Add default configuration file from OS specific location

Cargo.toml

@@ -5,9 +5,10 @@ edition = "2021"
 build = "build.rs"
 
 [dependencies]
-clap = { version = "4.5.13", features = ["derive", "env"] }
+clap = { version = "4.5.13", features = ["derive", "env", "string"] }
 clap-serde-derive = "0.2.1"
 color-eyre = "0.6.3"
+directories = "5.0.1"
 env_logger = "0.11.5"
 log = "0.4.22"
 nom = "7.1.3"

src/main.rs

@@ -12,11 +12,11 @@ use std::{
 #[command(version)]
 struct Args {
     /// The debug level.
-    #[arg(short, long, action = clap::ArgAction::Count)]
+    #[arg(short, long, env = "ELEPHANTINE_DEBUG", action = clap::ArgAction::Count)]
     debug: u8,
 
     /// Path to the configuration file.
-    #[arg(long, value_name = "FILE", default_value = "config.toml")]
+    #[arg(long, env = "ELEPHANTINE_CONFIG_FILE", value_name = "FILE", default_value = default_config_file())]
     config_file: PathBuf,
 
     /// The configuration options.
@@ -37,3 +37,15 @@ fn main() -> Result<()> {
     let mut output = stdout();
     Listener::new(config).listen(input, &mut output)
 }
+
+fn default_config_file() -> String {
+    directories::ProjectDirs::from("org", "elephantine", "elephantine").map_or_else(
+        || "elephantine.toml".to_string(),
+        |dirs| {
+            dirs.config_dir()
+                .join("elephantine.toml")
+                .to_string_lossy()
+                .to_string()
+        },
+    )
+}