README.md

@@ -522,7 +522,9 @@ token {w} is repeated, and uppercase operand "w" holds "w" to one draw per expan
 
 ### Performance
 
-Each file is parsed, validated and weight-indexed once, in `New`. A `Fake` call
+Each file is parsed, validated and weight-indexed once, in `New`. Proving the draw
+fences adds one pass over the loaded tree, and walks what a render reads only where
+data binds a reference, so a set that binds none pays for the pass alone. A `Fake` call
 then costs about what its output costs: an unweighted pick is O(1) whatever the
 list's length, a weighted one O(log n), and long formats, deep nesting and many
 tokens add cost in proportion to the output.
@@ -604,7 +606,9 @@ tokens add cost in proportion to the output.
   is two draws, as `{word} {word}` is, while every `{/p.first}` in one render reads
   one draw, and a bare `{/p}` beside them is a load error: a bare token
   is by contract an independent draw, a path pins its level, and a fresh draw of a
-  pinned level could show another row.
+  pinned level could show another row. A builtin's operand holds what it reads for
+  its expansion, references included, so `{uppercase(/p)} {/p}` is one draw — the
+  rule every operand follows.
 - **Reference sigils follow the filesystem.** `/` is the root, `.` this file's
   folder, `..` the folder above — what those spellings already mean to anyone who
   has typed a path. A locale's files reach each other without naming the locale,
@@ -695,8 +699,9 @@ tokens add cost in proportion to the output.
   columns always read through the render's draws, so making the maps where the set is
   declared keeps them on that frame's stack. A `Fake` often reads no reference path at
   all, and making them anyway cost about a fifth of the cheapest render, so it makes
-  them on the first read instead — which the allocation gate prices at two heap
+  them on the first read instead, at two heap allocations for a render that does share
-  allocations, paid only by a render that shares a draw.
+  a draw. The allocation gate over a repeat of a reference path and over a named draw
+  group prices that, and pins the two measures that keep a draw set off the heap.
 - **A category never references itself, and a record's fences run at load.** A category
   is one unit: a reference back into it — `{/users.first}` inside `users` — describes a
   draw other than the fields beside it, so `New` refuses it and the sibling path stays

draw.go

@@ -118,7 +118,6 @@ type drawCheck struct {
 	memo map[readsMemo]bool
 }
 
-// readsMemo is one answer reads has given: for a node, and for each place it stops.
 type readsMemo struct {
 	n           node
 	stopAtGroup bool
@@ -155,8 +154,14 @@ func (c *drawCheck) checkRecordDraws(path string, n node) error {
 	if !ok || !t.record {
 		return nil
 	}
+	// A record's columns are its fields, which its format need not render at all, so the
+	// reads to weigh are theirs rather than the template's own.
 	columns := recordColumns(t)
-	if len(columns) == 0 {
+	reads := false
+	for _, name := range columns {
+		reads = reads || c.readsPath(t.fields[name])
+	}
+	if !reads {
 		return nil
 	}
 	if err := checkColumnDraws(t, columns); err != nil {
@@ -182,7 +187,7 @@ func (c *drawCheck) reads(n node, stopAtGroup bool) bool {
 	}
 	r := false
 	for _, e := range renderEdges(n) {
-		if a, _, isRef := refRead(n, e.label); (isRef && len(a.tail) > 0) || (!repeats(e.to) && !(stopAtGroup && grouped(e.to)) && c.reads(e.to, stopAtGroup)) {
+		if a, isRef := refRead(n, e.label); (isRef && len(a.tail) > 0) || (!repeats(e.to) && !(stopAtGroup && grouped(e.to)) && c.reads(e.to, stopAtGroup)) {
 			r = true
 			break
 		}
@@ -201,18 +206,17 @@ func repeats(n node) bool {
 
 func grouped(n node) bool {
 	t, isTemplate := n.(*template)
-	return isTemplate && t.drawGroup != ""
+	return isTemplate && t.drawGroupKey != ""
 }
 
-// refRead is the reference an edge of n reads, and the node it is bound to; false when the edge
+// refRead is the reference an edge of n reads; false when the edge reads none.
-// reads none.
+func refRead(n node, label string) (arm, bool) {
-func refRead(n node, label string) (arm, node, bool) {
 	t, isTemplate := n.(*template)
 	if !isTemplate {
-		return arm{}, nil, false
+		return arm{}, false
 	}
 	a := splitArm(label, t.refs)
-	return a, t.fields[a.key], isRef(a.key)
+	return a, isRef(a.key)
 }
 
 // checkColumnDraws fences a record's columns as one render.
@@ -224,22 +228,17 @@ func checkColumnDraws(t *template, columns []string) error {
 	return w.check()
 }
 
-// drawWalk gathers what one render reads by reference and what it draws afresh, each by draw group,
+// drawWalk gathers the references one render reads, by draw group, for check to compare.
-// for check to compare.
 type drawWalk struct {
 	reads []pathRead
 	read  map[drawKey]bool
-	fresh []freshDraw
 	seen  map[drawVisit]bool
 }
 
-// drawAt is where a walk stands: the draw group it draws in, how the render's root reached it, the
+// drawAt is where a walk stands: the draw group it draws in, and how the render's root reached it.
-// reference it last crossed, and whether it renders inside a reference path's draw.
 type drawAt struct {
 	group string
 	route drawRoute
-	via   string
-	held  bool
 }
 
 // drawRoute is how a render reaches a draw: as its author spells it, and the root edge's label.
@@ -247,25 +246,18 @@ type drawRoute struct{ spelling, label string }
 
 // pathRead is one reference a render reads: a path, or a bare reference with no tail.
 type pathRead struct {
-	at     drawAt
-	a      arm
-	target node
-}
-
-type freshDraw struct {
 	at drawAt
-	n  node
+	a  arm
 }
 
 type drawVisit struct {
 	n     node
 	group string
-	held  bool
 }
 
 type drawKey struct {
 	group string
-	key   any
+	path  string
 }
 
 func newDrawWalk() *drawWalk {
@@ -275,14 +267,11 @@ func newDrawWalk() *drawWalk {
 // walk follows what rendering n renders. A repeat renders over draws of its own, so the walk stops
 // there.
 func (w *drawWalk) walk(n node, at drawAt) {
-	v := drawVisit{n, at.group, at.held}
+	v := drawVisit{n, at.group}
 	if w.seen[v] {
 		return
 	}
 	w.seen[v] = true
-	if !at.held {
-		w.fresh = append(w.fresh, freshDraw{at, n})
-	}
 	if repeats(n) {
 		return
 	}
@@ -295,21 +284,18 @@ func (w *drawWalk) walk(n node, at drawAt) {
 }
 
 func (w *drawWalk) edge(from node, e renderEdge, at drawAt) {
-	a, target, reads := refRead(from, e.label)
+	if a, reads := refRead(from, e.label); reads {
-	if !reads {
+		if k := (drawKey{at.group, a.path}); !w.read[k] {
-		w.walk(e.to, at)
+			w.read[k] = true
-		return
+			w.reads = append(w.reads, pathRead{at, a})
+		}
 	}
-	if k := (drawKey{at.group, a.path}); !w.read[k] {
-		w.read[k] = true
-		w.reads = append(w.reads, pathRead{at, a, target})
-	}
-	at.via, at.held = a.name, len(a.tail) > 0
 	w.walk(e.to, at)
 }
 
-// check refuses what one draw per reference path cannot answer for, reads compared in path order so
+// check refuses what one draw per reference path cannot answer for: a read of a level beside a path
-// which pair is reported does not vary.
+// another read takes into it. Reads are compared in path order, so which pair is reported does not
+// vary.
 func (w *drawWalk) check() error {
 	sort.SliceStable(w.reads, func(i, j int) bool {
 		if w.reads[i].at.group != w.reads[j].at.group {
@@ -317,14 +303,6 @@ func (w *drawWalk) check() error {
 		}
 		return w.reads[i].a.path < w.reads[j].a.path
 	})
-	if err := w.checkOverlaps(); err != nil {
-		return err
-	}
-	return w.checkFreshDraws()
-}
-
-// checkOverlaps refuses a read of a level beside a path another read takes into it.
-func (w *drawWalk) checkOverlaps() error {
 	for i, level := range w.reads {
 		for _, into := range w.reads[i+1:] {
 			if into.at.group == level.at.group && strings.HasPrefix(into.a.path, level.a.path+".") {
@@ -335,29 +313,6 @@ func (w *drawWalk) checkOverlaps() error {
 	return nil
 }
 
-// checkFreshDraws refuses a node drawn afresh where a reference path's draw holds it.
-func (w *drawWalk) checkFreshDraws() error {
-	pins := map[drawKey]pathRead{}
-	for _, r := range w.reads {
-		if len(r.a.tail) == 0 {
-			continue
-		}
-		held := map[node]bool{}
-		coverPath(r.target, r.a.tail, held)
-		for n := range held {
-			if _, pinned := pins[drawKey{r.at.group, n}]; !pinned {
-				pins[drawKey{r.at.group, n}] = r
-			}
-		}
-	}
-	for _, f := range w.fresh {
-		if r, pinned := pins[drawKey{f.at.group, f.n}]; pinned {
-			return overlapError(f.at.route, f.at.via, r)
-		}
-	}
-	return nil
-}
-
 func overlapError(route drawRoute, ref string, into pathRead) error {
 	return fmt.Errorf("%s renders a level that %s reads a path into; name the fields you want instead, or draw them apart with a drawGroup", route.spelled(ref), into.at.route.spelled(into.a.name))
 }

graph.go

@@ -197,14 +197,25 @@ func checkRenders(s nodeScope) error {
 		return err
 	}
 	fence := &drawCheck{}
-	if err := s(fence.checkDrawGroup); err != nil {
+	// One pass refuses a draw group that splits nothing and notes whether the scope binds a
+	// reference at all: weighing what a render's draws hold walks every node it renders, and
+	// with nothing bound there is no draw to share, so the shipped set pays for neither walk.
+	refs := false
+	if err := s(func(path string, n node) error {
+		if t, ok := n.(*template); ok && len(t.refs) > 0 {
+			refs = true
+		}
+		return fence.checkDrawGroup(path, n)
+	}); err != nil {
 		return err
 	}
-	if err := s(fence.checkDraws); err != nil {
+	if refs {
-		return err
+		if err := s(fence.checkDraws); err != nil {
-	}
+			return err
-	if err := s(fence.checkRecordDraws); err != nil {
+		}
-		return err
+		if err := s(fence.checkRecordDraws); err != nil {
+			return err
+		}
 	}
 	return s((&valueProof{}).checkDatatype)
 }