# Advent of Code 2017, Day 2

This is my solution for Day 2 of this year’s Advent of Code.

You may also enjoy browsing the Day 2 solutions megathread on Reddit.

## PROBLEM

The spreadsheet consists of rows of apparently-random numbers. To make sure the recovery process is on the right track, they need you to calculate the spreadsheet’s checksum. For each row, determine the difference between the largest value and the smallest value; the checksum is the sum of all of these differences.

For example, given the following spreadsheet:

```5 1 9 5
7 5 3
2 4 6 8
```

The first row’s largest and smallest values are 9 and 1, and their difference is 8.

The second row’s largest and smallest values are 7 and 3, and their difference is 4.

The third row’s difference is 6.

In this example, the spreadsheet’s checksum would be 8 + 4 + 6 = 18.

## SOLUTION

```(define (line->list line)
;; String -> List
(in-port (make-string-input-port line)))
(map string->number fields))))

;; File -> List[List[Number]]
(call-with-input-file file
(lambda (port)
(results '()))
(if (eof-object? line)
results
(loop (read-line port) (cons line results)))))))

(define (main prog+args)
(write (apply + (map
(lambda (row)
(let* ((xs (line->list row))
(min (apply min xs))
(max (apply max xs)))
(- max min)))
rows)))
(newline)))
```

# Advent of Code 2017, Day 1

This is my solution for Day 1 of this year’s Advent of Code.

You may also enjoy browsing the Day 1 solutions megathread on Reddit.

## PROBLEM

The captcha requires you to review a sequence of digits (your puzzle input) and find the sum of all digits that match the next digit in the list. The list is circular, so the digit after the last digit is the first digit in the list.

For example:

• 1122 produces a sum of 3 (1 + 2) because the first digit (1) matches the second digit and the third digit (2) matches the fourth digit.

• 1111 produces 4 because each digit (all 1) matches the next.

• 1234 produces 0 because no digit matches the next.

• 91212129 produces 9 because the only digit that matches the next one is the last digit, 9.

## SOLUTION

```(define captcha-input "5994521226795838")

(define (gather-matches s)
;; String -> List
(let ((in-port (make-string-input-port s)) (count 0) (head #f) (vals '()))
(let loop ((cur (read-char in-port)) (next (peek-char in-port)) (count count) (vals vals))
(if (eof-object? next)
(cons cur vals)
vals)
(cond ((= count 0)
(begin
(loop cur next (+ 1 count) vals)))
((char=? cur next)
(loop (read-char in-port) (peek-char in-port) (+ 1 count) (cons cur vals)))
(else (loop (read-char in-port) (peek-char in-port) (+ 1 count) vals)))))))

(define (main prog+args)
(matches* (map (lambda (c) (string->number (string c))) matches))
(sum (apply + matches*)))
(begin
(format #t "MATCHES*: ~A~%" matches*)
(format #t "SUM: ~A~%" sum))))
```

# Advent of Code, Day 2

This post describes my solution for Day 2 of the Advent of Code.

## Problem Description

First, the problem description (copied from the website):

Day 2: I Was Told There Would Be No Math

The elves are running low on wrapping paper, and so they need to submit an order for more. They have a list of the dimensions (length l, width w, and height h) of each present, and only want to order exactly as much as they need.

Fortunately, every present is a box (a perfect right rectangular prism), which makes calculating the required wrapping paper for each gift a little easier: find the surface area of the box, which is 2 x l x w + 2 x w x h + 2 x h x l. The elves also need a little extra paper for each present: the area of the smallest side.

For example:

• A present with dimensions 2x3x4 requires 2 x 6 + 2 x 12 + 2 x 8 = 52 square feet of wrapping paper plus 6 square feet of slack, for a total of 58 square feet.
• A present with dimensions 1x1x10 requires 2 x 1 + 2 x 10 + 2 x 10 = 42 square feet of wrapping paper plus 1 square foot of slack, for a total of 43 square feet.

All numbers in the elves’ list are in feet. How many total square feet of wrapping paper should they order?

## Solution

Once again, we’ll be working in Scheme.

For this problem, I decided to create a “box” data type. In addition to the automatically generated accessors (thanks SRFI-9!), I wrote several procedures to perform calculations on boxes, namely:

• `SURFACE-AREA`: Calculate the box’s surface area.
• `SMALLEST-SIDE`: Determine which of the box’s sides has the smallest surface area (the extra material makes it easier to wrap).

`WRAPPING-PAPER` is just a “wrapper” (pun intended) around the first two.

`LINE->BOX`, `READ-BOXES`, and `SUM-BOXES` are all about parsing the input file contents and shuffling them into the box data type that we use to do the actual calculation. The only part that required a bit of thought was the line with `STRING-TOKENIZE` in `LINE->BOX`. In Perl I’d use `my @params = split /x/, \$line` without even thinking, but I was less familiar with Scheme’s facility for solving this problem, so it took a few minutes to puzzle out the right part of Scheme’s “API”. (`STRING-TOKENIZE` was helpfully provided by SRFI-13.)

Abstract data types FTW! I’ll be using them more as the month’s challenges progress.

```;; ,open srfi-9 srfi-13 sort

(define-record-type box
(make-box l w h)
box?
(l box-length set-box-length!)
(w box-width set-box-width!)
(h box-height set-box-height!))

(define (surface-area box)
;; Box -> Int
(let ((l (box-length box))
(w (box-width box))
(h (box-height box)))
(+ (* 2 l w)
(* 2 w h)
(* 2 h l))))

(define (smallest-side box)
;; Box -> Int
(define (smallest-two xs)
;; List -> List
(let ((sorted (sort-list xs <)))
(list (first sorted)
(second sorted))))
(let ((l (box-length box))
(w (box-width box))
(h (box-height box)))
(apply * (smallest-two (list l w h)))))

(define (wrapping-paper box)
;; Box -> Int
(let ((minimum (surface-area box))
(extra (smallest-side box)))
(+ minimum extra)))

(define (line->box line)
;; String -> Box
(define (line->lon s)
;; String -> List<Number>
(let ((xs (string-tokenize s (char-set-complement (char-set #\x)))))
(map string->number xs)))
(let* ((dims (line->lon line)))
(let ((l (first dims))
(w (second dims))
(h (third dims)))
(make-box l w h))))

;; Pathname -> List<Box>
(with-input-from-file file
(lambda ()
(ys '()))
(if (eof-object? line)
ys
(loop
(cons (line->box line) ys)))))))

(define (sum-boxes boxes)
;; List<Box> -> Int
(let ((xs (map wrapping-paper boxes)))
(apply + xs)))

;; eof
```

# Make your own iterators in Scheme using closures

## Introduction

I recently bought a copy of Higher Order Perl (also known as HOP) by Mark Dominus. In chapter 4, he introduces iterators. At a high level, an iterator is a black box with an input funnel, an output funnel, and a big red “NEXT” button. You put something into the funnel at time `T`; then, at some later time `T+n`, you press the “NEXT” button and something useful comes out.

Iterators are useful in cases when you want to access a potentially huge list of things one at a time, but you don’t have access to a computer with infinite memory.

In this post we’ll look at a moderately useful iterator example; a directory walker that recurses down a directory tree on your file system. The implementation is given in Scheme (specifically, the scsh dialect); the code and the descriptions thereof are heavily inspired by Dominus’ excellent book. However, this post contains the work of an amateur. Any errors or omissions in the following code and explanations are mine.

## Iterators are made with closures

In order to build a useful iterator, we need to have a basic understanding of closures. A closure is just a function packaged up with an environment. The environment allows us to store up state between function calls. As noted above, it’s easiest just to think of it as a little black box that we can pass around in our program.

In Scheme and Common Lisp, you build functions with `LAMBDA`. Here’s a very simple closure example in Scheme, translated from the first example given in HOP:

```(define (upto m n)
(set! m (- m 1)) ;needed since we don't have post-increment (\$i++)
(lambda ()
(if (< m n)
(begin (set! m (+ m 1)) m)
#f)))
```

This is a function that returns a function (a closure). To use the closure returned by `UPTO`, assign it to a variable and call it whenever you want the next value. When the iterator is exhausted, it will return `#f`.

```> (define 1to10 (upto 1 10))
> (1to10)
1
> (1to10)
2
...
> (1to10)
9
> (1to10)
10
> (1to10)
#f
```

## A trivial file tree iterator

A more interesting and practical use of this technique is to walk a directory tree on your computer. In the Scheme procedure below, we take a list of directories and then build and return a closure that, when called, will walk the directory tree breadth-first, printing out the names of files as we go.

```(define (dir-walk queue)
(let ((file #f))
(lambda ()
(if (not (null? queue))
(begin (set! file (car queue))
(set! queue (cdr queue))
(cond ((file-directory? file)
(let ((new-files (directory-files file)))
(set! queue (append queue (map (lambda (filename)
(string-append file "/" filename))
new-files)))
file))
((file-regular? file) file)
(else #f)))))))
```

The important part to notice is the inner `LAMBDA`. This is where we create a closure by packaging up a procedure with an environment. The closure’s environment remembers the contents of the variable `QUEUE` in memory, so we can ask for the elements of `QUEUE` at our leisure.

Here it is running on my machine:

```> (define dir-iter1 (dir-walk '("/Users/rloveland/Desktop/current/")))
> (dir-iter1)
"/Users/rloveland/Desktop/current/XProlog.jar"
> (dir-iter1)
"/Users/rloveland/Desktop/current/all-jira-issues.txt"
> (dir-iter1)
"/Users/rloveland/Desktop/current/automate-campaign-setup.txt"
> (dir-iter1)
"/Users/rloveland/Desktop/current/buflocal.scm"
> (dir-iter1)
"/Users/rloveland/Desktop/current/bundle.js"
```

Just for fun, here’s a version of the same function in Common Lisp. It’s essentially the same as the scsh version, save that it uses the nice CL pathname machinery instead of appending raw strings, and also makes use of the CL-FAD convenience library.

```(defun dir-walk (queue)
(let ((file nil))
(lambda ()
(if (not (null queue))
(progn
(setf file (car queue))
(setf queue (cdr queue))
(setf queue (append queue (mapcar #'(lambda (filename)
(merge-pathnames filename file))
new-files)))
file))
(t nil)))))))
```

Usage is also essentially the same:

```CL-USER> (defvar *tree-walker* (dir-walk '(#P"/Users/rloveland/Desktop/current/")))
*TREE-WALKER*
CL-USER> (type-of *tree-walker*)
FUNCTION
CL-USER> (funcall *tree-walker*)
#P"/Users/rloveland/Desktop/current/"
CL-USER> (funcall *tree-walker*)
#P"/Users/rloveland/Dropbox/current/.debuggerDefaults"
CL-USER> (funcall *tree-walker*)
#P"/Users/rloveland/Dropbox/current/.DS_Store"
CL-USER> (funcall *tree-walker*)
#P"/Users/rloveland/Dropbox/current/A"
CL-USER> (funcall *tree-walker*)
#P"/Users/rloveland/Dropbox/current/A.hi"
CL-USER> (funcall *tree-walker*)
#P"/Users/rloveland/Dropbox/current/A.hs"
```

## A slightly less trivial file tree iterator

Listing out files and directories is nice, but not that useful. We’d like a way to see only those files and directories that have some interesting property.

This can be accomplished by passing in another argument to our `DIR-WALK` function: this argument will be yet another function that will test the current file to see whether it’s interesting to us. It’s pretty easy to change `DIR-WALK` to accept a function argument, `INTERESTING?`. This arbitrary function is used to check the file to see if we care about it.

This time around, when we build our internal queue, we use a call to `FILTER` to make sure that only interesting files get added.

```(define (dir-walk* interesting? queue)
(let ((file #f))
(lambda ()
(if (not (null? queue))
(begin (set! file (car queue))
(set! queue (cdr queue))
(cond
((file-directory? file)
(let ((new-files (directory-files file)))
(set! queue (append queue (filter interesting? (map (lambda (filename)
(string-append file "/" filename))
new-files))))
(if (interesting? file) file)))
((interesting? file) file)
(else #f)))
#f))))
```

And here it is in use; in this example, we pass an `INTERESTING?` function that asks for only files that are not marked as executable:

```> (define dir-iter2 (dir-walk* (lambda (f) (file-not-executable? f)) '("/home/rml/Desktop/current")))
> (dir-iter2)
> (dir-iter2)
"/home/rml/Desktop/current/A.hi"
> (dir-iter2)
"/home/rml/Desktop/current/A.hs"
> (dir-iter2)
"/home/rml/Desktop/current/A.o"
> (dir-iter2)
"/home/rml/Desktop/current/ABBREV.xlsx"
> (dir-iter2)
> (dir-iter2)
```

## Conclusion

There are still bugs and unexpected behavior in `DIR-WALK*`. For example, on the first call to the resulting iterator there is no result due to the one-armed `IF`. There are also strange things that happen if we want to filter out directories from our results, since we mix together the `FILE-DIRECTORY?` check and the `INTERESTING?` check inside the iterator. However, despite these small nits, it can still do useful work on my machine 1, and it’s a good enough example of using closures to build iterators.

Here’s hoping you’ll enjoy playing with closures and iterators in your own code!

(Image courtesy zeitfanger.at under a Creative Commons License.)

## Footnotes:

1 In the grandest internet tradition, “It works on my machine” ™.

# Scsh is UNIX, Part 1: Sockets

Inspired by words from Ryan Tomayko and Jacob Kaplan-Moss, I set out to see how difficult it would be to implement a tiny UNIX echo server in scsh. It turns out to be fairly easy. In this post, we’ll cover:

• How the UNIX socket programming model works at a high level
• How to write a basic echo server in scsh

Unlike the echo servers of Tomayko and Kaplan-Moss, this one doesn’t use `fork` (yet). We will add forking to the server in a follow-up post. Hopefully this will make it easier for less experienced UNIX programmers (like me) to get started with.

# The UNIX socket programming model for dummies

At a high level, the steps to open a server-side socket are:

• create a socket “thing”
• toggle some settings on it so the OS knows it’s for interweb use
• bind it to an address
• listen for connections on it
• start accepting those connections

scsh does its part to make this easy by providing a high-level Scheme procedure, `BIND-LISTEN-ACCEPT-LOOP`, which handles this stuff for you, in addition to some nifty error handling and other bookkeeping chores. But we’re going to ignore that for now and write everything by hand to see how it’s done.

You should be able to enter all of the code in the next section directly in the scsh top-level. You don’t need to open any additional packages; this is just plain old scsh.

# Our echo server

Our server takes two arguments: the `PROC` is a procedure that causes the server to actually do something; we’ll write in a minute. The `PORT` is the port you want to serve on, e.g., 49995:

``````(define (server proc port)
(let* ((sock (create-socket protocol-family/internet socket-type/stream))
(listen-socket sock 5)
(let loop ()
(lambda () (accept-connection sock))
proc
(loop))))
``````

The first thing you’ll notice is that it’s pretty sequential and quite simple really. We just go through the socket opening dance we described earlier: open, configure, bind, listen, and accept.

Since this is an echo server, we’ll just, like, echo stuff:

``````(define (echo socket address)
(let* ((in (socket:inport socket))
(out (socket:outport socket))
(format out "~A~%" message)
(force-output out)))
``````

As you can see, our `ECHO` procedure takes a socket and an address. (We don’t do anything with the address in this example, but our procedure needs to “implement this interface”, as they say, in order to work. You can see this for yourself in the scsh 0.6.7 tarball; it’s in `scsh/network.scm`.)

In our `LET*`-binding we create some convenient locally bound names for the socket structure’s input and output ports, and then we read a line from the socket’s input port.

Since this is echo server, we just write the same data back out with `FORMAT`. We call `FORCE-OUTPUT` to flush output to the terminal immediately. Otherwise Scheme will wait for the operating system’s output buffer to fill before writing out, and it will appear to the user that nothing is happening.

# Trying it out

Let’s start it up and see if it works. Assuming you’ve loaded the procedures above in your scsh image somehow, enter this at the REPL:

``````> (server echo 49995)
``````

The REPL should appear to “hang” while the server is running. Now go to your terminal and connect to the echo server. There are several ways to do it; here’s what I used:

``````~ \$ telnet localhost 49995
Trying ::1...
telnet: connect to address ::1: Connection refused
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
HELLO
HELLO
who's there?
who's there?
i don't know
i don't know
could you stop that?
could you stop that?
fine then
fine then
goodbye
goodbye
``````

Hopefully that was enough to get you started playing with sockets in scsh. I’ll write a followup post where we talk about the UNIX forking process model and update this example to make it a “preforking” server.

(Image courtesy Hope for Gorilla under a Creative Commons license.)