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

package core

import "testing"

// TestCumProdHalfNarrowsCarryPerStep pins the float16 cumulative
// product's carry discipline: every step narrows the running product to
// half before combining, exactly as the stored element narrows, so each
// step multiplies the rounded carry. The input 67, 65, 6 makes the
// discipline visible: 67 times 65 is 4355 in float64, which narrows to
// 4356, and 4356 times 6 rounds to 26144; a carry left in float64 would
// give 4355 times 6 = 26130, which rounds to 26128.
func TestCumProdHalfNarrowsCarryPerStep(t *testing.T) {
	a := mustFromHalves(t, []uint16{
		HalfFromFloat64(67), HalfFromFloat64(65), HalfFromFloat64(6),
	}, 3)
	out, err := CumProd(a, 0)
	if err != nil {
		t.Fatalf("CumProd: %v", err)
	}
	want := []float64{67, 4356, 26144}
	for i, w := range want {
		if got := HalfToFloat64(out.RawHalves()[i]); got != w {
			t.Fatalf("element %d = %v, want %v", i, got, w)
		}
	}
}