//! Support for common configuration behaviour and loading them from various sources.
//!
//! # Direct configuration example
//!
//! This example shows how to use the direct configuration. Please note that the direct
//! configuration is usually useful in testing scenarios. Production usage is discouraged.
//!
//! ```
//! # #[cfg(feature = "configuration")]
//! # {
//! # use blockz::prelude::*;
//! #[derive(Configuration)]
//! #[configuration(direct)]
//! struct ServerConfig {
//!     address: String,
//!     port: u16,
//! }
//!
//! # #[tokio::main]
//! # async fn main() {
//! let config = <ServerConfig as Configuration>::load(ServerConfig {
//!     address: "127.0.0.1".to_string(),
//!     port: 58732,
//! }).await;
//! # assert_eq!(config.address.as_str(), "127.0.0.1");
//! # assert_eq!(config.port, 58732);
//! println!("Server binding to {}:{}.", config.address, config.port);
//! // Server binding to 127.0.0.1:58732.
//! # }
//! # }
//! # #[cfg(not(feature = "configuration"))]
//! # fn main() {}
//! ```
//!
//! If the configuration type implements Default, you can use EasyConfiguration to load the default
//! value, like so:
//!
//! ```
//! # use blockz::prelude::*;
//! #[derive(Configuration)]
//! #[configuration(direct)]
//! struct ServerConfig {
//!     address: String,
//!     port: u16,
//! }
//!
//! impl Default for ServerConfig {
//!     fn default() -> ServerConfig {
//!         ServerConfig {
//!             address: "0.0.0.0".to_string(),
//!             port: 9999,
//!         }
//!     }
//! }
//!
//! # #[tokio::main]
//! # async fn main() {
//! let config = <ServerConfig as EasyConfiguration>::load().await;
//! # assert_eq!(config.address.as_str(), "0.0.0.0");
//! # assert_eq!(config.port, 9999);
//! println!("Server binding to {}:{}.", config.address, config.port);
//! // Server binding to 0.0.0.0:9999.
//! # }
//! ```
//!
//! # Env configuration examples
//!
//! **NOTE**: These examples require the __env_configuration__ feature.
//!
//! The first example shows how to use the env configuration with a prefix.
//!
//! ```
//! # #[cfg(feature = "env_configuration")]
//! # {
//! # use blockz::prelude::*;
//! # use serde::Deserialize;
//! #[derive(Configuration, Deserialize)]
//! #[configuration(env(prefix = "COOL_APP_"))]
//! struct ServerConfig {
//!     #[serde(rename = "bind_addr")]
//!     address: String,
//!     #[serde(rename = "bind_port")]
//!     port: u16,
//! }
//!
//! # #[tokio::main]
//! # async fn main() {
//! # std::env::set_var("COOL_APP_BIND_ADDR", "0.0.0.0");
//! # std::env::set_var("COOL_APP_BIND_PORT", "58732");
//! let config = <ServerConfig as EasyConfiguration>::load()
//!     .await
//!     .expect("Failed to load configuration from the environment!");
//! # assert_eq!(config.address.as_str(), "0.0.0.0");
//! # assert_eq!(config.port, 58732);
//! println!("Server binding to {}:{}.", config.address, config.port);
//! // Server binding to aaa.bbb.ccc.ddd:ppppp.
//! # }
//! # }
//! # #[cfg(not(feature = "env_configuration"))]
//! # fn main() {}
//! ```
//!
//! The second example shows how to use the env configuration with a prefix source in the form of a
//! constant.
//!
//! ```
//! # #[cfg(feature = "env_configuration")]
//! # {
//! # use blockz::prelude::*;
//! # use serde::Deserialize;
//! /// This is our prefix.
//! const PREFIX: &str = "COOL_APP_";
//!
//! #[derive(Configuration, Deserialize)]
//! #[configuration(env(prefix_source = "PREFIX.to_string()"))]
//! struct ServerConfig {
//!     #[serde(rename = "bind_addr")]
//!     address: String,
//!     #[serde(rename = "bind_port")]
//!     port: u16,
//! }
//!
//! # #[tokio::main]
//! # async fn main() {
//! # std::env::set_var("COOL_APP_BIND_ADDR", "0.0.0.0");
//! # std::env::set_var("COOL_APP_BIND_PORT", "58732");
//! let config = <ServerConfig as EasyConfiguration>::load()
//!     .await
//!     .expect("Failed to load configuration from the environment!");
//! # assert_eq!(config.address.as_str(), "0.0.0.0");
//! # assert_eq!(config.port, 58732);
//! println!("Server binding to {}:{}.", config.address, config.port);
//! // Server binding to aaa.bbb.ccc.ddd:ppppp.
//! # }
//! # }
//! # #[cfg(not(feature = "env_configuration"))]
//! # fn main() {}
//! ```
//!
//! The third example shows how to use the env configuration with a prefix source in the form of a
//! function that can return an error. The error must be mapped to an `envy::Error`.
//!
//! ```
//! # #[cfg(feature = "env_configuration")]
//! # {
//! # use blockz::prelude::*;
//! # use serde::Deserialize;
//! /// This function sources our prefix.
//! fn source_prefix() -> Result<String, envy::Error> {
//!     std::env::var("COOL_APP_PREFIX")
//!         .map_err(|err| envy::Error::Custom(format!("failed to source the prefix: {}", err)))
//! }
//!
//! #[derive(Configuration, Deserialize)]
//! #[configuration(env(prefix_source = "source_prefix()?"))]
//! struct ServerConfig {
//!     #[serde(rename = "bind_addr")]
//!     address: String,
//!     #[serde(rename = "bind_port")]
//!     port: u16,
//! }
//!
//! # #[tokio::main]
//! # async fn main() {
//! # std::env::set_var("COOL_APP_PREFIX", "COOL_APP_");
//! # std::env::set_var("COOL_APP_BIND_ADDR", "0.0.0.0");
//! # std::env::set_var("COOL_APP_BIND_PORT", "58732");
//! let config = <ServerConfig as EasyConfiguration>::load()
//!     .await
//!     .expect("Failed to load configuration from the environment!");
//! # assert_eq!(config.address.as_str(), "0.0.0.0");
//! # assert_eq!(config.port, 58732);
//! println!("Server binding to {}:{}.", config.address, config.port);
//! // Server binding to aaa.bbb.ccc.ddd:ppppp.
//! # }
//! # }
//! # #[cfg(not(feature = "env_configuration"))]
//! # fn main() {}
//! ```

#[cfg(feature = "env_configuration")]
use serde::Deserialize;

use std::marker::PhantomData;

/// Common behaviour of configurations.
#[async_trait::async_trait]
pub trait Configuration {
    /// The type of options container this Configuration accepts for the purpose of loading the
    /// configuration.
    type Opts: Send;

    /// The result type that can be produced by loading the configuration.
    type Result: Send;

    /// Load the configuration.
    async fn load(opts: Self::Opts) -> Self::Result;
}

/// An easy configuration is a configuration that can be loaded without other parameters.
#[async_trait::async_trait]
pub trait EasyConfiguration {
    /// The result type that can be produced by loading the configuration.
    type Result: Send;

    /// Load the configuration.
    async fn load() -> Self::Result;
}

/// Automatically implement EasyConfiguration on Configuration implementations whose Opts type is
/// Default.
#[async_trait::async_trait]
impl<C, O, R> EasyConfiguration for C
where
    C: Configuration<Opts = O, Result = R>,
    O: Default + Send + 'static,
    R: Send + 'static,
{
    type Result = R;

    async fn load() -> Self::Result {
        C::load(O::default()).await
    }
}

/// Direct configuration that just returns the passed value.
pub struct DirectConfiguration<T>
where
    T: Send,
{
    _phantom: PhantomData<T>,
}

#[async_trait::async_trait]
impl<T> Configuration for DirectConfiguration<T>
where
    T: Send,
{
    type Opts = T;
    type Result = T;

    async fn load(opts: Self::Opts) -> Self::Result {
        opts
    }
}

/// Configuration that can be sourced from environment variables.
#[cfg(feature = "env_configuration")]
#[cfg_attr(docsrs, doc(cfg(feature = "env_configuration")))]
pub struct EnvConfiguration<T>
where
    T: for<'de> Deserialize<'de> + Send,
{
    _phantom_t: PhantomData<T>,
}

#[cfg(feature = "env_configuration")]
#[cfg_attr(docsrs, doc(cfg(feature = "env_configuration")))]
#[async_trait::async_trait]
impl<T> Configuration for EnvConfiguration<T>
where
    T: for<'de> Deserialize<'de> + Send,
{
    type Opts = Option<String>;
    type Result = Result<T, envy::Error>;

    async fn load(opts: Self::Opts) -> Self::Result {
        if let Some(prefix) = opts {
            envy::prefixed(prefix).from_env::<T>()
        } else {
            envy::from_env::<T>()
        }
    }
}