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pre-allocate the buffer, including a correctness test #6

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pre-allocate the buffer, including a correctness test #6

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17 changes: 11 additions & 6 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -13,15 +13,19 @@ pub fn left_pad(s: &str, pad: usize) -> String
/// If str.len() is not less than pad, then the string is returned verbatim
pub fn left_pad_char(s: &str, pad: usize, padchar: char) -> String
{
let mut out = String::new();
// pad_len is the number of padchar that actually need to be put before the string.
let pad_len = pad.saturating_sub(s.len());
let required_capacity = s.as_bytes().len() + (pad_len as usize) * padchar.len_utf8();
let mut out = String::with_capacity(required_capacity);

let len = s.len();
if pad > len {
for _ in 0..pad-len {
out.push(padchar);
}
for _ in 0..pad_len {
out.push(padchar);
}
out.push_str(s);
// Assert a correct prediction of the required capacity.
debug_assert_eq!(out.as_bytes().len(), required_capacity);
// Assert no further allocation was done.
debug_assert_eq!(out.capacity(), required_capacity);

out
}
Expand All @@ -36,5 +40,6 @@ fn pad_test() {
assert_eq!(left_pad("foo", 5), " foo");
assert_eq!(left_pad_char("foo", 7, 'X'), "XXXXfoo");
assert_eq!(left_pad_char("bar", 5, '-'), "--bar");
assert_eq!(left_pad_char("rust", 7, '\u{2764}'), "❤❤❤rust");
}