Add Day 25

Author: sergree

aoc25/Cargo.lock

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+[package]
+name = "aoc25"
+version = "0.1.0"
+authors = ["Sergree <[email protected]>"]
+license = "MIT OR Apache-2.0"
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+anyhow = "1.0.40"

aoc25/Cargo.toml

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+15335876
+15086442

aoc25/input/input.txt

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+use anyhow::Result;
+use std::io::{self, Error, ErrorKind, Read, Write};
+
+const DIVISOR: u64 = 20201227;
+const SUBJECT_NUMBER: u64 = 7;
+
+fn main() -> Result<()> {
+    let mut input = String::new();
+    io::stdin().read_to_string(&mut input)?;
+
+    part1(&input)?;
+    Ok(())
+}
+
+fn part1(input: &str) -> Result<()> {
+    let mut input_iter = input.lines().map(|l| l.parse());
+
+    let card_pub_key = input_iter
+        .next()
+        .ok_or_else(|| Error::from(ErrorKind::InvalidData))??;
+    let door_pub_key = input_iter
+        .next()
+        .ok_or_else(|| Error::from(ErrorKind::InvalidData))??;
+
+    let mut value = 1;
+    let mut loop_size = 0;
+    while value != card_pub_key {
+        value = value * SUBJECT_NUMBER % DIVISOR;
+        loop_size += 1;
+    }
+
+    writeln!(
+        io::stdout(),
+        "{}",
+        mod_pow(door_pub_key, loop_size, DIVISOR)
+    )?;
+    Ok(())
+}
+
+/// https://en.wikipedia.org/wiki/Modular_exponentiation
+fn mod_pow(mut base: u64, mut exp: u64, modulus: u64) -> u64 {
+    if modulus == 1 {
+        return 0;
+    }
+    let mut result = 1;
+    base %= modulus;
+    while exp > 0 {
+        if exp % 2 == 1 {
+            result = result * base % modulus;
+        }
+        exp >>= 1;
+        base = base * base % modulus
+    }
+    result
+}