Lisplog

Going Supersonic in Elon Musk's Test Hyperloop Tube

Submitted by Bill St. Clair on Mon, 28 Aug 2017 12:43:10 GMT

Elon Musk tweeted that it "Might be possible to go supersonic in our test Hyperloop tube, even though it's only 0.8 miles long." This post computes the acceleration necessary for that.

Speed of sound: 767 mph = 1125 feet/second
1/2 test track length = 0.4 * 5280 feet = 2112 feet

t = time in seconds
a = acceleration in feet/second

1. 1/2 * a * t^2 = 2112 (distance from acceleration and time)
2. t * a = 1125 (speed from acceleration and time)

3. t = 1125 / a (divide both sides of 2 by a)
4. 1/2 * a * (1125/a)^2 = 2112 (plug 3 into 1)
5. 1/2 * a * 1,265,625 / a^2 = 2112 (do the square in 4)
6. 632812.5 / a = 2112 (do the arithmetic in 5)
7. a = 632812.5 / 2112 (multiply both sides of 6 by a and divide by 2112)
8. a = 299.6 feet/second
9. g = 32 feet/second^2 (acceleration due to gravity)
10. a = 299.6 / 32 = 9.4 g (divide 8 by (32 feet/second)/g)
11. t = 2 * 1125 / a = 7.6 seconds (time to accelerate AND decelerate)

So a Hyperloop vehicle could accelerate to the speed of sound and decelerate back to a standstill in 0.8 miles by accelerating at 9.4 g for 3.8 seconds and then decelerating at 9.4 g for 3.8 seconds.

Wikipedia's G-force page says that untrained humans can survive 10 g horizontal, eyes in or out, for 1 minute, so it's survivable.

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Spokes is Done!

Submitted by Bill St. Clair on Sat, 29 Jul 2017 11:19:08 GMT

I have only worked on it occasionally since getting a new full-time, paying Lisp gig, but this morning, I fixed the last known bug in Spokes, the board game that my son invented, and that I've been implementing in Elm. There are still some desirable features to add, but I'll wait until users beat down my door with requests before spending more time on it. For now, I'm going to switch my spare-time Elm programming back to Xossbow.

Spokes now has chat, voting on impossible resolution (it was too time-consuming and difficult to do that automatically), public games, and a textual game state representation that allows you to restart a saved game.

You can play at gibgoygames.com/spokes.

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Mirroring a Blogspot Site

Submitted by Bill St. Clair on Thu, 13 Apr 2017 10:58:31 GMT

William Norman Grigg died yesterday. RIP.

When an important blogger passes, I often mirror their web site(s). I've been doing that with Mr. Grigg's Pro Libertate. site. It's on Blogspot, so doing a simple "wget -mk" pulls a separate file for each comment to each post, even though all those copies are identical. I finally figured out how to tell wget to NOT keep those files. It still downloads them all, scans for links that it already knows about, and then deletes them, but at least they don't stay to waste disk space. I have found no way to tell it to completely ignore those files. Mirroring would be much faster if that were possible. It took a few minutes to pull the 1,030 html files, and then a long time to pull and discard all the "?showComment" files.

I named the script that does this mirror-blogspot. The important line is:

wget -mk -R "*?showComment*" -pH
 -D "$DOMAIN,1.bp.blogspot.com,2.bp.blogspot.com,3.bp.blogspot.com,4.bp.blogspot.com" $1

-m is the standard wget mirror command. It enables recursive download, disables limits on that, and ensures that no links outside of the initial argument will be followed.

-k is --convert-links. It causes internal links to be changed from absolute to relative, so <a href='$1/foo'> becomes <a href='foo'>, with all the right stuff done to make that work correctly. Unfortunately, there's no way to tell wget to do that process on an existing mirror, so if your mirror quits before it's done, you're SOL.

-R is --reject. It's the important thing I learned yesterday. It tells wget to reject files whose names match the argument, which is either a list of file types or a pattern (not a regular expression).

-p is --page-requisites. It tells wget to download inline images, but will NOT by itself make it download from another domain.

-H is --span-hosts.

-D is --domains. If not specified with -r, then only files from the domain mentioned as the final argument will be downloaded. It allows you to add other domains, but also requires that you include the domain on the command line; hence $DOMAIN in that list. The *.bp.blogspot.com domains are where Blogspot stores images.

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Diceware Passphrase Generator Updated

Submitted by Bill St. Clair on Mon, 10 Apr 2017 03:22:51 GMT

I updated my Diceware Passphrase Generator, adding the EFF's two new (as of last July) wordlists for random passphrases.

The default is now six words from their short (4-die) table, instead of five words from the original (5-die) Diceware table.

I also improved the user interface for rolling your own physical dice. Instead of separate boxes for the dice rolls, there is now just one input box, it limits input to the number of dice necessary (4 or 5), and pressing the Enter/Return key will look up a word.

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Web Pages from JSON Templates in Elm

Submitted by Bill St. Clair on Fri, 24 Mar 2017 16:30:26 GMT

One of the problems I find with Elm is that the HTML is compiled into the JavaScript code for a web site. I wrote a template package to enable more data-driven HTML generation.

It's in the Elm repository at package.elm-lang.org/packages/billstclair/elm-html-template.

There's a live example website at lisplog.org/elm-html-template.

I'm switching now to a WordPress-like blogging package built on top of it. It will be similar to Lisplog (this web site), but will do all the rendering in the client browser, just as is done by the example, but with forms to create and edit content.

It's called XosBow, pronounced "Crossbow". That web site is a one-page place-holder for now, but once enough works, it will itself be done in XosBow.

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