/// Wrapper around the `bevy::ui::Val` enum
///
/// # Syntax
/// * `unit!(num1 px)` ⇒ `Val::Px(num1 as f32)`
/// * `unit!(num1 pct)` ⇒ `Val::Percent(num1 as f32)`
/// * `unit!(auto)` ⇒ `Val::Auto`
/// * `unit!(undefined)` ⇒ `Val::Undefined`
#[macro_export]
macro_rules! unit {
    (@with_value px $value:literal) => ( bevy::ui::Val::Px($value as f32));
    (@with_value pct $value:literal) => ( bevy::ui::Val::Percent($value as f32));
    (auto) => ( bevy::ui::Val::Auto );
    (undefined) => ( bevy::ui::Val::Undefined );
    ($value:literal $val_unit:ident) => ( unit!(@with_value $val_unit $value));
}

/// Wrapper around `bevy::ui::Style`
///
/// ```rust,ignore
/// style! {
///     param1: something,
///     param2: something_else,
/// }
/// // Is strictly equivalent to
/// bevy::ui::Style {
///     param1: something,
///     param2: something_else,
///     ..Default.default()
/// }
/// ```
#[macro_export]
macro_rules! style {
    (@default ($default:expr) $($field:ident : $content:expr),*) => (
        bevy::ui::Style { $($field : $content,)* .. $default }
    );
    ($($field:ident : $content:expr,)*) => (
        style!(@default (Default::default()) $($field : $content),*)
    );
}

/// Wrapper around `bevy::ui::Size::new`
///
/// # Syntax
/// * `size!(num1 val1, num2 val2)` ⇒ `Size::new(unit!(num1 val1), unit!(num2 val2))`
#[macro_export]
macro_rules! size {
    ($x:tt $($x_unit:ident)?, $y:tt $($y_unit:ident)?) => (
        bevy::ui::Size::new(unit!($x $($x_unit)?), unit!($y $($y_unit)?))
    );
}

/// Define a `bevy::ui::UiRect` similarly to how you would define it in CSS.
///
/// # Syntax
/// ```rust,ignore
/// // one argument
/// rect!(num1 val1) == Rect::all(unit!(num1 val1))
///
/// // two arguments
/// rect!(num1 val1, num2 val2) == Rect {
///     left: unit!(num1 val1),
///     right: unit!(num1 val1),
///     top: unit!(num2 val2),
///     bottom: unit!(num2 val2),
/// }
///
/// // four arguments
/// rect!(num1 val1, num2 val2, num3 val3, num4 val4) == Rect {
///     left: unit!(num1 val1),
///     top: unit!(num2 val2),
///     right: unit!(num3 val3),
///     bottom: unit!(num4 val4),
/// }
/// ```
#[macro_export]
macro_rules! rect {
    ($x:tt $($x_unit:ident)?) => (
        bevy::ui::UiRect::all(unit!($x $($x_unit)?))
    );
    (
        $left:tt $($left_unit:ident)?, $top:tt $($top_unit:ident)?,
        $right:tt $($right_unit:ident)?, $bottom:tt $($bottom_unit:ident)?,
    ) => (
        bevy::ui::UiRect {
            left: unit!($left $($left_unit)?),
            top: unit!($top $($top_unit)?),
            right: unit!($right $($right_unit)?),
            bottom: unit!($bottom $($bottom_unit)?),
        }
    );
    ($x:tt $($x_unit:ident)?, $y:tt $($y_unit:ident)?) => (
        bevy::ui::UiRect {
            left: unit!($x $($x_unit)?),
            top: unit!($y $($y_unit)?),
            right: unit!($x $($x_unit)?),
            bottom: unit!($y $($y_unit)?),
        }
    );
}

/// Define a bevy UI and spawns it using `cmd`
///
/// # Syntax
/// ```rust,ignore
/// use bevy::prelude::*;
/// let commands: Commands;
/// let my_id: Entity;
/// build_ui! {
///     // The bevy `Commands`
///     #[cmd(commands)]
///     // The "preset" is an identifier, see doc
///     $entity
///         // Style modifiers. Supposing $entity is a `NodeBundle`, does:
///         // $entity.style = style!{ flex_whatever: Whatever }
///         // Leads to a compilation error if $entity doesn't have a `style`
///         // field
///         { flex_whatever: Whatever }
///         // Additional components and bundles. Translates to
///         // $entity.insert_bundle(bundle1).insert_bundle(bundle2).insert(comp1).insert(comp2)
///         // If you don't care for bundles or comp, just leave the left or
///         // right of the ; blank
///         [bundle1, bundl2 ;comp1, comp2]
///         // Children entities, may have {..}, [..;..] and (..)
///         (
///             entity[ButtonBundle](square),
///             id(my_id)
///         )
/// }
/// ```
///
/// The `$entity` in the macro may be one of the following:
/// * `id(Entity)`: inserts a pre-existing entity as child of containing entity
/// * `$ident`: where `$ident` is the name of a local variable of type
///   `T: ComponentBundle`. Spawn the bundle as base to insert extra components
///   to. Useful to not repeat yourself.
/// * `entity`: spawn an empty bundle as base to insert extra components to.
///
/// # Example
///
/// ```rust,ignore
/// build_ui! {
///     #[cmd(commands)]
///     vertical{size:size!(100 pct, 100 pct)}(
///         horizontal{justify_content: FlexStart, flex_basis: unit!(10 pct)}(
///             tab_square[;focus], tab_square[;focus], tab_square[;focus],
///         ),
///         column_box(
///             column[;red](
///                 vertical(select_square, select_square),
///                 horizontal{flex_wrap: Wrap}[gray](
///                     square[;focus], square[;focus], square[;focus], square[;focus],
///                     square[;focus], square[;focus], square[;focus], square[;focus],
///                     square[;focus], square[;focus], square[;focus], square[;focus],
///                 ),
///                 horizontal{flex_wrap: Wrap}[gray](
///                     square[;focus], square[;focus], square[;focus], square[;focus],
///                     square[;focus], square[;focus], square[;focus], square[;focus],
///                 ),
///             ),
///         ),
///     )
/// }
/// // Basically becomes
/// commands.spawn_bundle(NodeBundle {
///     style: Style { size: size!(100 pct, 100 pct), .. vertical.style },
///     .. vertical
/// })
///   .with_children(|cmds| {
///     cmds.spawn_bundle(NodeBundle {
///         style: Style {justify_content: FlexStart, flex_basis: unit!(10 pct), .. horizontal.style },
///         .. horizontal
///     })
///       .with_children(|cmds| {
///         cmds.spawn_bundle(tab_square).insert(focus);
///         cmds.spawn_bundle(tab_square).insert(focus);
///         cmds.spawn_bundle(tab_square).insert(focus);
///       });
///     cmds.spawn_bundle(column_box)
///       .with_children(|cmds| {
///         cmds.spawn_bundle(column).insert(red)
///           .with_children(|cmds| {
///             vertical.with_children(|cmds| {
///               cmds.spawn_bundle(select_square);
///               cmds.spawn_bundle(select_square);
///             });
///             cmds.spawn_bundle(NodeBundle {
///                 style: Style {flex_wrap: Wrap, ..horizontal.style},
///                 .. horizontal
///             }).insert(gray)
///               .with_children(|cmds| {
///                 for _ in 0..12 {
///                   cmds.spawn_bundle(square).insert(focus);
///                 }
///               });
///             cmds.spawn_bundle(NodeBundle {
///                 style: Style {flex_wrap: Wrap, ..horizontal.style},
///                 .. horizontal
///             }).insert(gray)
///               .with_children(|cmds| {
///                 for _ in 0..8 {
///                   cmds.spawn_bundle(square).insert(focus);
///                 }
///               });
///           });
///       });
///   });
/// ```
#[macro_export]
macro_rules! build_ui {
    (@preset entity) => (());
    (@preset $anything_else:ident) => ($anything_else);
    (@preset $node:ident {$($styles:tt)*}) => (
        bevy::ui::entity::NodeBundle {
            style: style!(@default ($node.style.clone()) $($styles)*),
            .. $node.clone()
        }
    );
    // if-else terminal
    (@child_list list: (if ($predicate:expr) { $( $if_true:tt )* } else { $( $if_false:tt )* } $(,)?),
        cmds: $cmds:expr, prefix: ($( $prefix:tt )*),
    ) => (
        $( $prefix )*
        if $predicate {
            build_ui!(@child_list list: ($( $if_true )*), cmds: $cmds, prefix: (),);
        } else {
            build_ui!(@child_list list: ($( $if_false )*), cmds: $cmds, prefix: (),);
        }
    );
    // if terminal
    (@child_list list: (if ($predicate:expr) { $( $if_true:tt )* } $(,)?),
        cmds: $cmds:expr, prefix: ($( $prefix:tt )*),
    ) => (
        $( $prefix )*
        if $predicate {
            build_ui!(@child_list list: ($( $if_true )*), cmds: $cmds, prefix: (),);
        }
    );
    // if-else with tail
    (@child_list list: (
            if ($predicate:expr) { $( $if_true:tt )* } else { $( $if_false:tt )* }
            , $( $tail:tt )+
        ),
        cmds: $cmds:expr, prefix: ($( $prefix:tt )*),
    ) => (
        build_ui! ( @child_list
            list: ( $( $tail )+ ),
            cmds: $cmds,
            prefix: ($( $prefix )*
                if $predicate {
                    build_ui!(@child_list list: ($( $if_true )*), cmds: $cmds, prefix: (),);
                } else {
                    build_ui!(@child_list list: ($( $if_false )*), cmds: $cmds, prefix: (),);
                }
            ),
        )
    );
    // if with tail
    (@child_list list: (if ($predicate:expr) { $( $if_true:tt )* } , $( $tail:tt )+),
        cmds: $cmds:expr, prefix: ($( $prefix:tt )*),
    ) => (
        build_ui! ( @child_list
            list: ( $( $tail )+ ),
            cmds: $cmds,
            prefix: ($( $prefix )*
                if $predicate {
                    build_ui!(@child_list list: ($( $if_true )*), cmds: $cmds, prefix: (),);
                }
            ),
        )
    );
    // just terminal
    (@child_list list: ($preset:ident $( { $($syl:tt)* } )? $( [ $($bc:tt)* ] )? $( ( $( $c:tt )* ) )? $(,)?),
        cmds: $cmds:expr, prefix: ($( $prefix:tt )*),
    ) => (
        $( $prefix )*
        build_ui!{ #[cmd($cmds)] $preset $( { $($syl)* } )? $( [ $($bc)* ] )? $( ( $($c)* ) )? }
    );
    // just has a tail
    (@child_list list: (
            $preset:ident $( { $($syl:tt)* } )? $( [ $($bc:tt)* ] )? $( ( $( $c:tt )* ) )?
            , $( $tail:tt )+
        ),
        cmds: $cmds:expr, prefix: ($( $prefix:tt )*),
    ) => (
        build_ui! ( @child_list
            list: ( $( $tail )+ ),
            cmds: $cmds,
            prefix: ($( $prefix )*
                build_ui!{ #[cmd($cmds)] $preset $( { $($syl)* } )? $( [ $($bc)* ] )? $( ( $($c)* ) )? };
            ),
        )
    );
    (#[cmd($cmds:expr)] id ( $id:expr )) => ({
        use bevy::ecs::system::Insert;
        let parent = $cmds.parent_entity();
        let insert = bevy::hierarchy::AddChild {
            child: $id,
            parent,
        };
        $cmds.add_command(insert);
    });
    (#[cmd($cmds:expr)] $preset:ident
        $( {$($styles:tt)*} )? // {..} style modifiers
        $( [$($bundles:expr),* ; $($components:expr),*] )? // [..] components
        $( ( $( $children_list:tt )* ) )?
    ) => (
        $cmds.spawn_bundle(build_ui!(@preset $preset $({$($styles)*})?).clone())
            $($(.insert_bundle($bundles.clone()))*
            $(.insert($components.clone()))*)?
            $(.with_children(|cmds| {
                build_ui!(@child_list
                    list: ( $( $children_list )* ),
                    cmds: cmds,
                    prefix: (),
                );
            }))?
    );
}