1 files changed, 111 insertions, 0 deletions
diff --git a/src/bin/day25.rs b/src/bin/day25.rs
new file mode 100644
index 0000000..4350d7d
--- /dev/null
+++ b/src/bin/day25.rs
@@ -0,0 +1,111 @@
+static DAY: u8 = 25;
+
+fn main() {
+    let input = advent::read_lines(DAY);
+    println!("{DAY}a: {}", console_number(&input));
+}
+
+fn snafu_to_number(input: &str) -> i64 {
+    let mut number = 0;
+    for c in input.chars() {
+        number *= 5;
+        number += match c {
+            '0' => 0,
+            '1' => 1,
+            '2' => 2,
+            '-' => -1,
+            '=' => -2,
+            _ => unimplemented!(),
+        };
+    }
+    number
+}
+
+fn number_to_snafu(input: i64) -> String {
+    let mut number = input;
+
+    let mut digits = Vec::new();
+    while number > 0 {
+        digits.push(number % 5);
+        number /= 5;
+    }
+    digits.push(0);
+
+    for i in 0 .. digits.len() - 1 {
+        if digits[i] == 3 {
+            digits[i+1] += 1;
+            digits[i] = -2;
+        } else if digits[i] == 4 {
+            digits[i+1] += 1;
+            digits[i] = -1;
+        } else if digits[i] == 5 {
+            digits[i+1] += 1;
+            digits[i] = 0;
+        }
+    }
+
+    let mut snafu = String::new();
+    for digit in digits.iter().rev().skip_while(|&x| *x == 0) {
+        match digit {
+            -1 => snafu.push('-'),
+            -2 => snafu.push('='),
+            0 => snafu.push('0'),
+            1 => snafu.push('1'),
+            2 => snafu.push('2'),
+            _ => unimplemented!(),
+        }
+    }
+    snafu
+}
+
+fn console_number(input: &[String]) -> String {
+    let number = input.iter()
+                      .map(|x| snafu_to_number(x))
+                      .sum();
+    number_to_snafu(number)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        let input = [
+            "1=-0-2",
+            "12111",
+            "2=0=",
+            "21",
+            "2=01",
+            "111",
+            "20012",
+            "112",
+            "1=-1=",
+            "1-12",
+            "12",
+            "1=",
+            "122",
+        ].iter().map(|&x| String::from(x)).collect::<Vec<_>>();
+
+        assert_eq!(console_number(&input), "2=-1=0");
+    }
+
+    #[test]
+    fn test_number_to_snafu() {
+        assert_eq!(number_to_snafu(1), "1");
+        assert_eq!(number_to_snafu(2), "2");
+        assert_eq!(number_to_snafu(3), "1=");
+        assert_eq!(number_to_snafu(4), "1-");
+        assert_eq!(number_to_snafu(5), "10");
+        assert_eq!(number_to_snafu(6), "11");
+        assert_eq!(number_to_snafu(7), "12");
+        assert_eq!(number_to_snafu(8), "2=");
+        assert_eq!(number_to_snafu(9), "2-");
+        assert_eq!(number_to_snafu(10), "20");
+        assert_eq!(number_to_snafu(15), "1=0");
+        assert_eq!(number_to_snafu(20), "1-0");
+        assert_eq!(number_to_snafu(2022), "1=11-2");
+        assert_eq!(number_to_snafu(12345), "1-0---0");
+        assert_eq!(number_to_snafu(314159265), "1121-1110-1=0");
+    }
+}