/// \file
/// POM configuration parser for C++.
///
/// ## Thread-safety
///
/// Of course, you should not \ref pom::Configuration::merge into
/// a configuration while another thread is using it.
///
/// Otherwise, libpom is fully thread-safe
/// provided that C11 atomics are available
/// (`__STDC_VERSION__ >= 201112L && !defined(__STDC_NO_ATOMICS__)`).
/// But beware of race conditions when using \ref pom::Configuration::unread_keys
/// — this will not lead to UB but you may get unexpected results —
/// ensure there is synchronization so that all threads
/// have certainly read their keys before calling it.
///
/// If C11 atomics are not available, you can almost certainly still get away
/// with sharing configurations across threads, as long as you use proper
/// synchronization for \ref pom::Configuration::unread_keys.
/// (Essentially, libpom may end up writing the same value to the same address
///  from separate threads, which is *technically* undefined behaviour, but will
///  likely never be an issue on any real machine.)
/// Even if you are extremely paranoid, you can still use
/// distinct configurations in different threads without worry.
///
/// ## Notes
///
/// Every libpom++ function may change the value of `errno` arbitrarily
/// (its value after any libpom++ call should be ignored).

/// \mainpage libpom++ doxygen documentation
///
/// See \ref pom.hpp for all types/functions.
#ifndef POM_HPP_
#define POM_HPP_

#include <exception>
#include <cstdint>
#include <string_view>
#include <string>
#include <vector>
#include <memory>
#include <optional>

/// libpom++ namespace
namespace pom {

/// A libpom++ error.
class Error: public std::exception {
public:
	~Error();
	Error(Error &other) = delete;
	/// Get file where error occured.
	///
	/// Returned string view lives for as long as `this`.
	std::string_view file() const noexcept;
	/// Get line number where error occurred.
	uint64_t line() const noexcept;
	/// Get error message. You probably want \ref to_string instead.
	///
	/// This only gets a single error message from this entry in an error list,
	/// and doesn't include the file name or line number.
	///
	/// Returned string view lives for as long as `this`.
	std::string_view message() const noexcept;
	/// Get next error
	///
	/// Returned pointer lives for as long as `this`.
	inline const Error *next() const noexcept { return m_next.get(); }
	/// Get description of error.
	///
	/// Returned string view lives for as long as `this`.
	std::string_view to_string() noexcept;
	/// Get description of error (equivalent to \ref to_string — use that instead if you can).
	///
	/// You should only call this on the first error in an error list
	/// (i.e. don't call it on an error gotten from \ref next);
	/// otherwise you will get a valid but mostly useless string.
	/// (Unlike \ref to_string, this can't be enforced with
	/// constness because this method needs to override `std::exception::what`.)
	///
	/// Returned pointer lives for as long as `this`.
	virtual const char *what() const noexcept override;
private:
	friend class Configuration;
	friend class Parser;
	Error() = delete;
	Error(void *C_error);
	Error(const void *C_error);
	void *C;
	bool m_is_original;
	std::unique_ptr<const Error> m_next;
};
/// Print error.
std::ostream &operator<<(std::ostream &, Error &);

/// Settings for configuration parsing.
class Settings {
public:
	/// Default settings.
	inline Settings() {}
	/// Set language for error messages.
	///
	/// Currently supported languages: `en`, `fr`.
	///
	/// \param lang IETF-like language tag.
	/// The closest supported language will be used
	/// (e.g. `fr-CA` will currently redirect to `fr`).
	void set_error_language(std::string_view lang);
private:
	void check_version() const;
	void to_C(void *C) const;
	friend class Configuration;
	char m_error_lang[16] = {};
	// to allow for future extensions without breaking backwards compatibility
	const uint32_t version = 1;
};

/// Abstract base class for a file reader.
///
/// This can be passed to \ref Configuration::Configuration(std::string_view, Reader &, const Settings *)
class Reader {
public:
	/// Read up to `count` bytes of data into `buf`.
	///
	/// This method will not be called excessively/with lots of tiny reads—it's
	/// okay to do unbuffered reads in it.
	///
	/// \param buf Place to put data.
	/// \param count (Non-zero) number of bytes to read
	///
	/// \returns 0 to indicate the end of the file,
	/// or a positive value ≤ `count`, to indicate the number of bytes read.
	virtual size_t read(char *buf, size_t count) = 0;
};

/// An item in a configuration (returned by \ref Configuration::items).
class Item {
public:
	/// Name of the key.
	///
	/// The returned string view is valid for the lifetime of `this`.
	std::string_view key() const noexcept { return m_key; }
	/// Value of the key.
	///
	/// The returned string view is valid for the lifetime of `this`.
	std::string_view value() const noexcept { return m_value; }
	/// File name where key was defined.
	///
	/// The returned string view is valid for the lifetime of `this`.
	std::string_view file() const noexcept { return m_file; }
	/// Line number where key was defined.
	uint64_t line() const noexcept { return m_line; }
private:
	Item(std::string_view key, std::string_view value, std::string_view file, uint64_t line):
		m_key(key), m_value(value), m_file(file), m_line(line) {}
	friend class Configuration;
	std::string m_key, m_value, m_file;
	uint64_t m_line;
};

/// A source location.
class Location {
public:
	/// File name
	///
	/// The returned string view is valid for the lifetime of `this`.
	inline std::string_view file() const { return m_file; }
	/// Line number
	inline uint64_t line() const { return m_line; }
private:
	inline Location(std::string file, uint64_t line):
		m_file(file), m_line(line) {}
	friend class Configuration;
	std::string m_file;
	uint64_t m_line;
	// to allow for future extensions without breaking backwards compatibility
	const uint32_t version = 1;
};
/// Print location as `file``:``line` (exact format may change in future).
std::ostream &operator<<(std::ostream &, const Location &);

/// A POM configuration.
class Configuration {
public:
	/// Create empty configuration.
	Configuration();
	/// Set `this` to `other`, deleting the current configuration in `this`.
	Configuration &operator=(const Configuration &other);
	/// Copy configuration from `other`.
	inline Configuration(const Configuration &other) { *this = other; };
	/// Load configuration from abstract \ref Reader.
	///
	/// Most of the time, you will be able to use another constructor to load a configuration.
	/// But if you have special functions for performing reads, you may need this.
	///
	/// \param filename File name for error messages.
	/// \param source Abstract reader to get file data from.
	/// \param settings Settings for parsing the file, or `nullptr` to use defaults.
	Configuration(std::string_view filename, Reader &source, const Settings *settings = nullptr);
	/// Load configuration from a `std::istream`.
	///
	/// \param filename File name for error messages.
	/// \param stream Stream to read file from.
	/// \param settings Settings for parsing the file, or `nullptr` to use defaults.
	Configuration(std::string_view filename, std::istream &stream, const Settings *settings = nullptr);
	/// Load configuration from a file path.
	///
	/// \param path File name for error messages and to load configuration from.
	/// \param settings Settings for parsing the file, or `nullptr` to use defaults.
	Configuration(std::string_view path, const Settings *settings = nullptr);
	/// Load configuration from a string.
	///
	/// \param filename File name for error messages.
	/// \param string String containing the configuration.
	/// \param settings Settings for parsing the file, or `nullptr` to use defaults.
	Configuration(std::string_view filename, std::string_view string, const Settings *settings = nullptr);
	~Configuration();
	/// Get value of `key` in configuration.
	std::optional<std::string> get(std::string_view key) const;
	/// Get value of `key` in configuration, or `dflt` if `key` isn't defined.
	std::string get_or_default(std::string_view key, std::string_view dflt) const;
	/// Get signed integer value of `key` in configuration.
	///
	/// Throws an \ref Error if `key` exists but isn't a valid signed integer.
	std::optional<int64_t> get_int(std::string_view key) const;
	/// Get signed integer value of `key` in configuration, or `dflt` if `key` isn't defined.
	///
	/// Throws an \ref Error if `key` exists but isn't a valid signed integer.
	int64_t get_int_or_default(std::string_view key, int64_t dflt) const;
	/// Get unsigned integer value of `key` in configuration.
	///
	/// Throws an \ref Error if `key` exists but isn't a valid unsigned integer.
	std::optional<uint64_t> get_uint(std::string_view key) const;
	/// Get unsigned integer value of `key` in configuration, or `dflt` if `key` isn't defined.
	///
	/// Throws an \ref Error if `key` exists but isn't a valid unsigned integer.
	uint64_t get_uint_or_default(std::string_view key, uint64_t dflt) const;
	/// Get floating-point value of `key` in configuration.
	///
	/// Throws an \ref Error if `key` exists but isn't a valid floating-point number.
	std::optional<double> get_float(std::string_view key) const;
	/// Get floating-point value of `key` in configuration, or `dflt` if `key` isn't defined.
	///
	/// Throws an \ref Error if `key` exists but isn't a valid floating-point number.
	double get_float_or_default(std::string_view key, double dflt) const;
	/// Get boolean value of `key` in configuration.
	///
	/// Throws an \ref Error if `key` exists but isn't a valid boolean (`on`/`off`/`yes`/`no`/`true`/`false`).
	std::optional<bool> get_bool(std::string_view key) const;
	/// Get boolean value of `key` in configuration, or `dflt` if `key` isn't defined.
	///
	/// Throws an \ref Error if `key` exists but isn't a valid boolean (`on`/`off`/`yes`/`no`/`true`/`false`).
	bool get_bool_or_default(std::string_view key, bool dflt) const;
	/// Get list value of `key` in configuration.
	std::optional<std::vector<std::string>> get_list(std::string_view key) const;
	/// Get list value of `key` in configuration, or `dflt` if `key` isn't defined.
	std::vector<std::string> get_list_or_default(std::string_view key, const std::vector<std::string> &dflt) const;
	/// Returns whether `key` is in this configuration.
	bool has(std::string_view key) const;
	/// Returns location of `key` in this configuration.
	std::optional<Location> location(std::string_view key) const;
	/// Extract section of configuration consisting of all keys starting with `name.` and their values.
	Configuration section(std::string_view name) const;
	/// Get list of keys which haven't been the target of a `get_*` method.
	std::vector<std::string> unread_keys() const;
	/// Get list of all "direct" keys (unique first components of keys) in this configuration.
	std::vector<std::string> keys() const;
	/// Get all key-value pairs in this configuration.
	///
	/// This returns a vector of pointers, so that more fields can be added to items in the future
	/// without breaking binary backwards compatibility.
	std::vector<std::unique_ptr<Item>> items() const;
	/// Merge `other` configuration into `this`.
	///
	/// Puts all the key-value pairs of `other` into this configuration.
	/// If `this` and `other` both have a value for a key, the one
	/// in `other` is preferred.
	void merge(const Configuration &other);
private:
	void load(std::string_view filename, Reader &source, const Settings *settings);
	explicit Configuration(void *c): C(c) {}
	void *C;
};
/// Print configuration.
std::ostream &operator<<(std::ostream &, const Configuration &);

} // namespace pom

#endif // POM_HPP_