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fix(sampling): format _dd.p.ksr to 6 decimal places, not 6 significant digits #2086

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Merged
gh-worker-dd-mergequeue-cf854d merged 1 commit into from
Jun 5, 2026
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fix(sampling): format _dd.p.ksr to 6 decimal places, not 6 significant digits #2086

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43 changes: 17 additions & 26 deletions libdd-sampling/src/datadog_sampler.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -187,45 +187,27 @@ impl DatadogSampler {
}
}

/// Formats a sampling rate with up to 6 significant digits, stripping trailing zeros.
/// Formats a sampling rate with up to 6 decimal places, stripping trailing zeros.
///
/// This matches the Go behavior of `strconv.FormatFloat(rate, 'g', 6, 64)`.
/// Matches `strconv.FormatFloat(rate, 'f', 6, 64)` in Go, after trailing-zero
/// stripping. Rates below 5e-7 round to `"0"`.
///
/// # Examples
/// - `1.0` → `Some("1")`
/// - `0.5` → `Some("0.5")`
/// - `0.7654321` → `Some("0.765432")`
/// - `0.100000` → `Some("0.1")`
/// - `0.0000001` → `Some("0")` (rounds below the 6th decimal place)
/// - `0.00000051` → `Some("0.000001")` (rounds up to one millionth)
/// - `-0.1` → `None`
/// - `1.1` → `None`
fn format_sampling_rate(rate: f64) -> Option<String> {
if rate.is_nan() || !(0.0..=1.0).contains(&rate) {
return None;
}

if rate == 0.0 {
return Some("0".to_string());
}

let digits = 6_i32;
let magnitude = rate.abs().log10().floor() as i32;
let scale = 10f64.powi(digits - 1 - magnitude);
let rounded = (rate * scale).round() / scale;

// Determine decimal places needed for 6 significant digits
let decimal_places = if magnitude >= digits - 1 {
0
} else {
(digits - 1 - magnitude) as usize
};

let s = format!("{:.prec$}", rounded, prec = decimal_places);
// Strip trailing zeros after decimal point
Some(if s.contains('.') {
s.trim_end_matches('0').trim_end_matches('.').to_string()
} else {
s
})
let s = format!("{rate:.6}");
Some(s.trim_end_matches('0').trim_end_matches('.').to_string())
}

pub struct TraceRootSamplingInfo {
Expand Down Expand Up @@ -1178,7 +1160,7 @@ mod tests {
assert_eq!(format_sampling_rate(0.100000), Some("0.1".to_string()));
assert_eq!(format_sampling_rate(0.500000), Some("0.5".to_string()));

// Truncation to 6 significant digits
// Truncation to 6 decimal places
assert_eq!(
format_sampling_rate(0.7654321),
Some("0.765432".to_string())
Expand All @@ -1190,6 +1172,15 @@ mod tests {

// Small values
assert_eq!(format_sampling_rate(0.001), Some("0.001".to_string()));
assert_eq!(format_sampling_rate(0.000001), Some("0.000001".to_string()));

// Below the 6th decimal place rounds to "0"
assert_eq!(format_sampling_rate(0.0000001), Some("0".to_string()));
// Just above the round-up threshold lands on one millionth
assert_eq!(
format_sampling_rate(0.00000051),
Some("0.000001".to_string())
);

// Boundary values
assert_eq!(format_sampling_rate(0.75), Some("0.75".to_string()));
Expand Down
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