feat(notify): resolve theme icon names to files

qt6ct's icon_theme is authoritative on this desktop, with the GTK3
setting and hicolor as fallbacks. The lookup prefers scalable, then the
largest raster, and follows the theme's Inherits chain, so an app that
passes a name instead of a path gets an icon.

1 files changed, 158 insertions, 0 deletions

diff --git a/internal/notify/icons.go b/internal/notify/icons.go
new file mode 100644
index 0000000..b874581
--- /dev/null
+++ b/internal/notify/icons.go
@@ -0,0 +1,158 @@
+// Copyright (C) 2026 Danilo M. <danix@danix.xyz>
+//
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU General Public License version 2 as
+// published by the Free Software Foundation.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+package notify
+
+import (
+	"os"
+	"path/filepath"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+// IconThemeName reads the desktop's current icon theme: qt6ct is the truth on
+// this desktop, then the GTK3 setting, then hicolor. A missing or empty value
+// falls through.
+func IconThemeName() string {
+	if v := iniValue(filepath.Join(homeDir(), ".config", "qt6ct", "qt6ct.conf"), "icon_theme"); v != "" {
+		return v
+	}
+	if v := iniValue(filepath.Join(homeDir(), ".config", "gtk-3.0", "settings.ini"), "gtk-icon-theme-name"); v != "" {
+		return v
+	}
+	return "hicolor"
+}
+
+// ResolveIcon turns an app_icon or image-path value into a file path. A URI is
+// trimmed, a path is returned unchanged, and a bare name is looked up in the
+// icon theme. Nothing found is an empty string, which renders no image.
+func ResolveIcon(value string) string {
+	if value == "" {
+		return ""
+	}
+	if strings.HasPrefix(value, "file://") {
+		return strings.TrimPrefix(value, "file://")
+	}
+	if strings.Contains(value, "/") {
+		return value
+	}
+	return lookupThemeIcon(value, IconThemeName())
+}
+
+// XDGIconDirs is the icon search path: the user's dir then each data dir.
+func XDGIconDirs() []string {
+	home := os.Getenv("XDG_DATA_HOME")
+	if home == "" {
+		home = filepath.Join(homeDir(), ".local", "share")
+	}
+	dirs := []string{filepath.Join(home, "icons")}
+	for _, d := range filepath.SplitList(os.Getenv("XDG_DATA_DIRS")) {
+		if d != "" {
+			dirs = append(dirs, filepath.Join(d, "icons"))
+		}
+	}
+	if len(dirs) == 1 {
+		dirs = append(dirs, "/usr/local/share/icons", "/usr/share/icons")
+	}
+	return dirs
+}
+
+// lookupThemeIcon searches the theme, then its Inherits chain, then hicolor.
+func lookupThemeIcon(name, theme string) string {
+	seen := map[string]bool{}
+	for theme != "" && !seen[theme] {
+		seen[theme] = true
+		found, next := "", ""
+		for _, root := range XDGIconDirs() {
+			base := filepath.Join(root, theme)
+			if p := findInTheme(base, name); p != "" {
+				found = p
+				break
+			}
+			if next == "" {
+				next = inheritsOf(base)
+			}
+		}
+		if found != "" {
+			return found
+		}
+		theme = next
+	}
+	for _, root := range XDGIconDirs() {
+		if p := findInTheme(filepath.Join(root, "hicolor"), name); p != "" {
+			return p
+		}
+	}
+	return ""
+}
+
+// findInTheme prefers scalable then the largest raster under apps/.
+func findInTheme(base, name string) string {
+	sizes := []string{"scalable"}
+	matches, _ := filepath.Glob(filepath.Join(base, "apps", "[0-9]*"))
+	for _, m := range matches {
+		sizes = append(sizes, filepath.Base(m))
+	}
+	numeric := sizes[1:]
+	sort.Slice(numeric, func(i, j int) bool {
+		a, _ := strconv.Atoi(strings.TrimSuffix(numeric[i], "@2x"))
+		b, _ := strconv.Atoi(strings.TrimSuffix(numeric[j], "@2x"))
+		return a > b
+	})
+	for _, size := range sizes {
+		for _, ext := range []string{"svg", "png", "xpm"} {
+			p := filepath.Join(base, "apps", size, name+"."+ext)
+			if fileExists(p) {
+				return p
+			}
+		}
+	}
+	return ""
+}
+
+func inheritsOf(base string) string {
+	v := iniValue(filepath.Join(base, "index.theme"), "Inherits")
+	if i := strings.IndexByte(v, ','); i >= 0 {
+		v = v[:i]
+	}
+	return strings.TrimSpace(v)
+}
+
+func iniValue(path, key string) string {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return ""
+	}
+	for _, line := range strings.Split(string(data), "\n") {
+		line = strings.TrimSpace(line)
+		if strings.HasPrefix(line, "#") || !strings.Contains(line, "=") {
+			continue
+		}
+		k, v, _ := strings.Cut(line, "=")
+		if strings.TrimSpace(k) == key {
+			return strings.TrimSpace(v)
+		}
+	}
+	return ""
+}
+
+func homeDir() string {
+	if h := os.Getenv("HOME"); h != "" {
+		return h
+	}
+	return os.TempDir()
+}
+
+func fileExists(path string) bool {
+	info, err := os.Stat(path)
+	return err == nil && !info.IsDir()
+}