use std::env::{var, VarError::*};

use std::ffi::OsString;

use std::fmt;

use std::path::{is_separator, PathBuf};

const CREDENTIALS_DIRECTORY: &str = "CREDENTIALS_DIRECTORY";

/// Takes a systemd credential ID. If the environment variable

/// `CREDENTIALS_DIRECTORY` is set and valid Unicode, and the file corresponding

/// to the credential exists, returns the path of the file corresponding to the

/// credential.

///

/// Absence of the environment variable and inexistence of the file are handled

/// gracefully returning `Ok(None)`.

pub fn path(id: &str) -> Result<Option<PathBuf>, PathError> {

    use PathError::*;

    if id.contains(is_separator) {

        return Err(InvalidCredentialId { id: id.to_owned() });

    }

    let credential = match var(CREDENTIALS_DIRECTORY) {

        Err(NotUnicode(os)) => return Err(EnvVarNotUnicode { os }),

        Err(NotPresent) => return Ok(None),

        Ok(env) => PathBuf::from(env).join(id),

    };

    Ok(credential.exists().then_some(credential))

}

/// The error returned by [`path`].

#[derive(Debug)]

pub enum PathError {

    InvalidCredentialId { id: String },

    EnvVarNotUnicode { os: OsString },

}

impl fmt::Display for PathError {

    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {

        use PathError::*;

        match self {

		InvalidCredentialId { id } => write!(f, "The systemd credential ID '{id}' is invalid."),

		EnvVarNotUnicode { os } => write!(f, "The value of environment variable '{CREDENTIALS_DIRECTORY}' is not valid Unicode (it lossily translates to '{}').", os.to_string_lossy()),

	}

    }

}

pub mod credential;