1 files changed, 73 insertions, 0 deletions
diff --git a/src/day25.rs b/src/day25.rs
new file mode 100644
index 0000000..2d26028
--- /dev/null
+++ b/src/day25.rs
@@ -0,0 +1,73 @@
+fn from_snafu(s: &str) -> i64 {
+    // This is the easy bit... 
+    let mut i: i64 = 0;
+    let mut pow: i64 = 1;
+    for ch in s.chars().rev() {
+        match ch {
+            '2' => i += 2 * pow,
+            '1' => i += 1 * pow,
+            '0' => {}, // additive identity!
+            '-' => i += -1 * pow,
+            '=' => i += -2 * pow,
+            _ => panic!("unexpected input {}", ch)
+        }
+        pow *= 5;
+    }
+    i
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn test_from_snafu() {
+        let cases = vec![
+        ("1=-0-2",     "1747"),
+        ( "12111",      "906"),
+        (  "2=0=",      "198"),
+        (    "21",       "11"), // works
+        (  "2=01",      "201"), // 201,1 > 40,0 > 8,= > 1,1
+        (   "111",       "31"), // works
+        ( "20012",     "1257"), //works
+        (   "112",       "32"), // works
+        ( "1=-1=",      "353"), // broke
+        (  "1-12",      "107"),
+        (    "12",        "7"),
+        (    "1=",        "3"),
+        (   "122",       "37"),
+        ];
+        for (snafu, decimal) in cases {
+            let x = from_snafu(snafu);
+            assert_eq!(x.to_string().as_str(), decimal);
+            assert_eq!(to_snafu(x), snafu);
+        }
+    }
+}
+
+fn to_snafu(mut i: i64) -> String {
+    // how to do this :thinking: add two, mod 5,take away 2? // nope
+    let mut res = Vec::<char>::new();
+    while i != 0 {
+        let d = ((i + 2) % 5) - 2;
+        let ch = match d {
+            2 => '2',
+            1 => '1',
+            0 => '0',
+            -1 => '-',
+            -2 => '=',
+            _ => panic!("unexpected digit {}", d)
+        };
+        res.push(ch);
+        i = (i + 2) / 5;
+    }
+    res.reverse();
+    res.into_iter().collect::<String>()
+    
+}
+
+pub fn run(input: String) {
+    let s: i64 = input.lines()
+        .map(from_snafu)
+        .sum();
+    println!("Day 25: {}", to_snafu(s));
+}
+\ No newline at end of file