First pass at implementing the Flex Layout Algorithm

https://drafts.csswg.org/css-flexbox/#layout-algorithm

components/layout_2020/flexbox/geom.rs

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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! https://drafts.csswg.org/css-flexbox/#box-model
+
+use crate::geom::flow_relative::{Rect, Sides, Vec2};
+use style::properties::longhands::flex_direction::computed_value::T as FlexDirection;
+
+#[derive(Clone, Copy)]
+pub(super) struct FlexRelativeVec2<T> {
+    pub main: T,
+    pub cross: T,
+}
+
+#[derive(Clone, Copy)]
+pub(super) struct FlexRelativeSides<T> {
+    pub cross_start: T,
+    pub main_start: T,
+    pub cross_end: T,
+    pub main_end: T,
+}
+
+pub(super) struct FlexRelativeRect<T> {
+    pub start_corner: FlexRelativeVec2<T>,
+    pub size: FlexRelativeVec2<T>,
+}
+
+impl<T> std::ops::Add for FlexRelativeVec2<T>
+where
+    T: std::ops::Add,
+{
+    type Output = FlexRelativeVec2<T::Output>;
+    fn add(self, rhs: Self) -> Self::Output {
+        FlexRelativeVec2 {
+            main: self.main + rhs.main,
+            cross: self.cross + rhs.cross,
+        }
+    }
+}
+
+impl<T> std::ops::Sub for FlexRelativeVec2<T>
+where
+    T: std::ops::Sub,
+{
+    type Output = FlexRelativeVec2<T::Output>;
+    fn sub(self, rhs: Self) -> Self::Output {
+        FlexRelativeVec2 {
+            main: self.main - rhs.main,
+            cross: self.cross - rhs.cross,
+        }
+    }
+}
+
+impl<T> FlexRelativeSides<T> {
+    pub fn sum_by_axis(self) -> FlexRelativeVec2<T::Output>
+    where
+        T: std::ops::Add,
+    {
+        FlexRelativeVec2 {
+            main: self.main_start + self.main_end,
+            cross: self.cross_start + self.cross_end,
+        }
+    }
+}
+
+/// One of the two bits set by the `flex-direction` property
+/// (The other is "forward" v.s. reverse.)
+#[derive(Clone, Copy, PartialEq)]
+pub(super) enum FlexAxis {
+    /// The main axis is the inline axis of the container (not necessarily of flex items!),
+    /// cross is block.
+    Row,
+    /// The main axis is the block axis, cross is inline.
+    Column,
+}
+
+/// Which flow-relative sides map to the main-start and cross-start sides, respectively.
+/// See https://drafts.csswg.org/css-flexbox/#box-model
+#[derive(Clone, Copy)]
+pub(super) enum MainStartCrossStart {
+    InlineStartBlockStart,
+    InlineStartBlockEnd,
+    BlockStartInlineStart,
+    BlockStartInlineEnd,
+    InlineEndBlockStart,
+    InlineEndBlockEnd,
+    BlockEndInlineStart,
+    BlockEndInlineEnd,
+}
+
+impl FlexAxis {
+    pub fn from(flex_direction: FlexDirection) -> Self {
+        match flex_direction {
+            FlexDirection::Row | FlexDirection::RowReverse => FlexAxis::Row,
+            FlexDirection::Column | FlexDirection::ColumnReverse => FlexAxis::Column,
+        }
+    }
+
+    pub fn vec2_to_flex_relative<T>(self, flow_relative: Vec2<T>) -> FlexRelativeVec2<T> {
+        let Vec2 { inline, block } = flow_relative;
+        match self {
+            FlexAxis::Row => FlexRelativeVec2 {
+                main: inline,
+                cross: block,
+            },
+            FlexAxis::Column => FlexRelativeVec2 {
+                main: block,
+                cross: inline,
+            },
+        }
+    }
+
+    pub fn vec2_to_flow_relative<T>(self, flex_relative: FlexRelativeVec2<T>) -> Vec2<T> {
+        let FlexRelativeVec2 { main, cross } = flex_relative;
+        match self {
+            FlexAxis::Row => Vec2 {
+                inline: main,
+                block: cross,
+            },
+            FlexAxis::Column => Vec2 {
+                block: main,
+                inline: cross,
+            },
+        }
+    }
+}
+
+macro_rules! sides_mapping_methods {
+    (
+        $(
+            $variant: path => {
+                $( $flex_relative_side: ident <=> $flow_relative_side: ident, )+
+            },
+        )+
+    ) => {
+        pub fn sides_to_flex_relative<T>(self, flow_relative: Sides<T>) -> FlexRelativeSides<T> {
+            match self {
+                $(
+                    $variant => FlexRelativeSides {
+                        $( $flex_relative_side: flow_relative.$flow_relative_side, )+
+                    },
+                )+
+            }
+        }
+
+        pub fn sides_to_flow_relative<T>(self, flex_relative: FlexRelativeSides<T>) -> Sides<T> {
+            match self {
+                $(
+                    $variant => Sides {
+                        $( $flow_relative_side: flex_relative.$flex_relative_side, )+
+                    },
+                )+
+            }
+        }
+    }
+}
+
+impl MainStartCrossStart {
+    pub fn from(flex_direction: FlexDirection, flex_wrap_reverse: bool) -> Self {
+        match (flex_direction, flex_wrap_reverse) {
+            // See definition of each keyword in
+            // https://drafts.csswg.org/css-flexbox/#flex-direction-property and
+            // https://drafts.csswg.org/css-flexbox/#flex-wrap-property,
+            // or the tables (though they map to physical rather than flow-relative) at
+            // https://drafts.csswg.org/css-flexbox/#axis-mapping
+            (FlexDirection::Row, true) => MainStartCrossStart::InlineStartBlockEnd,
+            (FlexDirection::Row, false) => MainStartCrossStart::InlineStartBlockStart,
+            (FlexDirection::Column, true) => MainStartCrossStart::BlockStartInlineEnd,
+            (FlexDirection::Column, false) => MainStartCrossStart::BlockStartInlineStart,
+            (FlexDirection::RowReverse, true) => MainStartCrossStart::InlineEndBlockEnd,
+            (FlexDirection::RowReverse, false) => MainStartCrossStart::InlineEndBlockStart,
+            (FlexDirection::ColumnReverse, true) => MainStartCrossStart::BlockEndInlineEnd,
+            (FlexDirection::ColumnReverse, false) => MainStartCrossStart::BlockEndInlineStart,
+        }
+    }
+
+    sides_mapping_methods! {
+        MainStartCrossStart::InlineStartBlockStart => {
+            main_start <=> inline_start,
+            cross_start <=> block_start,
+            main_end <=> inline_end,
+            cross_end <=> block_end,
+        },
+        MainStartCrossStart::InlineStartBlockEnd => {
+            main_start <=> inline_start,
+            cross_start <=> block_end,
+            main_end <=> inline_end,
+            cross_end <=> block_start,
+        },
+        MainStartCrossStart::BlockStartInlineStart => {
+            main_start <=> block_start,
+            cross_start <=> inline_start,
+            main_end <=> block_end,
+            cross_end <=> inline_end,
+        },
+        MainStartCrossStart::BlockStartInlineEnd => {
+            main_start <=> block_start,
+            cross_start <=> inline_end,
+            main_end <=> block_end,
+            cross_end <=> inline_start,
+        },
+        MainStartCrossStart::InlineEndBlockStart => {
+            main_start <=> inline_end,
+            cross_start <=> block_start,
+            main_end <=> inline_start,
+            cross_end <=> block_end,
+        },
+        MainStartCrossStart::InlineEndBlockEnd => {
+            main_start <=> inline_end,
+            cross_start <=> block_end,
+            main_end <=> inline_start,
+            cross_end <=> block_start,
+        },
+        MainStartCrossStart::BlockEndInlineStart => {
+            main_start <=> block_end,
+            cross_start <=> inline_start,
+            main_end <=> block_start,
+            cross_end <=> inline_end,
+        },
+        MainStartCrossStart::BlockEndInlineEnd => {
+            main_start <=> block_end,
+            cross_start <=> inline_end,
+            main_end <=> block_start,
+            cross_end <=> inline_start,
+        },
+    }
+}
+
+/// The start corner coordinates in both the input rectangle and output rectangle
+/// are relative to some “base rectangle” whose size is passed here.
+pub(super) fn rect_to_flow_relative<T>(
+    flex_axis: FlexAxis,
+    main_start_cross_start_sides_are: MainStartCrossStart,
+    base_rect_size: FlexRelativeVec2<T>,
+    rect: FlexRelativeRect<T>,
+) -> Rect<T>
+where
+    T: Copy + std::ops::Add<Output = T> + std::ops::Sub<Output = T>,
+{
+    // First, convert from (start corner, size) to offsets from the edges of the base rectangle
+
+    let end_corner_position = rect.start_corner + rect.size;
+    let end_corner_offsets = base_rect_size - end_corner_position;
+    // No-ops, but hopefully clarifies to human readers:
+    let start_corner_position = rect.start_corner;
+    let start_corner_offsets = start_corner_position;
+
+    // Then, convert to flow-relative using methods above
+    let flow_relative_offsets =
+        main_start_cross_start_sides_are.sides_to_flow_relative(FlexRelativeSides {
+            main_start: start_corner_offsets.main,
+            cross_start: start_corner_offsets.cross,
+            main_end: end_corner_offsets.main,
+            cross_end: end_corner_offsets.cross,
+        });
+    let flow_relative_base_rect_size = flex_axis.vec2_to_flow_relative(base_rect_size);
+
+    // Finally, convert back to (start corner, size)
+    let start_corner = Vec2 {
+        inline: flow_relative_offsets.inline_start,
+        block: flow_relative_offsets.block_start,
+    };
+    let end_corner_position = Vec2 {
+        inline: flow_relative_base_rect_size.inline - flow_relative_offsets.inline_end,
+        block: flow_relative_base_rect_size.block - flow_relative_offsets.block_end,
+    };
+    let size = &end_corner_position - &start_corner;
+    Rect { start_corner, size }
+}