feat: initial release

Assisted-by: GLM 5.3 Flash

spmd/partition_test.go

@@ -0,0 +1,44 @@
+// 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)
+		}
+	}
+}