1 files changed, 77 insertions, 0 deletions
diff --git a/gui/internal/ui/highlight.go b/gui/internal/ui/highlight.go
new file mode 100644
index 0000000..8a4acd4
--- /dev/null
+++ b/gui/internal/ui/highlight.go
@@ -0,0 +1,77 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package ui
+
+import (
+	"github.com/diamondburned/gotk4/pkg/gtk/v4"
+
+	"krino/gui/internal/model"
+)
+
+// The colours a configuration is painted in. They are chosen to read on a
+// light and a dark theme alike, since the window follows whatever GTK theme
+// is in use (his request, 2026-09-16).
+var spanColours = map[model.SpanKind]string{
+	model.SpanComment: "#8b8b8b",
+	model.SpanString:  "#2e8b57",
+	model.SpanHead:    "#3584e4",
+	model.SpanAction:  "#c06014",
+	model.SpanParen:   "#9a9a9a",
+}
+
+// highlighter paints a configuration in a TextView.
+type highlighter struct {
+	buf  *gtk.TextBuffer
+	tags map[model.SpanKind]*gtk.TextTag
+	on   bool
+}
+
+// newHighlighter registers one tag per kind, once per buffer.
+func newHighlighter(buf *gtk.TextBuffer) *highlighter {
+	h := &highlighter{buf: buf, tags: map[model.SpanKind]*gtk.TextTag{}, on: true}
+	table := buf.TagTable()
+	names := map[model.SpanKind]string{
+		model.SpanComment: "krino-comment",
+		model.SpanString:  "krino-string",
+		model.SpanHead:    "krino-head",
+		model.SpanAction:  "krino-action",
+		model.SpanParen:   "krino-paren",
+	}
+	for kind, name := range names {
+		t := gtk.NewTextTag(name)
+		t.SetObjectProperty("foreground", spanColours[kind])
+		if kind == model.SpanComment {
+			t.SetObjectProperty("style", 2) // PANGO_STYLE_ITALIC
+		}
+		table.Add(t)
+		h.tags[kind] = t
+	}
+	return h
+}
+
+// setEnabled turns the colours on or off.
+func (h *highlighter) setEnabled(on bool) {
+	h.on = on
+	if !on {
+		h.clear()
+	}
+}
+
+// clear takes every colour off the buffer.
+func (h *highlighter) clear() {
+	start, end := h.buf.Bounds()
+	h.buf.RemoveAllTags(start, end)
+}
+
+// paint colours the text now in the buffer.
+func (h *highlighter) paint(text string) {
+	h.clear()
+	if !h.on {
+		return
+	}
+	for _, s := range model.Spans(text) {
+		if tag, ok := h.tags[s.Kind]; ok {
+			h.buf.ApplyTag(tag, h.buf.IterAtOffset(s.From), h.buf.IterAtOffset(s.To))
+		}
+	}
+}