// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: MIT

package core

import "testing"

// TestInterpolateQueryOnKnotTakesTheLeftSegment pins the segment choice
// for a query landing exactly on a knot: the bisection takes the first
// knot at or above the query, so the left segment is used and the query
// sits at its upper end with t exactly 1. With a lower y of zero the
// interpolation is exactly the knot's own y. A repeated knot makes the
// choice observable: the query returns the first of the repeated ys,
// where the right segment would return the second.
func TestInterpolateQueryOnKnotTakesTheLeftSegment(t *testing.T) {
	xs := mustFloats(t, []float64{0, 1, 2, 3})
	ys := mustFloats(t, []float64{0, 10, 20, 30})
	q := mustFloats(t, []float64{1, 2})
	out, err := Interpolate(xs, ys, q)
	if err != nil {
		t.Fatalf("Interpolate: %v", err)
	}
	for i, w := range []float64{10, 20} {
		if got := out.FloatAt(i); got != w {
			t.Fatalf("query on knot %v answered %v, want exactly %v", q.FloatAt(i), got, w)
		}
	}
	dup, err := Interpolate(
		mustFloats(t, []float64{0, 1, 1, 2}),
		mustFloats(t, []float64{10, 20, 30, 40}),
		mustFloats(t, []float64{1}))
	if err != nil {
		t.Fatalf("Interpolate: %v", err)
	}
	if got := dup.FloatAt(0); got != 20 {
		t.Fatalf("query on a repeated knot answered %v, want the left segment's 20", got)
	}
}