big_integer/multiply: improve docs, validation, and clarity

Signed-off-by: KushalMeghani1644 <kushalmeghani108@gmail.com>

Author: KushalMeghani1644

src/big_integer/multiply.rs

@@ -1,72 +1,94 @@
-/// Performs long multiplication on string representations of non-negative numbers.
+/// Performs long multiplication on string representations of non-negative base-10 integers.
+///
+/// # Panics
+/// Panics if either input contains non-ASCII digits, leading zeros (except "0"),
+/// or is empty.
+///
+/// # Examples
+/// ```
+/// use crate::big_integer::multiply;
+/// assert_eq!(multiply("123", "456"), "56088");
+/// assert_eq!(multiply("99", "99"), "9801");
+/// ```
 pub fn multiply(num1: &str, num2: &str) -> String {
-    if !is_valid_nonnegative(num1) || !is_valid_nonnegative(num2) {
-        panic!("String does not conform to specification")
+    if !is_valid_nonnegative(num1) {
+        panic!("Invalid input: `{num1}` is not a valid non-negative integer string");
+    }
+    if !is_valid_nonnegative(num2) {
+        panic!("Invalid input: `{num2}` is not a valid non-negative integer string");
     }
 
     if num1 == "0" || num2 == "0" {
         return "0".to_string();
     }
-    let output_size = num1.len() + num2.len();
 
+    let output_size = num1.len() + num2.len();
     let mut mult = vec![0; output_size];
+
     for (i, c1) in num1.chars().rev().enumerate() {
         for (j, c2) in num2.chars().rev().enumerate() {
             let mul = c1.to_digit(10).unwrap() * c2.to_digit(10).unwrap();
-            // It could be a two-digit number here.
-            mult[i + j + 1] += (mult[i + j] + mul) / 10;
-            // Handling rounding. Here's a single digit.
-            mult[i + j] = (mult[i + j] + mul) % 10;
+            let sum = mult[i + j] + mul;
+
+            mult[i + j + 1] += sum / 10;
+            mult[i + j] = sum % 10;
         }
     }
+
     if mult[output_size - 1] == 0 {
         mult.pop();
     }
     mult.iter().rev().map(|&n| n.to_string()).collect()
 }
 
+/// Checks whether a string represents a valid non-negative base-10 integer.
+/// Disallows empty strings, leading zeros (except "0"), and non-ASCII digits.
 pub fn is_valid_nonnegative(num: &str) -> bool {
-    num.chars().all(char::is_numeric) && !num.is_empty() && (!num.starts_with('0') || num == "0")
+    !num.is_empty()
+        && num.chars().all(|c| c.is_ascii_digit())
+        && (!num.starts_with('0') || num == "0")
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
+
     macro_rules! test_multiply {
         ($($name:ident: $inputs:expr,)*) => {
-        $(
-            #[test]
-            fn $name() {
-                let (s, t, expected) = $inputs;
-                assert_eq!(multiply(s, t), expected);
-                assert_eq!(multiply(t, s), expected);
-            }
-        )*
+            $(
+                #[test]
+                fn $name() {
+                    let (s, t, expected) = $inputs;
+                    assert_eq!(multiply(s, t), expected, "multiply({s}, {t})");
+                    assert_eq!(multiply(t, s), expected, "multiply({t}, {s})");
+                }
+            )*
         }
     }
 
     test_multiply! {
-        multiply0: ("2", "3", "6"),
-        multiply1: ("123", "456", "56088"),
+        multiply_small: ("2", "3", "6"),
+        multiply_basic: ("123", "456", "56088"),
         multiply_zero: ("0", "222", "0"),
-        other_1: ("99", "99", "9801"),
-        other_2: ("999", "99", "98901"),
-        other_3: ("9999", "99", "989901"),
-        other_4: ("192939", "9499596", "1832842552644"),
+        multiply_double_digits: ("99", "99", "9801"),
+        multiply_triple_digits: ("999", "99", "98901"),
+        multiply_four_digits: ("9999", "99", "989901"),
+        multiply_large: ("192939", "9499596", "1832842552644"),
     }
 
     macro_rules! test_multiply_with_wrong_input {
         ($($name:ident: $inputs:expr,)*) => {
-        $(
-            #[test]
-            #[should_panic]
-            fn $name() {
-                let (s, t) = $inputs;
-                multiply(s, t);
-            }
-        )*
+            $(
+                #[test]
+                #[should_panic]
+                fn $name() {
+                    let (s, t) = $inputs;
+                    multiply(s, t);
+                }
+            )*
         }
     }
+
     test_multiply_with_wrong_input! {
         empty_input: ("", "121"),
         leading_zero: ("01", "3"),