altui_core/layout/

layout.rs

use std::time::Instant;
#[cfg(feature = "layout-cache")]
use std::{cell::RefCell, collections::HashMap};

use cassowary::strength::{MEDIUM, REQUIRED, STRONG, WEAK};
use cassowary::WeightedRelation::*;
use cassowary::{Constraint as CassowaryConstraint, Solver};

use crate::layout::elements::{LineBox, Variables};
use crate::layout::{
    elements::{ElConstraint, Element},
    flex::Flex,
    rect::Rect,
};

const SUPER: f64 = 1_001_001_001_000.0;

#[derive(Default)]
struct Line {
    elements: Vec<Element>,
}

#[derive(Copy, Clone)]
pub(crate) enum Attr {
    Start,
    CrossStart,
    Size,
    CrossSize,
    Gap,
}

#[derive(Debug, Hash, Clone, Copy, PartialEq, Eq)]
pub enum Corner {
    TopLeft,
    TopRight,
    BottomRight,
    BottomLeft,
}

#[derive(Debug, Hash, Clone, Copy, PartialEq, Eq)]
pub enum Direction {
    Horizontal,
    Vertical,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Constraint {
    // TODO: enforce range 0 - 100
    Percentage(u16),
    Ratio(u32, u32),
    Length(u16),
    Max(u16),
    Min(u16),
}

impl Constraint {
    pub fn apply(&self, length: u16) -> u16 {
        match *self {
            Constraint::Percentage(p) => length * p / 100,
            Constraint::Ratio(num, den) => {
                let r = num * u32::from(length) / den;
                r as u16
            }
            Constraint::Length(l) => length.min(l),
            Constraint::Max(m) => length.min(m),
            Constraint::Min(m) => length.max(m),
        }
    }
}

#[derive(Debug, Default, Clone, Copy, PartialEq, Eq, Hash)]
pub struct Margin {
    pub vertical: u16,
    pub horizontal: u16,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Alignment {
    Left,
    Center,
    Right,
}

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub struct Layout {
    direction: Direction,
    margin: Margin,
    constraints: Vec<Constraint>,
    flex: Flex,
    cross_flex: Flex,
    cross_size: Constraint,
    overlap: u16,
    cross_overlap: u16,
    boxed: bool,
    external_scroll: bool,
    wrap: bool,
}

#[cfg(feature = "layout-cache")]
thread_local! {
    static LAYOUT_CACHE: RefCell<HashMap<(Rect, Layout, u16), Vec<Rect>>> = RefCell::new(HashMap::new());
}

impl Default for Layout {
    fn default() -> Layout {
        Layout {
            direction: Direction::Vertical,
            margin: Margin {
                horizontal: 0,
                vertical: 0,
            },
            constraints: Vec::new(),
            flex: Flex::Start,
            cross_flex: Flex::Start,
            cross_size: Constraint::Percentage(100),
            overlap: 0,
            cross_overlap: 0,
            boxed: false,
            external_scroll: false,
            wrap: false,
        }
    }
}

impl Layout {
    /// Creates a vertical layout with the given constraints.
    ///
    /// A vertical layout splits the available area from top to bottom
    /// into multiple horizontal slices.
    ///
    /// In other words:
    /// * `Constraint` values are applied to the `height`
    /// * All resulting rectangles share the same `x` and `width`
    /// * Rectangles are stacked along the `Y axis`
    ///
    /// This method is equivalent to `Layout::default().direction(Direction::Vertical)`
    /// and effectively replaces the use of `Layout::new(...)` for vertical layouts.
    ///
    /// ## Visual representation
    /// ```text
    /// +----------------------+
    /// |        Rect 0        |  ← Constraint[0]
    /// +----------------------+
    /// |        Rect 1        |  ← Constraint[1]
    /// +----------------------+
    /// |        Rect 2        |  ← Constraint[2]
    /// +----------------------+
    /// ```
    ///
    /// ## Example
    ///
    /// ```
    /// use altui_core::layout::{Rect, Constraint::*, Layout};
    ///
    /// let chunks = Layout::vertical([Length(2),Min(0)])
    ///     .split(Rect {
    ///         x: 0,
    ///         y: 0,
    ///         width: 10,
    ///         height: 6,
    ///     });
    ///
    /// // Splits the area into two rows:
    /// // height = 2 and height = 4
    /// assert_eq!(chunks.len(), 2);
    /// assert_eq!(chunks[0].height, 2);
    /// assert_eq!(chunks[1].height, 4);
    /// ```
    /// See also [`Layout::horizontal`] for left-to-right layouts.
    pub fn vertical<C>(constraints: C) -> Self
    where
        C: Into<Vec<Constraint>>,
    {
        Layout {
            constraints: constraints.into(),
            ..Default::default()
        }
    }

    /// Creates a horizontal layout with the given constraints.
    ///
    /// A horizontal layout splits the available area from left to right
    /// into multiple vertical slices.
    ///
    /// In other words:
    /// * Constraint values are applied to the `width`
    /// * All resulting rectangles share the same `y` and `height`
    /// * Rectangles are stacked along the `X axis`
    ///
    /// This method is equivalent to
    /// `Layout::default().direction(Direction::Horizontal)`
    /// and effectively replaces the use of `Layout::new(...)` for horizontal layouts.
    ///
    /// ## Visual representation
    /// ```text
    /// +---------+---------+---------+
    /// | Rect 0  | Rect 1  | Rect 2  |
    /// |         |         |         |
    /// +---------+---------+---------+
    ///   ↑           ↑
    /// Constraint[0] Constraint[1]
    /// ```
    ///
    /// ## Example
    ///
    /// ```
    /// use altui_core::layout::{Rect, Constraint::*, Layout};
    ///
    /// let chunks = Layout::horizontal([Length(3),Min(0)])
    ///     .split(Rect {
    ///         x: 0,
    ///         y: 0,
    ///         width: 10,
    ///         height: 4,
    ///     });
    ///
    /// // Splits the area into two columns:
    /// // width = 3 and width = 7
    /// assert_eq!(chunks.len(), 2);
    /// assert_eq!(chunks[0].width, 3);
    /// assert_eq!(chunks[1].width, 7);
    /// ```
    /// See also [`Layout::vertical`] for top-to-bottom layouts.
    pub fn horizontal<C>(constraints: C) -> Self
    where
        C: Into<Vec<Constraint>>,
    {
        Layout {
            direction: Direction::Horizontal,
            constraints: constraints.into(),
            ..Default::default()
        }
    }

    pub fn constraints<C>(mut self, constraints: C) -> Layout
    where
        C: Into<Vec<Constraint>>,
    {
        self.constraints = constraints.into();
        self
    }

    pub fn margin(mut self, margin: u16) -> Layout {
        self.margin = Margin {
            horizontal: margin,
            vertical: margin,
        };
        self
    }

    pub fn horizontal_margin(mut self, horizontal: u16) -> Layout {
        self.margin.horizontal = horizontal;
        self
    }

    pub fn vertical_margin(mut self, vertical: u16) -> Layout {
        self.margin.vertical = vertical;
        self
    }

    pub fn direction(mut self, direction: Direction) -> Layout {
        self.direction = direction;
        self
    }

    /// Sets the amount of allowed overlap between adjacent elements
    /// along the main axis.
    ///
    /// The `overlap` value specifies how many neighboring characters are
    /// allowed to overlap between adjacent layout items.
    ///
    /// Useful for compact layouts with shared borders.
    ///
    /// ```text
    /// ┌────────┌────────┌────────┐
    /// │ Item 1 │ Item 2 │ Item 3 │
    /// └────────└────────└────────┘
    /// ```
    ///
    /// # Warning: Unstable API
    ///
    /// ⚠️ **Known Issue**: This method may cause incorrect rendering
    /// when used together with [`Layout::split_ext`], [`Flex::SpaceBetween`]
    /// or [`Flex::SpaceAround`].
    pub fn overlap(mut self, overlap: u16) -> Layout {
        self.overlap = overlap;
        self
    }

    /// Sets the amount of allowed overlap between adjacent elements
    /// along the cross axis.
    ///
    /// The `overlap` value specifies how many neighboring symbols are
    /// allowed to overlap between adjacent layout items.
    ///
    /// Useful for compact layouts with shared borders.
    ///
    /// ```text
    /// ┌────────┐
    /// │ Item 1 │
    /// ┌────────┐
    /// │ Item 2 │
    /// ┌────────┐
    /// │ Item 3 │
    /// └────────┘
    /// ```
    ///
    /// # Warning: Unstable API
    ///
    /// ⚠️ **Known Issue**: This method may cause incorrect rendering
    /// when used together with [`Layout::split_ext`], [`Flex::SpaceBetween`]
    /// or [`Flex::SpaceAround`].
    pub fn cross_overlap(mut self, cross_overlap: u16) -> Layout {
        self.cross_overlap = cross_overlap;
        self
    }

    /// Sets how free space is distributed between elements along the main axis.
    ///
    /// `Flex` controls the alignment and spacing of elements **after all constraints
    /// have been resolved**.
    ///
    /// This affects only the main axis:
    /// * horizontal layouts → X axis
    /// * vertical layouts → Y axis
    ///
    /// The default value is [`Flex::Start`].
    ///
    /// See [`Flex`] for a detailed description of each mode.
    pub fn flex(mut self, flex: Flex) -> Layout {
        self.flex = flex;
        self
    }

    /// Sets how free space is distributed between elements along the cross axis.
    ///
    /// `Flex` controls the alignment and spacing of elements **after all constraints
    /// have been resolved**.
    ///
    /// This affects only the cross axis:
    /// * horizontal layouts → Y axis
    /// * vertical layouts → X axis
    ///
    /// The default value is [`Flex::Start`].
    pub fn cross_flex(mut self, flex: Flex) -> Layout {
        self.cross_flex = flex;
        self
    }

    /// Enables or disables *boxed layout mode*.
    ///
    /// When `boxed` is set to `true`, fixed-size constraints
    /// (`Constraint::Length`, `Constraint::Min`) take precedence and are resolved first.
    ///
    /// Remaining space is then distributed between:
    /// * `Constraint::Ratio`
    /// * `Constraint::Percentage`
    ///
    /// When boxed is `false` (default), all constraints participate in layout
    /// solving on equal terms.
    ///
    /// ## Important notes
    ///
    /// * Using `boxed = true` together with scrolling or wrapping may produce
    ///   unintuitive results.
    /// * Percentage and ratio constraints are strongly discouraged when
    ///   `boxed` is enabled.
    ///
    /// ## Default
    ///
    /// `false`
    pub fn boxed(mut self, boxed: bool) -> Layout {
        self.boxed = boxed;
        self
    }

    /// Marks the layout as externally scroll-controlled.
    ///
    /// When enabled, the layout does not compute its own scroll bounds.
    /// Instead, the scroll size is assumed to be managed by the caller.
    ///
    /// This is mainly useful when:
    /// * multiple layouts share a single scroll state
    /// * scroll limits are computed elsewhere
    ///
    /// This option is only meaningful when using [`Layout::split_ext`].
    pub fn external_scroll(mut self) -> Layout {
        self.external_scroll = true;
        self
    }

    /// Enables wrapping into multiple lines when the accumulated main-axis length
    /// exceeds the given value. Sets line/column size on cross-axis.
    ///
    /// Wrapping splits elements into multiple lines/columns, similar to text wrapping.
    ///
    /// * Wrapping is performed based on main-axis length
    /// * Line/column size along the cross-axis is fixed (Min/Max Constraints work as Length)
    /// * Enabling wrapping implicitly changes the scroll direction
    ///
    /// ## Scroll direction
    ///
    /// * Without wrapping:
    ///   * horizontal layout → horizontal scroll
    ///   * vertical layout → vertical scroll
    ///
    /// * With wrapping enabled:
    ///   * horizontal layout → vertical scroll
    ///   * vertical layout → horizontal scroll
    ///
    /// ## Example
    ///
    /// Wrapping a horizontal layout into multiple rows:
    ///
    /// ```text
    /// [A][B][C]  → wrap
    /// [A][B]
    /// [C]
    /// ```
    /// See also [`cross_size`](Layout::cross_size).
    pub fn wrap(mut self, cross_size: Constraint) -> Layout {
        self.wrap = true;
        self.cross_size = cross_size;
        self
    }

    /// Sets the size of Rect along the cross-axis (or cross-direction) without wrapping
    ///
    /// Note!
    ///  * `Min/Max` constraints work as `Length`
    ///  * Default is `Percentage(100)`
    pub fn cross_size(mut self, cross_size: Constraint) -> Layout {
        self.cross_size = cross_size;
        self
    }

    #[cfg(not(feature = "layout-cache"))]
    pub fn cache_eq(&self, other: &Layout) -> bool {
        self.constraints == other.constraints
            && self.cross_size == other.cross_size
            && self.flex == other.flex
            && self.cross_flex == other.cross_flex
            && self.direction == other.direction
            && self.margin == other.margin
            && self.boxed == other.boxed
            && self.wrap == other.wrap
            && self.external_scroll == other.external_scroll
    }

    /// Splits the given area into multiple [`Rect`]s according to the layout
    /// configuration, direction, wrapping rules and constraints.
    ///
    /// This function is a high-level wrapper around an internal Cassowary-based
    /// solver. The layout process consists of two distinct phases:
    ///
    /// 1. Line/column construction (preparation phase)
    /// 2. Constraint solving (layout phase)
    ///
    /// ## Wrapping behavior
    ///
    /// * Wrapping is enabled when [`Layout::wrap`] method is used.
    /// * Each line/column has a fixed cross size determined in the method parameter.
    /// * When wrapping is enabled, a new line/column is started whenever adding the
    ///   next constraint would exceed the available main axis (direction) size.
    /// * Elements with `Max` constraint and `Percentage` and `Ratio` constraints
    ///   if layout is `boxed` do not contribute to line/column size and will be
    ///   collapsed by the solver.
    ///
    /// # Examples
    ///
    /// ### Vertical layout
    /// ```
    /// use altui_core::layout::{Rect, Constraint, Layout};
    ///
    /// let chunks = Layout::vertical([Constraint::Length(5), Constraint::Min(0)])
    ///     .split(Rect {
    ///         x: 2,
    ///         y: 2,
    ///         width: 10,
    ///         height: 10,
    ///     });
    /// assert_eq!(
    ///     chunks,
    ///     vec![
    ///         Rect {
    ///             x: 2,
    ///             y: 2,
    ///             width: 10,
    ///             height: 5
    ///         },
    ///         Rect {
    ///             x: 2,
    ///             y: 7,
    ///             width: 10,
    ///             height: 5
    ///         }
    ///     ]
    /// );
    /// ```
    ///
    /// ## Horizontal layout with ratios
    /// ```
    /// use altui_core::layout::{Rect, Constraint, Layout};
    ///
    /// let chunks = Layout::horizontal([Constraint::Ratio(1, 3), Constraint::Ratio(2, 3)])
    ///     .split(Rect {
    ///         x: 0,
    ///         y: 0,
    ///         width: 9,
    ///         height: 2,
    ///     });
    /// assert_eq!(
    ///     chunks,
    ///     vec![
    ///         Rect {
    ///             x: 0,
    ///             y: 0,
    ///             width: 3,
    ///             height: 2
    ///         },
    ///         Rect {
    ///             x: 3,
    ///             y: 0,
    ///             width: 6,
    ///             height: 2
    ///         }
    ///     ]
    /// );
    /// ```
    ///
    /// ### Wrapping into multiple lines by length
    /// ```
    /// use altui_core::layout::{Rect, Constraint, Layout};
    ///
    /// let chunks = Layout::horizontal([
    ///         Constraint::Length(3),
    ///         Constraint::Length(3),
    ///         Constraint::Length(3),
    ///     ])
    ///     .wrap(Constraint::Length(2))
    ///     .split(Rect {
    ///         x: 0,
    ///         y: 0,
    ///         width: 6,
    ///         height: 4,
    ///     });
    ///
    /// // Produces two rows with height `2` each:
    /// // [3][3]
    /// // [3]
    ///
    /// assert_eq!(
    ///     chunks,
    ///     vec![
    ///         Rect { x: 0, y: 0, width: 3, height: 2 },
    ///         Rect { x: 3, y: 0, width: 3, height: 2 },
    ///         Rect { x: 0, y: 2, width: 3, height: 2 },
    ///     ]
    /// );
    /// ```
    ///
    /// ### Wrapping into multiple lines by percentage
    /// ```
    /// use altui_core::layout::{Rect, Constraint, Layout};
    ///
    /// let chunks = Layout::horizontal([
    ///         Constraint::Length(3),
    ///         Constraint::Length(3),
    ///         Constraint::Length(3),
    ///     ])
    ///     .wrap(Constraint::Percentage(50))
    ///     .split(Rect {
    ///         x: 0,
    ///         y: 0,
    ///         width: 6,
    ///         height: 8,
    ///     });
    ///
    /// // Produces two rows with height `4` each:
    /// // [3][3]
    /// // [3]
    ///
    /// assert_eq!(
    ///     chunks,
    ///     vec![
    ///         Rect { x: 0, y: 0, width: 3, height: 4 },
    ///         Rect { x: 3, y: 0, width: 3, height: 4 },
    ///         Rect { x: 0, y: 4, width: 3, height: 4 },
    ///     ]
    /// );
    /// ```
    pub fn split(&self, area: Rect) -> Vec<Rect> {
        let dest_area = area.inner(&self.margin);

        // TODO: Maybe use a fixed size cache ?
        #[cfg(feature = "layout-cache")]
        {
            LAYOUT_CACHE.with(|c| {
                c.borrow_mut()
                    .entry((dest_area, self.clone(), 0))
                    .or_insert_with(|| self.split_inner(dest_area))
                    .clone()
            })
        }

        #[cfg(not(feature = "layout-cache"))]
        self.split_inner(dest_area)
    }

    fn split_inner(&self, dest_area: Rect) -> Vec<Rect> {
        let stopwatch = Instant::now();
        let (lines, _, line_cross_size, _, variables) = self.split_preparations(&dest_area);

        let result = Results::new(self.direction, self.constraints.len()).split(
            dest_area,
            self,
            0.0,
            lines,
            line_cross_size,
            variables,
        );

        tracing::trace!(
            "Altui split: {} µs",
            stopwatch.elapsed().as_nanos() as f64 / 1_000.0
        );
        result
    }

    /// Splits the given area into multiple [`Rect`]s with scrolling support.
    ///
    /// This method extends [`Layout::split`] by introducing a scroll offset and
    /// automatically computing the scrollable range based on the layout
    /// configuration.
    ///
    /// The layout process is identical to split, but the final positioning
    /// is shifted by a computed scroll offset along the active scroll axis.
    ///
    /// ## Scrolling model
    ///
    /// Scrolling is single-axis and its direction depends on whether wrapping
    /// is enabled:
    ///
    /// * Without wrapping:
    ///   * Scrolling occurs along the main axis of the layout.
    ///   * For example:
    ///     * Direction::Horizontal → horizontal scrolling
    ///     * Direction::Vertical → vertical scrolling
    ///
    /// * With wrapping enabled:
    ///   * Scrolling occurs along the cross axis.
    ///   * The layout grows by adding multiple lines/columns, and scrolling moves
    ///     between those lines/columns.
    ///
    /// This behavior mirrors common UI patterns where wrapping turns a
    /// one-dimensional layout into a multi-line structure with perpendicular
    /// scrolling.
    ///
    /// ## Scroll parameters
    ///
    /// * scroll — total scrollable length (output)
    ///   * Updated by this function unless external_scroll is enabled.
    ///   * Represents the full scroll range in layout units.
    ///
    /// * scrollstate — current scroll position (input/output)
    ///   * Clamped automatically to the valid scroll range.
    ///   * Used to compute the effective scroll offset.
    ///
    /// ## Wrapping and scrolling interaction
    ///
    /// * When wrapping is enabled:
    ///   * The scroll range is computed from the total cross-axis size of all
    ///     lines/columns.
    ///
    /// * When wrapping is disabled:
    ///   * The scroll range is computed from the total content length along the
    ///     main axis.
    ///
    /// ## External scrolling
    ///
    /// If external_scroll is enabled:
    /// * The scroll range is taken from the scroll parameter.
    /// * This allows integrating the layout with an external scrolling container.
    ///
    /// ## Constraints and limitations
    ///
    /// ⚠️ Important:
    /// Do not use the following constraint combinations with split_ext:
    ///
    /// * [`Constraint::Percentage`] or [`Constraint::Ratio`] when the layout is boxed
    /// * [`Constraint::Max`] when relying on internal scrolling
    ///
    /// These combinations may produce undefined or unintuitive scrolling behavior.
    ///
    /// ## Notes
    ///
    /// * Line construction and wrapping are performed before scrolling is
    ///   applied.
    /// * The solver does not decide the scroll direction.
    /// * Margins are applied before layout and scrolling.
    /// * Scrolling offsets are quantized to integer values.
    ///
    /// # Examples
    ///
    /// ### Horizontal scrolling without wrapping
    ///
    ///```
    /// use altui_core::layout::{Rect, Constraint, Layout};
    ///
    /// let mut scroll = 0;
    /// let mut scrollstate = 0;
    ///
    /// let chunks = Layout::horizontal([
    ///         Constraint::Length(5),
    ///         Constraint::Length(5),
    ///         Constraint::Length(5),
    ///     ])
    ///     .split_ext(
    ///         Rect { x: 0, y: 0, width: 8, height: 2 },
    ///         &mut scrollstate,
    ///         &mut scroll,
    ///     );
    ///
    /// // Content width exceeds available width, so scrolling is horizontal.
    /// assert!(scroll == 15);
    /// ```
    ///
    /// ### Vertical scrolling with wrapping enabled
    /// ```
    /// use altui_core::layout::{Rect, Constraint, Layout};
    ///
    /// let mut scroll = 0;
    /// let mut scrollstate = 0;
    ///
    /// let chunks = Layout::horizontal([
    ///         Constraint::Length(3),
    ///         Constraint::Length(3),
    ///         Constraint::Length(3),
    ///     ])
    ///     .wrap(Constraint::Length(8))
    ///     .split_ext(
    ///         Rect { x: 0, y: 0, width: 6, height: 8 },
    ///         &mut scrollstate,
    ///         &mut scroll,
    ///     );
    ///
    /// // Wrapping creates multiple rows; scrolling is now vertical.
    /// assert!(scroll == 16);
    /// ```
    /// See [`Layout::split`] for a non-scrolling variant.
    pub fn split_ext(&self, area: Rect, scrollstate: &mut u16, scroll: &mut u16) -> Vec<Rect> {
        let dest_area = area.inner(&self.margin);

        if *scroll > 0 && scrollstate > scroll {
            *scrollstate = *scroll;
        }

        // TODO: Maybe use a fixed size cache ?
        #[cfg(feature = "layout-cache")]
        {
            LAYOUT_CACHE.with(|c| {
                c.borrow_mut()
                    .entry((dest_area, self.clone(), *scrollstate))
                    .or_insert_with(|| self.split_ext_inner(dest_area, scrollstate, scroll))
                    .clone()
            })
        }

        #[cfg(not(feature = "layout-cache"))]
        self.split_ext_inner(dest_area, scrollstate, scroll)
    }

    fn split_ext_inner(
        &self,
        dest_area: Rect,
        scrollstate: &mut u16,
        scroll: &mut u16,
    ) -> Vec<Rect> {
        let stopwatch = std::time::Instant::now();
        let gap: f64;
        let (lines, size, line_cross_size, content_length, variables) =
            self.split_preparations(&dest_area);

        if !self.external_scroll && !self.wrap {
            gap = (content_length - size).max(0.0).floor();
            *scroll = content_length.max(size).floor() as u16;
        } else if !self.external_scroll {
            let cross_size = dest_area.cross_end(self.direction);
            let cross_length = *line_cross_size * lines.len() as f64;
            gap = (cross_length - cross_size).max(0.0).floor();
            *scroll = cross_length.max(cross_size).floor() as u16;
        } else {
            gap = (f64::from(*scroll) - size).max(0.0);
        }

        let scroll_inner = match gap == 0.0 {
            true => 0.0,
            false => f64::from(*scrollstate) / f64::from(*scroll) * gap,
        };

        let result = Results::new(self.direction, self.constraints.len()).split(
            dest_area,
            self,
            scroll_inner.floor(),
            lines,
            line_cross_size,
            variables,
        );

        tracing::trace!(
            "Altui scrollable split: {} µs",
            stopwatch.elapsed().as_nanos() as f64 / 1_000.0
        );
        result
    }

    #[inline(always)]
    fn split_preparations(
        &self,
        dest_area: &Rect,
    ) -> (Vec<Line>, f64, ElConstraint, f64, Variables) {
        use Constraint::*;
        let mut content_length = 0.0;
        let mut variables = Variables::default();
        let size = dest_area.size(self.direction);
        let line_cross_size = match self.cross_size {
            Percentage(100) => ElConstraint::Length(dest_area.cross_size(self.direction)),
            Percentage(v) => {
                ElConstraint::Length(dest_area.cross_size(self.direction) * v as f64 / 100.0)
            }
            Ratio(n, d) => ElConstraint::Ratio(size * f64::from(n) / f64::from(d)),
            Length(v) | Min(v) | Max(v) => ElConstraint::Length(f64::from(v)),
        };
        let mut lines = Vec::new();
        let mut line = Line::default();

        let boxedf64 = if self.boxed { STRONG } else { MEDIUM };
        let mut used = 0.0;

        for el in &self.constraints {
            let (boxed, strength, constraint) = match el {
                Length(v) => (false, boxedf64, ElConstraint::Length(f64::from(*v))),
                Min(v) => (false, boxedf64, ElConstraint::Min(f64::from(*v))),
                Percentage(v) => (
                    self.boxed,
                    MEDIUM,
                    ElConstraint::Percentage(size * f64::from(*v) / 100.0),
                ),
                Max(v) => (true, WEAK, ElConstraint::Max(f64::from(*v))),
                Ratio(n, d) => (
                    self.boxed,
                    MEDIUM,
                    ElConstraint::Ratio(size * f64::from(*n) / f64::from(*d)),
                ),
            };

            let w = if boxed { 0.0 } else { *constraint };
            content_length += w;

            if self.wrap && used > 0.0 && used + w > size {
                lines.push(std::mem::take(&mut line));
                used = 0.0;
            }

            line.elements
                .push(Element::new(strength, constraint, &mut variables));
            used += w;
        }

        if !line.elements.is_empty() {
            lines.push(line);
        }

        (lines, size, line_cross_size, content_length, variables)
    }
}

#[derive(Debug)]
struct Results {
    v: Vec<Rect>,
    axis: Direction,
}

impl Results {
    fn new(axis: Direction, len: usize) -> Self {
        Results {
            v: vec![Rect::default(); len],
            axis,
        }
    }

    #[inline(always)]
    fn main_axis(&mut self, index: usize, attr: Attr, value: u16) {
        let r = &mut self.v[index];
        match (self.axis, attr) {
            (Direction::Horizontal, Attr::Start) => r.x = value,
            (Direction::Horizontal, Attr::Size) => r.width = value,
            (Direction::Vertical, Attr::Start) => r.y = value,
            (Direction::Vertical, Attr::Size) => r.height = value,
            _ => {}
        }
    }

    #[inline(always)]
    fn cross_axis(&mut self, index: usize, line: usize, lines: &[(u16, u16)]) {
        let r = &mut self.v[index];
        let (start, size) = lines[line];
        match self.axis {
            Direction::Horizontal => {
                r.y = start;
                r.height = size;
            }
            Direction::Vertical => {
                r.x = start;
                r.width = size;
            }
        }
    }

    #[inline(always)]
    fn split(
        mut self,
        dest_area: Rect,
        layout: &Layout,
        mut offset: f64,
        lines: Vec<Line>,
        cross_size: ElConstraint,
        mut variables: Variables,
    ) -> Vec<Rect> {
        use crate::layout::Flex::*;

        // Gap constraints for each line + rows constraints + flex constraints
        let mut ccs: Vec<CassowaryConstraint> =
            Vec::with_capacity(lines.len() + layout.constraints.len() * 6 + lines.len() * 2);
        // Cross gap + line/column constraints + flex constraints
        let mut cross_ccs: Vec<CassowaryConstraint> = Vec::with_capacity(1 + lines.len() * 5 + 2);

        // Calc quantity of Cassowary Variables, that must be eq with the index of the last Variable:
        // Constraints Variables + Lines Variables + Gaps Variables + Cross gap variable
        let vars_indexes = layout.constraints.len() * 2 + lines.len() * 2 + lines.len() + 1;
        // Line, index, attr
        let mut ivars: Vec<Option<(usize, usize, Attr)>> = vec![None; vars_indexes];

        // Lines processing
        let elmlines = &lines
            .iter()
            .map(|_| LineBox::new(&mut variables))
            .collect::<Vec<LineBox>>();

        let overlap = (layout.overlap as f64).min(dest_area.size(self.axis));
        let cross_overlap = (layout.cross_overlap as f64).min(dest_area.cross_size(self.axis));

        // Cross axis gap for lines
        let cross_gap = variables.add();
        ivars[variables.get_id(cross_gap)] = Some((0, 0, Attr::Gap));
        cross_ccs.push(match layout.cross_flex {
            SpaceBetween | SpaceAround => cross_gap | GE(REQUIRED) | 0f64,
            _ => cross_gap | EQ(REQUIRED) | 0f64,
        });

        let mut j: usize = 0;
        // Lines
        for ((l, line), elmline) in lines.iter().enumerate().zip(elmlines) {
            elmline.vars(&mut ivars, l, &variables);
            // Main axis gap constraint for SpaceBetween or SpaceAround
            let gap = variables.add();
            ivars[variables.get_id(gap)] = Some((0, 0, Attr::Gap));
            ccs.push(match layout.flex {
                SpaceBetween | SpaceAround => gap | GE(REQUIRED) | 0f64,
                _ => gap | EQ(SUPER) | 0f64,
            });

            // calc offset for cross axis scroll
            let cross_size_with_offset = if layout.wrap {
                scroll(*cross_size, &mut offset)
            } else {
                *cross_size
            };

            // spacing between lines
            if let Some(next) = elmlines.get(l + 1) {
                cross_ccs.push(
                    next.cross_start - elmline.cross_end()
                        | EQ(REQUIRED)
                        | cross_gap - cross_overlap,
                );
            }

            cross_ccs.extend([
                elmline.cross_size | GE(REQUIRED) | 0.0,
                elmline.cross_start | GE(REQUIRED) | dest_area.cross_start(self.axis),
                elmline.cross_end() | LE(REQUIRED) | dest_area.cross_end(self.axis),
                elmline.cross_size | EQ(MEDIUM) | cross_size_with_offset,
            ]);
            // Elements
            for (i, elm) in line.elements.iter().enumerate() {
                elm.vars(&mut ivars, l, j, &variables);

                // calc offset for main axis scroll
                let v = match elm.constraint {
                    ElConstraint::Min(v) => {
                        ccs.push(elm.size | EQ(WEAK) | dest_area.size(self.axis));
                        if layout.wrap {
                            v
                        } else {
                            scroll(v, &mut offset)
                        }
                    }
                    ElConstraint::Percentage(v)
                    | ElConstraint::Ratio(v)
                    | ElConstraint::Length(v) => {
                        if layout.wrap {
                            v
                        } else {
                            scroll(v, &mut offset)
                        }
                    }
                    ElConstraint::Max(v) => v,
                };

                // spacing between elements
                if let Some(next_elm) = line.elements.get(i + 1) {
                    ccs.push(next_elm.start - elm.end() | EQ(REQUIRED) | gap - overlap);
                }

                ccs.extend([
                    elm.size | GE(REQUIRED) | 0.0,
                    elm.start | GE(REQUIRED) | dest_area.start(self.axis),
                    elm.end() | LE(REQUIRED) | dest_area.end(self.axis),
                    match elm.constraint {
                        ElConstraint::Min(_) => elm.size | GE(elm.strength) | v,
                        _ => elm.size | EQ(elm.strength) | v,
                    },
                ]);
                j += 1;
            }
            // flex constraints
            if let (Some(first), Some(last)) = (line.elements.first(), line.elements.last()) {
                match layout.flex {
                    Legacy | SpaceBetween => ccs.extend([
                        first.start | EQ(SUPER) | dest_area.start(self.axis),
                        last.end() | EQ(SUPER) | dest_area.end(self.axis),
                    ]),
                    Start => ccs.push(first.start | EQ(SUPER) | dest_area.start(self.axis)),
                    End => ccs.push(last.end() | EQ(SUPER) | dest_area.end(self.axis)),
                    Center => ccs.push(
                        first.start + last.size + last.start
                            | EQ(SUPER)
                            | dest_area.start(self.axis) + dest_area.end(self.axis),
                    ),
                    SpaceAround => ccs.extend([
                        first.start - dest_area.start(self.axis) | EQ(SUPER) | gap,
                        dest_area.end(self.axis) - last.end() | EQ(SUPER) | gap,
                    ]),
                }
            }
        }

        if let (Some(first), Some(last)) = (elmlines.first(), elmlines.last()) {
            match layout.cross_flex {
                Legacy | Start => {
                    cross_ccs.push(first.cross_start | EQ(SUPER) | dest_area.cross_start(self.axis))
                }
                End => {
                    cross_ccs.push(last.cross_end() | EQ(SUPER) | dest_area.cross_end(self.axis))
                }
                Center => cross_ccs.push(
                    first.cross_start + last.cross_size + last.cross_start
                        | EQ(SUPER)
                        | dest_area.cross_start(self.axis) + dest_area.cross_end(self.axis),
                ),
                SpaceBetween => cross_ccs.extend([
                    first.cross_start | EQ(SUPER) | dest_area.cross_start(self.axis),
                    last.cross_end() | EQ(SUPER) | dest_area.cross_end(self.axis),
                ]),
                SpaceAround => cross_ccs.extend([
                    first.cross_start - dest_area.cross_start(self.axis) | EQ(SUPER) | cross_gap,
                    dest_area.cross_end(self.axis) - last.cross_end() | EQ(SUPER) | cross_gap,
                ]),
            }
        }

        assert!(ccs.len() < layout.constraints.len() * 13 + lines.len() + 1);

        let mut solver = Solver::new();
        solver.add_constraints(&cross_ccs).unwrap();
        // (Start, Size)
        let mut lines = vec![(0, 0); lines.len()];
        for &(var, value) in solver.fetch_changes() {
            let (line, index, attr) = ivars
                .get(variables.get_id(var))
                .expect("Cassowary variable and usize must have the same size and alignment")
                .expect("Total quantity of Cassowary Variables used in altui must be eq the last Variable index");
            // We have one gap var in line, that can be eq 0, but indexs can not
            assert!(index == 0);
            let value = norm(value);

            match attr {
                Attr::CrossStart => lines[line].0 = value,
                Attr::CrossSize => lines[line].1 = value,
                _ => {}
            }
        }

        solver.reset();

        solver.add_constraints(&ccs).unwrap();
        for &(var, value) in solver.fetch_changes() {
            let (line, index, attr) = ivars
                .get(variables.get_id(var))
                .expect("Cassowary variable and usize must have the same size and alignment")
                .expect("Total quantity of Cassowary Variables used in altui must be eq the last Variable index");
            let value = norm(value);

            self.main_axis(index, attr, value);
            self.cross_axis(index, line, &lines);
        }

        self.v
    }
}

#[inline]
fn norm(v: f64) -> u16 {
    if v.is_sign_negative() {
        0
    } else {
        v as u16
    }
}

#[inline(always)]
fn scroll(mut x: f64, scroll: &mut f64) -> f64 {
    if *scroll > 0.0 {
        if *scroll >= x {
            *scroll -= x;
            return 0.0;
        } else {
            x -= *scroll;
            *scroll = 0.0;
        }
    }
    x
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_boxed() {
        let err_msg = "Failed to resolve area with boxed param in Layout";
        let area = Rect {
            x: 0,
            y: 0,
            width: 10,
            height: 10,
        };

        let layout = Layout::vertical([Constraint::Percentage(50), Constraint::Length(8)])
            .boxed(true)
            .split(area);

        assert_eq!(layout[0].height, 2, "{}, {}", err_msg, "Percentage");
        assert_eq!(layout[1].height, 8, "{}, {}", err_msg, "Length");
    }

    #[test]
    fn test_vertical_split_by_height() {
        let target = Rect {
            x: 2,
            y: 2,
            width: 10,
            height: 10,
        };

        let chunks = Layout::vertical([
            Constraint::Percentage(10),
            Constraint::Max(5),
            Constraint::Min(1),
        ])
        .split(target);

        assert_eq!(target.height, chunks.iter().map(|r| r.height).sum::<u16>());
        chunks.windows(2).for_each(|w| assert!(w[0].y <= w[1].y));
    }

    #[test]
    fn test_rect_size_truncation() {
        for width in 256u16..300u16 {
            for height in 256u16..300u16 {
                let rect = Rect::new(0, 0, width, height);
                rect.area(); // Should not panic.
                assert!(rect.width < width || rect.height < height);
                // The target dimensions are rounded down so the math will not be too precise
                // but let's make sure the ratios don't diverge crazily.
                assert!(
                    (f64::from(rect.width) / f64::from(rect.height)
                        - f64::from(width) / f64::from(height))
                    .abs()
                        < 1.0
                )
            }
        }

        // One dimension below 255, one above. Area above max u16.
        let width = 900;
        let height = 100;
        let rect = Rect::new(0, 0, width, height);
        assert_ne!(rect.width, 900);
        assert_ne!(rect.height, 100);
        assert!(rect.width < width || rect.height < height);
    }

    #[test]
    fn test_rect_size_preservation() {
        for width in 0..256u16 {
            for height in 0..256u16 {
                let rect = Rect::new(0, 0, width, height);
                rect.area(); // Should not panic.
                assert_eq!(rect.width, width);
                assert_eq!(rect.height, height);
            }
        }

        // One dimension below 255, one above. Area below max u16.
        let rect = Rect::new(0, 0, 300, 100);
        assert_eq!(rect.width, 300);
        assert_eq!(rect.height, 100);
    }
}