feat: initial release

Assisted-by: GLM 5.3 Flash

internal/core/interpolate_knot_test.go

@@ -0,0 +1,38 @@
+// 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)
+	}
+}