float_formatting.rs

fn float_precision(v: f64, sig_figs: u32) -> usize {
    let v = v.abs();
    if v == 0f64 {
        return 0;
    }
    let prec = (v.log10().floor() - (sig_figs as f64)) as isize + 1;
    if prec >= 0 {
        0
    } else {
        -prec as usize
    }
}

#[cfg(test)]
mod tests {
    use super::float_precision;

    parametrize_test!{test_float_precision_formatting, [
        (v: f64, s: u32, r: &'static str),
        (100f64,       2, "100"),
        ( 10f64,       2, "10"),
        (  1f64,       2, "1.0"),
        (  0f64,       2, "0"),
        (  0.1f64,     2, "0.10"),
        (123.456f64,   2, "123"),
        ( 23.456f64,   2, "23"),
        (  3.456f64,   2, "3.5"),
        (  0.456f64,   2, "0.46"),
        (  0.056f64,   2, "0.056"),
        (  0.00078f64, 2, "0.00078"),

        (100f64,       3, "100"),
        ( 10f64,       3, "10.0"),
        (  1f64,       3, "1.00"),
        (  0f64,       3, "0"),
        (  0.1f64,     3, "0.100"),
        (123.456f64,   3, "123"),
        ( 23.456f64,   3, "23.5"),
        (  3.456f64,   3, "3.46"),
        (  0.456f64,   3, "0.456"),
        (  0.056f64,   3, "0.0560"),
        (  0.00078f64, 3, "0.000780"),
    ], {
        assert_eq!(format!("{:.*}", float_precision(v, s), v), r);
    }}
}