Re: correct way of processing cache

On Mon, 13 Dec 2010 08:06:49 -0800 (PST), mark jason
<markjason72@gmail.com> wrote, quoted or indirectly quoted someone who
said :


here is some code I use to check the sizes of images which I keep in a
cache to avoid recomputing them. Note how the client of the method
does not concern itself with whether the info is cached on not. That
is none of its business. That is a purely implementation detail.

   /**
     * capacity of HashMap to store dimensions of cached images
     */
    private static final int IMAGE_CACHE_CAPACITY = 4000;

    /**
     * to avoid work of checking image dimensions on disk, we cache
results of good images found earlier
     * we store two shorts in the width x height.
     */
    private static final HashMap<String, Integer> doneBefore = new
HashMap<String, Integer>( IMAGE_CACHE_CAPACITY );

    /**
     * Find out width and height of an image in the project image
tree. Do not confuse with the more general
     * ImageInfo.getImageDimensions.
     *
     * @param projectImage name of image file in the image tree, e.g.
navigate/home.png
     *
     * @return array[2] [0[=width [1]=height 0 0 means invalid. Does
not throw exception.
     */
    public static int[] getProjectImageDimensions( String projectImage
)
    {
    // see if we have done this one before
    Integer wxh = doneBefore.get( projectImage );
    final int width;
    final int height;

    if ( wxh != null )
        {
        // use previous dimensions, saving time to look up image.
        // packed width in msw and height in lsw
        width = wxh >>> 16;
        height = wxh & 0xffff;
        return new int[] { width, height };
        }
    else
        {
        int[] dimensions = ImageInfo.getImageDimensions(
configuration.getWebrootOnEWithSlashes() + "/image/" + projectImage );

        width = dimensions[ 0 ];
        height = dimensions[ 1 ];

        if ( width == 0 || height == 0 )
            {
            return new int[] { 0, 0 };
            }

        // new good one
        // pack width in msw and height in lsw
        doneBefore.put( projectImage, width << 16 | height );
        return dimensions;
        }
    }
--
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