Revert "remove project_euler"

This reverts commit cdc0536.

Author: maltejur

CONTRIBUTING.md

@@ -50,6 +50,7 @@ Algorithms in this repo should not be how-to examples for existing Ruby packages
 
 #### Other Requirements for Submissions
 
+- If you are submitting code in the `project_euler/` directory, please also read [the dedicated Guideline](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/README.md) before contributing to our Project Euler library.
 - Strictly use snake_case (underscore_separated) in your file_name, as it will be easy to parse in future using scripts.
 - Please avoid creating new directories if at all possible. Try to fit your work into the existing directory structure.
 - If possible, follow the standard *within* the folder you are submitting to.

DIRECTORY.md

@@ -103,6 +103,26 @@
 ## Other
   * [Fisher Yates](https://github.com/TheAlgorithms/Ruby/blob/master/other/fisher_yates.rb)
 
+## Project Euler
+  * Problem 1
+    * [Sol1](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/problem_1/sol1.rb)
+  * Problem 2
+    * [Sol1](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/problem_2/sol1.rb)
+  * Problem 20
+    * [Sol1](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/problem_20/sol1.rb)
+  * Problem 21
+    * [Sol1](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/problem_21/sol1.rb)
+  * Problem 22
+    * [Sol1](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/problem_22/sol1.rb)
+  * Problem 3
+    * [Sol1](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/problem_3/sol1.rb)
+    * [Sol2](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/problem_3/sol2.rb)
+  * Problem 4
+    * [Sol1](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/problem_4/sol1.rb)
+    * [Sol2](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/problem_4/sol2.rb)
+  * Problem 5
+    * [Sol1](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/problem_5/sol1.rb)
+
 ## Searches
   * [Binary Search](https://github.com/TheAlgorithms/Ruby/blob/master/searches/binary_search.rb)
   * [Depth First Search](https://github.com/TheAlgorithms/Ruby/blob/master/searches/depth_first_search.rb)

project_euler/README.md

@@ -0,0 +1,20 @@
+# Project Euler
+
+Problems are taken from https://projecteuler.net/, the Project Euler. [Problems are licensed under CC BY-NC-SA 4.0](https://projecteuler.net/copyright).
+
+Project Euler is a series of challenging mathematical/computer programming problems that require more than just mathematical
+insights to solve. Project Euler is ideal for mathematicians who are learning to code.
+
+## Solution Guidelines
+
+Welcome to [TheAlgorithms/Ruby](https://github.com/TheAlgorithms/Ruby)! Before reading the solution guidelines, make sure you read the whole [Contributing Guidelines](https://github.com/TheAlgorithms/Ruby/blob/master/CONTRIBUTING.md) as it won't be repeated in here. If you have any doubt on the guidelines, please feel free to [state it clearly in an issue](https://github.com/TheAlgorithms/Ruby/issues/new) or ask the community in [Gitter](https://gitter.im/TheAlgorithms). Be sure to read the [Coding Style](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/README.md#coding-style) before starting solution.
+
+### Coding Style
+
+* Please maintain consistency in project directory and solution file names. Keep the following points in mind:
+  * Create a new directory only for the problems which do not exist yet.
+  * Please name the project **directory** as `problem_<problem_number>` where `problem_number` should be filled with 0s so as to occupy 3 digits. Example: `problem_001`, `problem_002`, `problem_067`, `problem_145`, and so on.
+
+* You can have as many helper functions as you want but there should be one main function called `solution` which should satisfy the conditions as stated below:
+  * It should contain positional argument(s) whose default value is the question input. Example: Please take a look at [Problem 1](https://projecteuler.net/problem=1) where the question is to *Find the sum of all the multiples of 3 or 5 below 1000.* In this case the main solution function will be `solution(limit = 1000)`.
+  * When the `solution` function is called without any arguments like so: `solution()`, it should return the answer to the problem.

project_euler/problem_1/sol1.rb

@@ -0,0 +1,14 @@
+# If we list all the natural numbers below 10 that are multiples of 3 or 5,
+# we get 3, 5, 6 and 9. The sum of these multiples is 23.
+# Find the sum of all the multiples of 3 or 5 below 1000.
+
+def divisible_by_three_or_five?(number)
+  (number % 3).zero? || (number % 5).zero?
+end
+
+sum = 0
+(1...1000).each do |i|
+  sum += i if divisible_by_three_or_five?(i)
+end
+
+p sum

project_euler/problem_2/sol1.rb

@@ -0,0 +1,17 @@
+# Each new term in the Fibonacci sequence is generated by adding the previous two terms.
+# By starting with 1 and 2, the first 10 terms will be:
+#                                           1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
+# By considering the terms in the Fibonacci sequence whose values do not exceed four million,
+# find the sum of the even-valued terms.
+
+even_fib_sum = 0
+fib_first = 1
+fib_second = 2
+
+while fib_second < 4_000_000
+  even_fib_sum += fib_second if fib_second.even?
+  fib_second += fib_first
+  fib_first = fib_second - fib_first
+end
+
+p even_fib_sum

project_euler/problem_20/sol1.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+# n! means n x (n - 1) x ... x 3 x 2 x 1
+
+# For example, 10! = 10 x 9 x ... x 3 x 2 x 1 = 3628800,
+# and the sum of the digits in the number 10! is 3 + 6 + 2 + 8 + 8 + 0 + 0 = 27.
+#
+# Find the sum of the digits in the number 100!
+
+# method to calculate factorial of a number
+def factorial(number)
+  number.downto(1).reduce(:*)
+end
+
+# fetch digits of factorial of `number` and find
+# sum of all those digits, and prints the result on the console
+number = 100
+puts factorial(number).digits.sum

project_euler/problem_21/sol1.rb

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+# Let d(n) be defined as the sum of proper divisors of n
+# (numbers less than n which divide evenly into n).
+# If d(a) = b and d(b) = a, where a & b, then a and b are an amicable pair.
+# and each of a and b are called amicable numbers.
+#
+# For example,
+#
+# The proper divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55 and 110;
+# therefore d(220) = 284.
+#
+# The proper divisors of 284 are 1, 2, 4, 71 and 142; so d(284) = 220.
+#
+# Evaluate the sum of all the amicable numbers under 10000.
+
+# get list of all divisors of `number`
+def get_divisors(number)
+  divisors = []
+  (1..(Math.sqrt(number).to_i)).each do |num|
+    if (number % num).zero?
+      divisors << num
+      divisors << number / num
+    end
+  end
+  divisors
+end
+
+# get list of all proper divisors of `number` i.e. removing `number` from
+# the list of divisors
+def get_proper_divisors(number)
+  divisors = get_divisors(number)
+  divisors.delete(number)
+  divisors
+end
+
+# implementation of a method `d` as mentioned in the question
+# i.e. finding sum of all proper divisors of `number`
+def d(number)
+  get_proper_divisors(number).sum
+end
+
+# given an upper `limit`, this method finds all amicable numbers
+# under this `limit`
+def find_amicable_numbers(limit)
+  result = []
+  (1...limit).each do |a|
+    b = d(a)
+    res = d(b)
+    result.push(a) if (a == res) && (a != b)
+  end
+  result
+end
+
+# calling `find_amicable_numbers` method and finding sum of all such numbers
+# below 10000, and printing the result on the console
+puts find_amicable_numbers(10_000).sum

project_euler/problem_22/p022_names.txt

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+# Problem 22
+# Using names.txt (right click and 'Save Link/Target As...'),
+# a 46K text file containing over five-thousand first names,
+# begin by sorting it into alphabetical order.
+# Then working out the alphabetical value for each name,
+# multiply this value by its alphabetical position in the list to obtain a name score.
+
+# For example, when the list is sorted into alphabetical order,
+# COLIN, which is worth 3 + 15 + 12 + 9 + 14 = 53,
+# is the 938th name in the list. So, COLIN would obtain a score of 938 * 53 = 49714.
+
+# What is the total of all the name scores in the file?
+
+# reading the contents of the file
+file_contents = File.read('p022_names.txt')
+
+# replacing the occuerance of \" to '' and spliting the result by ','
+# to get an array of sorted words
+words = file_contents.tr('\"', '').split(',').sort
+
+# this method calculates the worth of a word based on the ASCII
+# values of the characters
+def word_worth(word)
+  word.chars.sum { |char| char.ord - 'A'.ord + 1 }
+end
+
+# this method takes the words as an input
+# calls `word_worth` method on each word
+# to that value multiply that with the index of the word in the array
+# add the same to the result
+def total_rank(words)
+  result = 0
+  words.each_with_index { |word, index| result += word_worth(word) * (index + 1) }
+  result
+end
+
+# outputs total rank on the console
+puts total_rank(words)

project_euler/problem_22/sol1.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+# The prime factors of 13195 are 5, 7, 13 and 29.
+# What is the largest prime factor of the number 600851475143
+
+# find all factors of the given number
+def get_factors(number)
+  factors = []
+  (1..Math.sqrt(number).to_i).each do |num|
+    if (number % num).zero?
+      factors << num
+      factors << number / num
+    end
+  end
+  factors
+end
+
+# determine if a given number is a prime number
+def prime?(number)
+  get_factors(number).length == 2
+end
+
+# find the largest prime
+def largest_prime_factor(number)
+  prime_factors = get_factors(number).select { |factor| prime?(factor) }
+  prime_factors.max
+end
+
+puts largest_prime_factor(600_851_475_143)

project_euler/problem_3/sol1.rb

@@ -0,0 +1,18 @@
+# The prime factors of 13195 are 5, 7, 13 and 29.
+# What is the largest prime factor of the number 600851475143 ?
+
+def solution(n)
+  prime = 1
+  i = 2
+  while i * i <= n
+    while (n % i).zero?
+      prime = i
+      n = n.fdiv i
+    end
+    i += 1
+  end
+  prime = n if n > 1
+  prime.to_i
+end
+
+puts solution(600_851_475_143)

project_euler/problem_3/sol2.rb

@@ -0,0 +1,11 @@
+# A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.
+# Find the largest palindrome made from the product of two 3-digit numbers.
+
+answer = 0
+999.downto(99) do |i|
+  999.downto(99) do |j|
+    t = (i * j)
+    answer = i * j if (t.to_s == t.to_s.reverse) && (t > answer) && (t > answer)
+  end
+end
+puts answer

project_euler/problem_4/sol1.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+# A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.
+# Find the largest palindrome made from the product of two 3-digit numbers.
+
+class Integer
+  def parindrome?
+    self == reverse
+  end
+
+  # 123.reverse == 321
+  # 100.reverse == 1
+  def reverse
+    result = 0
+    n = self
+    loop do
+      result = result * 10 + n % 10
+      break if (n /= 10).zero?
+    end
+    result
+  end
+end
+
+factors = (100..999).to_a
+products = factors.product(factors).map { _1 * _2 }
+puts products.select(&:parindrome?).max

project_euler/problem_4/sol2.rb

@@ -0,0 +1,22 @@
+# 2520 is the smallest number that can be divided
+# by each of the numbers from 1 to 10 without any remainder.
+# What is the smallest positive number that is evenly
+# divisible by all of the numbers from 1 to 20?
+
+# Euclid's algorithm for the greatest common divisor
+def gcd(a, b)
+  b.zero? ? a : gcd(b, a % b)
+end
+
+# Calculate the LCM using GCD
+def lcm(a, b)
+  (a * b) / gcd(a, b)
+end
+
+result = 1
+
+20.times do |i|
+  result = lcm(result, i + 1)
+end
+
+p result