//! Native persistence boundary for the Linux client.
//!
//! Settings are deliberately limited to non-secret connection metadata. The
//! session credential is stored as a `user` key in the Linux kernel keyring
//! through the `keyctl` utility. If keyring access fails, callers receive an
//! error and must ask for credentials again; no plaintext fallback is allowed.

use serde::{Deserialize, Serialize};
use sha2::{Digest, Sha256};
use std::{
    fs, io,
    path::{Path, PathBuf},
    process::Command,
};

pub const KEYRING_SERVICE: &str = "trixie.server.session";

#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
pub struct Settings {
    pub server_address: String,
    pub username: String,
}

impl Settings {
    pub fn load(path: impl AsRef<Path>) -> io::Result<Self> {
        let bytes = fs::read(path)?;
        serde_json::from_slice(&bytes)
            .map_err(|error| io::Error::new(io::ErrorKind::InvalidData, error))
    }

    pub fn save(&self, path: impl AsRef<Path>) -> io::Result<()> {
        let path = path.as_ref();
        if let Some(parent) = path.parent() {
            fs::create_dir_all(parent)?;
        }
        let bytes = serde_json::to_vec_pretty(self)
            .map_err(|error| io::Error::new(io::ErrorKind::InvalidData, error))?;
        let temporary = path.with_extension("json.tmp");
        fs::write(&temporary, bytes)?;
        fs::rename(temporary, path)
    }
}

pub fn default_settings_path() -> PathBuf {
    let base = std::env::var_os("XDG_CONFIG_HOME")
        .map(PathBuf::from)
        .or_else(|| std::env::var_os("HOME").map(|home| PathBuf::from(home).join(".config")))
        .unwrap_or_else(|| PathBuf::from("."));
    base.join("trixie").join("settings.json")
}

fn key_description(settings: &Settings) -> String {
    let mut digest = Sha256::new();
    digest.update(settings.server_address.as_bytes());
    digest.update([0]);
    digest.update(settings.username.as_bytes());
    let digest = digest.finalize();
    format!("{KEYRING_SERVICE}:{}", hex_encode(&digest))
}

fn hex_encode(bytes: &[u8]) -> String {
    const HEX: &[u8; 16] = b"0123456789abcdef";
    let mut result = String::with_capacity(bytes.len() * 2);
    for byte in bytes {
        result.push(HEX[(byte >> 4) as usize] as char);
        result.push(HEX[(byte & 0x0f) as usize] as char);
    }
    result
}

fn keyctl(args: &[&str]) -> io::Result<Vec<u8>> {
    let output = Command::new("keyctl").args(args).output()?;
    if !output.status.success() {
        return Err(io::Error::new(
            io::ErrorKind::PermissionDenied,
            "Linux kernel keyring access failed",
        ));
    }
    Ok(output.stdout)
}

pub fn store_session(settings: &Settings, session: &str) -> io::Result<()> {
    if session.is_empty() {
        return Err(io::Error::new(io::ErrorKind::InvalidInput, "empty session"));
    }
    let description = key_description(settings);
    keyctl(&["add", "user", &description, session, "@s"]).map(|_| ())
}

pub fn load_session(settings: &Settings) -> io::Result<Option<String>> {
    let description = key_description(settings);
    let id = match keyctl(&["search", "@s", "user", &description]) {
        Ok(value) => String::from_utf8_lossy(&value).trim().to_string(),
        Err(_) => return Ok(None),
    };
    if id.is_empty() {
        return Ok(None);
    }
    let value = keyctl(&["pipe", &id])?;
    String::from_utf8(value)
        .map(Some)
        .map_err(|error| io::Error::new(io::ErrorKind::InvalidData, error))
}

pub fn clear_session(settings: &Settings) -> io::Result<()> {
    let description = key_description(settings);
    let id = String::from_utf8_lossy(&keyctl(&["search", "@s", "user", &description])?)
        .trim()
        .to_string();
    if id.is_empty() {
        return Ok(());
    }
    keyctl(&["unlink", &id, "@s"]).map(|_| ())
}

#[cfg(test)]
mod tests {
    use super::*;

    fn settings() -> Settings {
        Settings {
            server_address: "https://trixie.example".into(),
            username: "admin".into(),
        }
    }

    #[test]
    fn settings_round_trip_contains_no_password_field() {
        let directory = tempfile::tempdir().expect("temporary directory");
        let path = directory.path().join("settings.json");
        settings().save(&path).expect("save settings");
        let text = fs::read_to_string(&path).expect("read settings");
        assert!(!text.contains("password"));
        assert_eq!(Settings::load(path).expect("load settings"), settings());
    }

    #[test]
    fn key_description_is_stable_but_scoped_to_connection_and_user() {
        let first = key_description(&settings());
        assert_eq!(first, key_description(&settings()));
        let mut different = settings();
        different.username = "moderator".into();
        assert_ne!(first, key_description(&different));
        assert!(first.starts_with(KEYRING_SERVICE));
    }
}