add day 20

day20/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "day20"
+version = "0.1.0"

day20/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "day20"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]

day20/src/main.rs

@@ -0,0 +1,45 @@
+const LIMIT: i64 = 36_000_000;
+
+fn main() {
+    let input = 1_000_000;
+
+    part1(input); // 831600
+    part2(input); // 110880 too low
+}
+
+fn part1(input: i64) {
+    let mut result = vec![0_i64; input as usize];
+
+    for i in 1..input {
+        for j in (i..input).step_by(i as usize) {
+            result[j as usize] += 10 * i;
+        }
+    }
+
+    for (i, value) in result.iter().enumerate() {
+        if value > &LIMIT {
+            println!("{i}");
+            break;
+        }
+    }
+}
+
+fn part2(input: i64) {
+    let mut result = vec![0_i64; input as usize];
+
+    for i in 1..input {
+        for j in (i..=50 * i)
+            .step_by(i as usize)
+            .take_while(|idx| *idx < input)
+        {
+            result[j as usize] += 11 * i;
+        }
+    }
+
+    for (i, value) in result.iter().enumerate() {
+        if value > &LIMIT {
+            println!("{i}");
+            break;
+        }
+    }
+}