multishoot 1.2.0

//! Helpers for states held as PARTS arrays of f64 (part q of unknown i at `z[q][i]`), in any of multidouble's
//! precisions (`precision::Precision`: `Double`, `Quad`, `Octo`), which the residual, the Jacobian and Newton's
//! method are written over.
use multidouble::precision::Precision;
use multidouble::simd::Native;

/// The PARTS slices of a state held as owned part arrays
pub fn parts(z: &[Vec<f64>]) -> Vec<&[f64]> {
    z.iter().map(|p| p.as_slice()).collect()
}

/// The larger of a and b, NaN when either is (`f64::max` returns the other operand for a NaN, so a fold with it
/// would drop them)
pub fn nan_max(a: f64, b: f64) -> f64 {
    if a.is_nan() || b.is_nan() {
        f64::NAN
    } else {
        a.max(b)
    }
}

/// The smaller of a and b, NaN when either is (as `nan_max`)
pub fn nan_min(a: f64, b: f64) -> f64 {
    if a.is_nan() || b.is_nan() {
        f64::NAN
    } else {
        a.min(b)
    }
}

/// max |x| over the scalars (hi part of the double-double rounding); NaN when any is NaN, so a residual with a NaN
/// never passes a tolerance
pub fn max_abs<P: Precision<Native>>(x: &[P::Scalar]) -> f64 {
    x.iter().map(|&v| P::to_dd(v).hi.abs()).fold(0.0, nan_max)
}

/// A state in more parts: the given part arrays, then zero arrays up to `parts` (a double-double state as the start
/// of a quad-double Newton, a quad-double one for octo-double)
pub fn widen(z: &[Vec<f64>], parts: usize) -> Vec<Vec<f64>> {
    assert!(parts >= z.len(), "widen: {} parts into {parts}", z.len());
    let l = z.first().map_or(0, |p| p.len());
    z.iter()
        .cloned()
        .chain((z.len()..parts).map(|_| vec![0.0; l]))
        .collect()
}