SV: Re: Referencing a row in a two dimensional array

>
> However the time delta between a single call to READ-SEQUENCE and my
> previous loop which called READ-BYTE for each byte read is
> negligable. In CLL, READ-SEQUENCE seems to be implemented as a loop
> calling READ-BYTE.
>
> I was hoping that READ-SEQUENCE actually read an entire block of
> bytes in one shot, similar to "fread" in C, as this would probably
> increase performance (in my case) by a factor of 100 or more.
>

Corman Lisp cannot read from file directly to Lisp data because
blocks of Lisp data may be moved by garbage collector.

Corman Lisp first reads to a "foreign block", than copies this block
to file buffer, and read-byte reads from buffer to the target data.

So it is hardly possible to increase performance by a factor of 100.

Here is a function that reads the block 3 times faster:

(defun read-bytes (vector stream &optional (start 0) end)
    (let (length buf buf-pos buf-end)
        (multiple-value-setq (length end) (cl::validate-bounding-indices
vector start end))
    	(unless (and (cl::binary-stream-p stream)(input-stream-p stream))
	       (error "Expected a binary input stream, got ~A" stream))
        (setq buf (cl::stream-input-buffer stream))
        (do ((p start))
            (nil)
            (setf buf-pos (cl::stream-input-buffer-pos stream)
                  buf-end (cl::stream-input-buffer-num stream))
            (when (= buf-pos buf-end)
                (funcall (cl::stream-underflow-function stream) stream)
                (setf buf-pos (cl::stream-input-buffer-pos stream)
                      buf-end (cl::stream-input-buffer-num stream))
                (when (= buf-pos buf-end)
                    (return-from read-bytes p)))
            (do ((i buf-pos (+ i 1)))
                ((= i buf-end) (setf (cl::stream-input-buffer-pos stream)
buf-end))
                (when (= p end)
                    (setf (cl::stream-input-buffer-pos stream) i)
                    (return-from read-bytes end))
                (setf (elt vector p) (elt buf i))
                (incf p)))))


By redefining function elt I can read blocks 5 time faster.

I am planning also to write functions for fast copying data;
they may be used both to copy from "foreign buffer" to stream buffer
and from stream buffer to target data (if no data conversion required).

Pavel Grozman


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