//! User settings: the optional `config.yml` in the pkh configuration
//! directory (the same `~/.config/pkh/` that holds `contexts.json`),
//! and their defaults.
//!
//! A missing file carries the defaults — the file is written only when
//! a user creates it, never generated. A file that cannot be read or
//! parsed must not take the whole program down: like the context
//! configuration, it is backed up to `<path>.bak` (best effort) so a
//! later save cannot silently destroy it, and the defaults apply. Every
//! struct level is individually defaulted (`#[serde(default)]`), so any
//! hand-written file may carry a subset of the settings:
//!
//! ```yaml
//! deb:
//!   incremental:
//!     staged_trees: 2
//! ```

use serde::Deserialize;
use std::path::PathBuf;

/// The pkh configuration directory (the one holding `contexts.json`
/// and `config.yml`).
pub fn config_dir() -> Option<PathBuf> {
    directories::ProjectDirs::from("com", "pkh", "pkh").map(|dirs| dirs.config_dir().to_path_buf())
}

/// The settings file in [`config_dir`].
fn config_path() -> Option<PathBuf> {
    config_dir().map(|dir| dir.join("config.yml"))
}

/// The user-adjustable settings, as stored in `config.yml`. New fields
/// must default — the file always may carry a subset.
#[derive(Debug, Clone, Deserialize, Default)]
#[serde(default)]
pub struct Settings {
    /// `pkh deb` settings.
    pub deb: DebSettings,
}

/// The `pkh deb` settings.
#[derive(Debug, Clone, Deserialize, Default)]
#[serde(default)]
pub struct DebSettings {
    /// `pkh deb --incremental` settings.
    pub incremental: IncrementalSettings,
}

/// The `pkh deb --incremental` settings.
#[derive(Debug, Clone, Deserialize, Default)]
#[serde(default)]
pub struct IncrementalSettings {
    /// How many staged trees `pkh deb` retains per target: the newer
    /// ones first, with the newest successful one and the newest one
    /// always among them.
    pub staged_trees: Option<usize>,
}

/// How many staged trees `pkh deb` retains per target: the setting's
/// value, or the default (2 — the newest tree and the newest successful
/// one). A configured 0 cannot honor the retention invariant (the
/// newest successful tree always survives), so it clamps to 1 with a
/// warning.
pub fn staged_trees() -> usize {
    const DEFAULT: usize = 2;
    let Some(path) = config_path() else {
        return DEFAULT;
    };
    let Ok(text) = std::fs::read_to_string(&path) else {
        return DEFAULT;
    };
    let settings: Settings = match serde_yaml::from_str(&text) {
        Ok(settings) => settings,
        Err(e) => {
            log::error!(
                "Settings file {} is corrupt ({e}); using the defaults",
                path.display()
            );
            backup(&path);
            return DEFAULT;
        }
    };
    match settings.deb.incremental.staged_trees {
        Some(0) => {
            log::warn!(
                "Settings file {} carries staged_trees: 0; the newest \
                 successful staged tree is always retained, so 1 applies",
                path.display()
            );
            1
        }
        Some(count) => count,
        None => DEFAULT,
    }
}

/// Back up a corrupt settings file next to itself (best effort), so a
/// later save cannot silently destroy its content.
fn backup(path: &std::path::Path) {
    let mut os = path.as_os_str().to_os_string();
    os.push(".bak");
    let backup_path = PathBuf::from(os);
    match std::fs::copy(path, &backup_path) {
        Ok(_) => log::warn!(
            "Corrupt settings file backed up to {}",
            backup_path.display()
        ),
        Err(e) => log::warn!(
            "Could not back up corrupt settings file to {}: {e}",
            backup_path.display()
        ),
    }
}

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

    /// The nested setting parses; every level may be missing and every
    /// missing level defaults.
    #[test]
    fn settings_parse_the_nested_shape() {
        let settings: Settings =
            serde_yaml::from_str("deb:\n  incremental:\n    staged_trees: 3").unwrap();
        assert_eq!(settings.deb.incremental.staged_trees, Some(3));

        let settings: Settings = serde_yaml::from_str("deb: {}").unwrap();
        assert_eq!(settings.deb.incremental.staged_trees, None);

        let settings: Settings = serde_yaml::from_str("{}").unwrap();
        assert_eq!(settings.deb.incremental.staged_trees, None);
    }

    /// A value of the wrong type is a file error, not a default for the
    /// field: the whole file is corrupt (the caller backs it up and
    /// applies the defaults).
    #[test]
    fn a_mistyped_value_is_a_corrupt_file() {
        let result =
            serde_yaml::from_str::<Settings>("deb:\n  incremental:\n    staged_trees: two\n");
        assert!(result.is_err());
    }
}