feat: initial release

Assisted-by: GLM 5.3 Flash

integrate/odegrid_pin_test.go

@@ -0,0 +1,41 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: MIT
+
+package integrate
+
+import (
+	"math"
+	"testing"
+
+	"sourcedock.dev/petrbalvin/tensor/internal/core"
+)
+
+// TestFixedStepTimeGrid pins the exact time grid of the fixed-step
+// solvers: the stage times come from t0 + i·h, never from an
+// accumulated t += h, whose rounding walks over a long run. On
+// y' = cos t from t0 = 1e6 the walk used to hold the answer at an
+// error of 1.1e-8 that no step count refined away; the grid puts RK4
+// at its own truncation floor and lets the first-order schemes show
+// their h. The tolerances sit an order below the old walk, so the pin
+// fails on an accumulated-time build.
+func TestFixedStepTimeGrid(t *testing.T) {
+	const t0, span, steps = 1e6, 1e-1, 1e5
+	cos := func(t float64, y *core.Array) (*core.Array, error) {
+		return core.FromFloats([]float64{math.Cos(t)}, 1)
+	}
+	exact := math.Sin(t0 + span)
+	end, err := IntegrateRK4(cos, t0, t0+span, mustFloats(t, []float64{math.Sin(t0)}), steps)
+	if err != nil {
+		t.Fatalf("IntegrateRK4: %v", err)
+	}
+	if got := math.Abs(end.FloatAt(0) - exact); got > 1e-11 {
+		t.Fatalf("RK4 error %.3e exceeds 1e-11: the stage times have left the exact grid", got)
+	}
+	end, err = IntegrateBackwardEuler(cos, t0, t0+span, mustFloats(t, []float64{math.Sin(t0)}), 1e6, ODEOptions{})
+	if err != nil {
+		t.Fatalf("IntegrateBackwardEuler: %v", err)
+	}
+	if got := math.Abs(end.FloatAt(0) - exact); got > 5e-9 {
+		t.Fatalf("backward Euler error %.3e exceeds 5e-9: the step times have left the exact grid", got)
+	}
+}