[Hacker Rank]: Project Euler #1: Multiples of 3 and 5 solved ✓

docs/hackerrank/projecteuler/euler001.md

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+# [Multiples of 3 and 5](https://www.hackerrank.com/contests/projecteuler/challenges/euler001)
+
+- Difficulty: #easy
+- Category: #ProjectEuler+
+
+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 $ N $.
+
+## Input Format
+
+First line contains $ T $ that denotes the number of test cases.
+This is followed by $ T $ lines, each containing an integer, $ N $.
+
+## Constraints
+
+- 1 $ \leq T \leq 10^5 $
+- 1 $ \leq N \leq 10^9 $
+
+## Output Format
+
+For each test case, print an integer that denotes the sum of all the multiples
+of $ 3 $ or $ 5 $ below $ N $.
+
+## Sample Input 0
+
+```text
+2
+10
+100
+```
+
+## Sample Output 0
+
+```text
+23
+2318
+```
+
+## Explanation 0
+
+For $ N = 10 $, 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 $.
+
+Similarly for $ N = 100 $, we get $ 2318 $.

src/lib/exercises/include/exercises/hackerrank/projecteuler/euler001.hpp

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+#pragma once
+
+namespace hackerrank::projecteuler {
+
+int euler001(int a, int b, int n);
+
+}

src/lib/exercises/src/hackerrank/projecteuler/euler001.cpp

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+#include <exercises/hackerrank/projecteuler/euler001.hpp>
+
+/**
+ * @link Problem definition [[docs/hackerrank/projecteuler/euler001.md]]
+ */
+
+#include <algorithm>
+
+namespace hackerrank::projecteuler {
+
+// Function to return gcd of a and b
+int gcd(int a, int b) {
+  // Find Minimum of a and b
+  int result = std::min(a, b);
+  while (result > 0) {
+    if (a % result == 0 && b % result == 0) {
+      break;
+    }
+    result--;
+  }
+
+  // Return gcd of a and b
+  return result;
+}
+
+// Function to find sum of Arithmetic Progression series
+int sum_of_arithmetic_progression(int n, int d) {
+  // Number of terms
+  n = n / d;
+  return n * (1 + n) * d / 2;
+}
+
+// Function to find the sum of all multiples of a and b below n
+int euler001(int a, int b, int n) {
+  // Since, we need the sum of multiples less than N
+  n = n - 1;
+
+  int lcm = (a * b) / gcd(a, b);
+
+  return sum_of_arithmetic_progression(n, a) +
+         sum_of_arithmetic_progression(n, b) -
+         sum_of_arithmetic_progression(n, lcm);
+}
+
+} // namespace hackerrank::projecteuler

src/tests/unit/lib/hackerrank/projecteuler/euler001.test.cpp

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+#include <catch2/catch_test_macros.hpp>
+
+#include <exercises/hackerrank/projecteuler/euler001.hpp>
+#include <iostream>
+#include <vector>
+
+#include <filesystem>
+#include <fstream>
+#include <nlohmann/json.hpp>
+using json = nlohmann::json;
+
+TEST_CASE("euler001 JSON Test Cases",
+          "[hackerrank] [jsontestcase] [projecteuler]") {
+  std::filesystem::path cwd = std::filesystem::current_path();
+  std::string path =
+      cwd.string() +
+      "/unit/lib/hackerrank/projecteuler/euler001.testcases.json";
+
+  INFO("euler001 JSON test cases FILE: " << path);
+
+  std::ifstream f(path);
+  json data = json::parse(f);
+
+  for (auto testcase : data) {
+    int result = hackerrank::projecteuler::euler001(
+        testcase["a"], testcase["b"], testcase["n"]);
+    CHECK(result == testcase["expected"]);
+  }
+}

src/tests/unit/lib/hackerrank/projecteuler/euler001.testcases.json

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+[
+  { "a": 3, "b": 5, "n": 10, "expected": 23 },
+  { "a": 3, "b": 5, "n": 100, "expected": 2318 },
+  { "a": 3, "b": 5, "n": 1000, "expected": 233168 }
+]