literal braces are text, not placeholders, for containment and walk exclusion

1 files changed, 24 insertions, 0 deletions

diff --git a/internal/plan/placeholder.go b/internal/plan/placeholder.go
index b947236..5922b07 100644
--- a/internal/plan/placeholder.go
+++ b/internal/plan/placeholder.go
@@ -30,6 +30,30 @@ func splitExt(name string) (stem, ext string) {
 	return name[:i], name[i:]
 }
 
+// StaticPrefix returns the literal text of s before its first placeholder,
+// with "{{" and "}}" read as the braces they stand for, and whether s has a
+// placeholder at all. It scans like Expand, so a literal brace never counts
+// as a placeholder (triage 28b).
+func StaticPrefix(s string) (prefix string, templated bool) {
+	var b strings.Builder
+	for i := 0; i < len(s); {
+		switch {
+		case s[i] == '{' && i+1 < len(s) && s[i+1] == '{':
+			b.WriteByte('{')
+			i += 2
+		case s[i] == '}' && i+1 < len(s) && s[i+1] == '}':
+			b.WriteByte('}')
+			i += 2
+		case s[i] == '{':
+			return b.String(), true
+		default:
+			b.WriteByte(s[i])
+			i++
+		}
+	}
+	return b.String(), false
+}
+
 // Expand replaces every placeholder in s. It returns an error naming the
 // first placeholder it could not expand. It scans s once: "{{" and "}}"
 // each emit one literal brace, and any other "{" opens a placeholder that