//! Decides where pkgwatch's config, state, and work directories live on
//! disk — the only module that reads the environment to answer that; every
//! other module takes the directories it needs as parameters. (The pacman
//! repo dir, `pipeline::custom_repo_dir`, is resolved separately.)
//!
//! These follow the XDG base-directory spec instead of the current working
//! directory, so an installed `/usr/bin/pkgwatch` behaves the same
//! wherever it's launched from (a systemd unit, a shell, another checkout)
//! instead of only working from inside the repo.

use anyhow::{Context, Result};
use std::path::{Path, PathBuf};

const APP_DIR: &str = "pkgwatch";

#[derive(Debug, PartialEq, Eq)]
pub struct Paths {
    /// `*.toml` package declarations. Config: hand-edited, worth backing up.
    pub packages_dir: PathBuf,
    /// Last-published/pending versions. State: small, but losing it makes
    /// every package look new, so it isn't cache.
    pub state_dir: PathBuf,
    /// Downloaded artifacts and build trees. Cache: safe to delete.
    pub work_dir: PathBuf,
}

impl Paths {
    pub fn from_env() -> Result<Self> {
        Self::resolve(|key| std::env::var(key).ok())
    }

    /// `getenv` is injectable so tests don't mutate the process-global
    /// environment, which would race with `cargo test`'s parallel threads.
    ///
    /// Each directory has a pkgwatch-specific override, then the matching
    /// XDG variable, then the XDG default under `$HOME`; empty counts as
    /// unset. The overrides exist for dry runs against scratch
    /// directories, like `PKGWATCH_REPO_DIR` does for the pacman repo, so
    /// they're used verbatim. The XDG variables and `$HOME` must be
    /// absolute: the spec says to ignore a relative XDG value, and honoring
    /// one would bring back the cwd dependence this module exists to remove.
    fn resolve(getenv: impl Fn(&str) -> Option<String>) -> Result<Self> {
        let base = |override_var: &str, xdg_var: &str, home_subpath: &str| -> Result<PathBuf> {
            if let Some(dir) = non_empty(&getenv, override_var) {
                return Ok(PathBuf::from(dir));
            }
            if let Some(dir) = absolute(&getenv, xdg_var) {
                return Ok(Path::new(&dir).join(APP_DIR));
            }
            let home = absolute(&getenv, "HOME").context("HOME is not set to an absolute path")?;
            Ok(Path::new(&home).join(home_subpath).join(APP_DIR))
        };

        Ok(Self {
            packages_dir: base("PKGWATCH_CONFIG_DIR", "XDG_CONFIG_HOME", ".config")?
                .join("packages.d"),
            state_dir: base("PKGWATCH_STATE_DIR", "XDG_STATE_HOME", ".local/state")?,
            work_dir: base("PKGWATCH_WORK_DIR", "XDG_CACHE_HOME", ".cache")?,
        })
    }
}

/// The XDG spec says an empty variable must be treated as unset.
fn non_empty(getenv: &impl Fn(&str) -> Option<String>, key: &str) -> Option<String> {
    getenv(key).filter(|v| !v.is_empty())
}

/// Like `non_empty`, but also drops relative values (an empty string isn't
/// absolute either, so this subsumes the empty check).
fn absolute(getenv: &impl Fn(&str) -> Option<String>, key: &str) -> Option<String> {
    getenv(key).filter(|v| Path::new(v).is_absolute())
}

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

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

    #[test]
    fn defaults_live_under_home() {
        let paths = Paths::resolve(env(&[("HOME", "/home/u")])).unwrap();
        assert_eq!(
            paths,
            Paths {
                packages_dir: "/home/u/.config/pkgwatch/packages.d".into(),
                state_dir: "/home/u/.local/state/pkgwatch".into(),
                work_dir: "/home/u/.cache/pkgwatch".into(),
            }
        );
    }

    #[test]
    fn xdg_variables_override_home_defaults() {
        let paths = Paths::resolve(env(&[
            ("HOME", "/home/u"),
            ("XDG_CONFIG_HOME", "/xdg/config"),
            ("XDG_STATE_HOME", "/xdg/state"),
            ("XDG_CACHE_HOME", "/xdg/cache"),
        ]))
        .unwrap();
        assert_eq!(
            paths.packages_dir,
            Path::new("/xdg/config/pkgwatch/packages.d")
        );
        assert_eq!(paths.state_dir, Path::new("/xdg/state/pkgwatch"));
        assert_eq!(paths.work_dir, Path::new("/xdg/cache/pkgwatch"));
    }

    #[test]
    fn pkgwatch_overrides_win_and_are_used_verbatim() {
        let paths = Paths::resolve(env(&[
            ("HOME", "/home/u"),
            ("XDG_STATE_HOME", "/xdg/state"),
            ("PKGWATCH_CONFIG_DIR", "/scratch/cfg"),
            ("PKGWATCH_STATE_DIR", "/scratch/state"),
            ("PKGWATCH_WORK_DIR", "/scratch/work"),
        ]))
        .unwrap();
        // No `pkgwatch/` suffix appended to an explicit override.
        assert_eq!(paths.packages_dir, Path::new("/scratch/cfg/packages.d"));
        assert_eq!(paths.state_dir, Path::new("/scratch/state"));
        assert_eq!(paths.work_dir, Path::new("/scratch/work"));
    }

    #[test]
    fn empty_variables_are_treated_as_unset() {
        let paths = Paths::resolve(env(&[
            ("HOME", "/home/u"),
            ("XDG_STATE_HOME", ""),
            ("PKGWATCH_WORK_DIR", ""),
        ]))
        .unwrap();
        assert_eq!(paths.state_dir, Path::new("/home/u/.local/state/pkgwatch"));
        assert_eq!(paths.work_dir, Path::new("/home/u/.cache/pkgwatch"));
    }

    #[test]
    fn relative_xdg_variables_are_ignored() {
        let paths = Paths::resolve(env(&[
            ("HOME", "/home/u"),
            ("XDG_CONFIG_HOME", "rel/config"),
            ("XDG_STATE_HOME", "./state"),
            ("XDG_CACHE_HOME", "cache"),
        ]))
        .unwrap();
        assert_eq!(
            paths.packages_dir,
            Path::new("/home/u/.config/pkgwatch/packages.d")
        );
        assert_eq!(paths.state_dir, Path::new("/home/u/.local/state/pkgwatch"));
        assert_eq!(paths.work_dir, Path::new("/home/u/.cache/pkgwatch"));
    }

    #[test]
    fn relative_home_is_an_error() {
        let err = Paths::resolve(env(&[("HOME", "relative/home")])).unwrap_err();
        assert!(err.to_string().contains("HOME"));
    }

    #[test]
    fn errors_when_nothing_locates_home() {
        let err = Paths::resolve(env(&[])).unwrap_err();
        assert!(err.to_string().contains("HOME"));
    }

    #[test]
    fn no_home_needed_when_every_dir_is_overridden() {
        let paths = Paths::resolve(env(&[
            ("PKGWATCH_CONFIG_DIR", "/c"),
            ("PKGWATCH_STATE_DIR", "/s"),
            ("PKGWATCH_WORK_DIR", "/w"),
        ]))
        .unwrap();
        assert_eq!(paths.state_dir, Path::new("/s"));
    }
}