Friday, March 25, 2016


In the wake of an epic ragequit where Azer Ko├žulu removed all of his modules from npm (the node.js package manager), there have been so many entertaining discussions and explanations covering what happened.

Today, Programming Praxis posted the leftpad challenge, pointing out that the original solution ran in quadratic time due to it's use of character-by-character string concatenation (but not pointing out that it only works with strings).

First, the original code in Javascript:

function leftpad (str, len, ch) {
  str = String(str);
  var i = -1;
  if (!ch && ch !== 0) ch = ' ';
  len = len - str.length;
  while (++i < len) {
    str = ch + str;
  return str;

Now, a (simpler? faster? more general?) version in Factor:

:: left-pad ( seq n elt -- newseq )
    seq n seq length [-] elt <repetition> prepend ;

Using it, you can see it works:

IN: scratchpad "hello" 3 CHAR: h left-pad .

IN: scratchpad "hello" 10 CHAR: h left-pad .

And it even works with other types of sequences:

IN: scratchpad { 1 2 3 } 3 0 left-pad .
{ 1 2 3 }

IN: scratchpad { 1 2 3 } 10 0 left-pad .
{ 0 0 0 0 0 0 0 1 2 3 }

I should also point out that Factor has pad-head that does this in the standard library and node.js has a pad-left module that solves the quadratic time problem (but still only works with strings).

Saturday, September 19, 2015


Most books are issued a unique International Standard Book Number (ISBN) number. Often different formats of the same book will have different ISBN numbers. On a print book, you can usually find the ISBN on a barcode on the back cover.

Most countries seem to have a national ISBN registration agency. In some countries this is a free service provided by a government agency. In other countries, this is operated by a commercial entity. In the United States, one company (R.R. Bowker LLC) has an apparent monopoly on issuing ISBN numbers which can cost $125 for one (less if you buy in bulk).

The ISBN is 13 digits long if assigned starting in 2007, and 10 digits long if assigned before 2007. Each ISBN contains a check digit which is used for basic error detection We are going to build a few words in Factor to calculate the check digits and validate ISBNs.

We need to turn an ISBN (which might include spaces or dashes) into numeric digits:

: digits ( str -- digits )
    [ digit? ] filter string>digits ;

For ISBN-10, the check digit is the sum of each of 10 digits multiplied by a weight (descending from 10 to 1) modulo 11.

: isbn-10-check ( digits -- n )
    0 [ 10 swap - * + ] reduce-index 11 mod ;

For ISBN-13, the check digit is the sum of each of 13 digits multiplied by a weight (alternating between 1 and 3) modulo 10.

: isbn-13-check ( digits -- n )
    0 [ even? 1 3 ? * + ] reduce-index 10 mod ;

We can validate an ISBN by grabbing the digits and running either the ISBN-10 or ISBN-13 check and verifying that the result is zero.

: valid-isbn? ( str -- ? )
    digits dup length {
        { 10 [ isbn-10-check ] }
        { 13 [ isbn-13-check ] }
    } case 0 = ;

The code (and some tests) for this is on my GitHub.

Friday, September 11, 2015

Pig Latin

Pig Latin is a somewhat ridiculous language game which modifies words in such a funny way that is hard to figure out if you don't know how it works but easy if you do. Using Factor, we will build a converter from English to Pig Latin words.

There are two basic rules we should implement:

  1. For words that begin with consonant sounds, the initial consonant or consonant cluster is moved to the end of the word, and "ay" is added to the end.
{ "igpay" } [ "pig" pig-latin ] unit-test
{ "ananabay" } [ "banana" pig-latin ] unit-test
{ "ashtray" } [ "trash" pig-latin ] unit-test
{ "appyhay" } [ "happy" pig-latin ] unit-test
{ "uckday" } [ "duck" pig-latin ] unit-test
{ "oveglay" } [ "glove" pig-latin ] unit-test
  1. For words that begin with a vowel sounds or silent letter, add "way" to the end.
{ "eggway" } [ "egg" pig-latin ] unit-test
{ "inboxway" } [ "inbox" pig-latin ] unit-test
{ "eightway" } [ "eight" pig-latin ] unit-test

We can implement our two basic rules:

: pig-latin ( str -- str' )
    dup [ "aeiou" member? ] find drop [
        "way" append
    ] [
        cut swap "ay" 3append
    ] if-zero ;

We could improve this by:

  • better handling of words that start with capital vowels or are all consonants
  • reverse the rules to convert Pig Latin back to English
  • variations such as adding "yay" (or "i") instead of "way"
  • different rules like adding "ag" before each vowel ("pagig lagatagin")
  • support language games in other languages

Anyway, this is available on my GitHub.

Sunday, August 30, 2015

Bowling Scores

Today we are going to explore building a bowling score calculator using Factor. In particular, we will be scoring ten-pin bowling.

There are a lot of ways to "golf" this, including this short version in F#, but we will build this in several steps through transformations of the input. The test input is a string representation of the hits, misses, spares, and strikes. The output will be a number which is your total score. We will assume valid inputs and not do much error-checking.

A sample game might look like this:


Our first transformation is to convert each character to a number of pins that have been knocked down for each ball. Strikes are denoted with X, spares with /, misses with -, and normal hits with a number.

: pin ( last ch -- pin )
        { CHAR: X [ 10 ] }
        { CHAR: / [ 10 over - ] }
        { CHAR: - [ 0 ] }
        [ CHAR: 0 - ]
    } case nip ;

We use this to convert the entire string into a series of pins knocked down for each ball.

: pins ( str -- pins )
    f swap [ pin dup ] { } map-as nip ;

A single frame will be either one ball, if a strike, or two balls. We are going to use cut-slice instead of cut because it will be helpful later.

: frame ( pins -- rest frame )
    dup first 10 = 1 2 ? short cut-slice swap ;

A game is 9 "normal" frames and then a last frame that could have up to three balls in it.

: frames ( pins -- frames )
    9 [ frame ] replicate swap suffix ;

Some frames will trigger a bonus. Strikes add the value of the next two balls. Spares add the value of the next ball. We build this by "un-slicing" the frame and calling sum on the next balls.

: bonus ( frame -- bonus )
    [ seq>> ] [ to>> tail ] [ length 3 swap - ] tri head sum ;

We can score the frames by checking for frames where all ten pins are knocked down (either spares or strikes) and adding their bonus.

: scores ( frames -- scores )
    [ [ sum ] keep over 10 = [ bonus + ] [ drop ] if ] map ;

We can solve the original goal by just adding all the scores:

: bowl ( str -- score )
    pins frames scores sum ;

And write a bunch of unit tests to make sure it works:

{ 0 } [ "---------------------" bowl ] unit-test
{ 11 } [ "------------------X1-" bowl ] unit-test
{ 12 } [ "----------------X1-" bowl ] unit-test
{ 15 } [ "------------------5/5" bowl ] unit-test
{ 20 } [ "11111111111111111111" bowl ] unit-test
{ 20 } [ "5/5-----------------" bowl ] unit-test
{ 20 } [ "------------------5/X" bowl ] unit-test
{ 40 } [ "X5/5----------------" bowl ] unit-test
{ 80 } [ "-8-7714215X6172183-" bowl ] unit-test
{ 83 } [ "12X4--3-69/-98/8-8-" bowl ] unit-test
{ 150 } [ "5/5/5/5/5/5/5/5/5/5/5" bowl ] unit-test
{ 144 } [ "XXX6-3/819-44X6-" bowl ] unit-test
{ 266 } [ "XXXXXXXXX81-" bowl ] unit-test
{ 271 } [ "XXXXXXXXX9/2" bowl ] unit-test
{ 279 } [ "XXXXXXXXXX33" bowl ] unit-test
{ 295 } [ "XXXXXXXXXXX5" bowl ] unit-test
{ 300 } [ "XXXXXXXXXXXX" bowl ] unit-test
{ 100 } [ "-/-/-/-/-/-/-/-/-/-/-" bowl ] unit-test
{ 190 } [ "9/9/9/9/9/9/9/9/9/9/9" bowl ] unit-test

This is available on my GitHub.

Wednesday, August 26, 2015


The Haikunator is a project to provide "Heroku-like memorable random names". These names usually consist of an adjective, a noun, and a random number or token. The original repository is implemented in Ruby, with ports to Go, Javascript, Python, PHP, Elixer, .NET, Java, and Dart.

We will be implementing this in Factor using the qw vocabulary that provides a simple way to make "arrays of strings" using the qw{ syntax.

First, a list of adjectives:

CONSTANT: adjectives qw{
    autumn hidden bitter misty silent empty dry dark summer icy
    delicate quiet white cool spring winter patient twilight
    dawn crimson wispy weathered blue billowing broken cold
    damp falling frosty green long late lingering bold little
    morning muddy old red rough still small sparkling throbbing
    shy wandering withered wild black young holy solitary
    fragrant aged snowy proud floral restless divine polished
    ancient purple lively nameless lucky odd tiny free dry
    yellow orange gentle tight super royal broad steep flat
    square round mute noisy hushy raspy soft shrill rapid sweet
    curly calm jolly fancy plain shinny

Next, a list of nouns:

CONSTANT: nouns qw{
    waterfall river breeze moon rain wind sea morning snow lake
    sunset pine shadow leaf dawn glitter forest hill cloud
    meadow sun glade bird brook butterfly bush dew dust field
    fire flower firefly feather grass haze mountain night pond
    darkness snowflake silence sound sky shape surf thunder
    violet water wildflower wave water resonance sun wood dream
    cherry tree fog frost voice paper frog smoke star atom band
    bar base block boat term credit art fashion truth disk
    math unit cell scene heart recipe union limit bread toast
    bonus lab mud mode poetry tooth hall king queen lion tiger
    penguin kiwi cake mouse rice coke hola salad hat

We will make a token out of digits:

CONSTANT: token-chars "0123456789"

Finally, a simple haikunate implementation:

: haikunate ( -- str )
    adjectives random
    nouns random
    4 [ token-chars random ] "" replicate-as
    "%s-%s-%s" sprintf ;

We can try it a few times, to see how it works:

IN: scratchpad haikunate .

IN: scratchpad haikunate .

IN: scratchpad haikunate .

IN: scratchpad haikunate .

IN: scratchpad haikunate .

IN: scratchpad haikunate .

Some versions of "haikunate" in other languages include features such as:

  • allow customization of the delimiter (dots are popular)
  • allow the token to be specified as a range of possible numbers
  • allow the token to be restricted to a maximum length
  • allow the token to be represented using hex digits
  • allow the token to be represented with custom character sets
  • etc.

This is available on my GitHub.

Monday, August 17, 2015

Random Desktop Background

As a follow-up to my Desktop Background post, I wanted to show how to set your desktop background to random images from various online image sites. We will be downloading a URL to a local file and then setting the desktop picture to that file:

: download-and-set-desktop-picture ( url -- )
    dup "/" split1-last nip cache-file
    [ download-to ] [ set-desktop-picture ] bi ;

Okay, now we need some random images!

Imgur is a huge image-hosting website frequently used on sites like Reddit.

: random-imgur ( -- url )
    "" scrape-html nip
    "image_src" "rel" find-by-attribute-key-value
    first "href" attribute ;

XKCD has some fun comics. Maybe they would look good on the desktop!

: random-xkcd ( -- url )
    "" http-get nip
    R@ http://imgs\.xkcd\.com/comics/[^\.]+\.(png|jpg)@
    first-match >string ;

WallpaperStock has a bunch of more traditional desktop images. We will scrape their random wallpaper page, find the first wallpaper thumbnail, load that wallpaper page, find the default image page, and then load that to find the image URL.

: random-wallpaperstock ( -- url )
    scrape-html nip "wallpaper_thumb" find-by-class-between
    "a" find-by-name nip "href" attribute
    "" prepend scrape-html nip
    "the_view_link" find-by-id nip "href" attribute
    "http:" prepend scrape-html nip "myImage" find-by-id nip
    "src" attribute "http:" prepend ;

Using this is as easy as:

IN: scratchpad random-imgur

IN: scratchpad random-xkcd

IN: scratchpad random-wallpaperstock

This is available on my GitHub.

Friday, August 14, 2015

Desktop Background

One of the benefits of learning to program is learning how to automate tasks performed with a computer. I thought it might be fun to build a simple vocabulary to allow getting and setting of the desktop background picture. Since Factor makes it pretty easy to build cross-platform vocabularies, we will implement this on Mac OS X, Linux, and Windows.

Our API consists of two words, one that gets the current desktop picture and one that sets a new desktop picture, dispatching based on which operating system we are running on (technically, based on the value of the os variable).

HOOK: get-desktop-picture os ( -- path )

HOOK: set-desktop-picture os ( path -- )

Mac OS X

On Mac OS X, we use AppleScript to ask the Finder what the path to the current desktop picture is, or tell it to set the desktop picture to a specific path.

M: macosx get-desktop-picture
        "osascript" "-e"
        "tell app \"Finder\" to get posix path of (get desktop picture as alias)"
    } utf8 [ readln ] with-process-reader ;

M: macosx set-desktop-picture
    "tell application \"Finder\" to set desktop picture to POSIX file \"%s\""
    sprintf run-apple-script ;


On Windows, we use the SystemParametersInfo function to get and set the desktop wallpaper.



M: windows get-desktop-picture
        0 SystemParametersInfo win32-error<>0
    ] keep alien>native-string ;

M: windows set-desktop-picture
    [ SPI_SETDESKWALLPAPER 0 ] dip utf16n encode
    0 SystemParametersInfo win32-error<>0 ;


On Linux, which has many different desktops, we are going to assume a GNOME environment. Other window managers have different ways to change the desktop background.

M: linux get-desktop-picture
    } utf8 [ readln ] with-process-reader
    "'file://" ?head drop "'" ?tail drop ;

M: linux set-desktop-picture
    } swap absolute-path "file://" prepend suffix try-process ;

This is available on my GitHub.