Function ctr 

pub fn ctr(clicks: u64, impressions: u64) -> Result<f64, SerpError>

Click-through rate as a fraction in 0.0..=1.0.

Errors

Returns SerpError::NoImpressions for zero impressions and SerpError::ClicksExceedImpressions when the export is internally inconsistent.

assert_eq!(serp_ctr::ctr(0, 500).unwrap(), 0.0);
assert!(serp_ctr::ctr(1, 0).is_err());