Give scope, binding, link, pick and head one meaning each: renderEnv, nodeSet, pipeline, resolvedRef, resolveTemplates, drawItem and a read's start

env.go

@@ -0,0 +1,49 @@
+package fejkdata
+
+import "github.com/larvit/fejkdata/internal/invariant"
+
+// readValue is what one read drew: its text, and whether it landed on a null.
+type readValue struct {
+	text string
+	null bool
+}
+
+// renderEnv is where a render reads: the frames of the name scopes rendering; and row, the row
+// of the table rendering, which its columns read. It passes by value, and what is read from it
+// reaches a map key, an interface or a func value only as a copy; else env, and with it every
+// render's frames, moves to the heap.
+type renderEnv struct {
+	frames *frameStack
+	row    renderedRow
+	// base is the depth of the frame stack when the read rendering the category entered it,
+	// below which no frame is the category's.
+	base int
+	// pick is the named pick being rendered, nil outside one; pickAt is the key in it of what renders.
+	pick   *namedPick
+	pickAt pickKey
+}
+
+type renderedRow struct {
+	t     *table
+	index int
+}
+
+// at returns the env n renders in, n being the leaf of a path whose rows sit in pins; where n
+// is a table's row or column, the env carries the row pins holds for that table.
+func (env renderEnv) at(n node, pins *pinSet) renderEnv {
+	switch n := n.(type) {
+	case *tableRow:
+		env.row = renderedRow{n.t, pins.MustRow(n.t.rows)}
+	case *tableColumn:
+		env.row = renderedRow{n.t, pins.MustRow(n.t.rows)}
+	}
+	return env
+}
+
+// rowOf is the row of t its columns render from.
+func (env renderEnv) rowOf(t *table) int {
+	if env.row.t != t {
+		panic(invariant.Broken("a column of %s renders in a scope holding no row of it", t.rows.Segment()))
+	}
+	return env.row.index
+}