use std::{
    default::default,
    env,
    ffi::{OsStr, OsString},
    fs::File,
    io::Read,
};

// from https://doc.rust-lang.org/std/ffi/struct.OsString.html
fn concat_os_strings(a: &OsStr, b: &OsStr) -> OsString {
    let mut ret = OsString::with_capacity(a.len() + b.len()); // This will allocate
    ret.push(a); // This will not allocate further
    ret.push(b); // This will not allocate further
    ret
}

/// Check for an environment variable named for the given key with _FILE
/// appended to the end. If that exists, return the contents of the file, or
/// panic if it doesn't exist. If the `"${key}_FILE"` environment variable is
/// not set, return the value of the environment variable set by the given key,
/// or `None` if it's not set.
///
/// Panics:
///   - if the given file variable doesn't exist
///   - if there's an error reading the specified file
///   - if the environment variable string contains invalid unicode.
fn get_env_var_or_file<A: AsRef<OsStr>>(key: A) -> Option<String> {
    let key = key.as_ref();
    let file_key = concat_os_strings(key, "_FILE".as_ref());
    if let Some(path) = env::var_os(file_key) {
        // open the file and read it
        let mut file = File::open(&path).unwrap_or_else(|_| panic!("no such file at {path:?}"));
        let mut val: String = default();
        file.read_to_string(&mut val)
            .unwrap_or_else(|_| panic!("reading file at {path:?}"));
        Some(val)
    } else {
        env::var_os(key).map(|val| {
            val.to_str()
                .expect(&format!("value for ${key:?} contains invalid unicode"))
                .to_string()
        })
    }
}

/// Connect to the database using environment variables for configuration.
/// Panics on any failure.
pub(crate) fn connection_url() -> String {
    let user = get_env_var_or_file("POSTGRES_USER").expect("$POSTGRES_USER");
    let password = get_env_var_or_file("POSTGRES_PASSWORD").expect("$POSTGRES_PASSWORD");
    let db = get_env_var_or_file("POSTGRES_DB").expect("$POSTGRES_DB");
    let host = get_env_var_or_file("POSTGRES_HOST").unwrap_or_else(|| "localhost".into());
    let port = get_env_var_or_file("POSTGRES_PORT")
        .map(|port| {
            port.parse::<u16>()
                .expect("$POSTGRES_PORT is not a valid port number")
        })
        .unwrap_or(5432_u16);
    format!("postgres://{user}:{password}@{host}:{port}/{db}")
}