add day 25

solved by claude

day25/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "day25"
+version = "0.1.0"

day25/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "day25"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]

day25/src/main.rs

@@ -0,0 +1,27 @@
+fn main() {
+    let row: u64 = 3010;
+    let col: u64 = 3019;
+
+    let d = row + col - 1;
+    let index = d * (d - 1) / 2 + col; // 1-indexed position, (1,1) -> 1
+
+    const START: u64 = 20151125;
+    const MUL: u64 = 252533;
+    const MOD: u64 = 33554393;
+
+    let code = (START * mod_pow(MUL, index - 1, MOD)) % MOD;
+    println!("{}", code);
+}
+
+fn mod_pow(mut base: u64, mut exp: u64, modulus: u64) -> u64 {
+    let mut result = 1u64;
+    base %= modulus;
+    while exp > 0 {
+        if exp & 1 == 1 {
+            result = result * base % modulus;
+        }
+        exp >>= 1;
+        base = base * base % modulus;
+    }
+    result
+}