G-10 §479: a per-branch state's first view is named <owner>.<state>.<branch>

FollowBranch on the Reader and the ProcessReader, BranchState names the view with the
branch escaped as a key's name is; one row per branch in the first views.

Builds: design 47 G §479

Claude-Session: https://claude.ai/code/session_01JG73KEq4zoDfM9WtYpcQgZ

go/views/process_reader.go

@@ -104,6 +104,13 @@ func (r *ProcessReader) Follow(owner, state, tool string) (*Picture, error) {
 	return p, nil
 }
 
+// FollowBranch is Follow for a state kept per branch: the view of one branch is named `<owner>.<state>.<branch>` (the
+// branch escaped as a key's name is, BranchState) and its picture `<owner>_<state>.<branch>`, so a process follows
+// several branches of one state, each its own view (design 47 G §479). Call it before Start.
+func (r *ProcessReader) FollowBranch(owner, state, branch, tool string) (*Picture, error) {
+	return r.Follow(owner, BranchState(state, branch), tool)
+}
+
 // Adopt takes a follower a seat's client made (its Ask a seat's verb, such as the catalog half's view) as one of the
 // process's first views, so the client's callers write no glue: the reader sets its conditions (the unanswered first
 // view and view-waits, in the reader's findings), its log and clock, and its picture (`<owner>_<state>`) hands each

go/views/reader.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	stdio "git.novox.be/novox/mesh-sdk/go"
+	"git.novox.be/novox/mesh-sdk/go/keys"
 )
 
 // Reader is a module's pictures of the owners' states it used to read (novox/hq ADR 0615 §1): it
@@ -86,6 +87,17 @@ func (r *Reader) Follow(owner, state, tool string) *Follower {
 	return f
 }
 
+// BranchState is the name of a per-branch state's view: `<state>.<branch>`, the branch escaped as a key's name is (design
+// 47 G §479), so a reader names the view `<owner>.<state>.<branch>`. A state that is not per branch keeps its name.
+func BranchState(state, branch string) string { return state + "." + keys.EscapeName(branch) }
+
+// FollowBranch is Follow for a state kept per branch: the view of one branch is named `<owner>.<state>.<branch>`
+// (BranchState), so a reader follows several branches of one state, each its own view and its own row of
+// `<module>_first_views` (design 47 G §479). Call it before Start.
+func (r *Reader) FollowBranch(owner, state, branch, tool string) *Follower {
+	return r.Follow(owner, BranchState(state, branch), tool)
+}
+
 // Adopt takes a follower a seat's client made (its Ask a seat's verb, such as the catalog half's view) as one of the
 // reader's first views, so the client's callers write no glue: the reader sets its conditions (the unanswered first view
 // and view-waits, in the reader's findings), its log and clock, and hands each change to Watch, Changed and the

go/views/views_test.go

@@ -595,6 +595,36 @@ func TestViews(t *testing.T) {
 	// A seat's client makes the follower (its Ask a seat's verb) and both readers adopt it, so its callers write no glue:
 	// the reader's findings (unanswered, view-waits), Keys, Get, Watch, statuses and tools, and the seat's event handed to
 	// the client's handler.
+	// Design 47 G §479: a per-branch state's view is named <owner>.<state>.<branch>, several branches in one reader.
+	t.Run("a per-branch state is followed once per branch, each view its own row named with its escaped branch", func(t *testing.T) {
+		if got := BranchState("catalog-revisions", "feat/x y"); got != "catalog-revisions.feat/x=20y" {
+			t.Errorf("BranchState = %q", got)
+		}
+		r := &Reader{Module: "reader", Now: now}
+		a, b := r.FollowBranch("s", "catalog-revisions", "main", "s.read"), r.FollowBranch("s", "catalog-revisions", "feat/x", "s.read")
+		r.Follow("s", "plain", "s.read")
+		if a.State != "catalog-revisions.main" || b.State != "catalog-revisions.feat/x" {
+			t.Errorf("states %q %q", a.State, b.State)
+		}
+		for _, n := range []string{"s.catalog-revisions.main", "s.catalog-revisions.feat/x", "s.plain"} {
+			if !r.Follows(n) {
+				t.Errorf("%s is not followed", n)
+			}
+		}
+		if r.Follows("s.catalog-revisions") {
+			t.Errorf("the unnamed per-branch state is followed")
+		}
+		if st := r.Statuses(); len(st) != 3 || st[0].State != "catalog-revisions.main" || st[1].State != "catalog-revisions.feat/x" || st[2].State != "plain" {
+			t.Errorf("rows = %+v", st)
+		}
+		pr := &ProcessReader{Module: "reader"}
+		p1, err1 := pr.FollowBranch("s", "catalog-revisions", "main", "s.read")
+		p2, err2 := pr.FollowBranch("s", "catalog-revisions", "feat/x", "s.read")
+		if err1 != nil || err2 != nil || p1 == p2 || p1.Bucket() != "s_catalog-revisions.main" || pr.Picture("s_catalog-revisions.feat/x") != p2 {
+			t.Errorf("pictures %v %v %v", p1, p2, err1)
+		}
+	})
+
 	t.Run("a follower a seat's client made is adopted: the reader's findings, reads and tools hold it, and its seat event goes to the client", func(t *testing.T) {
 		ended := make(chan bool, 1)
 		kept := waitBound