/// Like an `if let Ok`,
/// but also reassigns variables with the value in the `Ok` variant.
///
/// Note: the `Ok` variant can only be destructured into a single variable or a tuple.
///
/// # Let pattern
///
/// You can declare variables usable inside this macro with `let` patterns, like this:
/// ```rust
/// # use konst::rebind_if_ok;
/// #
/// # let mut bar = 0;
/// # let mut number = 10;
/// let res: Result<_, ()> = Ok((10, 20));
/// rebind_if_ok!{(let foo, bar) = res =>
///     number += foo;
/// }
/// # assert_eq!(bar, 20);
/// ```
/// `foo` in this invocation of `rebind_if_ok` is a macro-local variable initialized with `10`,
/// while `bar` is a pre-existing variable that is assigned `20`.
///
/// This pattern only works when destructuring tuples.
///
/// # Example
///
/// ```rust
/// use konst::{
///     parsing::{Parser, ParseValueResult},
///     rebind_if_ok,
/// };
///
/// #[derive(Debug, PartialEq)]
/// struct Struct {
///     foo: bool,
///     bar: bool,
///     baz: Option<u64>,
/// }
///
/// const fn parse_struct(mut parser: Parser<'_>) -> (Struct, Parser<'_>) {
///     let mut flags = Struct {
///         foo: false,
///         bar: false,
///         baz: None,
///     };
///
///     // `parser` is reassigned if the `strip_prefix` method returns an `Ok` variant.
///     // (this also happens in every other invocation of `rebind_if_ok` in this example)
///     rebind_if_ok!{parser = parser.strip_prefix("foo,") =>
///         flags.foo = true;
///     }
///     rebind_if_ok!{parser = parser.strip_prefix("bar,") =>
///         flags.bar = true;
///     }
///     // `num` is only visible inside this macro invocation
///     rebind_if_ok!{(let num, parser) = parser.parse_u64() =>
///         flags.baz = Some(num);
///     }
///     (flags, parser)
/// }
///
/// const XX: [Struct; 2] = {
///     [
///         parse_struct(Parser::new("foo,1000")).0,
///         parse_struct(Parser::new("bar,")).0,
///     ]
/// };
///
/// assert_eq!(
///     XX,
///     [
///         Struct{foo: true, bar: false, baz: Some(1000)},
///         Struct{foo: false, bar: true, baz: None},
///     ]
/// );
///
///
/// ```
#[cfg_attr(feature = "docsrs", doc(cfg(feature = "parsing")))]
#[macro_export]
macro_rules! rebind_if_ok {
    (
        $pattern:tt $(:$ty:ty)? = $expression:expr
        $( => $($code:tt)* )?
    ) => {
        if let $crate::__::v::Ok(tuple) = $expression {
            $crate::__priv_ai_preprocess_pattern!{tuple, ($pattern $(:$ty)?)}
            $($($code)*)?
        }
    };
}

#[doc(hidden)]
#[macro_export]
macro_rules! __priv_ai_preprocess_pattern {
    ( $var:ident, (($($pat:tt)*))) => {
        $crate::__priv_assign_tuple!{$var, (0 1 2 3 4 5) , $($pat)*}
    };
    ( $var:ident, ($($pat:tt)*)) => {
        $crate::__priv_assign_tuple!{$var, (0 1 2 3 4 5) , $($pat)*}
    };
}
#[doc(hidden)]
#[macro_export]
macro_rules! __priv_assign_tuple {
    ($var:ident, $fields:tt, let $pat:tt: $ty:ty $(, $($rem:tt)*)?) => {
        $crate::__priv_next_ai_access!{
            (let $pat: $ty) $var, $fields, $($($rem)*)?
        }
    };
    ($var:ident, $fields:tt, let $pat:pat_param $(, $($rem:tt)*)?) => {
        $crate::__priv_next_ai_access!{
            (let $pat) $var, $fields, $($($rem)*)?
        }
    };
    ($var:ident, $fields:tt, _ $(: $ty:ty)? $(, $($rem:tt)*)?) => {
        $crate::__priv_next_ai_access!{
            (let _ $(: $ty)? ) $var, $fields, $($($rem)*)?
        }
    };
    ($var:ident, $fields:tt, $e:tt $(: $ty:ty)? $(, $($rem:tt)*)?) => {
        $(let _: $ty = $var;)?

        $crate::__priv_next_ai_access!{
            ($e) $var, $fields, $($($rem)*)?
        }
    };
    ($var:ident, $fields:tt, $e:expr $(, $($rem:tt)*)?) => {
        $crate::__priv_next_ai_access!(
            ($e) $var, $fields, $($($rem)*)?
        )
    };
}

#[doc(hidden)]
#[macro_export]
macro_rules! __priv_next_ai_access {
    ( ($($lhs:tt)*) $var:ident , (0 $($rem_fields:tt)*),  ) => {
        $($lhs)* = $var;
    };
    ( ($($lhs:tt)*) $var:ident , ($field:tt $($rem_fields:tt)*),  ) => {
        $($lhs)* = $var.$field;
    };
    ( ($($lhs:tt)*) $var:ident , ($field:tt $($rem_fields:tt)*), $($rem:tt)+ ) => {
        $($lhs)* = $var.$field;
        $crate::__priv_assign_tuple!($var,($($rem_fields:tt)*), $($rem)+)
    };
}

///////////////////////////////////////////////////////////////////////////////

/// Like the `?` operator,
/// but also reassigns variables with the value in the `Ok` variant.
///
/// Note: the `Ok` variant can only be destructured into a single variable or a tuple.
///
/// # Let pattern
///
/// You can declare new variables with `let` patterns like this:
/// ```text
/// try_rebind!{(let foo, bar) = Ok((10, 20))}
/// try_rebind!{(let (a, b, c), bar) = Ok(((10, 10, 10), 20))}
/// ```
/// `foo` in here is a new variable initialized with `10` (same for `a`, `b`, and `c`),
/// while `bar` is a pre-existing variable that is assigned `20`.
///
/// This pattern only works when destructuring tuples.
///
/// # Examples
///
/// ### Parsing
///
/// Inside of `parse_int_pair`, `parser` always refers to the same variable.
///
/// ```rust
/// use konst::{
///     parsing::{Parser, ParseValueResult},
///     try_rebind, unwrap_ctx,
/// };
///
/// const fn parse_int_pair(mut parser: Parser<'_>) -> ParseValueResult<'_, (u64, u64)> {
///
///     // `parser` is reassigned if the `parse_u64` method returns an `Ok`.
///     // (this also happens in every other invocation of `try_rebind` in this example)
///     try_rebind!{(let aa, parser) = parser.parse_u64()}
///
///     try_rebind!{parser = parser.strip_prefix(',')}
///
///     try_rebind!{(let bb, parser) = parser.parse_u64()}
///
///     Ok(((aa, bb), parser))
/// }
///
/// const PAIR: (u64, u64) = {
///     let parser = Parser::new("100,200");
///     unwrap_ctx!(parse_int_pair(parser)).0
/// };
///
/// assert_eq!(PAIR, (100, 200));
///
/// ```
///
#[cfg_attr(feature = "docsrs", doc(cfg(feature = "parsing")))]
#[macro_export]
macro_rules! try_rebind {
    (
        $pattern:tt = $expression:expr $(,)?
    ) => {
        let tuple = match $expression {
            $crate::__::v::Ok(tuple) => tuple,
            $crate::__::v::Err(_e) => return $crate::__::v::Err(_e),
        };
        $crate::__priv_ai_preprocess_pattern!(tuple, ($pattern));
    };
}

///////////////////////////////////////////////////////////////////////////////

macro_rules! partial{
    (
        $new_macro:ident = $macro:ident ! ($($prefix:tt)*)
    ) => {
        __priv_partial!(
            ($)
            $new_macro = $macro ! ($($prefix)*)
        )
    };
}

macro_rules! __priv_partial {
    (
        ($_:tt)
        $new_macro:ident = $macro:ident ! ($($prefix:tt)*)
    ) => {
        macro_rules! $new_macro {
            ($_($_ args:tt)*) => {
                $macro!($($prefix)* $_($_ args)* )
            }
        }
    }
}

///////////////////////////////////////////////////////////////////////////////

macro_rules! try_parsing {
    ( $parser:ident, $parse_direction:ident $(,$ret:ident)*; $($code:tt)* ) => ({
        #![allow(unused_parens, unused_labels, unused_macros)]

        $parser.parse_direction = ParseDirection::$parse_direction;
        let copy = $parser;

        let ($($ret),*) = 'ret: {
            partial!{throw = throw_out!(copy, $parse_direction,)}

            #[allow(unreachable_code)]
            {
                $($code)*
            }
        };

        enable_if_start!{$parse_direction,
            $parser.start_offset += (copy.str.len() - $parser.str.len()) as u32;
        }

        Ok(($($ret,)* $parser))
    })
}

macro_rules! parsing {
    ( $parser:ident, $parse_direction:ident $(,$ret:ident)*; $($code:tt)* ) => ({
        #![allow(unused_parens, unused_labels, unused_macros)]

        $parser.parse_direction = ParseDirection::$parse_direction;
        enable_if_start!{$parse_direction, let copy = $parser; }

        let ($($ret),*) = { $($code)* };

        enable_if_start!{$parse_direction,
            $parser.start_offset += (copy.str.len() - $parser.str.len()) as u32;
        }

        ($($ret,)* $parser)
    })
}

macro_rules! throw_out {
    ($copy:ident, $parse_direction:ident, $kind:expr) => {
        return Err(crate::parsing::ParseError::new($copy, $kind))
    };
    ($copy:ident, $parse_direction:ident, $kind:expr, map_err = $func:ident) => {
        return Err($func(crate::parsing::ParseError::new($copy, $kind)))
    };
}

macro_rules! enable_if_start{
    (FromEnd, $($tokens:tt)*) => { };
    (FromStart, $($tokens:tt)*) => {
        $($tokens)*
    };
    (FromBoth, $($tokens:tt)*) => {
        $($tokens)*
    };
}