Silly (but powerful) .htaccess handling of script-generated web thumbnails

.htaccess

    #############################
    # START: THUMBNAIL HANDLING #
    #############################

    # Since handling thumbnails in Nette is painfully slow (loading the whole framework 
    # just to find out that the thumbnail has already existed), there is a workaround
    # that finds all the generated thumbnails using mod_rewrite and only redirects to the
    # script if generating a thumbnail is really needed.

    # Even for generating thumbs the standard Nette usage is slow, so there is another workaround.
    # The ThumbnailPresenter is not really a presenter. Its entry point isn't index.php, but
    # document_root/thumbnail.php, which skips the most of the framework and then calls
    # app/thumbnail.php. That file contains the thumbnail generating code. However, there is
    # also app/DefaultModule/ThumbnailPresenter.php in which the signature of the default 
    # action has to be. And there is also needed a record in Nette router to be able to generate
    # the thumbnail links.

    # The basic idea of these rules is the following:
    # 1. thumbnail/path/and/name.jpg/160/120/123456789/1
    # 2. convert the url-parameter to only contain alphanumeric chars and -_ (and end with an extension like .jpg)
    #       thumbnail-webalizing/path_and_name.jpg/160/120/123456789
    # 3. data/temp/preview-path_and_name-160x120-123456789.jpg.toolarge
    # 4. If file from 3. exists, redirect either to the original image or the too-large-placeholder
    # 5. Skip .toolarge from 3.
    # 6. If the file exists, we've found the thumbnail, so serve it
    # 7. Otherwise redirect to thumbnail.php to generate the thumbnail

    # make sure the left part corresponds to the thumbnail router defined in APP_DIR/bootstrap-application.php
    # the E=...:... flags mean setting environment variables which are accessible further in the script via %{ENV:VARNAME}
    # the DPI flag means Discard PathInfo and is needed - if it missed, mod_rewrite would append the whole original URL to the rewritten one (WTF???)
    RewriteRule ^thumbnail/(.*)/([0-9]+)/([0-9]+)/([0-9]+)/([01])/?$ thumbnail-webalizing/$1/$2/$3/$4 [E=ORIGURL:$1,E=FALLBACKTOLARGE:$5,DPI]

    # translate all characters in the url-parameter other than a-zA-Z0-9_- to an underscore
    # this is achieved via the N flag which tells "restart the rewriting from the first rule again (but restart with the current rewritten address)
    # you can imagine this as a while loop substituting one character at a time
    RewriteRule ^thumbnail-webalizing/([a-zA-Z0-9_-]*)[^a-zA-Z0-9_-](.*)\.(.{3,4})/([0-9]+)/([0-9]+)/([0-9]+)$ thumbnail-webalizing/$1_$2.$3/$4/$5/$6 [N,DPI]

    # now we have the url-parameter in the correct form, so we can create the thumbnail filename
    # and we right away append the .toolarge extension to first check if we don't have to send a substitute image
    RewriteRule ^thumbnail-webalizing/(.*)\.(.{3,4})/([0-9]+)/([0-9]+)/([0-9]+)$ data/temp/preview-$1-$3x$4-$5.$2.toolarge [DPI]

    # recalling the environment variable FALLBACKTOLARGE (set above) we decide whether to return the original image or a too-large-substitute image
    # but only if the .toolarge file exists
    RewriteCond %{ENV:FALLBACKTOLARGE} ^1$
    RewriteCond %{REQUEST_FILENAME} -f
    RewriteRule ^data/temp/preview-.*-[0-9]+x[0-9]+-[0-9]+\..{3,4}\.toolarge$ %{ENV:ORIGURL} [L]

    RewriteCond %{ENV:FALLBACKTOLARGE} ^0$
    RewriteCond %{REQUEST_FILENAME} -f
    RewriteRule ^data/temp/preview-.*-[0-9]+x[0-9]+-[0-9]+\..{3,4}\.toolarge$ images/toolarge_placeholder.png [L]

    # the .toolarge file doesn't exist, so we can move on an remove the .toolarge extension
    RewriteRule ^(data/temp/preview-.*-[0-9]+x[0-9]+-[0-9]+\..{3,4})\.toolarge$ $1

    # if the thumbnail file exists, return it
    RewriteCond %{REQUEST_FILENAME} -f
    RewriteRule ^(data/temp/preview-.*-[0-9]+x[0-9]+-[0-9]+\..{3,4})$ $1 [L]

    # if the thumbnail file doesn't exist, call the creating script
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteRule ^data/temp/preview-.*-([0-9]+)x([0-9]+)-([0-9]+)\..{3,4}$ thumbnail.php?url=%{ENV:ORIGURL}&width=$1&height=$2&timestamp=$3&fallbackToLarge=%{ENV:FALLBACKTOLARGE} [L]

    ###########################
    # END: THUMBNAIL HANDLING #
    ###########################