From 529993aa5c72366ef8f5a6754822f366fa378267 Mon Sep 17 00:00:00 2001
From: newt <hi@newty.dev>
Date: Wed, 9 Oct 2024 18:02:45 +0100
Subject: [PATCH] feat(euler): 23 - non-abundant sums

---
 challenges/euler/readme.md                    |  2 +-
 .../euler/src/23 - Non-abundant sums.ts       | 56 +++++++++++++++++++
 2 files changed, 57 insertions(+), 1 deletion(-)
 create mode 100644 challenges/euler/src/23 - Non-abundant sums.ts

diff --git a/challenges/euler/readme.md b/challenges/euler/readme.md
index 60a1e0f..5729488 100644
--- a/challenges/euler/readme.md
+++ b/challenges/euler/readme.md
@@ -34,7 +34,7 @@ The source code can be found in the [src](src) directory. My thoughts about some
 -   [x] [20 - Factorial digit sum](src/20%20-%20Factorial%20digit%20sum.ts)
 -   [x] [21 - Amicable numbers](src/21%20-%20Amicable%20numbers.ts)
 -   [x] [22 - Names scores](src/22%20-%20Names%20scores.ts)
--   [ ] 23 - Non-abundant sums
+-   [x] [23 - Non-abundant sums](src/23%20-%20Non-abundant%20sums.ts)
 -   [x] [24 - Lexicographic permutations](src/24%20-%20Lexicographic%20permutations.ts)
 -   [x] [25 - 1000-digit Fibonacci number](src/25%20-%201000-digit%20Fibonacci%20number.ts)
 -   [ ] 26 - Reciprocal cycles
diff --git a/challenges/euler/src/23 - Non-abundant sums.ts b/challenges/euler/src/23 - Non-abundant sums.ts
new file mode 100644
index 0000000..97ad8ea
--- /dev/null
+++ b/challenges/euler/src/23 - Non-abundant sums.ts	
@@ -0,0 +1,56 @@
+// A perfect number is a number for which the sum of its proper divisors is exactly equal to the number. For example, the sum of the proper divisors of 28 would be 1 + 2 + 4 + 7 + 14 = 28, which means that 28 is a perfect number.
+// A number n is called deficient if the sum of its proper divisors is less than n and it is called abundant if this sum exceeds n.
+//
+// As 12 is the smallest abundant number, 1 + 2 + 3 + 4 + 6 = 16, the smallest number that can be written as the sum of two abundant numbers is 24. By mathematical analysis, it can be shown that all integers greater than 28123 can be written as the sum of two abundant numbers. However, this upper limit cannot be reduced any further by analysis even though it is known that the greatest number that cannot be expressed as the sum of two abundant numbers is less than this limit.
+// Find the sum of all the positive integers which cannot be written as the sum of two abundant numbers.
+export = {};
+
+const findProperDivisors = (number: number) => {
+    const divisors: number[] = [1];
+
+    for (let i = 2; i < number; i++) {
+        if (number % i === 0) {
+            divisors.push(i);
+        }
+    }
+
+    return divisors;
+};
+
+const isNumberAbundant = (number: number) => {
+    const divisors = findProperDivisors(number);
+    return divisors.reduce((a, b) => a + b) > number;
+};
+
+const abundantNumbers = (lowerBound: number, upperBound: number) => {
+    const numbers: number[] = [];
+
+    for (let i = lowerBound; i < upperBound; i++) {
+        if (isNumberAbundant(i)) numbers.push(i);
+    }
+
+    return numbers;
+};
+
+const upperLimit = 28123;
+const numbers = abundantNumbers(1, upperLimit);
+const sums = Array(upperLimit + 1).fill(0);
+
+for (let i = 0; i < numbers.length; i++) {
+    for (let j = i; j < numbers.length; j++) {
+        const sum = numbers[i] + numbers[j];
+
+        if (sum <= upperLimit) {
+            if (sums[sum] == 0) sums[sum] = sum;
+        }
+    }
+}
+
+let answer = 0;
+
+for (let i = 1; i < sums.length; i++) {
+    if (sums[i] === 0) answer += i;
+}
+
+// Output
+console.log(answer);