rust 58 lines · 8 steps

Parsing duration strings safely in Rust

A single-pass parser turns strings like "1h30m" into a Duration while catching every malformed or overflowing input.

Explained by highlit
Intermediate parsing error-handling checked-arithmetic state-machine enums
1use std::time::Duration;
2 
3#[derive(Debug, PartialEq)]
4pub enum ParseDurationError {
5 Empty,
6 UnexpectedChar(char),
7 MissingUnit,
8 Overflow,
9}
10 
11pub fn parse_duration(input: &str) -> Result<Duration, ParseDurationError> {
12 let input = input.trim();
13 if input.is_empty() {
14 return Err(ParseDurationError::Empty);
15 }
16 
17 let mut total_secs: u64 = 0;
18 let mut current: u64 = 0;
19 let mut has_digits = false;
20 
21 for ch in input.chars() {
22 match ch {
23 '0'..='9' => {
24 current = current
25 .checked_mul(10)
26 .and_then(|v| v.checked_add((ch as u8 - b'0') as u64))
27 .ok_or(ParseDurationError::Overflow)?;
28 has_digits = true;
29 }
30 _ => {
31 if !has_digits {
32 return Err(ParseDurationError::UnexpectedChar(ch));
33 }
34 let multiplier = match ch {
35 'd' => 86_400,
36 'h' => 3_600,
37 'm' => 60,
38 's' => 1,
39 other => return Err(ParseDurationError::UnexpectedChar(other)),
40 };
41 let segment = current
42 .checked_mul(multiplier)
43 .ok_or(ParseDurationError::Overflow)?;
44 total_secs = total_secs
45 .checked_add(segment)
46 .ok_or(ParseDurationError::Overflow)?;
47 current = 0;
48 has_digits = false;
49 }
50 }
51 }
52 
53 if has_digits {
54 return Err(ParseDurationError::MissingUnit);
55 }
56 
57 Ok(Duration::from_secs(total_secs))
58}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Modeling each failure mode as an enum variant makes error handling exhaustive and self-documenting.
  2. 2Checked arithmetic converts silent overflow into an explicit, recoverable error.
  3. 3A couple of accumulator variables let you parse in one pass without allocating substrings.

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