Complex(?) htaccess rewriting / redirecting - apache

Question

It seems every few weeks I have to ask more .htaccess rewriting/redirecting questions. Every time I think I understand it, another wrench gets thrown into my project that shows that I don't.
EDIT: My original question wasn't very clear so the following is an attempt to be more concise.
As it stands, all of the .html files live in the root directory. eg: http://example.com/about.html
There aren't any sub-directories with the exception of normal ones like img, css, etc.
For tracking purposes, if someone types in http://example.com/random/ where "random" can be any string of characters, I'd want them to see the index.html file, without modifying the url. The directory "random" doesn't actually exist on the server at all.
The same goes for other pages like about.html. If someone types in http://example.com/random/about.html I'd want them to see the about.html page.
At the same time, I'd like http://example.com/random/about or http://example.com/about (missing file extension) to also show the about page.
However, if someone typed in a page that doesn't exist, I'd like for it to use the ErrorDocument
Example: I don't have a file named "pickups.html" so the following would all be 404s:
http://example.com/pickups.html
http://example.com/pickups
http://example.com/random/pickups.html
http://example.com/random/pickups
It would be nice if the end redirect/rewrite did have the file extension stripped off (because it looks nicer).
My thoughts are that any request ending with a / would just serve up the index.html file that exists at the site root. So that leaves the files.
My thought process is:
strip the file extension off of the request
check if that file with an extension exists at site root
if yes, display that page.
if no, 404.
My initial code (had help on it) was this:
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.+)/(.*)$ /$2 [R=301,L]
I understand that in that code I'm grabbing everything after the last slash and serving it from the document root. Unfortunately, it doesn't account for files that do not exist.

Answer 1

Starting with existing files, they will be passed through unchanged. This also prevents rewrite loops.
RewriteCond %{REQUEST_FILENAME} -f
RewriteRule ^ - [L]
Next are existing files, requested as part of an optional, virtual subdirectory
RewriteCond %{DOCUMENT_ROOT}/$2 -f
RewriteRule ^(.+/)?(.+)$ /$2 [L]
RewriteCond %{DOCUMENT_ROOT}/$2.html -f
RewriteRule ^(.+/)?(.+)$ /$2.html [L]
This splits the request into an optional prefix (.+/)? and the file part. If this file part exists, maybe with an appended .html, you're done.
Next comes anything with a trailing slash, just rewrite to index.html
RewriteRule /$ /index.html [L]
Anything else will be requests for non-existing files, which yield a 404 status code.
In order to remove an optional .html extension and remove an optional trailing slash / for existing files, we must insert two rules at the beginning
RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteCond %{DOCUMENT_ROOT}/$2.html -f
RewriteRule ^(.+/)?(.+?)\.html/?$ /$1$2 [R,L]
RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteCond %{DOCUMENT_ROOT}/$2.html -f
RewriteRule ^(.+/)?(.+?)/$ /$1$2 [R,L]
These rules are similar to the other rules, except they do a redirect R|redirect instead of a rewrite, and have an additional condition to prevent a rewrite loop.
Putting everything together gives
RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteCond %{DOCUMENT_ROOT}/$2.html -f
RewriteRule ^(.+/)?(.+?)\.html/?$ /$1$2 [R,L]
RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteCond %{DOCUMENT_ROOT}/$2.html -f
RewriteRule ^(.+/)?(.+?)/$ /$1$2 [R,L]
RewriteCond %{REQUEST_FILENAME} -f
RewriteRule ^ - [L]
RewriteCond %{DOCUMENT_ROOT}/$2 -f
RewriteRule ^(.+/)?(.+)$ /$2 [L]
RewriteCond %{DOCUMENT_ROOT}/$2.html -f
RewriteRule ^(.+/)?(.+)$ /$2.html [L]
RewriteRule /$ /index.html [L]