Day 25, part 1

src/bin/d25p1.rs

@@ -0,0 +1,8 @@
+use aoc22::{day25, util};
+
+pub fn main() {
+    let numbers = day25::parse_snafus(&util::parse_input());
+    let sum: i64 = numbers.iter().sum();
+    println!("Sum of numbers: {}", sum);
+    println!("Sum of numbers, SNAFU'd: {}", day25::num_to_snafu(sum));
+}

src/day25.rs

@@ -0,0 +1,83 @@
+use std::collections::VecDeque;
+
+pub fn parse_snafus(input: &str) -> Vec<i64> {
+    input.lines().map(snafu_to_num).collect()
+}
+
+pub fn snafu_to_num(snafu: &str) -> i64 {
+    let mut pow = 1;
+    let mut result = 0;
+    for c in snafu.chars().rev() {
+        let digit = match c {
+            '=' => -2,
+            '-' => -1,
+            '0' => 0,
+            '1' => 1,
+            '2' => 2,
+            _ => panic!("Unknown digit {}", c),
+        };
+        result += pow * digit;
+        pow *= 5;
+    }
+    result
+}
+
+pub fn num_to_snafu(num: i64) -> String {
+    assert!(num > 0);
+
+    let exp = (num as f64).log(5.0).floor() as u32 + 1;
+    let mut result = VecDeque::with_capacity((exp as usize) + 1);
+    let mut pow = 5_i64.pow(exp);
+    let mut rem = num;
+
+    while pow >= 1 {
+        let digit = rem / pow;
+        rem %= pow;
+        result.push_back(digit);
+        pow /= 5;
+    }
+
+    for i in (0..result.len()).rev() {
+        if result[i] > 2 {
+            result[i] -= 5;
+            result[i - 1] += 1;
+        }
+    }
+
+    result
+        .iter()
+        .skip_while(|d| **d == 0)
+        .map(|d| match d {
+            -2 => '=',
+            -1 => '-',
+            0 => '0',
+            1 => '1',
+            2 => '2',
+            _ => panic!("Unknown digit {}", d),
+        })
+        .collect()
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn num_to_snafu_examples() {
+        use crate::day25::num_to_snafu;
+        assert_eq!(num_to_snafu(1), "1");
+        assert_eq!(num_to_snafu(2), "2");
+        assert_eq!(num_to_snafu(3), "1=");
+        assert_eq!(num_to_snafu(4), "1-");
+        assert_eq!(num_to_snafu(5), "10");
+        assert_eq!(num_to_snafu(6), "11");
+        assert_eq!(num_to_snafu(7), "12");
+        assert_eq!(num_to_snafu(8), "2=");
+        assert_eq!(num_to_snafu(9), "2-");
+        assert_eq!(num_to_snafu(10), "20");
+        assert_eq!(num_to_snafu(15), "1=0");
+        assert_eq!(num_to_snafu(20), "1-0");
+        assert_eq!(num_to_snafu(2022), "1=11-2");
+        assert_eq!(num_to_snafu(12345), "1-0---0");
+        assert_eq!(num_to_snafu(314159265), "1121-1110-1=0");
+        assert_eq!(num_to_snafu(4890), "2=-1=0");
+    }
+}

src/lib.rs

@@ -15,6 +15,7 @@ pub mod day21;
 pub mod day22;
 pub mod day23;
 pub mod day24;
+pub mod day25;
 pub mod day3;
 pub mod day4;
 pub mod day5;