use tera::Tera;
use serde::Deserialize;

use std::fmt;

use crate::HumusProtoEngine;
use crate::read_toml_from_file;
use crate::TomlError;

/// Helps loading Templates from disk along with a handy configuration file.
///
/// Templates should be placed in a flat file structure in one directory.
/// They may be accompanied by an `extra.toml` file.
/// (by default, the location is configurable)
///
/// It also helps out a bit with deriving template realted configuration
/// for the rest of whatever you are building.
/// 
/// Example:
/// ```
/// let template_loader = TemplateEngineLoader::new(
/// 	config.template.template_location.clone(),
/// 	config.template.extra_config.clone()
/// )
/// 	.cli_template_location(cli_args.template_location)
/// 	.cli_extra_config_location(cli_args.extra_config);
/// 	
/// 
/// let templating_engine = match template_loader.load_templates() {
/// 	Ok(t) => t.into(),
/// 	Err(e) => {
/// 		println!("{e}");
/// 		::std::process::exit(1);
/// 	}
/// };
/// ```
///
#[derive(Deserialize, Clone)]
pub struct TemplateEngineLoader {
	/// The path to the directory where the templates are.
	pub template_location: String,
	/// The path to the extra configuration
	/// (relative to the current pwd, not to the templates)
	pub extra_config_location: Option<String>,
}

impl TemplateEngineLoader {

	/// Creates a new `TemplateEngineLoader` with minimal typing.
	pub fn new(
		template_location: String,
		extra_config_location: Option<String>
	) -> Self {
		Self {
			template_location: template_location,
			extra_config_location: extra_config_location,
		}
	}

	/// Overrides the template location with a new location if it is set.
	///
	/// Intended for processing cli-options.
	pub fn cli_template_location(mut self, location: Option<String>) -> Self {
		if let Some(location) = location {
			self.template_location = location;
		}
		self
	}

	/// Overrides the extra configuration location with a new location if it is set.
	///
	/// Intended for processing cli-options.
	pub fn cli_extra_config_location(mut self, location: Option<String>) -> Self {
		if let Some(location) = location {
			self.extra_config_location = Some(location);
		}
		self
	}

	/// Returns the template base directory.
	///
	/// Contructed by ensuring the template_location ends in a "/".
	pub fn base_dir(&self) -> String {
		if self.template_location.ends_with("/") {
			self.template_location.clone()
		} else {
			self.template_location.clone()+"/"
		}
	}

	/// Initialize a [HumusProtoEngine] with the given templates and extra configuration.
	///
	/// Failure Modes:
	/// * The `extra.toml` was not found and the path was explicitly set.
	/// * The `extra.toml` was found and is not valid toml.
	/// * The `extra.toml` passes, but tera finds an error in the templates.
	///
	/// If `extra_config_location` is `None` no error is returned if the `extra.toml`
	/// was not found as the template might not require one.
	/// 
	/// [HumusProtoEngine]: ./struct.HumusProtoEngine.html
	pub fn load_templates(
		&self
	) -> Result<HumusProtoEngine,TemplateEngineLoaderError> {
		let template_base_dir = self.base_dir();
		let template_extra_config_res = match &self.extra_config_location {
			Some(path) => read_toml_from_file(path),
			None => {
				read_toml_from_file(&(template_base_dir.clone()+"extra.toml"))
			}
		};
		let template_extra_config = match template_extra_config_res {
			Ok(c) => Some(c),
			Err(e) => match &e {
				TomlError::FileError{..} => {
					// Only fatal if the file was explicitly requested.
					// An implicit request could also mean that
					// the template doesn't need a config file.
					if self.extra_config_location.is_some() {
						return Err(TemplateEngineLoaderError::TomlError(e));
					}
					None
				},
				TomlError::ParseError{..} => {
					return Err(TemplateEngineLoaderError::TomlError(e));
				}
			},
		};
		let template_glob = template_base_dir.clone()+"*";
		println!("Parsing Templates from '{}' ...", &template_glob);
		let res = Tera::new((template_glob).as_str());
		let tera = match res {
			Ok(t) => t,
			Err(e) => {
				return Err(TemplateEngineLoaderError::TemplateParseError{
					path: template_glob,
					tera_error: e,
				});
			}
		};
		Ok(HumusProtoEngine {
			tera: tera,
			template_config: template_extra_config,
		})
	}
}

/// Returned when loading a template using the [TemplateEngineLoader] fails.
///
/// [TemplateEngineLoader]: ./struct.TemplateEngineLoader.html
pub enum TemplateEngineLoaderError {
	/// An error occourred while loading the template configuration file.
	TomlError(TomlError),
	/// An error occourred while parsing the templates.
	TemplateParseError{
		/// Path to the template directory
		path: String,
		/// what went wrong
		tera_error: tera::Error
	},
}

impl fmt::Display for TemplateEngineLoaderError {
	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
		match self {
			Self::TomlError(e) =>
				write!(f,"Error with template extra configuration:\n{e}"),
			Self::TemplateParseError{path, tera_error} =>
				write!(f,"Error parsing template '{path}':\n{tera_error}"),
		}
	}
}