project_euler_1.d

/*
 * Project Euler: Classic, "Even Fibonacci numbers" solution
 * Implementation by A. S. "Aleksey" Ahmann <hackermaneia@riseup.net>
 * - https://github.com/Alekseyyy
 *
 * Problem link: https://projecteuler.net/problem=2
 */

import std.stdio : writeln;
import std.algorithm.iteration : sum;

void main() {
	int[2] fibonacci = [1, 2];
	int[] even_fibonacci = [2];

	do {
		int fib_k = fibonacci[0] + fibonacci[1];
		if (fib_k % 2 == 0)
			even_fibonacci ~= fib_k;
		fibonacci[0] = fibonacci[1];
		fibonacci[1] = fib_k;
	} while (fibonacci[0] + fibonacci[1] <= 3999999);

	writeln(sum(even_fibonacci));
}

project_euler_2.d

/*
 * Project Euler: Classic, "10001st prime" solution
 * Implementation by A. S. "Aleksey" Ahmann <hackermaneia@riseup.net>
 * - https://github.com/Alekseyyy
 *
 * Problem link: https://projecteuler.net/problem=7
 * I ported some code from here: https://www.geeksforgeeks.org/python-program-for-sieve-of-eratosthenes/
 */

import std.stdio : writeln, printf;
import std.algorithm.mutation : fill;

int[] sieve(int n) {

	//printf("On sieve function with n = %i\n", n);
	bool[] bsieve = new bool[n + 1];
	fill(bsieve, true);

	for (int p = 2; p * p <= bsieve.length - 1; p++) {
		if (bsieve[p] == true) {
			for (int i = p * p; i <= bsieve.length - 1; i += p)
				bsieve[i] = false;
		}
	}

	int[] result;
	for (int p = 2; p <= bsieve.length - 1; p++) {
		if (bsieve[p])
			result ~= p;
	}
	return result;
}

void main() {

	int i = 0;
	int[] primes;
	do {
		primes = sieve(i);
		i = i + 10000;
	} while (primes.length <= 10001);
	writeln(primes[10000]);
}

project_euler_3.d

/*
 * Project Euler: Classic, "Largest product in a series" solution
 * Implementation by A. S. "Aleksey" Ahmann <hackermaneia@riseup.net>
 * - https://github.com/Alekseyyy
 *
 * Problem link: https://projecteuler.net/problem=8
 * Thanks to StackOverflowers for helping me with that code: https://stackoverflow.com/questions/68245264/inconsitent-behaviour-with-computation-of-greatest-product-given-n-adjacent-d
 */

import std.stdio : writeln;
import std.array : replace;
import std.conv : to;

string number = "73167176531330624919225119674426574742355349194934
96983520312774506326239578318016984801869478851843
85861560789112949495459501737958331952853208805511
12540698747158523863050715693290963295227443043557
66896648950445244523161731856403098711121722383113
62229893423380308135336276614282806444486645238749
30358907296290491560440772390713810515859307960866
70172427121883998797908792274921901699720888093776
65727333001053367881220235421809751254540594752243
52584907711670556013604839586446706324415722155397
53697817977846174064955149290862569321978468622482
83972241375657056057490261407972968652414535100474
82166370484403199890008895243450658541227588666881
16427171479924442928230863465674813919123162824586
17866458359124566529476545682848912883142607690042
24219022671055626321111109370544217506941658960408
07198403850962455444362981230987879927244284909188
84580156166097919133875499200524063689912560717606
05886116467109405077541002256983155200055935729725
71636269561882670428252483600823257530420752963450";

void main() {

	number = replace(number, "\n", "");
	long[] x = format_adjacent(number);
	long largest_product = largestProduct(x, 13);

	writeln(largest_product);
}

long[] format_adjacent(string number) {

	long[] adjacent_numbers;
	for (long i = 0; i < number.length; i++)
		adjacent_numbers ~= to!long(number[i] - to!char("0"));
	return adjacent_numbers;
}

long computeProduct(long[] sequence) {

	long product = sequence[0];
	for (long i = 1; i < sequence.length; i++)
		product = product * sequence[i];
	return product;
}

long largestProduct(long[] sequence, long slice) {

	long largest_product = 0;

	long n = 0;
	long fin = to!long(sequence.length) - (slice - 1);

	while (n < fin) {
		long[] seq = sequence[n .. n + slice];
		long product = computeProduct(seq);
		if (product > largest_product)
			largest_product = product;
		n = n + 1;
	}
	return largest_product;
}