//
//  require.hpp
//  gameutils
//
//  Created by Sam Jaffe on 8/19/16.
//

#pragma once

#include <stdexcept>

#undef CONCAT2
#define CONCAT2(A, B) A##B
#undef CONCAT
#define CONCAT(A, B) CONCAT2(A, B)

#undef STRING2
#define STRING2(A) #A
#undef STRING
#define STRING(A) STRING2(A)

/* expands to the first argument */
#define FIRST(...) FIRST_HELPER(__VA_ARGS__, throwaway)
#define FIRST_HELPER(first, ...) first

#define NUM(...) SELECT_5TH(__VA_ARGS__, FOURPLUS, THREE, TWO, ONE, throwaway)
#define SELECT_5TH(a1, a2, a3, a4, a5, ...) a5

namespace contract {
  template <typename expect>
  class ensure_t {
  private:
    std::function<bool()> passes_;
    std::string message_;
  public:
    template <typename F>
    ensure_t(F && passes, std::string const & msg, char const * = "")
      : passes_(passes), message_(msg) {}
    ~ensure_t() noexcept(false) {
      if (!std::uncaught_exception() && !passes_()) {
        throw expect{message_};
      }
    }
  };

  template <typename except>
  void _contract_impl(bool expr, std::string const & msg, char const * = "") {
    if ( ! expr ) throw except{ msg };
  }
  
  template <typename except>
  void _contract_impl(bool expr, char const * msg, char const * = "") {
    if ( ! expr ) throw except{ msg };
  }
}

#define EXCEPT_T_TWO(X, Y) Y
#define EXCEPT_T_THREE(X, Y, Z) Z
#define EXCEPT_T_FOURPLUS(X, Y, Z, ...) Y

#define EXCEPT_T(...) EXCEPT_T_HELPER(NUM(__VA_ARGS__), __VA_ARGS__)
#define EXCEPT_T_HELPER(N, ...) EXCEPT_T_HELPER2(N, __VA_ARGS__)
#define EXCEPT_T_HELPER2(N, ...) EXCEPT_T_##N(__VA_ARGS__)

#define EXCEPT_MSG_TWO(X, Y) Y
#define EXCEPT_MSG_THREE(X, Y, Z) Y
#define EXCEPT_MSG_FOURPLUS(X, Y, Z, ...) Z

#define EXCEPT_MSG(...) EXCEPT_MSG_HELPER(NUM(__VA_ARGS__), __VA_ARGS__)
#define EXCEPT_MSG_HELPER(N, ...) EXCEPT_MSG_HELPER2(N, __VA_ARGS__)
#define EXCEPT_MSG_HELPER2(N, ...) EXCEPT_MSG_##N(__VA_ARGS__)

#if defined( __clang__ ) || defined( __GNUC__ )
# define FUNCTION STRING(__PRETTY_FUNCTION__)
#elif defined( _MSC_VER )
# define FUNCTION STRING(__FUNCTION__)
#else
# define FUNCTION "???"
#endif

#define LOCATION_MSG ". in " FUNCTION "(" __FILE__ ":" STRING( __LINE__ ) ")"

#define DEF_MSG( header, expr ) \
  header " failed: " STRING(expr) LOCATION_MSG

#define SUB_MSG( header, bool_expr, rval_expr ) \
  header " failed: " STRING(bool_expr) " [ with _ = " STRING(rval_expr) " ]" LOCATION_MSG

/*
 * Usage: 
 *  expects( bool-expr )
 *  expects( bool-expr, custom_msg )
 *  expects( bool-expr, error_type, custom_msg )
 */
#define expects( ... ) \
  contract::_contract_impl<EXCEPT_T( __VA_ARGS__, std::logic_error)>( \
    FIRST(__VA_ARGS__), \
    EXCEPT_MSG(__VA_ARGS__, \
               DEF_MSG("precondition", FIRST(__VA_ARGS__))) \
  )

/*
 * Usage:
 *  expects_graceful( bool-expr[, rval] )
 *
 *  rval        - The value to return if the expression is false.
 *                Skip rval for a function returning void.
 */
#define expects_graceful( expr, ... ) \
  if ( ! bool(expr) ) { return __VA_ARGS__; }

/*
 * Usage:
 *  ensures( bool-expr )
 *  ensures( bool-expr, custom_msg )
 *  ensures( bool-expr, error_type, custom_msg )
 */
#define ensures( ... ) \
  contract::ensure_t<EXCEPT_T(__VA_ARGS__, std::runtime_error)>\
    CONCAT(contract_ensures_, __LINE__)( \
    [&]() { return FIRST(__VA_ARGS__); }, \
    EXCEPT_MSG(__VA_ARGS__ , \
               DEF_MSG("postcondition", FIRST(__VA_ARGS__))) \
  )


/**
 * RVO works if expr is an rvalue, but not if it is an
 * lvalue. e.g:
 * return ensures( expression, condition );
 *   will rvo the object (if expression is rvo-able)
 * but
 * return ensures( var, condition );
 *   will not, instead copies
 * By making _ an argument, both options will move
 *
 * Usage:
 *  return_ensures( rval-expr, ensure-expr )
 *  return_ensures( rval-expr, ensure-expr, custom_msg )
 *  return_ensures( rval-expr, ensure-expr, error_type, custom_msg )
 *
 *  rval-expr   - An expression that can be cast to the return type
 *                of the invoking function. Bound to the token '_'.
 *  ensure-expr - A special boolean expression using the token '_'.
 */
#if __cplusplus < 201300
#define return_ensures( expr, ... ) \
  [ & ]( ) { \
    decltype((expr)) _ = expr; \
    contract::_contract_impl<EXCEPT_T(__VA_ARGS__, std::runtime_error)>( \
        FIRST(__VA_ARGS__), \
        EXCEPT_MSG(__VA_ARGS__, \
                   SUB_MSG("postcondition", FIRST(__VA_ARGS__), expr))); \
    return _; \
  }( )
#else
#define return_ensures( expr, cond, ... ) \
  [ _ = expr ]( ) { \
    contract::_contract_impl<EXCEPT_T(-, ##__VA_ARGS__, std::runtime_error)>( \
        cond, \
        EXCEPT_MSG(cond, ##__VA_ARGS__, \
                   SUB_MSG("postcondition", cond, expr))); \
    return _; \
  }( )
#endif