//! TRACES: FR-PLAT-LIN-1 | FR-PLAT-WIN-1 | NFR-PORT-1
//! Where this application's files belong on this platform.
//!
//! One place for the rule, because it was in five. Each site read
//! `XDG_*_HOME` and fell back to `$HOME/.local/...` on its own, which is fine
//! on Linux and wrong everywhere else: Windows sets neither variable, so every
//! one of them degraded to a path relative to the working directory — for a
//! Start Menu launch, `C:\Windows\System32`. The callers keep their own
//! override (`set_state_dir`, `set_data_dir`), which is how Android names its
//! app-private directory; this answers the question those overrides do not.
//!
//! # The rules
//!
//! | | Config | Data | State |
//! |---|---|---|---|
//! | Unix | `$XDG_CONFIG_HOME` | `$XDG_DATA_HOME` | `$XDG_STATE_HOME` |
//! | Unix default | `~/.config` | `~/.local/share` | `~/.local/state` |
//! | Windows | `%APPDATA%` | `%LOCALAPPDATA%` | `%LOCALAPPDATA%`, then `state` |
//! | Windows default | `%USERPROFILE%\AppData\Roaming` | `…\AppData\Local` | `…\AppData\Local` |
//! | macOS default | `~/.config` | `~/.local/share` | `~/Library/Logs` |
//!
//! macOS follows the Unix rules except for the one directory a user is asked
//! to find by hand: the log. Finder hides `~/.local`, and `~/Library/Logs`
//! is where Console.app and a Mac user already look (docs/dev/macos.md).
//!
//! then `darkroom` under each. Config roams on Windows and the rest does not,
//! which is the same split XDG makes between config and everything else, and
//! the reason `settings.json` is small and the thumbnail store is not.
//!
//! A relative value is ignored rather than resolved: the XDG specification
//! says so, and the alternative is a `darkroom/` directory wherever the app
//! was launched from. With nothing usable set at all the answer is the
//! temporary directory — a container or a service unit with no home — because
//! writing a log into `/tmp` is a poor outcome and refusing to write one, on
//! exactly the machine nobody is sitting in front of, is a worse one.

use std::ffi::OsString;
use std::path::{Path, PathBuf};

/// Which kind of directory. The distinction is the one every platform makes:
/// what the user edits, what the application builds, and what it records.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Base {
    /// Settings and accounts. Small, and the user may back it up or edit it.
    Config,
    /// Catalogs, thumbnails, models. Large, rebuildable, never edited by hand.
    Data,
    /// Logs and crash records. Survives a cache sweep; not configuration.
    State,
}

/// The directory of this kind on this machine, ending in `darkroom`.
///
/// Not created here: whoever writes into it creates it, so that merely asking
/// leaves nothing behind.
pub fn base_dir(kind: Base) -> PathBuf {
    resolve(kind, |name| std::env::var_os(name))
}

/// Directories a package may have installed shared data into, most specific
/// first, each ending in `darkroom`.
///
/// Unix: `$XDG_DATA_DIRS`, defaulting to `/usr/local/share:/usr/share` — what
/// the Arch package and the Flatpak install under. Windows: the directory the
/// executable is in, which is where the installer puts the models
/// (FR-PLAT-WIN-2). Android: none; the APK's copy is unpacked into the user
/// data directory instead, because an asset inside a package is not a path
/// anything can read from.
pub fn system_data_dirs() -> Vec<PathBuf> {
    if cfg!(target_os = "android") {
        return Vec::new();
    }
    if cfg!(windows) {
        return std::env::current_exe()
            .ok()
            .and_then(|exe| exe.parent().map(Path::to_path_buf))
            .into_iter()
            .collect();
    }
    let dirs = std::env::var("XDG_DATA_DIRS")
        .ok()
        .filter(|v| !v.is_empty())
        .unwrap_or_else(|| "/usr/local/share:/usr/share".into());
    dirs.split(':')
        .filter(|d| !d.is_empty())
        .map(|d| PathBuf::from(d).join("darkroom"))
        .collect()
}

/// The resolution as a function of its inputs rather than of the process
/// environment, so it can be tested without `set_var` racing every other
/// test in the binary — and so both platforms' rules are tested on either.
fn resolve(kind: Base, env: impl Fn(&str) -> Option<OsString>) -> PathBuf {
    let base = if cfg!(windows) {
        windows_base(kind, &env)
    } else {
        xdg_base(kind, &env)
    };
    let dir = base.unwrap_or_else(std::env::temp_dir).join("darkroom");
    match (kind, cfg!(windows)) {
        // Data and state share `%LOCALAPPDATA%`; state gets its own level so
        // a log and a catalog do not sit side by side.
        (Base::State, true) => dir.join("state"),
        _ => dir,
    }
}

/// Where state goes under `$HOME` when `XDG_STATE_HOME` does not say.
const STATE_UNDER_HOME: &str = if cfg!(target_os = "macos") {
    "Library/Logs"
} else {
    ".local/state"
};

fn xdg_base(kind: Base, env: &impl Fn(&str) -> Option<OsString>) -> Option<PathBuf> {
    let (var, under_home) = match kind {
        Base::Config => ("XDG_CONFIG_HOME", ".config"),
        Base::Data => ("XDG_DATA_HOME", ".local/share"),
        Base::State => ("XDG_STATE_HOME", STATE_UNDER_HOME),
    };
    absolute(env(var)).or_else(|| absolute(env("HOME")).map(|h| h.join(under_home)))
}

fn windows_base(kind: Base, env: &impl Fn(&str) -> Option<OsString>) -> Option<PathBuf> {
    let (var, under_profile) = match kind {
        Base::Config => ("APPDATA", "AppData\\Roaming"),
        Base::Data | Base::State => ("LOCALAPPDATA", "AppData\\Local"),
    };
    absolute(env(var)).or_else(|| absolute(env("USERPROFILE")).map(|p| p.join(under_profile)))
}

/// A value only if it names an absolute path. `is_absolute` is the host's
/// notion, so a test of the Windows rule on Linux uses Unix-shaped paths;
/// what is tested is the variable and the suffix, which is the part that
/// was wrong.
fn absolute(value: Option<OsString>) -> Option<PathBuf> {
    value
        .filter(|v| Path::new(v).is_absolute())
        .map(PathBuf::from)
}

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

    fn env(pairs: &[(&str, &str)]) -> impl Fn(&str) -> Option<OsString> {
        let map: HashMap<String, OsString> = pairs
            .iter()
            .map(|(k, v)| (k.to_string(), OsString::from(v)))
            .collect();
        move |name| map.get(name).cloned()
    }

    #[test]
    fn xdg_honours_each_variable_and_defaults_under_home() {
        let e = env(&[("XDG_CONFIG_HOME", "/etc/me"), ("HOME", "/home/someone")]);
        assert_eq!(xdg_base(Base::Config, &e), Some(PathBuf::from("/etc/me")));
        assert_eq!(
            xdg_base(Base::Data, &e),
            Some(PathBuf::from("/home/someone/.local/share"))
        );
        assert_eq!(
            xdg_base(Base::State, &e),
            Some(PathBuf::from("/home/someone").join(STATE_UNDER_HOME))
        );
    }

    #[test]
    fn a_relative_value_is_ignored_rather_than_resolved() {
        // The specification requires this, and the failure it prevents is a
        // `darkroom/` directory appearing in whatever the working directory
        // happened to be — including, on a desktop launcher, `/`.
        let e = env(&[("XDG_STATE_HOME", "state"), ("HOME", "/home/someone")]);
        assert_eq!(
            xdg_base(Base::State, &e),
            Some(PathBuf::from("/home/someone").join(STATE_UNDER_HOME))
        );
        assert_eq!(xdg_base(Base::State, &env(&[("HOME", "")])), None);
    }

    #[test]
    fn windows_splits_config_from_the_rest() {
        // Config roams, data and state do not. The variable is what was wrong
        // before this module: neither XDG_* nor HOME exists on Windows.
        let e = env(&[("APPDATA", "/r"), ("LOCALAPPDATA", "/l")]);
        assert_eq!(windows_base(Base::Config, &e), Some(PathBuf::from("/r")));
        assert_eq!(windows_base(Base::Data, &e), Some(PathBuf::from("/l")));
        assert_eq!(windows_base(Base::State, &e), Some(PathBuf::from("/l")));
    }

    #[test]
    fn windows_falls_back_to_the_profile() {
        let e = env(&[("USERPROFILE", "/u")]);
        assert_eq!(
            windows_base(Base::Config, &e),
            Some(PathBuf::from("/u").join("AppData\\Roaming"))
        );
        assert_eq!(
            windows_base(Base::Data, &e),
            Some(PathBuf::from("/u").join("AppData\\Local"))
        );
    }

    #[test]
    fn nothing_set_is_still_absolute() {
        // A container or a service unit: the answer is somewhere writable,
        // never a relative path.
        let dir = resolve(Base::State, env(&[]));
        assert!(dir.is_absolute());
        assert!(dir.ends_with("darkroom") || dir.ends_with("state"));
    }

    #[test]
    fn every_kind_ends_in_the_application_name() {
        let e = env(&[
            ("HOME", "/home/someone"),
            ("APPDATA", "/r"),
            ("LOCALAPPDATA", "/l"),
        ]);
        for kind in [Base::Config, Base::Data, Base::State] {
            let dir = resolve(kind, &e);
            assert!(
                dir.components().any(|c| c.as_os_str() == "darkroom"),
                "{kind:?} resolved to {}",
                dir.display()
            );
        }
    }
}