## Computing Eulerian paths is harder than you think

Everyone who has studied any graph theory at all knows the celebrated story of the Seven Bridges of Königsberg, and how Euler gave birth to modern graph theory while solving the problem.

Euler’s proof is clever, incisive, not hard to understand, and a great introduction to the kind of abstract reasoning we can do about graphs. There’s little wonder that it is often used as one of the first nontrivial graph theory results students are introduced to, e.g. in a discrete mathematics course. (Indeed, I will be teaching discrete mathematics in the spring and certainly plan to talk about Eulerian paths!)

Euler’s 1735 solution was not constructive, and in fact he really only established one direction of the “if and only if”:

If a graph has an Eulerian path, then it has exactly zero or two vertices with odd degree.

This can be used to rule out the existence of Eulerian paths in graphs without the right vertex degrees, which was Euler’s specific motivation. However, one suspects that Euler knew it was an if and only if, and didn’t write about the other direction (if a graph has exactly zero or two vertices with odd degree, then it has an Eulerian path) because he thought it was trivial.1

The first person to publish a full proof of both directions, including an actual algorithm for finding an Eulerian path, seems to be Carl Hierholzer, whose friend published a posthumous paper in Hierholzer’s name after his untimely death in 1871, a few weeks before his 31st birthday.2 (Notice that this was almost 150 years after Euler’s original paper!) If the vertex degrees cooperate, finding an Eulerian path is almost embarrassingly easy according to Hierholzer’s algorithm: starting at one of the odd-degree vertices (or anywhere you like if there are none), just start walking through the graph—any which way you please, it doesn’t matter!—visiting each edge at most once, until you get stuck. Then pick another part of the graph you haven’t visited, walk through it randomly, and splice that path into your original path. Repeat until you’ve explored the whole graph. And generalizing all of this to directed graphs isn’t much more complicated.

So, in summary, this is a well-studied problem, solved hundreds of years ago, that we present to students as a first example of a nontrivial yet still simple-to-understand graph proof and algorithm. So it should be pretty easy to code, right?

## So what’s the problem?

Recently I came across the eulerianpath problem on Open Kattis, and I realized that although I have understood this algorithm on a theoretical level for almost two decades (I almost certainly learned it as a young undergraduate), I have never actually implemented it! So I set out to solve it.

Right away the difficulty rating of 5.7 tells us that something strange is going on. “Easy” problems—the kind of problems you can give to an undergraduate at the point in their education when they might first be presented with the problem of finding Eulerian paths—typically have a difficulty rating below 3. As I dove into trying to implement it, I quickly realized two things. First of all, given an arbitrary graph, there’s a lot of somewhat finicky work that has to be done to check whether the graph even has an Eulerian path, before running the algorithm proper:

1. Calculate the degree of all graph vertices (e.g. by iterating through all the edges and incrementing appropriate counters for the endpoints of each edge).
2. Check if the degrees satisfy Euler’s criteria for the existence of a solution, by iterating through all vertices and making sure their degrees are all even, but also counting the number of vertices with an odd degree to make sure it is either zero or two. At the same time, if we see an odd-degree vertex, remember it so we can be sure to start the path there.
3. If all vertices have even degree, pick an arbitrary node as the start vertex.
4. Ensure the graph is connected (e.g. by doing a depth-first search)—Euler kind of took this for granted, but this technically has to be part of a correct statement of the theorem. If we have a disconnected graph, each component could have an Eulerian path or cycle without the entire graph having one.

And if the graph is directed—as it is in the eulerianpath problem on Kattis—then the above steps get even more finicky. In step 1, we have to count the in- and outdegree of each vertex separately; in step 2, we have to check that the in- and outdegrees of all vertices are equal, except for possibly two vertices where one of them has exactly one more outgoing than incoming edge (which must be the start vertex), and vice versa for the other vertex; in step 4, we have to make sure to start the DFS from the chosen start vertex, because the graph need not be strongly connected, it’s enough for the entire graph to be reachable from the start vertex.

The second thing I realized is that Hierholzer’s algorithm proper—walk around until getting stuck, then repeatedly explore unexplored parts of the graph and splice them into the path being built—is still rather vague, and it’s nontrivial to figure out how to do it, and what data structures to use, so that everything runs in time linear in the number of edges. For example, we don’t want to iterate over the whole graph—or even just the whole path built so far—to find the next unexplored part of the graph every time we get stuck. We also need to be able to do the path splicing in constant time; so, for example, we can’t just store the path in a list or array, since then splicing in a new path segment would require copying the entire path after that point to make space. I finally found a clever solution that pushes the nodes being explored on a stack; when we get stuck, we start popping nodes, placing them into an array which will hold the final path (starting from the end), and keep popping until we find a node with an unexplored outgoing edge, then switch back into exploration mode, pushing things on the stack until we get stuck again, and so on. But this is also nontrivial to code correctly since there are many lurking off-by-one errors and so on. And I haven’t even talked about how we keep track of which edges have been explored and quickly find the next unexplored edge from a vertex.

I think it’s worth writing another blog post or two with more details of how the implementation works, both in an imperative language and in a pure functional language, and I may very well do just that. But in any case, what is it about this problem that results in such a large gap between the ease of understanding its solution theoretically, and the difficulty of actually implementing it?

1. Actually, the way I have stated the other direction of the if and only if is technically false!—can you spot the reason why?

2. Though apparently someone named Listing published the basic idea of the proof, with some details omitted, some decades earlier. I’ve gotten all this from Herbert Fleischner, Eulerian Graphs and Related Topics, Annals of Discrete Mathematics 45, Elsevier 1990. Fleischner reproduces Euler’s original paper as well as Hierholzer’s, together with English translations.

Posted in learning | Tagged , , , , | 3 Comments

In my last post in this series, we looked at building a small Scanner combinator library for lightweight input parsing. It uses String everywhere, and usually this is fine, but occasionally it’s not.

A good example is the Kattis problem Army Strength (Hard). There are a number of separate test cases; each test case consists of two lines of positive integers which record the strengths of monsters in two different armies. Supposedly the armies will have a sequence of battles, where the weakest monster dies each time, with some complex-sounding rules about how to break ties. It sounds way more complicated than it really is, though: a bit of thought reveals that to find out who wins we really just need to see which army’s maximum-strength monster is strongest.

So our strategy for each test case is to read in the two lists of integers, find the maximum of each list, and compare. Seems pretty straightforward, right? Something like this:

import           Control.Arrow
import           Data.List.Split

main = interact $lines >>> drop 1 >>> chunksOf 4 >>> map (drop 2 >>> map (words >>> map read) >>> solve) >>> unlines solve :: [[Int]] -> String solve [gz, mgz] = case compare (maximum gz) (maximum mgz) of LT -> "MechaGodzilla" _ -> "Godzilla" Note I didn’t actually use the Scanner abstraction here, though I could have; it’s actually easier to just ignore the numbers telling us how many test cases there are and the length of each line, and just split up the input by lines and go from there. This seems straightforward enough, but sadly, it results in a Time Limit Exceeded (TLE) error on the third of three test cases. Apparently this program takes longer than the allowed 1 second. What’s going on? If we look carefully at the limits for the problem, we see that there could be up to 50 test cases, each test case could have two lists of length $10^5$, and the numbers in the lists can be up to $10^9$. If all those are maxed out (as they probably are in the third, secret test case), we are looking at an input file many megabytes in size. At this point the time to simply read the input is a big factor. Reading the input as a String has a lot of overhead: each character gets its own cons cell; breaking the input into lines and words requires traversing over these cons cells one by one. We need a representation with less overhead. Now, if this were a real application, we would reach for Text, which is made for representing textual information and can correctly handle unicode encodings and all that good stuff. However, this isn’t a real application: competitive programming problems always limit the input and output strictly to ASCII, so characters are synonymous with bytes. Therefore we will commit a “double no-no”: not only are we going to use ByteString to represent text, we’re going to use Data.ByteString.Lazy.Char8 which simply assumes that each 8 bits is one character. As explained in a previous post, however, I think this is one of those things that is usually a no-no but is completely justified in this context. Let’s start by just replacing some of our string manipulation with corresponding ByteString versions: import Control.Arrow import qualified Data.ByteString.Lazy.Char8 as C import Data.List.Split main = C.interact$
C.lines >>> drop 1 >>> chunksOf 4 >>>
map (drop 2 >>> map (C.words >>> map (C.unpack >>> read)) >>> solve) >>>
C.unlines

solve :: [[Int]] -> C.ByteString
solve [gz, mgz] = case compare (maximum gz) (maximum mgz) of
LT -> C.pack "MechaGodzilla"
_  -> C.pack "Godzilla"

This already helps a lot: this version is actually accepted, taking 0.66 seconds. (Note there’s no way to find out how long our first solution would take if allowed to run to completion: once it goes over the time limit Kattis just kills the process. So we really don’t know how much of an improvement this is, but hey, it’s accepted!)

But we can do even better: it turns out that read also has a lot of overhead, and if we are specifically reading Int values we can do something much better. The ByteString module comes with a function

readInt :: C.ByteString -> Maybe (Int, C.ByteString)

Since, in this context, we know we will always get an integer with nothing left over, we can replace C.unpack >>> read with C.readInt >>> fromJust >>> fst. Let’s try it:

import           Control.Arrow
import qualified Data.ByteString.Lazy.Char8 as C
import           Data.List.Split
import           Data.Maybe (fromJust)

main = C.interact $C.lines >>> drop 1 >>> chunksOf 4 >>> map (drop 2 >>> map (C.words >>> map readInt) >>> solve) >>> C.unlines where readInt = C.readInt >>> fromJust >>> fst solve :: [[Int]] -> C.ByteString solve [gz, mgz] = case compare (maximum gz) (maximum mgz) of LT -> C.pack "MechaGodzilla" _ -> C.pack "Godzilla" Now we’re talking — this version completes in a blazing 0.04 seconds! We can take these principles and use them to make a variant of the Scanner module from last time which uses (lazy, ASCII) ByteString instead of String, including the use of the readInt functions to read Int values quickly. You can find it here. Posted in haskell | | Leave a comment ## Lightweight invertible enumerations in Haskell In a previous post I introduced a new Haskell library for enumerations (now on Hackage as simple-enumeration). The Data.Enumeration module defines a type Enumeration a, represented simply by a function Integer -> a which picks out the value of type a at a given index. This representation has a number of advantages, including the ability to quickly index into very large enumerations, and the convenience that comes from having Functor, Applicative, and Alternative instances for Enumeration. I’ve just uploaded version 0.2 of the package, which adds a new Data.Enumeration.Invertible module with a new type, IEnumeration a, representing invertible enumerations. Whereas a normal enumeration is just a function from index to value, an invertible enumeration is a bijection between indices and values. In particular, alongside the Integer -> a function for picking out the value at an index, an invertible enumeration also stores an inverse function a -> Integer (called locate) for finding the index of a given value. On the one hand, this comes at a cost: because the type parameter a now occurs both co- and contravariantly, IEnumeration i s no longer an instance of Functor, Applicative, or Alternative. There is a mapE combinator provided for mapping IEnumeration a to IEnumeration b, but in order to work it needs both an a -> b function and an inverse b -> a. On the other hand, we also gain something: of course the ability to look up the index of a value is nifty, and beyond that we also get a combinator functionOf :: IEnumeration a -> IEnumeration b -> IEnumeration (a -> b) which works as long as the IEnumeration a is finite. This is not possible to implement with normal, non-invertible enumerations: we have to take an index and turn it into a function a -> b, but that function has to take an a as input and decide what to do with it. There’s nothing we can possibly do with a value of type a unless we have a way to connect it back to the IEnumeration a it came from. Here’s a simple example of using the functionOf combinator to enumerate all Bool -> Bool functions, and then locating the index of not: >>> bbs = functionOf (boundedEnum @Bool) (boundedEnum @Bool) >>> card bbs Finite 4 >>> locate bbs not 2 >>> map (select bbs 2) [False, True] [True,False] And here’s an example of enumerating recursive trees, which is parallel to an example given in my previous post. Note, however, how we can no longer use combinators like <$>, <*>, and <|>, but must explicitly use <+> (disjoint sum of enumerations) and >< (enumeration product) in combination with mapE. In return, though, we can find the index of any given tree in addition to selecting trees by index.

data Tree = L | B Tree Tree
deriving Show

toTree :: Either () (Tree, Tree) -> Tree
toTree = either (const L) (uncurry B)

fromTree :: Tree -> Either () (Tree, Tree)
fromTree L       = Left ()
fromTree (B l r) = Right (l,r)

trees :: IEnumeration Tree
trees = infinite $mapE toTree fromTree (unit <+> (trees >< trees)) >>> locate trees (B (B L (B L L)) (B (B L (B L L)) (B L (B L L)))) 123 >>> select trees 123 B (B L (B L L)) (B (B L (B L L)) (B L (B L L))) Of course, the original Data.Enumeration module remains available; there is clearly an inherent tradeoff to invertibility, and you are free to choose either style depending on your needs. Other than the tradeoffs outlined above and a couple other minor exceptions, the two modules export largely identical APIs. Posted in combinatorics, haskell, projects | | 1 Comment ## Competitive Programming in Haskell: Scanner In my previous post I explored solving a simple competitive programming problem in Haskell. The input of the problem just consisted of a bunch of lines containing specific data, so that we could parse it using lines and words. There is another common class of problems, however, which follow this pattern: The first line of the input consists of an integer $T$. Each of the next $T$ lines consists of… That is, the input contains integers which are not input data per se but just tell you how many things are to follow. This is really easy to process in an imperative language like Java or C++. For example, in Java we might write code like this: Scanner in = new Scanner(System.in); int T = in.nextInt(); for (int i = 0; i < T; i++) { // process each line } Occasionally, we can get away with completely ignoring the extra information in Haskell. For example, if the input consists of a number $T$ followed by $T$ lines, each of which contains a number $n$ followed by a list of $n$ numbers, we can just write main = interact$
lines >>> drop 1 >>> map (words >>> drop 1 >>> map read) >>> ...

That is, we can ignore the first line containing $T$ since the end-of-file will tell us how many lines there are; and we can ignore the $n$ at the beginning of each line, since the newline character tells us when the list on that line is done.

Sometimes, however, this isn’t possible, especially when there are multiple test cases, or when a single test case has multiple parts, each of which can have a variable length. For example, consider Popular Vote, which describes its input as follows:

The first line of input contains a single positive integer $T \leq 500$ indicating the number of test cases. The first line of each test case also contains a single positive integer $n$ indicating the number of candidates in the election. This is followed by $n$ lines, with the $i$th line containing a single nonnegative integer indicating the number of votes candidate $i$ received.

How would we parse this? We could still ignore $T$—just keep reading until the end of the file—but there’s no way we can ignore the $n$ values. Since the values for each test case are all on separate lines instead of on one line, there’s otherwise no way to know when one test case ends and the next begins.

Once upon a time, I would have done this using splitAt and explicit recursion, like so:

type Election = [Int]

readInput = lines >>> drop 1 {- ignore T -} >>> map read >>> go
where
go :: [Int] -> [Election]
go []     = []
go (n:xs) = votes : go rest
where (votes,rest) = splitAt n xs

However, this is really annoying to write and easy to get wrong. There are way too many variable names to keep track of (n, xs, votes, rest, go) and for more complex inputs it becomes simply unmanageable. You might think we should switch to using a real parser combinator library—parsec is indeed installed in the environment Kattis uses to run Haskell solutions—and although sometimes a full-blown parser combinator library is needed, in this case it’s quite a bit more heavyweight than we would like. I can never remember which modules I have to import to get parsec set up; there’s a bunch of boilerplate needed to set up a lexer; and so on. Using parsec is only worth it if we’re parsing something really complex.

## Scanner

The heart of the issue is that we want to be able to specify a high-level description of the sequence of things we expect to see in the input, without worrying about managing the stream of tokens explicitly. Another key insight is that 99% of the time, we don’t need the ability to deal with parse failure or the ability to parse multiple alternatives. With these insights in mind, we can create a very simple Scanner abstraction, which is just a Stateful computation over a list of tokens:

type Scanner = State [String]

runScanner :: Scanner a -> String -> a
runScanner s = evalState s . words

To run a scanner, we just feed it the entire input as a String, which gets chopped into tokens using words. (Of course in some scenarios we might want to use lines instead of words, or even do more complex tokenization.)

Note since Scanner is just a type synonym for State [String], it is automatically an instance of Functor, Applicative, and Monad (but not Alternative).

So let’s develop a little Scanner DSL. The most fundamental thing we can do is read the next token.

str :: Scanner String
str = get >>= \case { s:ss -> put ss >> return s }

(This uses the LambdaCase extension, though we could easily rewrite it without.) str gets the current list of tokens, puts it back without the first token, and returns the first token. Note that I purposely didn’t include a case for the empty list. You might think we want to include a case for the empty token list and have it return the empty string or something like that. But since the input will always be properly formatted, if this scenario ever happens it means my program has a bug—e.g. perhaps I misunderstood the description of the input format. In this scenario I want it to crash loudly, as soon as possible, rather than continuing on with some bogus data.

We can now add some scanners for reading specific token types other than String, simply by mapping the read function over the output of str:

int :: Scanner Int
int = read <$> str integer :: Scanner Integer integer = read <$> str

double :: Scanner Double
double = read <$> str Again, these will crash if they see a token in an unexpected format, and that is a very deliberate choice. Now, as I explained earlier, a very common pattern is to have an integer $n$ followed by $n$ copies of something. So let’s make a combinator to encapsulate that pattern: numberOf :: Scanner a -> Scanner [a] numberOf s = int >>= flip replicateM s numberOf s expects to first see an Int value $n$, and then it runs the provided scanner $n$ times, returning a list of the results. It’s also sometimes useful to have a way to repeat a Scanner some unknown number of times until encountering EOF (for example, the input for some problems doesn’t specify the number of test cases up front the way that Popular Vote does). This is similar to the many combinator from Alternative. many :: Scanner a -> Scanner [a] many s = get >>= \case { [] -> return []; _ -> (:) <$> s <*> many s }

many s repeats the scanner s as many times as it can, returning a list of the results. In particular it first peeks at the current token list to see if it is empty. If so, it returns the empty list of results; if there are more tokens, it runs s once and then recursively calls many s, consing the results together.

Finally, it’s quite common to want to parse a specific small number of something, e.g. two double values representing a 2D coordinate pair. We could just write replicateM 2 double, but this is common enough that I find it helpful to define dedicated combinators with short names:

two, three, four :: Scanner a -> Scanner [a]
[two, three, four] = map replicateM [2..4]

The complete file can be found on GitHub. As I continue this series I’ll be putting more code into that repository. Note I do not intend to make this into a Hackage package, since that wouldn’t be useful: you can’t tell Kattis to go download a package from Hackage before running your submission. However, it is possible to submit multiple files at once, so you can include Scanner.hs in your submission and just import Scanner at the top of your main module.

## Examples

So what have we gained? Writing the parser for Popular Vote is now almost trivial:

type Election = [Int]

main = interact $runScanner elections >>> ... elections :: Scanner [Election] elections = numberOf (numberOf int) In practice I would probably just inline the definition of elections directly: interact$ runScanner (numberOf (numberOf int)) >>> ...

As a slightly more involved example, chosen almost at random, consider Board Wrapping:

On the first line of input there is one integer, $N \leq 50$, giving the number of test cases (moulds) in the input. After this line, $N$ test cases follow. Each test case starts with a line containing one integer $n, 1 \leq n \leq 600$, which is the number of boards in the mould. Then $n$ lines follow, each with five floating point numbers $x,y,w,h,v$ where $0 \leq x,y,w,h \leq 10000$ and $-90^{\circ} < v \leq 90^{\circ}$. The $x$ and $y$ are the coordinates of the center of the board and $w$ and $h$ are the width and height of the board, respectively. $v$ is the angle between the height axis of the board to the $y$-axis in degrees, positive clockwise.

Here’s how I would set up the input, using Scanner and a custom data type to represent boards.

import Scanner

type V = [Double]     -- 2D vectors/points
newtype A = A Double  -- angle (radians)
-- newtype helps avoid conversion errors

fromDeg :: Double -> A
fromDeg d = A (d * pi / 180)

data Board = Board { boardLoc :: V, boardDims :: V, boardAngle :: A }

board :: Scanner Board
board = Board
<$> two double <*> two double <*> ((fromDeg . negate) <$> double)

main = interact $runScanner (numberOf (numberOf board)) >>> ... Posted in haskell | | 8 Comments ## Lightweight, efficiently sampleable enumerations in Haskell For another project I’m working on, I needed a way to enumerate and randomly sample values from various potentially infinite collections. There are plenty of packages in this space, but none of them quite fit my needs: • universe (and related packages) is very nice, but it’s focused on enumerating values of Haskell data types, not arbitrary sets: since it uses type classes, you have to make a new Haskell type for each thing you want to enumerate. It also uses actual Haskell lists of values, which doesn’t play nicely with sampling. • enumerable has not been updated in a long time and seems to be superseded by universe. • enumerate is likewise focused on generating values of Haskell data types, with accompanying generic deriving machinery. • size-based is used as the basis for the venerable testing-feat library, but these are again focused on generating values of Haskell data types. I’m also not sure I need the added complexity of size-indexed enumerations. • enumeration looks super interesting, and I might be able to use it for what I want, but (a) I’m not sure whether it’s maintained anymore, and (b) it seems rather more complex than I need. I really want something like Racket’s nice data/enumerate package, but nothing like that seems to exist in Haskell. So, of course, I made my own! For now you can find it on GitHub.1 Here’s the package in a nutshell: • Enumerations are represented by the parameterized type Enumeration, which is an instance of Functor, Applicative, and Alternative (but not Monad). • Enumerations keep track of their cardinality, which could be either countably infinite or a specific natural number. • Enumerations are represented as functions from index to value, so they can be efficiently indexed (which also enables efficient random sampling). • The provided combinators will always do something sensible so that every value in the resulting enumeration occurs at a finite index. For example, if you take the disjoint union of two infinite enumerations, the resulting enumeration will alternate between values from the two inputs. I wrote about something similar a few years ago. The main difference is that in that post I limited myself to only finite enumerations. There’s a lot more I could say but for now I think I will just show some examples: >>> enumerate empty [] >>> enumerate unit [()] >>> enumerate$ empty <|> unit <|> unit
[(),()]

>>> enumerate $finite 4 >< finiteList [27,84,17] [(0,27),(0,84),(0,17),(1,27),(1,84),(1,17),(2,27),(2,84),(2,17),(3,27),(3,84),(3,17)] >>> select (finite 4000000000000 >< finite 123456789) 0 (0,0) >>> select (finite 4000000000000 >< finite 123456789) 196598723084073 (1592449,82897812) >>> card (finite 4000000000000 >< finite 123456789) Finite 493827156000000000000 >>> :set -XTypeApplications >>> enumerate$ takeE 26 . dropE 65 $boundedEnum @Char "ABCDEFGHIJKLMNOPQRSTUVWXYZ" >>> take 10 . enumerate$ nat >< nat
[(0,0),(0,1),(1,0),(0,2),(1,1),(2,0),(0,3),(1,2),(2,1),(3,0)]
>>> take 10 . enumerate $cw [1 % 1,1 % 2,2 % 1,1 % 3,3 % 2,2 % 3,3 % 1,1 % 4,4 % 3,3 % 5] >>> take 15 . enumerate$ listOf nat
[[],[0],[0,0],[1],[0,0,0],[1,0],[2],[0,1],[1,0,0],[2,0],[3],[0,0,0,0],[1,1],[2,0,0],[3,0]]

data Tree = L | B Tree Tree
deriving (Eq, Show)

trees :: Enumeration Tree
trees = infinite $singleton L <|> B <$> trees <*> trees

>>> take 3 . enumerate $trees [L,B L L,B L (B L L)] >>> select trees 87239862967296 B (B (B (B (B L L) (B (B (B L L) L) L)) (B L (B L (B L L)))) (B (B (B L (B L (B L L))) (B (B L L) (B L L))) (B (B L (B L (B L L))) L))) (B (B L (B (B (B L (B L L)) (B L L)) L)) (B (B (B L (B L L)) L) L)) treesOfDepthUpTo :: Int -> Enumeration Tree treesOfDepthUpTo 0 = singleton L treesOfDepthUpTo n = singleton L <|> B <$> t' <*> t'
where t' = treesOfDepthUpTo (n-1)

>>> card (treesOfDepthUpTo 0)
Finite 1
>>> card (treesOfDepthUpTo 1)
Finite 2
>>> card (treesOfDepthUpTo 3)
Finite 26
>>> card (treesOfDepthUpTo 10)
Finite
14378219780015246281818710879551167697596193767663736497089725524386087657390556152293078723153293423353330879856663164406809615688082297859526620035327291442156498380795040822304677
>>> select (treesOfDepthUpTo 10) (2^50)
B L (B L (B L (B (B L (B (B L (B (B L L) L)) (B (B (B (B L L) (B L L)) (B L (B L L))) (B (B (B L L) L) (B (B L L) L))))) (B (B (B (B (B (B L L) L) (B (B L L) L)) (B L L)) (B (B (B (B L L) L) (B L (B L L))) (B (B (B L L) (B L L)) L))) (B (B (B (B L L) (B L L)) (B (B (B L L) L) L)) (B (B L L) (B (B (B L L) L) (B (B L L) L))))))))

1. I chose the name enumeration before I realized there was already a package of that name on Hackage! So now I have to come up with another name that’s not already taken. Suggestions welcome…

Posted in combinatorics, haskell, projects | Tagged , , , , | 10 Comments

## Code style and moral absolutes

In my previous post about my basic setup for solving competitive programming problems with Haskell, I (somewhat provocatively) used lists to represent pairs, and wrote a partial function to process them. Commenter Yom responded with a proposed alternative that was (less) partial. I was glad for the comment, because it gave me a good opportunity to think more about why I wrote the code in the way I did, and how it fits into larger issues of good coding practices and the reasons behind them.

## Good code style as moral behavior

What is good code style? You probably have some opinions about this. In fact, I’m willing to bet you might even have some very strong opinions about this; I know I do. Whether consciously or not, we tend to frame good coding practices as a moral issue. Following good coding practices makes us feel virtuous; ignoring them makes us feel guilty. I can guess that this is why Yom said “I don’t think I could bring myself to be satisfied with partial functions” [emphasis added]. And this is why we say “good code style”, not “optimal” or “rational” or “best practice” code style.

Why is this? Partly, it is just human: we like to have right and wrong ways to do everything (load the dishwasher, enforce grammar “rules”, use a text editor, etc.), and we naturally create and enforce community standards via subtle and not-so-subtle social cues. In the case of coding practices, I think we also sometimes do it consciously and explicitly, because the benefits can be unintuitive or only manifest in the long term. So the only way to get our students—or ourselves—to follow practices that are in our rational self-interest is by framing them in moral terms; rational arguments do not work in and of themselves. For example, I cannot get my students to write good comments by explaining to them how it will be beneficial to them in the future. It seems obvious to them that they will remember perfectly how their code works in the future, so any argument claiming the opposite falls on deaf ears. The only way to get them to write comments is to make it a moral issue: they should feel bad (i.e. lose points, lose respect, feel like they are “taking shortcuts”) if they don’t. Of course I do this “for their own good”: I trust that in the future they will come to appreciate this ingrained behavior on its own merits.

The problem is that things framed in moral terms become absolutes, and it is then difficult for us to assess them rationally. My students will never be able to write a [function without comments, partial function, goto statement, …] without feeling bad about it, and they probably won’t stop to think about why.

## Good code style as rational behavior

I ask again: what is good code style—and why? I have identified a few reasons for various “good” coding practices. Ultimately, we want our code to have properties such as:

• Robustness: it should handle unexpected or invalid inputs gracefully.
• Readability: it should be easy for others (or us in the future) to read and understand the program.
• Maintainability: it should be easy to modify the program as requirements change.
• Efficiency: in general, programs should not do anything obviously redundant, or use data structures with a lot of overhead when faster ones are available (e.g. String vs Text or ByteString).

Even in scenarios where one might initially think these properties are not needed (e.g. writing a one-off script for some sysadmin or data processing task), they often end up being important anyway (e.g. that one-off script gets copied and mutated until it becomes a key piece of some production system). And this is exactly one of the reasons for framing good coding style in moral terms! I won’t write comments or use good function decomposition in my one-off script just because I know, rationally, that it might end up in a production system someday. (I “know” that this particular script really is just a one-off script!) But I just might follow good coding practices anyway if I feel bad about not doing it (e.g. I would feel ashamed if other people saw it).

It seems to me that most things we would typically think of as good code style are geared towards producing code with some or all of the above properties (and perhaps some other properties as well), and most scenarios in which code is being written really do benefit from these properties.

## “Good” code style is context-dependent

But what if there was a scenario where these properties are actually, concretely of no benefit? As you can probably guess, I would argue that competitive programming is one such scenario:

• Robustness: we do not care what our program does when given unexpected or invalid inputs, since we are absolutely, 100% guaranteed that our program will only ever be run on inputs that exactly follow the given specification.
• Maintainability: the requirements for our program will never change.
• Efficiency: if you haven’t done much competitive programming you might be surprised to learn that we often don’t care about efficiency either. That is, although we certainly do care about asymptotic efficiency, i.e. choosing a good algorithm, problem time limits are typically set in such a way that constant factors don’t matter very much. A program that runs within 5x-10x of the optimal speed will often fit comfortably within the time limit.

So what do we care about?

• Readability: the one thing from my previous list that we do care about is readability. Debugging becomes quite difficult if you can’t read and understand the code you wrote (this becomes even more important if you’re working on a team). And insofar as a solution represents particular insights or techniques, you may want be able to read it much later in order to remember or share what you learned.
• Programmer time: programmer time is always valuable, of course, but with competitive programming this is taken to an extreme: it is almost always done under time pressure, so the whole point is to write a program to solve a given problem as fast as possible.

The combination of optimizing for speed and not caring about things like robustness, maintainability, and efficiency leads to a number of “best practices” for competitive programming that fly in the face of typical standards. For example:

• Adding code to deal gracefully with inputs that don’t follow the specification would just be a waste of time (a cardinal sin in this context!). My Haskell solutions are full of calls to partial functions like read, head, tail, fromJust, and so on, even though I would almost never use these functions in other contexts. This is also why I used a partial function that was only defined on lists of length two in my previous post (though as I argue in a comment, perhaps it’s not so much that the function is partial as that its type is too big).
• I often just use String for text processing, even though something like Text or ByteString (depending on the scenario) would be faster or more robust. (The exception is problems with a large amount of I/O, when the overhead of String really does become a problem; more on this in a future post.)
• Other than the simplest uses of foldr, foldl', and scanl, I don’t bother with generic recursion schemes; I tend to just write lots of explicit recursion, which I find quicker to write and easier to debug.

There are similar things I do in Java as well. It has taken me quite a while to become comfortable with these things and stop feeling bad about them, and I think I finally understand why.

I’m not sure I really have a main point, other than to encourage you to consider your coding practices, and why you consider certain practices to be good or bad (and whether it depends on the context!).

Next time, back to your regularly scheduled competitive programming tips!

## Competitive Programming in Haskell: Basic Setup

I am the coach of my school’s competitive programming team and enjoy solving problems on Open Kattis. Since Kattis accepts submissions in a wide variety of languages (including Haskell, OCaml, Rust, Common Lisp, and even Prolog), I often enjoy submitting solutions in Haskell. Of the 946 problems I have solved on Kattis1, I used Haskell for 607 of them (I used Java for the rest, except for one in C++).

After solving so many problems in Haskell, by now I’ve figured out some patterns that work well, identified some common pitfalls, developed some nice little libraries, and so forth. I thought it would be fun to write a series of blog posts sharing my experience for the benefit of others—and because I expect I will also learn things from the ensuing discussion!

## The Basics: I/O

As a basic running example I’ll use the same example problem that Kattis uses in its help section, namely, A Different Problem. In this problem, we are told that the input will consist of a number of pairs of integers between $0$ and $10^{15}$, one pair per line, and we should output the absolute value of the difference between each pair. The given example is that if the input looks like this:

10 12
71293781758123 72784
1 12345677654321

then our program should produce output that looks like this:

2
71293781685339
12345677654320

Kattis problems are always set up this way, with input of a specified format provided on standard input, and output to be written to standard output. To do this in Haskell, one might think we will need to use things like getLine and putStrLn to read and write the input. But wait! There is a much better way. Haskell’s standard Prelude has a function

interact :: (String -> String) -> IO ()

It takes a pure String -> String function, and creates an IO action which reads from standard input, feeds the input to the function, and then writes the function’s output to standard output. It uses lazy IO, so the reading and writing can be interleaved with computation of the function—a bit controversial and dangerous in general, but absolutely perfect for our use case! Every single Kattis problem I have ever solved begins with

main = interact $... (or the equivalent for ByteString, more on that in a future post) and that is the only bit of IO in the entire program. Yay! ## From Input to Output So now we need to write a pure function which transforms the input into the output. Of course, in true Haskell fashion, we will do this by constructing a chained pipeline of functions to do the job incrementally. The general plan of attack (for any Kattis problem) is as follows: 1. First, parse the input, that is, transform the raw String input into some more semantically meaningful representation—typically using a combination of functions like lines, words, read, map, and so on (or more sophisticated tools—see a later post). 2. Next, solve the problem, turning a semantically meaningful representation of the input into a semantically meaningful representation of the output. 3. Finally, format the output using things like show, unwords, unlines, and so on. Idiomatic Haskell uses the composition operator (.) to combine functions. However, when solving competitive programming problems, I much prefer to use the reverse composition operator, (>>>) from Control.Arrow (that is, (>>>) = flip (.)). The reason is that since I often end up constructing long function pipelines, I want to be able to think about the process of transforming input to output and type from left to right at the same time; having to add functions from right to left would be tedious. ## A Full Solution So here’s my solution to A Different Problem: main = interact$
lines >>> map (words >>> map read >>> solve >>> show) >>> unlines

solve :: [Integer] -> Integer
solve [a,b] = abs (a - b)

A few notes:

• Since each line is to be processed independently, notice how I put the processing of each line inside a single call to map.
• We could probably inline the solve function in this case, but I prefer to split it out explicitly in order to specify its type, which both prevents problems with read/show ambiguity and also serves as a sanity check on the parsing and formatting code.
• The machines on which our solution will run definitely have 64-bit architectures, so we could technically get away with using Int instead of Integer (maxBound :: Int64 is a bit more than $9 \times 10^{18}$, plenty big enough for inputs up to $10^{15}$), but there would be no benefit to doing so. If we use Integer we don’t even have to consider potential problems with overflow.

And one last thing: I said we were going to parse the input into a “semantically meaningful representation”, but I lied a teensy bit: the problem says we are going to get a pair of integers but I wrote my solve function as though it takes a list of integers. And even worse, my solve function is partial! Why did I do that?

The fact is that I almost never use actual Haskell tuples in my solutions, because they are too awkward and inconvenient. Representing homogeneous tuples as Haskell lists of a certain known length allows us to read and process “tuples” using standard functions like words and map, to combine them using zipWith, and so on. And since we get to assume that the input always precisely follows the specification—which will never change—this is one of the few situations where, in my opinion, we are fully justified in writing partial functions like this if it makes the code easier to write. So I always represent homogeneous tuples as lists and just pattern match on lists of the appropriate (known) length. (If I need heterogeneous tuples, on the other hand, I create an appropriate data type.)

Of course I’ve only scratched the surface here—I’ll have a lot more to say in future posts—but this should be enough to get you started! I’ll leave you with a few very easy problems, which can each be done with just a few lines of Haskell:

Of course you can also try solving any of the other problems (as of this writing, over 2400 of them!) on Kattis as well.

1. Did I mention that I really enjoy solving competitive programming problems?