Defer zeroes deletion to last steps

utils/fixed_decimal/src/decimal.rs

@@ -1062,13 +1062,12 @@ impl FixedDecimal {
         }
 
         // 3. If the rounding position is *in the middle* of the nonzero digits
-
         if magnitude < self.magnitude {
-            // 3-a. Calculate the number of digits to retain and remove the rest
+            // 3a. Calculate the number of digits to retain and remove the rest
             let digits_to_retain = crate::ops::i16_abs_sub(self.magnitude, magnitude) as usize;
             self.digits.truncate(digits_to_retain);
 
-            // 3-b. Truncate to the next coarseness.
+            // 3b. Truncate to the next coarseness.
             'coarseness: {
                 match coarseness {
                     RoundingCoarseness::MultiplesOf1 => {
@@ -1085,7 +1084,7 @@ impl FixedDecimal {
                             break 'coarseness;
                         };
 
-                        let num = match coarseness {
+                        *last_digit = match coarseness {
                             RoundingCoarseness::MultiplesOf2 => {
                                 // Equivalent to (n / 2) * 2, which truncates to the next
                                 // multiple of two
@@ -1095,12 +1094,6 @@ impl FixedDecimal {
                             RoundingCoarseness::MultiplesOf5 => 5,
                             _ => 0, // Unreachable
                         };
-
-                        if num == 0 {
-                            self.digits.pop();
-                        } else {
-                            *last_digit = num;
-                        }
                     }
                     RoundingCoarseness::MultiplesOf25 => {
                         // Temporarily extend the digits to have the correct
@@ -1116,21 +1109,23 @@ impl FixedDecimal {
                         let Some((second_last_digit, _)) = digits.split_last_mut() else {
                             // The number has no other digits aside from the last,
                             // making it strictly less than 25.
-                            self.digits.pop();
+                            *last_digit = 0;
                             break 'coarseness;
                         };
 
                         let number = *second_last_digit * 10 + *last_digit;
 
+                        // All zeroes will be removed below. We can defer the deletion
+                        // to that part.
                         if number < 25 {
-                            self.digits.truncate(self.digits.len() - 2);
+                            *second_last_digit = 0;
+                            *last_digit = 0;
                         } else if number < 50 {
                             *second_last_digit = 2;
                             *last_digit = 5;
                         } else if number < 75 {
                             *second_last_digit = 5;
-                            // Last digit can just be removed since it will be zero.
-                            self.digits.pop();
+                            *last_digit = 0;
                         } else {
                             *second_last_digit = 7;
                             *last_digit = 5;
@@ -1139,7 +1134,7 @@ impl FixedDecimal {
                 }
             }
 
-            // 3-c. Handle the case where `digits` has leading zeros after
+            // 3c. Handle the case where `digits` has leading zeros after
             // truncating to the next coarseness.
             let position_last_nonzero_digit = self
                 .digits
@@ -1149,7 +1144,7 @@ impl FixedDecimal {
                 .unwrap_or(0);
             self.digits.truncate(position_last_nonzero_digit);
 
-            // 3-d. If `digits` had only trailing zeroes after truncating,
+            // 3d. If `digits` had only trailing zeroes after truncating,
             // reset to zero.
             if self.digits.is_empty() {
                 self.magnitude = 0;
@@ -3596,7 +3591,7 @@ fn test_concatenate() {
 }
 
 #[test]
-fn test_rounding_increments() {
+fn test_rounding_coarseness() {
     // Test Truncate Right
     let mut dec = FixedDecimal::from(4235970).multiplied_pow10(-3);
     assert_eq!("4235.970", dec.to_string());