Draft custom layout engine based on `druid`

4 files changed, 522 insertions, 3 deletions

diff --git a/native/src/layout/DRUID_LICENSE b/native/src/layout/DRUID_LICENSE
new file mode 100644
index 00000000..d6456956
--- /dev/null
+++ b/native/src/layout/DRUID_LICENSE
@@ -0,0 +1,202 @@
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diff --git a/native/src/layout/flex.rs b/native/src/layout/flex.rs
new file mode 100644
index 00000000..98cc344e
--- /dev/null
+++ b/native/src/layout/flex.rs
@@ -0,0 +1,155 @@
+// This code is heavily inspired by the [`druid`] codebase.
+//
+// [`druid`]: https://github.com/xi-editor/druid
+//
+// Copyright 2018 The xi-editor Authors, Héctor Ramón
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+use crate::{
+    layout::{Limits, Node},
+    Element, Size,
+};
+
+#[derive(Debug)]
+pub enum Axis {
+    Horizontal,
+    Vertical,
+}
+
+impl Axis {
+    fn main(&self, size: Size) -> f32 {
+        match self {
+            Axis::Horizontal => size.width,
+            Axis::Vertical => size.height,
+        }
+    }
+
+    fn cross(&self, size: Size) -> f32 {
+        match self {
+            Axis::Horizontal => size.height,
+            Axis::Vertical => size.width,
+        }
+    }
+
+    fn pack(&self, main: f32, cross: f32) -> (f32, f32) {
+        match self {
+            Axis::Horizontal => (main, cross),
+            Axis::Vertical => (cross, main),
+        }
+    }
+}
+
+// TODO: Remove `Message` type parameter
+pub fn resolve<Message, Renderer>(
+    axis: Axis,
+    renderer: &Renderer,
+    limits: &Limits,
+    padding: f32,
+    spacing: f32,
+    children: &[Element<'_, Message, Renderer>],
+) -> Node
+where
+    Renderer: crate::Renderer,
+{
+    let limits = limits.pad(padding);
+
+    let mut total_non_fill = spacing as f32 * (children.len() - 1) as f32;
+    let mut fill_sum = 0;
+
+    let mut nodes: Vec<Node> = Vec::with_capacity(children.len());
+    nodes.resize(children.len(), Node::default());
+
+    for (i, child) in children.iter().enumerate() {
+        let fill_factor = match axis {
+            Axis::Horizontal => child.width(),
+            Axis::Vertical => child.height(),
+        }
+        .fill_factor();
+
+        if fill_factor == 0 {
+            let child_limits = Limits::new(Size::ZERO, limits.max());
+
+            let layout = child.layout(renderer, &child_limits);
+
+            total_non_fill += axis.main(layout.size());
+
+            nodes[i] = layout;
+        } else {
+            fill_sum += fill_factor;
+        }
+    }
+
+    let available = axis.main(limits.max());
+    let remaining = (available - total_non_fill).max(0.0);
+
+    for (i, child) in children.iter().enumerate() {
+        let fill_factor = match axis {
+            Axis::Horizontal => child.width(),
+            Axis::Vertical => child.height(),
+        }
+        .fill_factor();
+
+        if fill_factor != 0 {
+            let max_main = remaining * fill_factor as f32 / fill_sum as f32;
+            let min_main = if max_main.is_infinite() {
+                0.0
+            } else {
+                max_main
+            };
+
+            let (min_main, min_cross) =
+                axis.pack(min_main, axis.cross(limits.min()));
+
+            let (max_main, max_cross) =
+                axis.pack(max_main, axis.cross(limits.max()));
+
+            let child_limits = Limits::new(
+                Size::new(min_main, min_cross),
+                Size::new(max_main, max_cross),
+            );
+
+            let layout = child.layout(renderer, &child_limits);
+
+            nodes[i] = layout;
+        }
+    }
+
+    let mut main = padding;
+    let mut cross = axis.cross(limits.min());
+
+    for (i, node) in nodes.iter_mut().enumerate() {
+        if i > 0 {
+            main += spacing;
+        }
+
+        let (x, y) = axis.pack(main, padding);
+
+        node.bounds.x = x;
+        node.bounds.y = y;
+
+        let size = node.size();
+
+        main += axis.main(size);
+        cross = cross.max(axis.cross(size));
+    }
+
+    let (width, height) = axis.pack(main, cross);
+    let size = limits.resolve(Size::new(width, height));
+
+    let (padding_x, padding_y) = axis.pack(padding, padding * 2.0);
+
+    Node::with_children(
+        Size::new(size.width + padding_x, size.height + padding_y),
+        nodes,
+    )
+}
diff --git a/native/src/layout/limits.rs b/native/src/layout/limits.rs
index ad105119..e5cee5df 100644
--- a/native/src/layout/limits.rs
+++ b/native/src/layout/limits.rs
@@ -1,6 +1,131 @@
-#[derive(Debug)]
-pub struct Limits {}
+use crate::{Length, Size};
+
+#[derive(Debug, Clone, Copy)]
+pub struct Limits {
+    min: Size,
+    max: Size,
+    fill: Size,
+}
 
 impl Limits {
-    pub const NONE: Limits = Limits {};
+    pub const NONE: Limits = Limits {
+        min: Size::ZERO,
+        max: Size::INFINITY,
+        fill: Size::INFINITY,
+    };
+
+    pub fn new(min: Size, max: Size) -> Limits {
+        Limits {
+            min,
+            max,
+            fill: Size::INFINITY,
+        }
+    }
+
+    pub fn width(mut self, width: Length) -> Limits {
+        match width {
+            Length::Shrink => {
+                self.fill.width = self.min.width;
+            }
+            Length::Fill => {
+                self.fill.width = self.fill.width.min(self.max.width);
+            }
+            Length::Units(units) => {
+                let new_width =
+                    (units as f32).min(self.max.width).max(self.min.width);
+
+                self.min.width = new_width;
+                self.max.width = new_width;
+                self.fill.width = new_width;
+            }
+        }
+
+        self
+    }
+
+    pub fn height(mut self, height: Length) -> Limits {
+        match height {
+            Length::Shrink => {
+                self.fill.height = self.min.height;
+            }
+            Length::Fill => {
+                self.fill.height = self.fill.height.min(self.max.height);
+            }
+            Length::Units(units) => {
+                let new_height =
+                    (units as f32).min(self.max.height).max(self.min.height);
+
+                self.min.height = new_height;
+                self.max.height = new_height;
+                self.fill.height = new_height;
+            }
+        }
+
+        self
+    }
+
+    pub fn min_width(mut self, min_width: u32) -> Limits {
+        self.min.width =
+            self.min.width.max(min_width as f32).min(self.max.width);
+
+        self
+    }
+
+    pub fn max_width(mut self, max_width: u32) -> Limits {
+        self.max.width =
+            self.max.width.min(max_width as f32).max(self.min.width);
+
+        self
+    }
+
+    pub fn max_height(mut self, max_height: u32) -> Limits {
+        self.max.height =
+            self.max.height.min(max_height as f32).max(self.min.height);
+
+        self
+    }
+
+    pub fn resolve(&self, intrinsic_size: Size) -> Size {
+        Size::new(
+            intrinsic_size
+                .width
+                .min(self.max.width)
+                .max(self.fill.width),
+            intrinsic_size
+                .height
+                .min(self.max.height)
+                .max(self.fill.height),
+        )
+    }
+
+    pub fn min(&self) -> Size {
+        self.min
+    }
+
+    pub fn max(&self) -> Size {
+        self.max
+    }
+
+    pub fn pad(&self, padding: f32) -> Limits {
+        self.shrink(Size::new(padding * 2.0, padding * 2.0))
+    }
+
+    pub fn shrink(&self, size: Size) -> Limits {
+        let min = Size::new(
+            (self.min().width - size.width).max(0.0),
+            (self.min().height - size.height).max(0.0),
+        );
+
+        let max = Size::new(
+            (self.max().width - size.width).max(0.0),
+            (self.max().height - size.height).max(0.0),
+        );
+
+        let fill = Size::new(
+            (self.fill.width - size.width).max(0.0),
+            (self.fill.height - size.height).max(0.0),
+        );
+
+        Limits { min, max, fill }
+    }
 }
diff --git a/native/src/layout/node.rs b/native/src/layout/node.rs
new file mode 100644
index 00000000..7537ad3b
--- /dev/null
+++ b/native/src/layout/node.rs
@@ -0,0 +1,37 @@
+use crate::{Rectangle, Size};
+
+#[derive(Debug, Clone, Default)]
+pub struct Node {
+    pub bounds: Rectangle,
+    children: Vec<Node>,
+}
+
+impl Node {
+    pub fn new(size: Size) -> Self {
+        Self::with_children(size, Vec::new())
+    }
+
+    pub fn with_children(size: Size, children: Vec<Node>) -> Self {
+        Node {
+            bounds: Rectangle {
+                x: 0.0,
+                y: 0.0,
+                width: size.width,
+                height: size.height,
+            },
+            children,
+        }
+    }
+
+    pub fn size(&self) -> Size {
+        Size::new(self.bounds.width, self.bounds.height)
+    }
+
+    pub fn bounds(&self) -> Rectangle {
+        self.bounds
+    }
+
+    pub fn children(&self) -> &[Node] {
+        &self.children
+    }
+}