1 files changed, 97 insertions, 0 deletions
diff --git a/gui/internal/model/highlight.go b/gui/internal/model/highlight.go
new file mode 100644
index 0000000..e34d9d9
--- /dev/null
+++ b/gui/internal/model/highlight.go
@@ -0,0 +1,97 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package model
+
+// SpanKind is what a stretch of a configuration file is, for colouring it.
+type SpanKind int
+
+const (
+	SpanComment SpanKind = iota
+	SpanString
+	SpanHead   // the symbol a form starts with
+	SpanAction // a head that does something to a file
+	SpanParen
+)
+
+// Span is one stretch of text to colour, in character offsets - what a
+// GtkTextBuffer counts in, not bytes.
+type Span struct {
+	From, To int
+	Kind     SpanKind
+}
+
+// actionHeads are the forms that act on a file rather than describe one.
+var actionHeads = map[string]bool{
+	"copy": true, "move": true, "rename": true, "delete": true, "stop": true,
+}
+
+// Spans reads a configuration and says what to colour: a ";" outside a
+// string comments out the rest of its line; a string runs to its closing
+// quote, a backslash escaping the next character; the first symbol after
+// "(" is the form's head. Nothing here knows about widgets, so the rules of
+// the little language stay testable (his request, 2026-09-16).
+func Spans(text string) []Span {
+	var out []Span
+	runes := []rune(text)
+	afterParen := false
+	for i := 0; i < len(runes); i++ {
+		switch c := runes[i]; {
+		case c == ';':
+			j := i
+			for j < len(runes) && runes[j] != '\n' {
+				j++
+			}
+			out = append(out, Span{i, j, SpanComment})
+			i = j
+			afterParen = false
+		case c == '"':
+			j := i + 1
+			for j < len(runes) {
+				if runes[j] == '\\' {
+					j += 2
+					continue
+				}
+				if runes[j] == '"' {
+					j++
+					break
+				}
+				j++
+			}
+			if j > len(runes) {
+				j = len(runes)
+			}
+			out = append(out, Span{i, j, SpanString})
+			i = j - 1
+			afterParen = false
+		case c == '(' || c == ')':
+			out = append(out, Span{i, i + 1, SpanParen})
+			afterParen = c == '('
+		case c == ' ' || c == '\t' || c == '\n' || c == '\r':
+			// whitespace between "(" and the head is allowed
+		default:
+			j := i
+			for j < len(runes) && !isDelimiter(runes[j]) {
+				j++
+			}
+			if afterParen {
+				kind := SpanHead
+				if actionHeads[string(runes[i:j])] {
+					kind = SpanAction
+				}
+				out = append(out, Span{i, j, kind})
+			}
+			i = j - 1
+			afterParen = false
+		}
+	}
+	return out
+}
+
+// isDelimiter reports whether a character ends a symbol.
+func isDelimiter(r rune) bool {
+	switch r {
+	case ' ', '\t', '\n', '\r', '(', ')', '"', ';':
+		return true
+	}
+	return false
+}