Document Year 2022 Day 25

Author: maneatingape

src/year2022/day25.rs

@@ -1,17 +1,23 @@
-pub fn parse(input: &str) -> Vec<&[u8]> {
-    input.lines().map(str::as_bytes).collect()
+//! # Full of Hot Air
+//!
+//! The SNAFU numbers are balanced quinary, similar to
+//! [an actual base](https://en.wikipedia.org/wiki/Balanced_ternary)
+//! used by some experimental computers.
+pub fn parse(input: &str) -> Vec<&str> {
+    input.lines().collect()
 }
 
-pub fn part1(input: &[&[u8]]) -> String {
+pub fn part1(input: &[&str]) -> String {
     to_snafu(input.iter().map(from_snafu).sum())
 }
 
-pub fn part2(_input: &[&[u8]]) -> &'static str {
+pub fn part2(_input: &[&str]) -> &'static str {
     "n/a"
 }
 
-fn from_snafu(snafu: &&[u8]) -> i64 {
-    snafu.iter().fold(0, |acc, c| {
+/// Converting from SNAFU to decimal is straightforward.
+fn from_snafu(snafu: &&str) -> i64 {
+    snafu.bytes().fold(0, |acc, c| {
         let digit = match c {
             b'=' => -2,
             b'-' => -1,
@@ -24,8 +30,10 @@ fn from_snafu(snafu: &&[u8]) -> i64 {
     })
 }
 
-fn to_snafu(decimal: i64) -> String {
-    let mut n = decimal;
+/// Convert to decimal by first finding the result modulus 5 for each digit.
+/// If the answer is 3 or 4 then we must add a carry to the next digit so account for the
+/// subtraction.
+fn to_snafu(mut n: i64) -> String {
     let mut digits = String::new();
 
     while n > 0 {
@@ -38,6 +46,8 @@ fn to_snafu(decimal: i64) -> String {
             _ => unreachable!(),
         };
         digits.insert(0, next);
+        // If the remainder of n is 3 or higher then this will add a carry digit to account
+        // for the subtraction.
         n = (n + 2) / 5;
     }