/*
Program: Print numbers with a thousand separator as spaces in Rust.
Architecture notes:
-------------------
- Rust's standard formatting macros do not support custom thousand separators.
- We implement a manual formatter:
* Convert the number to a string.
* Handle negative numbers cleanly.
* Insert spaces every three digits from the left.
- The function is modular, reusable, and efficient.
Performance notes:
------------------
- O(n) time where n is the number of digits.
- Memory usage is minimal; only small strings are created.
- No external crates; uses only Rust's standard library.
Pitfalls:
---------
- Must preserve the minus sign for negative numbers.
- Must ensure correct grouping even for very large integers.
- Avoid formatting floats unless converted to integers first.
Security notes:
---------------
- No external input parsing; safe for demonstration.
- Avoid printing sensitive values in real systems.
Tests:
------
- Positive numbers
- Negative numbers
- Zero
- Very large integers
*/
/// Format an integer using spaces as thousand separators.
fn format_with_spaces(n: i64) -> String {
let s = n.to_string();
// Handle negative numbers
let is_negative = s.starts_with('-');
let digits = if is_negative { &s[1..] } else { &s };
let len_digits = digits.len();
// Determine how many digits before the first space
let first_group = if len_digits % 3 == 0 { 3 } else { len_digits % 3 };
let mut result = String::new();
if is_negative {
result.push('-');
}
// First group
result.push_str(&digits[..first_group]);
// Remaining groups
let mut pos = first_group;
while pos < len_digits {
result.push(' ');
result.push_str(&digits[pos..pos + 3]);
pos += 3;
}
result
}
fn main() {
let tests: [i64; 6] = [
0,
42,
1234,
987_654_321,
-1_234_567,
1_000_000_000_000_000_000, // 10^18
];
println!("Running test cases:\n");
for n in tests {
println!("Original: {}", n);
println!("Formatted: {}", format_with_spaces(n));
println!();
}
}
/*
run:
Running test cases:
Original: 0
Formatted: 0
Original: 42
Formatted: 42
Original: 1234
Formatted: 1 234
Original: 987654321
Formatted: 987 654 321
Original: -1234567
Formatted: -1 234 567
Original: 1000000000000000000
Formatted: 1 000 000 000 000 000 000
*/
