Say what relates two changes, so unrelated ones can walk beside each other

The helper decides, by repository, module (widened to dependents), delivery
group head or module graph edge, whether two changes are related, as novox/hq
ADR 0304 decision 1 reads in ADR 0306 decision 3. Nothing calls it yet; it is
the first step of issue 525's parallel walks.

cmd/mesh-controller/related_test.go

@@ -0,0 +1,92 @@
+package main
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/novox/mesh-controller/internal/inventory"
+	"github.com/novox/mesh-controller/internal/link"
+)
+
+// heardMerge is a kept merge of a repository from a head branch that moved modules.
+func heardMerge(repository, head string, moves ...string) inventory.BatchedMerge {
+	m := inventory.BatchedMerge{Repository: repository, Branch: "main", Commit: "c-" + repository}
+	m.Event, _ = json.Marshal(keptMerge{SourceMoved: link.SourceMoved{Head: head}, Moves: moves})
+	return m
+}
+
+// walkOf is a walk carrying one repository's commit and planning modules.
+func walkOf(repository string, modules ...string) inventory.Plan {
+	p := inventory.Plan{Repository: repository, Commit: "w-" + repository, Modules: map[string]*inventory.PlanModule{}}
+	for _, name := range modules {
+		p.Modules[name] = &inventory.PlanModule{}
+	}
+	return p
+}
+
+// TestRelatedChanges: two changes are related by a repository, a module, a group head or a graph edge between
+// their modules, and by a module reached only through dependents (novox/hq ADR 0304 decision 1, ADR 0306
+// decision 3); otherwise they are not.
+func TestRelatedChanges(t *testing.T) {
+	edges := []inventory.Edge{
+		{From: "nzbget", To: "base-image", Kind: inventory.EdgeStandsOn},
+		{From: "app", To: "build-machine", Kind: inventory.EdgeBuiltBy},
+	}
+	cases := []struct {
+		name string
+		a, b footprint
+		why  string
+	}{
+		{"repository, case aside",
+			footprintOfWalk(walkOf("novox/Mesh-Catalog", "notes"), nil),
+			footprintOfBatch([]inventory.BatchedMerge{heardMerge("NOVOX/mesh-catalog", "fix/a", "app")}, nil),
+			"both change novox/mesh-catalog"},
+		{"module",
+			footprintOfWalk(walkOf("novox/one", "nzbget"), nil),
+			footprintOfBatch([]inventory.BatchedMerge{heardMerge("novox/two", "fix/b", "nzbget")}, nil),
+			"both move nzbget"},
+		{"edge, walk depends on batch",
+			footprintOfWalk(walkOf("novox/one", "nzbget"), nil),
+			footprintOfBatch([]inventory.BatchedMerge{heardMerge("novox/two", "fix/b", "base-image")}, nil),
+			"nzbget depends on base-image (stands-on)"},
+		{"edge, batch depends on walk",
+			footprintOfWalk(walkOf("novox/one", "build-machine"), nil),
+			footprintOfBatch([]inventory.BatchedMerge{heardMerge("novox/two", "fix/b", "app")}, nil),
+			"app depends on build-machine (built-by)"},
+		{"group head",
+			footprintOfWalk(walkOf("novox/one", "notes"), []inventory.BatchedMerge{heardMerge("novox/one", "feat/x")}),
+			footprintOfBatch([]inventory.BatchedMerge{heardMerge("novox/two", "feat/x", "app")}, nil),
+			"both are members of group feat/x"},
+		{"module reached through dependents",
+			footprintOfWalk(walkOf("novox/one", "nzbget"), nil),
+			footprintOfBatch([]inventory.BatchedMerge{heardMerge("novox/two", "fix/b", "base-image")}, edges),
+			"both move nzbget"},
+		{"unrelated",
+			footprintOfWalk(walkOf("novox/one", "notes"), []inventory.BatchedMerge{heardMerge("novox/one", "feat/x")}),
+			footprintOfBatch([]inventory.BatchedMerge{heardMerge("novox/two", "feat/y", "app")}, edges),
+			""},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			for _, pair := range [][2]footprint{{c.a, c.b}, {c.b, c.a}} {
+				is, why := related(pair[0], pair[1], edges)
+				if is != (c.why != "") || why != c.why {
+					t.Errorf("related = %v, %q; want %q", is, why, c.why)
+				}
+			}
+		})
+	}
+}
+
+// TestFootprintOfBatchWidensToDependents: a batch's modules are what its merges moved and every module that
+// depends on them, along the edges that widen a walk.
+func TestFootprintOfBatchWidensToDependents(t *testing.T) {
+	edges := []inventory.Edge{
+		{From: "nzbget", To: "base-image", Kind: inventory.EdgeStandsOn},
+		{From: "base-image", To: "build-machine", Kind: inventory.EdgeBuiltBy},
+	}
+	f := footprintOfBatch([]inventory.BatchedMerge{heardMerge("novox/two", "", "base-image")}, edges)
+	if !f.modules["base-image"] || !f.modules["nzbget"] || f.modules["build-machine"] || len(f.heads) != 0 {
+		t.Errorf("footprint = %+v; want base-image and nzbget, no head", f)
+	}
+}