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

package spmd

import (
	"strings"
	"testing"
)

// mustPartition is Partition for the tests' fixed valid arguments: the
// piece, or a failure the tests can never meet. Benchmarks pass their
// own tb.
func mustPartition(tb testing.TB, globalN, size, rank int) Span {
	tb.Helper()
	span, err := Partition(globalN, size, rank)
	if err != nil {
		tb.Fatal(err)
	}
	return span
}

// TestPartitionRefusesInvalidArguments: the inputs Partition refuses
// come back as errors naming the input, never as panics.
func TestPartitionRefusesInvalidArguments(t *testing.T) {
	for _, tc := range []struct {
		name                string
		globalN, size, rank int
		want                string
	}{
		{"negative global length", -1, 4, 0, "Partition of a negative global length -1"},
		{"empty world", 100, 0, 0, "Partition of rank 0 in a world of 0"},
		{"negative rank", 100, 4, -1, "Partition of rank -1 in a world of 4"},
		{"rank past the world", 100, 4, 4, "Partition of rank 4 in a world of 4"},
	} {
		span, err := Partition(tc.globalN, tc.size, tc.rank)
		if err == nil {
			t.Fatalf("%s: Partition answered %v", tc.name, span)
		}
		if !strings.Contains(err.Error(), tc.want) {
			t.Fatalf("%s: the error does not name the input: %v", tc.name, err)
		}
	}
}