From 34af78c8534a3081487735a4aab904ddfb1f56cc Mon Sep 17 00:00:00 2001
From: Stephen Waits <steve@waits.net>
Date: Fri, 1 Dec 2023 19:45:00 -0700
Subject: [PATCH] refactor(day01): more idiomatic, cleaner, optimal (because
 fun!)

---
 src/day01.rs | 62 +++++++++++++++++++---------------------------------
 1 file changed, 23 insertions(+), 39 deletions(-)

diff --git a/src/day01.rs b/src/day01.rs
index 52d7740..3afa34f 100644
--- a/src/day01.rs
+++ b/src/day01.rs
@@ -9,23 +9,18 @@ inventory::submit! {
 }
 
 fn first_and_last_digit(s: &str) -> usize {
-    // Find the first character in the string that is an ASCII digit
-    let first = s
-        .chars()
-        .find(|c| c.is_ascii_digit()) // Use find to get an Option containing the first digit character
-        .unwrap() // Unwrap the Option, panicking if no digit is found
-        .to_digit(10) // Convert the character to a digit in base 10
-        .unwrap() as usize; // Convert the digit to a usize
+    // Find the first ASCII digit in the string
+    let first = s.chars().filter_map(|c| c.to_digit(10)).next().unwrap() as usize; // Safe to use unwrap as a digit is always present
 
-    // Find the last character in the string that is an ASCII digit
+    // Find the last ASCII digit in the string
     let last = s
         .chars()
-        .rfind(|c| c.is_ascii_digit()) // Use rfind to get an Option containing the last digit character
-        .unwrap() // Unwrap the Option, panicking if no digit is found
-        .to_digit(10) // Convert the character to a digit in base 10
-        .unwrap() as usize; // Convert the digit to a usize
+        .rev() // Reverse the iteration for the last digit
+        .filter_map(|c| c.to_digit(10))
+        .next()
+        .unwrap() as usize; // Safe to use unwrap here as well
 
-    // Return the result
+    // Combine the first and last digits to form a new number
     first * 10 + last
 }
 
@@ -35,34 +30,23 @@ lazy_static! {
 }
 
 fn first_and_last_digit_regex(s: &str) -> usize {
-    // Initialize variables to keep track of the positions and values of the first and last digits.
-    let mut first_start = usize::MAX;
-    let mut last_end = 0;
-    let mut first_digit = String::new();
-    let mut last_digit = String::new();
+    // Find the first digit in the string by testing against each sice from [0..] to [n-1..]
+    let first_digit = s
+        .char_indices()
+        .find_map(|(index, _)| REGEX.find(&s[index..]).map(|mat| mat.as_str().to_string()))
+        .unwrap(); // Safe to use unwrap because a digit is always expected
 
-    // Iterate over each character index in the string.
-    for (index, _) in s.char_indices() {
-        // Check if there is a digit at the current index or beyond.
-        if let Some(mat) = REGEX.find(&s[index..]) {
-            // Calculate the start and end positions of the matched digit.
-            let match_start = index + mat.start();
-            let match_end = index + mat.end();
+    // Iterate from the end of the string, checking each slice for the last match
+    let last_digit = s
+        .char_indices()
+        .rev()
+        .find_map(|(index, _)| {
+            // Check if the slice from this index to the end of the string matches the regex
+            REGEX.find(&s[index..]).map(|mat| mat.as_str().to_string())
+        })
+        .unwrap(); // Safe to use unwrap as a digit is always expected
 
-            // If the current match starts before any previously found digit, it's the new first digit.
-            if match_start < first_start {
-                first_start = match_start;
-                first_digit = mat.as_str().to_string();
-            }
-            // If the current match ends after any previously found digit, it's the new last digit.
-            if match_end > last_end {
-                last_end = match_end;
-                last_digit = mat.as_str().to_string();
-            }
-        }
-    }
-
-    // Convert the first and last digit strings to their numeric values and return the result
+    // Convert the found digits to numbers and calculate the result
     digit_value(&first_digit) * 10 + digit_value(&last_digit)
 }