conf/conf.v

module conf

import os
import toml

// get_env_var tries to read the contents of the given environment variable. It
// looks for either `${env.prefix}${field_name.to_upper()}` or
// `${env.prefix}${field_name.to_upper()}${env.file_suffix}`, returning the
// contents of the file instead if the latter. If both or neither exist, the
// function returns an error.
fn get_env_var(prefix string, field_name string, file_suffix string) !string {
	env_var_name := '$prefix$field_name.to_upper()'
	env_file_name := '$prefix$field_name.to_upper()$file_suffix'
	env_var := os.getenv(env_var_name)
	env_file := os.getenv(env_file_name)

	// If both are missing, we return an empty string
	if env_var == '' && env_file == '' {
		return ''
	}

	// If they're both set, we report a conflict
	if env_var != '' && env_file != '' {
		return error('Only one of $env_var_name or $env_file_name can be defined.')
	}

	// If it's the env var itself, we return it.
	// I'm pretty sure this also prevents variable ending in _FILE (e.g.
	// VIETER_LOG_FILE) from being mistakingely read as an _FILE suffixed env
	// var.
	if env_var != '' {
		return env_var
	}

	// Otherwise, we process the file
	return os.read_file(env_file) or {
		error('Failed to read file defined in $env_file_name: ${err.msg()}.')
	}
}

[params]
pub struct LoadConfig {
	prefix       string
	file_suffix  string = '_FILE'
	default_path string
}

// load<T> attempts to create an object of type T from the given path to a toml
// file & environment variables. For each field, it will select either a value
// given from an environment variable, a value defined in the config file or a
// configured default if present, in that order.
pub fn load<T>(conf LoadConfig) !T {
	mut res := T{}

	// This array allows us to determine later whether the variable is actually
	// zero or just a null'ed struct field
	mut has_value := map[string]bool{}

	// Later, this could be read from an env var as well.
	path := conf.default_path

	if os.exists(path) {
		// We don't use reflect here because reflect also sets any fields not
		// in the toml back to their zero value, which we don't want
		doc := toml.parse_file(path)!

		$for field in T.fields {
			s := doc.value(field.name)

			if s !is toml.Null {
				$if field.typ is string {
					res.$(field.name) = s.string()
				} $else $if field.typ is int {
					res.$(field.name) = s.int()
					// This seems to not work in V 0.3.2
					//} $else {
					//	$compile_error('Unsupported config struct field type detected.')
				}

				has_value[field.name] = true
			}
		}
	}

	$for field in T.fields {
		env_value := get_env_var(conf.prefix, field.name, conf.file_suffix)!

		// The value of an env var will always take precedence over the toml
		// file.
		if env_value != '' {
			$if field.typ is string {
				res.$(field.name) = env_value
			} $else $if field.typ is int {
				res.$(field.name) = env_value.int()
				// This seems to not work in V 0.3.2
				//} $else {
				//	$compile_error('Unsupported config struct field type detected.')
			}

			has_value[field.name] = true
		}

		if !(has_value[field.name] or { false }) {
			return error("Missing config variable '$field.name' with no provided default. Either add it to the config file or provide it using an environment variable.")
		}
	}
	return res
}