Parallel Build & Progress Reporting

ChebyshevSharp exposes two constructor-time options on all four interpolant classes:

• nWorkers (int?): null (sequential, default), -1 (Environment.ProcessorCount), or positive int (Parallel.For pool size).
• progress (IProgress<int>?): per-evaluation cumulative count for Approx/Spline/Slider; per-sweep for TT-Cross.

Thread-safety contract

When nWorkers is non-null, the user-supplied function may be invoked concurrently from multiple threads via Parallel.For. Functions that capture mutable state must use locks or external synchronization. Functions that are pure (no captured mutable state) need no special handling.

// Pure function: thread-safe by construction.
double F(double[] p, object? _) => Math.Sin(p[0]) + Math.Cos(p[1]);
var ap = new ChebyshevApproximation(F, 2,
    new[] { new[] { -1.0, 1.0 }, new[] { -1.0, 1.0 } },
    new[] { 32, 32 },
    nWorkers: -1);  // use all cores
ap.Build();

// Captures shared state — wrap in lock if used with nWorkers != null.
int callCount = 0;
object lockObj = new();
double F2(double[] p, object? _)
{
    lock (lockObj) callCount++;
    return p[0] * p[0];
}

Progress reporting

Progress reports are cumulative integer counts. The total is derivable upfront via the GetNumEvaluationPoints() getter on Approx/Spline/Slider. For ChebyshevTT, progress is only reported by the TT-Cross path and counts completed sweeps up to maxSweeps; TT-SVD and ALS do not currently report progress.

var counter = new Progress<int>(n => Console.Write($"\r{n} evaluations done"));
var ap = new ChebyshevApproximation(
    function: F,
    numDimensions: 2,
    domain: new[] { new[] { -1.0, 1.0 }, new[] { -1.0, 1.0 } },
    nNodes: new[] { 32, 32 },
    progress: counter);
ap.Build();

progress: null is a no-op; the parallel path skips the increment entirely.

TT and nWorkers

ChebyshevTT accepts nWorkers for API symmetry but ignores it: TT-Cross is an adaptive sampling algorithm, not a pre-grid evaluation, so per-grid parallelism does not apply. progress on TT fires once per completed TT-Cross sweep and may stop before maxSweeps when the tolerance or improvement stopping rule is reached.