2 files changed, 120 insertions, 0 deletions
diff --git a/internal/xdg/xdg.go b/internal/xdg/xdg.go
new file mode 100644
index 0000000..ed34838
--- /dev/null
+++ b/internal/xdg/xdg.go
@@ -0,0 +1,60 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+// Package xdg resolves the XDG base directories and expands ~ in paths.
+package xdg
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// ConfigHome is $XDG_CONFIG_HOME, or ~/.config.
+func ConfigHome() string { return base("XDG_CONFIG_HOME", ".config") }
+
+// StateHome is $XDG_STATE_HOME, or ~/.local/state.
+func StateHome() string { return base("XDG_STATE_HOME", filepath.Join(".local", "state")) }
+
+// DataHome is $XDG_DATA_HOME, or ~/.local/share.
+func DataHome() string { return base("XDG_DATA_HOME", filepath.Join(".local", "share")) }
+
+// base follows the XDG rule that a relative value is invalid and ignored.
+func base(env, fallback string) string {
+	if v := os.Getenv(env); filepath.IsAbs(v) {
+		return filepath.Clean(v)
+	}
+	return filepath.Join(Home(), fallback)
+}
+
+// Home is the user's home directory, or "/" when it is unknown.
+func Home() string {
+	if h, err := os.UserHomeDir(); err == nil && h != "" {
+		return filepath.Clean(h)
+	}
+	return "/"
+}
+
+// Expand replaces a leading "~" or "~/" with the home directory.
+// "~user" is left alone.
+func Expand(p string) string {
+	if p == "~" {
+		return Home()
+	}
+	if strings.HasPrefix(p, "~/") {
+		return filepath.Join(Home(), p[2:])
+	}
+	return p
+}
+
+// Abbrev replaces a leading home directory with "~", for display and for
+// paths written into config files.
+func Abbrev(p string) string {
+	h := Home()
+	if p == h {
+		return "~"
+	}
+	if h != "/" && strings.HasPrefix(p, h+"/") {
+		return "~/" + p[len(h)+1:]
+	}
+	return p
+}
diff --git a/internal/xdg/xdg_test.go b/internal/xdg/xdg_test.go
new file mode 100644
index 0000000..7c7de87
--- /dev/null
+++ b/internal/xdg/xdg_test.go
@@ -0,0 +1,60 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package xdg
+
+import "testing"
+
+func TestBaseDirs(t *testing.T) {
+	t.Setenv("HOME", "/home/u")
+	t.Setenv("XDG_CONFIG_HOME", "")
+	t.Setenv("XDG_STATE_HOME", "relative/ignored")
+	t.Setenv("XDG_DATA_HOME", "/data/")
+	if got := ConfigHome(); got != "/home/u/.config" {
+		t.Errorf("ConfigHome() = %q", got)
+	}
+	if got := StateHome(); got != "/home/u/.local/state" {
+		t.Errorf("StateHome() = %q, a relative value must be ignored", got)
+	}
+	if got := DataHome(); got != "/data" {
+		t.Errorf("DataHome() = %q", got)
+	}
+}
+
+// TestHomeTrailingSlash is item H: Home() must clean its result, or a
+// trailing slash from $HOME breaks Abbrev's prefix check.
+func TestHomeTrailingSlash(t *testing.T) {
+	t.Setenv("HOME", "/home/u/")
+	if got := Abbrev("/home/u/x"); got != "~/x" {
+		t.Errorf("Abbrev(/home/u/x) = %q, want ~/x", got)
+	}
+	if got := Expand("~/x"); got != "/home/u/x" {
+		t.Errorf("Expand(~/x) = %q, want /home/u/x", got)
+	}
+}
+
+func TestExpandAbbrev(t *testing.T) {
+	t.Setenv("HOME", "/home/u")
+	expand := map[string]string{
+		"~":        "/home/u",
+		"~/d/x":    "/home/u/d/x",
+		"~other/x": "~other/x",
+		"/abs":     "/abs",
+		"rel/x":    "rel/x",
+	}
+	for in, want := range expand {
+		if got := Expand(in); got != want {
+			t.Errorf("Expand(%q) = %q, want %q", in, got, want)
+		}
+	}
+	abbrev := map[string]string{
+		"/home/u":     "~",
+		"/home/u/d/x": "~/d/x",
+		"/home/ux/y":  "/home/ux/y",
+		"/etc":        "/etc",
+	}
+	for in, want := range abbrev {
+		if got := Abbrev(in); got != want {
+			t.Errorf("Abbrev(%q) = %q, want %q", in, got, want)
+		}
+	}
+}