is saved as a
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;;; single chunk, so the collector's chunks are not shared with client data.)
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;;;
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;;; MAKE-STRING-COLLECTOR allocates a new string collector data structure.
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;;; COLLECT-STRING! appends a string to the current collection.
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;;; CLEAR-STRING-COLLECTOR! clears out accumulated strings from a collector.
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;;; STRING-COLLECTOR->STRING converts a collector into a contiguous string.
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;;;
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;;; This facility makes it reasonably efficient to accumulate strings
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;;; of any size in increments of any size.
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(define-record string-collector
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(len 0) ; How many chars have we accumulated?
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(chunks '()) ; The chunk list.
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(chunk #f) ; The current chunk being filled, if any.
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(chunk-left 0)) ; How many chars left to fill in the current chunk.
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(define (clear-string-collector! sc)
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(set-string-collector:len sc 0)
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(set-string-collector:chunks sc '())
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(set-string-collector:chunk sc #f)
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sc)
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;;; (COLLECT-STRING! sc s)
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;;; ----------------------
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;;; S is a string. Append it to the string being collected in the
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;;; string-collector SC.
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;;;
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;;; The algorithm:
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;;; First, do nothing if S is the empty string. Otherwise:
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;;; If there is a current chunk:
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;;; Copy characters from S into it.
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;;; If we filled up the chunk
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;;; Put the chunk on the chunk list.
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;;; Look at the remaining chars from S we haven't copied yet.
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;;; If there a lot of characters left (>= 128)
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;;; Save them as a single chunk on the chunk list.
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;;; No current chunk.
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;;; Else if there a just a few characters left (> 0, < 128)
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;;; Start a new current chunk, copy the chars left into it.
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;;; Else if there aren't any characters left
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;;; No current chunk.
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;;;
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;;; If there is no current chunk:
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;;; If there are a lot of characters in S (>= 128)
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;;; Save a copy of S as a single chunk on the chunk list.
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;;; Still no current chunk.
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;;; Else if there are a few characters in S (> 0, < 128)
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;;; Start a new current chunk, copy the S into it.
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(define (collect-string! sc s)
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(let ((slen (string-length s))
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(chunk (string-collector:chunk sc))
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(chunk-left (string-collector:chunk-left sc))
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;; Add the chunk C to the collector's chunk list.
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(push-chunk! (lambda (c)
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(set-string-collector:chunks sc
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(cons c (string-collector:chunks sc)))))
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;; Copy nchars characters from src[j] to dest[i]
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;; *Way* too much bounds checking going on in this loop.
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(copy-substring! (lambda (dest i src j nchars)
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(do ((i i (+ i 1))
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(j j (+ j 1))
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(nchars nchars (- nchars 1)))
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((zero? nchars))
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(string-set! dest i (string-ref src j))))))
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(cond ((zero? slen)) ; Empty string, do nothing.
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(chunk
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(let ((ncopy (min slen chunk-left)))
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(copy-substring! chunk (- 128 chunk-left) s 0 ncopy)
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(if (> chunk-left slen)
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(set-string-collector:chunk-left sc (- chunk-left slen))
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;; Current chunk is full.
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(let ((s-left (- slen chunk-left)))
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(push-chunk! chunk) ; Push the current chunk.
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;; Handle remaining chars from S that weren't copied into
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;; the current chunk we just pushed:
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(cond ((>= s-left 128)
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;; A lot more chars left. Push them as one chunk.
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(push-chunk! (substring s chunk-left slen))
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(set-string-collector:chunk sc #f))
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((> s-left 0)
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;; A few more chars left. Start a new chunk.
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(let ((new-chunk (make-string 128)))
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(copy-substring! new-chunk 0 s chunk-left s-left)
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(set-string-collector:chunk sc new-chunk)
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(set-string-collector:chunk-left sc
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(- 128 s-left))))
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;; No more chars left. No current chunk.
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(else (set-string-collector:chunk sc #f)))))))
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(else ; No current chunk.
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(if (>= slen 128) ; How many chars is S?
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(push-chunk! (string-copy s)) ; A lot. Push as one chunk.
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(let ((chunk (make-string 128))) ; Not many. Start a new chunk.
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(set-string-collector:chunk sc chunk)
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(copy-substring! chunk 0 s 0 slen)
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(set-string-collector:chunk-left sc (- 128 slen))))))
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;; We don't actually do anything with this, but we keep it updated anyway.
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(set-string-collector:len sc (+ (string-collector:len sc) slen))
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sc))
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;;; A bummed version for collecting a single character.
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(define (collect-char! sc c)
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(let ((chunk (string-collector:chunk sc))
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(chunk-left (string-collector:chunk-left sc)))
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(cond (chunk
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(string-set! chunk (- 128 chunk-left) c)
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(cond ((> chunk-left 1)
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(set-string-collector:chunk-left (- chunk-left 1)))
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(else
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(set-string-collector:chunks sc
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(cons chunk (string-collector:chunks sc)))
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(set-string-collector:chunk sc #f))))
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(else
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(let ((new-chunk (make-string 128 c)))
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(set-string-collector:chunk-left 127)
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(set-string-collector:chunk sc new-chunk)))))
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;; We don't actually do anything with this, but we keep it updated anyway.
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(set-string-collector:len sc (+ (string-collector:len sc) 1))
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sc)
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;;; Convert the data in the string-collector SC to a single contiguous
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;;; string and return it.
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(define (string-collector->string sc)
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(let ((chunk (string-collector:chunk sc))
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(chunks (string-collector:chunks sc)))
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(apply string-append
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(reverse (if chunk
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(cons (substring chunk 0
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(- 128
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(string-collector:chunk-left sc)))
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chunks)
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chunks)))))
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;;; It's too bad we can't side-effect the string-collector's chunk list
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;;; to be a single chunk after this coalescing operation, but we don't
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;;; want to share the string we return -- the user might side-effect it.
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