1 files changed, 111 insertions, 0 deletions
diff --git a/2022/src/bin/day_25.rs b/2022/src/bin/day_25.rs
new file mode 100644
index 0000000..d55f30a
--- /dev/null
+++ b/2022/src/bin/day_25.rs
@@ -0,0 +1,111 @@
+use nom::{
+    branch::alt,
+    bytes::complete::tag,
+    character::complete::line_ending,
+    combinator::{map, value},
+    multi::{many1, separated_list1},
+    IResult,
+};
+use std::fs;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let input = fs::read_to_string("inputs/day_25.txt")?;
+    let parsed = SnafuList::parser(&input).unwrap().1;
+    dbg!(to_snafu(parsed.sum()));
+
+    Ok(())
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+struct SnafuList(Vec<i64>);
+
+impl SnafuList {
+    fn parser(input: &str) -> IResult<&str, SnafuList> {
+        map(
+            separated_list1(
+                line_ending,
+                map(
+                    many1(alt((
+                        value(2, tag("2")),
+                        value(1, tag("1")),
+                        value(0, tag("0")),
+                        value(-1, tag("-")),
+                        value(-2, tag("=")),
+                    ))),
+                    |digits| {
+                        let mut result: i64 = 0;
+                        for digit in digits {
+                            result *= 5;
+                            result += digit;
+                        }
+                        result
+                    },
+                ),
+            ),
+            SnafuList,
+        )(input)
+    }
+
+    fn sum(&self) -> i64 {
+        self.0.iter().sum()
+    }
+}
+
+fn to_snafu(mut remaining_value: i64) -> String {
+    let mut result = String::new();
+
+    let mut current_power = 5i64.pow(26);
+
+    while current_power > 0 {
+        let next_digit_range = {
+            let mut next_digit_power = current_power / 5;
+            let mut max_next_digit = 0;
+            while next_digit_power > 0 {
+                max_next_digit += 2 * next_digit_power;
+                next_digit_power /= 5;
+            }
+            -max_next_digit..=max_next_digit
+        };
+
+        let (digit, digit_value) = [('=', -2), ('-', -1), ('0', 0), ('1', 1), ('2', 2)]
+            .into_iter()
+            .filter_map(|(digit, digit_value)| {
+                let digit_value = digit_value * current_power;
+                let remaining_if_digit_set = remaining_value - digit_value;
+                if next_digit_range.contains(&remaining_if_digit_set) {
+                    Some((digit, digit_value))
+                } else {
+                    None
+                }
+            })
+            .next()
+            .expect("No digit found");
+
+        remaining_value -= digit_value;
+        result.push(digit);
+
+        current_power /= 5;
+    }
+
+    result.trim_start_matches("0").to_owned()
+}
+
+#[test]
+fn round_trip_works() {
+    let str_list = vec![
+        "1=-0-2", "12111", "2=0=", "21", "2=01", "111", "20012", "112", "1=-1=", "1-12", "12",
+        "1=", "122",
+    ];
+    let list = SnafuList::parser(&str_list.join("\n")).unwrap().1;
+
+    assert_eq!(
+        list,
+        SnafuList(vec![
+            1747, 906, 198, 11, 201, 31, 1257, 32, 353, 107, 7, 3, 37
+        ])
+    );
+
+    for (i, num) in list.0.iter().enumerate() {
+        assert_eq!(to_snafu(*num), str_list[i]);
+    }
+}