/// Formats `f64` with Java `Double.toString` semantics as closely as Rust's
/// shortest-roundtrip formatting allows.
pub fn format_java_double(value: f64) -> String {
    if value.is_nan() {
        return "NaN".to_string();
    }
    if value == f64::INFINITY {
        return "Infinity".to_string();
    }
    if value == f64::NEG_INFINITY {
        return "-Infinity".to_string();
    }
    if value == 0.0 {
        return if value.is_sign_negative() {
            "-0.0".to_string()
        } else {
            "0.0".to_string()
        };
    }

    let scientific = format!("{value:e}");
    let (mantissa, exp) = scientific
        .split_once('e')
        .expect("lowerexp formatting always contains e");
    let exponent: i32 = exp.parse().expect("lowerexp exponent is an integer");
    let negative = mantissa.starts_with('-');
    let mantissa = mantissa.trim_start_matches('-');
    let digits: String = mantissa.chars().filter(|c| *c != '.').collect();

    // Java uses plain decimal notation for 1e-3 <= abs(value) < 1e7.
    if (-3..7).contains(&exponent) {
        let point_pos = exponent + 1;
        let mut out = String::new();
        if negative {
            out.push('-');
        }
        if point_pos <= 0 {
            out.push_str("0.");
            for _ in 0..(-point_pos) {
                out.push('0');
            }
            out.push_str(&digits);
        } else if point_pos as usize >= digits.len() {
            out.push_str(&digits);
            for _ in 0..(point_pos as usize - digits.len()) {
                out.push('0');
            }
            out.push_str(".0");
        } else {
            let split = point_pos as usize;
            out.push_str(&digits[..split]);
            out.push('.');
            out.push_str(&digits[split..]);
        }
        out
    } else {
        let mut out = String::new();
        if negative {
            out.push('-');
        }
        out.push(digits.as_bytes()[0] as char);
        out.push('.');
        if digits.len() == 1 {
            out.push('0');
        } else {
            out.push_str(&digits[1..]);
        }
        out.push('E');
        out.push_str(&exponent.to_string());
        out
    }
}

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

    #[test]
    fn java_double_formatting() {
        assert_eq!(format_java_double(1.0), "1.0");
        assert_eq!(format_java_double(36.0), "36.0");
        assert_eq!(format_java_double(0.001), "0.001");
        assert_eq!(format_java_double(0.0001), "1.0E-4");
        assert_eq!(format_java_double(9_999_999.0), "9999999.0");
        assert_eq!(format_java_double(10_000_000.0), "1.0E7");
        assert_eq!(format_java_double(1e22), "1.0E22");
        assert_eq!(format_java_double(1e-7), "1.0E-7");
        assert_eq!(format_java_double(-2.5), "-2.5");
    }
}