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2024/rust/src/day3.rs

@@ -8,20 +8,43 @@ fn main() {
 
 fn sum_muls(input: &str) -> usize {
     let mut sum = 0;
+
+    // this string slice will be how we keep track of what is "left" to parse
+    // though it would likely be easier to use an index and slice as needed :shrug:
     let mut s = &input[..];
+
+    // look for the next valid mul operation
     while let Some(i) = s.find("mul(") {
+        // re-slice to the remaining string
         s = &s[i + 4..];
+
+        // variables to contain our operands
         let mut left: Option<usize> = None;
         let mut right: Option<usize> = None;
+
+        // variable to contain our currently-parsing token, as we will continue
+        // from here one character at a time
         let mut token = &s[0..0];
+
         for c in s.chars() {
             match c {
+                // any digit simply gets "appended" to our token (in reality, we are just increasing the length of the token slice)
                 '0'..='9' => {
                     token = &s[0..token.len() + 1];
+                    // the problem indicates that numbers may not be longer than
+                    // 3 digits, so if we encounter one, we break out of this
+                    // loop and head to the next mul operation
                     if token.len() > 3 {
                         break;
                     }
                 }
+
+                // once we hit a comma, we check that we don't already have
+                // a left operand then parse the number (which we can unwrap
+                // since we are assuring we are dealing with digits above,
+                // though we could easily optimize that out with bit of
+                // arithmetic), advance through our string slice, and reset our
+                // currently-parsing token
                 ',' => {
                     if left.is_some() {
                         break;
@@ -30,6 +53,9 @@ fn sum_muls(input: &str) -> usize {
                     s = &s[token.len() + 1..];
                     token = &s[0..0];
                 }
+
+                // pretty much the same logic for finding a comma, but for the
+                // right operand
                 ')' => {
                     if right.is_some() {
                         break;
@@ -49,21 +75,38 @@ fn sum_muls(input: &str) -> usize {
 }
 
 fn sum_muls_with_do_and_dont(input: &str) -> usize {
+    // the gist of this function is that we look for string segments that are
+    // between do() and don't() operations and only sum_muls for those NOT
+    // between a don't() and do()
     let mut sum = 0;
     let mut s = &input[..];
+
+    // since there may be a case where the last do() or don't() is a do(),
+    // we need to know whether or not to parse the "remainder" after we have
+    // processed the last don't() segment which may have ended with a do()
     let mut mul_rest = true;
     while let Some(i) = s.find("don't()") {
+        // once we find a don't, we know we want to sum_muls for everything up
+        // to the current position
         sum += sum_muls(&s[0..i]);
+
+        // then we can advance our slice
         s = &s[i..];
         mul_rest = false;
         if let Some(i) = s.find("do()") {
+            // if we find a do(), set our mul_rest flag appropriately, advance
+            // along the slice, and head to the next iteration (or be done in
+            // the loop)
             s = &s[i..];
             mul_rest = true;
         }
     }
+
+    // if we "ended" with a d(), we want to sum_muls for the rest of the slice
     if mul_rest {
         sum += sum_muls(s);
     }
+
     sum
 }