# The first 20 Project Euler problems

During my first week here at RC I decided to attend a session led by another recurser on Project Euler, an online coding challenge based on a series of discrete math problems. The problems are written to lend themselves to a computational solution. There are hundreds of problems and the problems get increasingly hard past the first 100. While the early problems are definitely easier than the later ones, they still take some doing to finish and are a great way to quickly work on a bunch of problems to learn a new programming language.

I’m getting more and more used to thinking about Rust code in an idiomatic way, but I don’t think I’m comfortable enough to call myself a rustacean yet. To further my goal of oxidizing my brain with rust knowledge, I decided to start working through project euler problems sequentially. I’ve recently finished the first 20 problems and I though I’d share the highlights of what I learned about rust along the way.

If you’d like to peruse my solutions, they’re all on GitHub.

## Functional processing pipelines

I’m used to Python, where computation tends to happen in an imperative way. Although Python does have functional programming features, real-world code I’ve seen tends to avoid it or use them sparingly instead of composing an entire computation or block of code as a functional data processing pipeline. Guido van Rossum is also of the opinion that some of these features should be removed from the language and tried to remove lambda, map, filter, from Python 3 and succeeded in removing reduce as a builtin and moving it to functools.

In Rust it’s much more natural to process a collection of things by creating an iterator over the collection and then applying transformations to the elements of the iterator as needed and, if necessary, doing a reduction step to process the transformed elements of the collection. One advantage of this approach is that if we’re handed an abstract iterator that lazily generates data, this approach will avoid needing to hold all the elements of the iterator in memory.

Problem 6 asks us to compute the difference between the sum of the squares and the square of the sum of the natural numbers up to 100. This lends itself to a nice straightforward functional solution:

fn main() {
let maxnum = 100;
let sumsquares: u64 = (1..maxnum+1).map(|x| x*x).sum();
let sum: u64 = (1..maxnum+1).sum();
let squaresum = sum*sum;
println!("{}", squaresum - sumsquares);
}


To me the rust map syntax looked a little strange at first but I’m starting to get used to it. The |foo| syntax defines a closure. Closures in rust are a bit like python’s lambda statement, it lets us name a temporary variable that is valid inside the scope of the map function call. In this case it represents a single value in the range iterator defined by 1..maxnum+1. After mapping the numbers to their square, we then sum the values. Since range iterators lazily generate data, our solution never needs to hold all of the values of the iterator in memory - cool!

## Generating prime numbers

Several of the Project Euler problems require generating large numbers of prime numbers quickly. The brute force approach to this is called trial division. For a natural number $n$, we check whether all natural numbers $1 < m < n$ have the property $n \mod m \ne 0$. There are some straightforward performance optizations one can apply to speed this up. If $n$ is divisible by 2 then it can’t be prime, so we reject it. If it’s not divisible by 2 then it can’t be divisible by any larger even number either, so we can skip all even numbers - a 2x performance boost. We can get an even bigger performance improvement for large $n$ by realizing that one need not check any integer bigger than $\sqrt{n}$, since if $n$ is divisible by a number bigger than $\sqrt{n}$ it must also be divisible by a number smaller than $\sqrt{n}$, which we would have already checked. We need to go all the way up to $\sqrt{n}$ in case $n$ is a perfect square.

Another approach is to use the Sieve of Eratosthenes, an ancient algorithm for generating all primes up to a limit. Start with a list of all the natural numbers from two to $n$ and then for each number, find all the multiples of $n$ in the list and mark them as not prime. Once all the numbers in the list up to $n/2$ have been tested, all the numbers that have not been discarded must be prime. For Problem #7 I ended up coding this up like so:

static MAX: usize = 1_000_000;

fn sieve(n: usize) -> usize {
let data: Vec<usize> = (0..MAX).collect();
let mut isprime: Vec<bool> = vec![true; data.len()];
isprime[0] = false;
for (i, d) in data.iter().enumerate() {
let offset = *d;
if !isprime[i] || offset == 1 {
continue
}
for j in ((i+offset)..isprime.len()).step_by(offset) {
isprime[j as usize] = false
}
}
let primes = isprime.iter().enumerate().fold(vec![], |mut acc, (index, value)| {
if *value {
acc.push(index)
}
acc
});
primes[n]
}


This approach uses a Vec<usize> called data to contain the prime numbers and a Vec<bool> to contain the list of numbers that are prime in data. We make use of enumerate in two places to generate an indexed iterator over a list of numbers, a pattern I like to use from Python where one array indexes another array. It’s nice how the approaches I like to use in Python tend to be adaptable in Rust despite the languages being very different.

It turns out that Problem 10 also needs a large number of prime numbers, so we can reuse this function with a small adaptation there.

## The num crate and the BigInt type

Many of the problems in Project Euler generate integers that cannot be represented using the integer data types built into rust. In Python, integers transparently get promoted from machine integers to arbitrary-precision integers (see PEP 237 for more details about python integers) transparently, so we don’t need to worry about the distinction. Rust is a lower-level language and wants you to carefully consider the memory cost of an operation, so it won’t automatically convert integers when they overflow. However, in debug mode, integer overflow does trigger a panic! I find this delightful compared with my experiences in other low-level languages like C or C++:

In this way we know immediately when the builtin integer types are no longer sufficient and we need to switch to an arbitrary-precision integer arithmetic library.

Currently, the most popular library for this sort of thing on crates.io is the num crate, in particular the num::bigint::BigInt type. Rather than coercing integers automatically to be instances of BigInt by overriding the arithmetic operators as a Python library probably would do, the approach here is to force users to specify which numbers exactly are BigInt instances by explicitly converting.

I used num for 5 of the first 20 problems, in problems 8, 13, 15, 16, and 20. The usage from Problem 20, which asks to compute the sum of the digits of $100!$, is probably the most readable:

use num::bigint::{BigInt, ToBigInt};
use num::traits::{One, Zero};

fn main() {
let mut num = 100.to_bigint().unwrap();
let mut ret: BigInt = One::one();
let zero: BigInt = Zero::zero();

while num != zero {
ret *= &num;
num -= 1;
}

let digits = format!("{}", ret)
.chars()
.map(|c| c.to_digit(10).expect("not a digit!"))
.collect::<Vec<u32>>();

println!("{}", digits.iter().sum::<u32>());
}


First, we convert 100 to a BigInt using the to_bigint method that the ToBigInt trait defines on rust’s buitin integer types. Adding new features to standard library types like this would be very strange in Python, but in rust it’s very natural. Since we explicitly bring the ToBigInt trait into scope it’s still explicit, however care must be taken to understand the side effects of bringing traits into scope.

Next we define bigints to represent one and zero. The num package helpfully provides traits to generate BigInt instances directly. From there the math looks more or less identical to how we’d write it for a builtin type, however with a BigInt this will not overflow. One difference is that the multiplication step needs to borrow the data in the BigInt with the & operator:

while num != zero {
ret *= &num;
num -= 1;
}


This borrow isn’t necessary for machine integers because they implement the Copy trait and can be cheaply copied. A BigInt is a wrapper for the byte buffer that could be arbitrarily long and stores that data on the heap, so we need to explicitly borrow or copy whenever we use the value.

Finally we format the bigint as a string to get a base 10 representation, convert the digits of the string to integers, and sum the digits.

## Problem 11: Largest product of four numbers in a grid

In this problem we are given a 20x20 grid of natural numbers and asked to find the sequence of 4 neighboring numbers (in any direction, including along diagonals) that has the largest product.

This problem lends itself to using a 2D array data structure to store the grid of numbers. Because I like NumPy, I decided to use the ndarray crate, which provides a ND array data structure that acts a lot like a NumPy array.

To generate the array, I first read the data into a Vec containing Vecs of u64 entries:

let rows = contents.split("\n").collect::<Vec<&str>>();

let rows = rows
.iter()
.map(|r| {
r.split(" ")
.filter_map(|d| d.parse::<u64>().ok())
.collect::<Vec<u64>>()
})
.collect::<Vec<Vec<u64>>>();


A tricky bit here is my usage of ok() to convert a Result into an Option, which allows me to use filter_map and avoid using unwrap. Since I know a priori that the input table doesn’t have blanks this is mostly a semantic dance and I could probably get away with using unwrap, but it’s nice to know of ways to generate code that provably cannot panic and crash.

Next I generate 2D 20x20 array and read the data into it:

let nrows = rows.len();
let ncols = rows[0].len();

let mut array = ndarray::Array::zeros((nrows, ncols));

for i in 0..nrows {
for j in 0..ncols {
array[(i, j)] = rows[i][j];
}
}


Now to actually check sequences, I defined a wrapper type for a 4-tuple and gave it a prod method to calculate the product of the numbers in the sequence:

struct Sequence((u64, u64, u64, u64));

impl Sequence {
fn prod(&self) -> u64 {
(self.0).0 * (self.0).1 * (self.0).2 * (self.0).3
}
}


And I wrote a check function that checks to make sure if the product of the sequence is greater than a number:

fn check(s: Option<Sequence>, largeprod: &mut u64) {
match s {
Some(x) => {
let product = x.prod();
if product > *largeprod {
*largeprod = product
}
}
None => {}
}
}


Note how this function accepts an Option<Sequence>. This lets us transparently handle cases where there isn’t a valid sequence and ignore them without carefully designing our code to avoid testing places near the edge of the array where we cannot generate a sequence 4 numbers long.

Finally we need code that generates sequences that go to the right, up, rising diagonally, or falling diagonally from a starting location defined by the tuple of indices (i, j). Here’s the code to generate horizontal sequences, the other types are analogous:

fn horizontal_sequence(i: usize, j: usize, m: usize, arr: &Array<u64, Ix2>) -> Option<Sequence> {
match (i, j) {
(i, j) if i + 3 >= m || j + 3 >= m => None,
_ => Some(Sequence((
arr[(i, j)],
arr[(i + 1, j)],
arr[(i + 2, j)],
arr[(i + 3, j)],
))),
}
}


If the sequence goes off the edge of the array, we return None, otherwise we return a Sequence to test.

Finally, we check all the possible sequences:

let mut largeprod = 0;

for i in 0..nrows {
for j in 0..ncols {
// horizontal sequences
check(horizontal_sequence(i, j, nrows, &array), &mut largeprod);
check(vertical_sequence(i, j, nrows, &array), &mut largeprod);
check(
rising_diagonal_sequence(i, j, nrows, &array),
&mut largeprod,
);
check(
falling_diagonal_sequence(i, j, nrows, &array),
&mut largeprod,
);
}
}

println!("{}", largeprod);


The slightly unsightly formatting here is generated for me automatically by rustfmt. Sometimes I disagree with it but it’s better in my opinion not to fight and just live with the standard automatic code formatting for the sake of sanity.

This is an example of dynamic programming, where we come up with a clever way of exhaustively but efficiently testing all possible cases without repeating work.

## Problem 17: Converting Numbers to English

In this problem we’re asked to generate English versions of all of the natural numbers up to 1000 and then count how many non-hyphen and non-whitespace characters are in all of the numbers.

I found it most natural in rust to use a match statement:

fn englishify(i: usize) -> String {
let ret = match i {
1 => "one".to_owned(),
2 => "two".to_owned(),
3 => "three".to_owned(),
4 => "four".to_owned(),
5 => "five".to_owned(),
6 => "six".to_owned(),
7 => "seven".to_owned(),
8 => "eight".to_owned(),
9 => "nine".to_owned(),
10 => "ten".to_owned(),
11 => "eleven".to_owned(),
12 => "twelve".to_owned(),
13 => "thirteen".to_owned(),
14 => "fourteen".to_owned(),
15 => "fifteen".to_owned(),
16 => "sixteen".to_owned(),
17 => "seventeen".to_owned(),
18 => "eighteen".to_owned(),
19 => "nineteen".to_owned(),
20 => "twenty".to_owned(),
30 => "thirty".to_owned(),
40 => "forty".to_owned(),
50 => "fifty".to_owned(),
60 => "sixty".to_owned(),
70 => "seventy".to_owned(),
80 => "eighty".to_owned(),
90 => "ninety".to_owned(),
1000 => "one thousand".to_owned(),
_ => {
let hundreds = i / 100;
let tens = (i - hundreds * 100) / 10;
let ones = i - hundreds * 100 - tens * 10;
let mut ret: String = String::new();
if hundreds > 0 {
ret += &(englishify(hundreds) + " hundred");
}
if tens < 2 {
let remainder = i - hundreds * 100;
if remainder != 0 {
ret += &(" and ".to_owned() + &englishify(i - hundreds * 100));
}
} else {
if hundreds > 0 {
ret += " and ";
}
ret += &englishify(tens * 10);
if ones > 0 {
ret += &("-".to_owned() + &englishify(ones))
}
}
ret
}
};
ret
}


For all of the numbers whose English spellings cannot be generated algorithmically, we have special cases in the match statement. For all other cases, we generate the phrase algorithmically by considering the hundreds, tens, and ones digits of the number and calling englishify recursively.

The usage of to_owned() in many of the match cases is a little ugly, but this way it’s clear that the return value of englishify owns the data for the string. I think it might also be possible to store references to 'static &[str] instances and then build the result by dereferencing the references, but that seemed more complicated than this approach, even if there’s more copying happening.

With this function, calculating the result is a straightforward functional processing pipeline on the range of numbers between one and 1000:

fn main() {
let numbers = (1..1001)
.map(|n| englishify(n).replace(" ", "").replace("-", ""))
.collect::<Vec<String>>();
dbg!(&numbers);
let nchars = numbers.iter().fold(0, |acc, x| acc + x.len());
println!("{}", nchars);
}


Here we made use of the fold function, which allows us to define custom accumulation logic, although we could have done the same thing with a map(|x| x.len()).sum().

## More Project Euler?

From here on I think I’m going to be a bit more sparing in my problem selection and not try to exhaustively do them all. Hopefully I’ll have some more blog posts about Project Euler problems as I run into particularly interesting ones or ones that teach me something new about rust.