Create own file for background calculations in layout

Move display_list_builder.rs and webrender_helpers.rs
along with the new file to components/layout/display_list/

Remove apparently unused IdType enum.
Only variant used was OverflowClip.

See #19676

components/layout/display_list/background.rs

@@ -0,0 +1,549 @@
+/* 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 http://mozilla.org/MPL/2.0/. */
+
+//! Calculations for CSS images and CSS backgrounds.
+
+#![deny(unsafe_code)]
+
+// FIXME(rust-lang/rust#26264): Remove GenericEndingShape and GenericGradientItem.
+
+use app_units::Au;
+use display_list::ToGfxColor;
+use euclid::{Point2D, Size2D, Vector2D};
+use gfx::display_list;
+use model::MaybeAuto;
+use style::values::computed::{Angle, GradientItem};
+use style::values::computed::{LengthOrPercentage, LengthOrPercentageOrAuto, Percentage};
+use style::values::computed::Position;
+use style::values::computed::image::{EndingShape, LineDirection};
+use style::values::generics::background::BackgroundSize;
+use style::values::generics::image::{Circle, Ellipse, ShapeExtent};
+use style::values::generics::image::EndingShape as GenericEndingShape;
+use style::values::generics::image::GradientItem as GenericGradientItem;
+use style::values::specified::background::RepeatKeyword;
+use style::values::specified::position::{X, Y};
+use webrender_api::GradientStop;
+
+/// A helper data structure for gradients.
+#[derive(Clone, Copy)]
+struct StopRun {
+    start_offset: f32,
+    end_offset: f32,
+    start_index: usize,
+    stop_count: usize,
+}
+
+/// For a given area and an image compute how big the
+/// image should be displayed on the background.
+pub fn compute_background_image_size(
+    bg_size: BackgroundSize<LengthOrPercentageOrAuto>,
+    bounds_size: Size2D<Au>,
+    intrinsic_size: Option<Size2D<Au>>,
+) -> Size2D<Au> {
+    match intrinsic_size {
+        None => match bg_size {
+            BackgroundSize::Cover | BackgroundSize::Contain => bounds_size,
+            BackgroundSize::Explicit { width, height } => Size2D::new(
+                MaybeAuto::from_style(width, bounds_size.width)
+                    .specified_or_default(bounds_size.width),
+                MaybeAuto::from_style(height, bounds_size.height)
+                    .specified_or_default(bounds_size.height),
+            ),
+        },
+        Some(own_size) => {
+            // If `image_aspect_ratio` < `bounds_aspect_ratio`, the image is tall; otherwise, it is
+            // wide.
+            let image_aspect_ratio = own_size.width.to_f32_px() / own_size.height.to_f32_px();
+            let bounds_aspect_ratio =
+                bounds_size.width.to_f32_px() / bounds_size.height.to_f32_px();
+            match (bg_size, image_aspect_ratio < bounds_aspect_ratio) {
+                (BackgroundSize::Contain, false) | (BackgroundSize::Cover, true) => Size2D::new(
+                    bounds_size.width,
+                    bounds_size.width.scale_by(image_aspect_ratio.recip()),
+                ),
+                (BackgroundSize::Contain, true) | (BackgroundSize::Cover, false) => Size2D::new(
+                    bounds_size.height.scale_by(image_aspect_ratio),
+                    bounds_size.height,
+                ),
+                (
+                    BackgroundSize::Explicit {
+                        width,
+                        height: LengthOrPercentageOrAuto::Auto,
+                    },
+                    _,
+                ) => {
+                    let width = MaybeAuto::from_style(width, bounds_size.width)
+                        .specified_or_default(own_size.width);
+                    Size2D::new(width, width.scale_by(image_aspect_ratio.recip()))
+                },
+                (
+                    BackgroundSize::Explicit {
+                        width: LengthOrPercentageOrAuto::Auto,
+                        height,
+                    },
+                    _,
+                ) => {
+                    let height = MaybeAuto::from_style(height, bounds_size.height)
+                        .specified_or_default(own_size.height);
+                    Size2D::new(height.scale_by(image_aspect_ratio), height)
+                },
+                (BackgroundSize::Explicit { width, height }, _) => Size2D::new(
+                    MaybeAuto::from_style(width, bounds_size.width)
+                        .specified_or_default(own_size.width),
+                    MaybeAuto::from_style(height, bounds_size.height)
+                        .specified_or_default(own_size.height),
+                ),
+            }
+        },
+    }
+}
+
+fn tile_image_round(
+    position: &mut Au,
+    size: &mut Au,
+    absolute_anchor_origin: Au,
+    image_size: &mut Au,
+) {
+    if *size == Au(0) || *image_size == Au(0) {
+        *position = Au(0);
+        *size = Au(0);
+        return;
+    }
+
+    let number_of_tiles = (size.to_f32_px() / image_size.to_f32_px()).round().max(1.0);
+    *image_size = *size / (number_of_tiles as i32);
+    tile_image(position, size, absolute_anchor_origin, *image_size);
+}
+
+fn tile_image_spaced(
+    position: &mut Au,
+    size: &mut Au,
+    tile_spacing: &mut Au,
+    absolute_anchor_origin: Au,
+    image_size: Au,
+) {
+    if *size == Au(0) || image_size == Au(0) {
+        *position = Au(0);
+        *size = Au(0);
+        *tile_spacing = Au(0);
+        return;
+    }
+
+    // Per the spec, if the space available is not enough for two images, just tile as
+    // normal but only display a single tile.
+    if image_size * 2 >= *size {
+        tile_image(position, size, absolute_anchor_origin, image_size);
+        *tile_spacing = Au(0);
+        *size = image_size;
+        return;
+    }
+
+    // Take the box size, remove room for two tiles on the edges, and then calculate how many
+    // other tiles fit in between them.
+    let size_remaining = *size - (image_size * 2);
+    let num_middle_tiles = (size_remaining.to_f32_px() / image_size.to_f32_px()).floor() as i32;
+
+    // Allocate the remaining space as padding between tiles. background-position is ignored
+    // as per the spec, so the position is just the box origin. We are also ignoring
+    // background-attachment here, which seems unspecced when combined with
+    // background-repeat: space.
+    let space_for_middle_tiles = image_size * num_middle_tiles;
+    *tile_spacing = (size_remaining - space_for_middle_tiles) / (num_middle_tiles + 1);
+}
+
+/// Tile an image
+fn tile_image(position: &mut Au, size: &mut Au, absolute_anchor_origin: Au, image_size: Au) {
+    // Avoid division by zero below!
+    // Images with a zero width or height are not displayed.
+    // Therefore the positions do not matter and can be left unchanged.
+    // NOTE: A possible optimization is not to build
+    // display items in this case at all.
+    if image_size == Au(0) {
+        return;
+    }
+
+    let delta_pixels = absolute_anchor_origin - *position;
+    let image_size_px = image_size.to_f32_px();
+    let tile_count = ((delta_pixels.to_f32_px() + image_size_px - 1.0) / image_size_px).floor();
+    let offset = image_size * (tile_count as i32);
+    let new_position = absolute_anchor_origin - offset;
+    *size = *position - new_position + *size;
+    *position = new_position;
+}
+
+/// For either the x or the y axis ajust various values to account for tiling.
+///
+/// This is done separately for both axes because the repeat keywords may differ.
+pub fn tile_image_axis(
+    repeat: RepeatKeyword,
+    position: &mut Au,
+    size: &mut Au,
+    tile_size: &mut Au,
+    tile_spacing: &mut Au,
+    offset: Au,
+    clip_origin: Au,
+    clip_size: Au,
+) {
+    let absolute_anchor_origin = *position + offset;
+    match repeat {
+        RepeatKeyword::NoRepeat => {
+            *position += offset;
+            *size = *tile_size;
+        },
+        RepeatKeyword::Repeat => {
+            *position = clip_origin;
+            *size = clip_size;
+            tile_image(position, size, absolute_anchor_origin, *tile_size);
+        },
+        RepeatKeyword::Space => {
+            tile_image_spaced(
+                position,
+                size,
+                tile_spacing,
+                absolute_anchor_origin,
+                *tile_size,
+            );
+            let combined_tile_size = *tile_size + *tile_spacing;
+            *position = clip_origin;
+            *size = clip_size;
+            tile_image(position, size, absolute_anchor_origin, combined_tile_size);
+        },
+        RepeatKeyword::Round => {
+            tile_image_round(position, size, absolute_anchor_origin, tile_size);
+            *position = clip_origin;
+            *size = clip_size;
+            tile_image(position, size, absolute_anchor_origin, *tile_size);
+        },
+    }
+}
+
+/// Determines the radius of a circle if it was not explictly provided.
+/// <https://drafts.csswg.org/css-images-3/#typedef-size>
+fn convert_circle_size_keyword(
+    keyword: ShapeExtent,
+    size: &Size2D<Au>,
+    center: &Point2D<Au>,
+) -> Size2D<Au> {
+    let radius = match keyword {
+        ShapeExtent::ClosestSide | ShapeExtent::Contain => {
+            let dist = get_distance_to_sides(size, center, ::std::cmp::min);
+            ::std::cmp::min(dist.width, dist.height)
+        },
+        ShapeExtent::FarthestSide => {
+            let dist = get_distance_to_sides(size, center, ::std::cmp::max);
+            ::std::cmp::max(dist.width, dist.height)
+        },
+        ShapeExtent::ClosestCorner => get_distance_to_corner(size, center, ::std::cmp::min),
+        ShapeExtent::FarthestCorner | ShapeExtent::Cover => {
+            get_distance_to_corner(size, center, ::std::cmp::max)
+        },
+    };
+    Size2D::new(radius, radius)
+}
+
+/// Returns the radius for an ellipse with the same ratio as if it was matched to the sides.
+fn get_ellipse_radius<F>(size: &Size2D<Au>, center: &Point2D<Au>, cmp: F) -> Size2D<Au>
+where
+    F: Fn(Au, Au) -> Au,
+{
+    let dist = get_distance_to_sides(size, center, cmp);
+    Size2D::new(
+        dist.width.scale_by(::std::f32::consts::FRAC_1_SQRT_2 * 2.0),
+        dist.height
+            .scale_by(::std::f32::consts::FRAC_1_SQRT_2 * 2.0),
+    )
+}
+
+/// Determines the radius of an ellipse if it was not explictly provided.
+/// <https://drafts.csswg.org/css-images-3/#typedef-size>
+fn convert_ellipse_size_keyword(
+    keyword: ShapeExtent,
+    size: &Size2D<Au>,
+    center: &Point2D<Au>,
+) -> Size2D<Au> {
+    match keyword {
+        ShapeExtent::ClosestSide | ShapeExtent::Contain => {
+            get_distance_to_sides(size, center, ::std::cmp::min)
+        },
+        ShapeExtent::FarthestSide => get_distance_to_sides(size, center, ::std::cmp::max),
+        ShapeExtent::ClosestCorner => get_ellipse_radius(size, center, ::std::cmp::min),
+        ShapeExtent::FarthestCorner | ShapeExtent::Cover => {
+            get_ellipse_radius(size, center, ::std::cmp::max)
+        },
+    }
+}
+
+fn convert_gradient_stops(gradient_items: &[GradientItem], total_length: Au) -> Vec<GradientStop> {
+    // Determine the position of each stop per CSS-IMAGES § 3.4.
+
+    // Only keep the color stops, discard the color interpolation hints.
+    let mut stop_items = gradient_items
+        .iter()
+        .filter_map(|item| match *item {
+            GenericGradientItem::ColorStop(ref stop) => Some(*stop),
+            _ => None,
+        })
+        .collect::<Vec<_>>();
+
+    assert!(stop_items.len() >= 2);
+
+    // Run the algorithm from
+    // https://drafts.csswg.org/css-images-3/#color-stop-syntax
+
+    // Step 1:
+    // If the first color stop does not have a position, set its position to 0%.
+    {
+        let first = stop_items.first_mut().unwrap();
+        if first.position.is_none() {
+            first.position = Some(LengthOrPercentage::Percentage(Percentage(0.0)));
+        }
+    }
+    // If the last color stop does not have a position, set its position to 100%.
+    {
+        let last = stop_items.last_mut().unwrap();
+        if last.position.is_none() {
+            last.position = Some(LengthOrPercentage::Percentage(Percentage(1.0)));
+        }
+    }
+
+    // Step 2: Move any stops placed before earlier stops to the
+    // same position as the preceding stop.
+    let mut last_stop_position = stop_items.first().unwrap().position.unwrap();
+    for stop in stop_items.iter_mut().skip(1) {
+        if let Some(pos) = stop.position {
+            if position_to_offset(last_stop_position, total_length) >
+                position_to_offset(pos, total_length)
+            {
+                stop.position = Some(last_stop_position);
+            }
+            last_stop_position = stop.position.unwrap();
+        }
+    }
+
+    // Step 3: Evenly space stops without position.
+    // Note: Remove the + 2 if fix_gradient_stops is changed.
+    let mut stops = Vec::with_capacity(stop_items.len() + 2);
+    let mut stop_run = None;
+    for (i, stop) in stop_items.iter().enumerate() {
+        let offset = match stop.position {
+            None => {
+                if stop_run.is_none() {
+                    // Initialize a new stop run.
+                    // `unwrap()` here should never fail because this is the beginning of
+                    // a stop run, which is always bounded by a length or percentage.
+                    let start_offset =
+                        position_to_offset(stop_items[i - 1].position.unwrap(), total_length);
+                    // `unwrap()` here should never fail because this is the end of
+                    // a stop run, which is always bounded by a length or percentage.
+                    let (end_index, end_stop) = stop_items[(i + 1)..]
+                        .iter()
+                        .enumerate()
+                        .find(|&(_, ref stop)| stop.position.is_some())
+                        .unwrap();
+                    let end_offset = position_to_offset(end_stop.position.unwrap(), total_length);
+                    stop_run = Some(StopRun {
+                        start_offset: start_offset,
+                        end_offset: end_offset,
+                        start_index: i - 1,
+                        stop_count: end_index,
+                    })
+                }
+
+                let stop_run = stop_run.unwrap();
+                let stop_run_length = stop_run.end_offset - stop_run.start_offset;
+                stop_run.start_offset +
+                    stop_run_length * (i - stop_run.start_index) as f32 /
+                        ((2 + stop_run.stop_count) as f32)
+            },
+            Some(position) => {
+                stop_run = None;
+                position_to_offset(position, total_length)
+            },
+        };
+        assert!(offset.is_finite());
+        stops.push(GradientStop {
+            offset: offset,
+            color: stop.color.to_gfx_color(),
+        })
+    }
+    stops
+}
+
+pub fn convert_linear_gradient(
+    size: Size2D<Au>,
+    stops: &[GradientItem],
+    direction: LineDirection,
+    repeating: bool,
+) -> display_list::Gradient {
+    let angle = match direction {
+        LineDirection::Angle(angle) => angle.radians(),
+        LineDirection::Horizontal(x) => match x {
+            X::Left => Angle::Deg(270.).radians(),
+            X::Right => Angle::Deg(90.).radians(),
+        },
+        LineDirection::Vertical(y) => match y {
+            Y::Top => Angle::Deg(0.).radians(),
+            Y::Bottom => Angle::Deg(180.).radians(),
+        },
+        LineDirection::Corner(horizontal, vertical) => {
+            // This the angle for one of the diagonals of the box. Our angle
+            // will either be this one, this one + PI, or one of the other
+            // two perpendicular angles.
+            let atan = (size.height.to_f32_px() / size.width.to_f32_px()).atan();
+            match (horizontal, vertical) {
+                (X::Right, Y::Bottom) => ::std::f32::consts::PI - atan,
+                (X::Left, Y::Bottom) => ::std::f32::consts::PI + atan,
+                (X::Right, Y::Top) => atan,
+                (X::Left, Y::Top) => -atan,
+            }
+        },
+    };
+
+    // Get correct gradient line length, based on:
+    // https://drafts.csswg.org/css-images-3/#linear-gradients
+    let dir = Point2D::new(angle.sin(), -angle.cos());
+
+    let line_length =
+        (dir.x * size.width.to_f32_px()).abs() + (dir.y * size.height.to_f32_px()).abs();
+
+    let inv_dir_length = 1.0 / (dir.x * dir.x + dir.y * dir.y).sqrt();
+
+    // This is the vector between the center and the ending point; i.e. half
+    // of the distance between the starting point and the ending point.
+    let delta = Vector2D::new(
+        Au::from_f32_px(dir.x * inv_dir_length * line_length / 2.0),
+        Au::from_f32_px(dir.y * inv_dir_length * line_length / 2.0),
+    );
+
+    // This is the length of the gradient line.
+    let length = Au::from_f32_px((delta.x.to_f32_px() * 2.0).hypot(delta.y.to_f32_px() * 2.0));
+
+    let mut stops = convert_gradient_stops(stops, length);
+
+    // Only clamped gradients need to be fixed because in repeating gradients
+    // there is no "first" or "last" stop because they repeat infinitly in
+    // both directions, so the rendering is always correct.
+    if !repeating {
+        fix_gradient_stops(&mut stops);
+    }
+
+    let center = Point2D::new(size.width / 2, size.height / 2);
+
+    display_list::Gradient {
+        start_point: center - delta,
+        end_point: center + delta,
+        stops: stops,
+        repeating: repeating,
+    }
+}
+
+pub fn convert_radial_gradient(
+    size: Size2D<Au>,
+    stops: &[GradientItem],
+    shape: EndingShape,
+    center: Position,
+    repeating: bool,
+) -> display_list::RadialGradient {
+    let center = Point2D::new(
+        center.horizontal.to_used_value(size.width),
+        center.vertical.to_used_value(size.height),
+    );
+    let radius = match shape {
+        GenericEndingShape::Circle(Circle::Radius(length)) => {
+            let length = Au::from(length);
+            Size2D::new(length, length)
+        },
+        GenericEndingShape::Circle(Circle::Extent(extent)) => {
+            convert_circle_size_keyword(extent, &size, &center)
+        },
+        GenericEndingShape::Ellipse(Ellipse::Radii(x, y)) => {
+            Size2D::new(x.to_used_value(size.width), y.to_used_value(size.height))
+        },
+        GenericEndingShape::Ellipse(Ellipse::Extent(extent)) => {
+            convert_ellipse_size_keyword(extent, &size, &center)
+        },
+    };
+
+    let mut stops = convert_gradient_stops(stops, radius.width);
+    // Repeating gradients have no last stops that can be ignored. So
+    // fixup is not necessary but may actually break the gradient.
+    if !repeating {
+        fix_gradient_stops(&mut stops);
+    }
+
+    display_list::RadialGradient {
+        center: center,
+        radius: radius,
+        stops: stops,
+        repeating: repeating,
+    }
+}
+
+#[inline]
+/// Duplicate the first and last stops if necessary.
+///
+/// Explanation by pyfisch:
+/// If the last stop is at the same position as the previous stop the
+/// last color is ignored by webrender. This differs from the spec
+/// (I think so). The  implementations of Chrome and Firefox seem
+/// to have the same problem but work fine if the position of the last
+/// stop is smaller than 100%. (Otherwise they ignore the last stop.)
+///
+/// Similarly the first stop is duplicated if it is not placed
+/// at the start of the virtual gradient ray.
+fn fix_gradient_stops(stops: &mut Vec<GradientStop>) {
+    if stops.first().unwrap().offset > 0.0 {
+        let color = stops.first().unwrap().color;
+        stops.insert(
+            0,
+            GradientStop {
+                offset: 0.0,
+                color: color,
+            },
+        )
+    }
+    if stops.last().unwrap().offset < 1.0 {
+        let color = stops.last().unwrap().color;
+        stops.push(GradientStop {
+            offset: 1.0,
+            color: color,
+        })
+    }
+}
+
+/// Returns the the distance to the nearest or farthest corner depending on the comperator.
+fn get_distance_to_corner<F>(size: &Size2D<Au>, center: &Point2D<Au>, cmp: F) -> Au
+where
+    F: Fn(Au, Au) -> Au,
+{
+    let dist = get_distance_to_sides(size, center, cmp);
+    Au::from_f32_px(dist.width.to_f32_px().hypot(dist.height.to_f32_px()))
+}
+
+/// Returns the distance to the nearest or farthest sides depending on the comparator.
+///
+/// The first return value is horizontal distance the second vertical distance.
+fn get_distance_to_sides<F>(size: &Size2D<Au>, center: &Point2D<Au>, cmp: F) -> Size2D<Au>
+where
+    F: Fn(Au, Au) -> Au,
+{
+    let top_side = center.y;
+    let right_side = size.width - center.x;
+    let bottom_side = size.height - center.y;
+    let left_side = center.x;
+    Size2D::new(cmp(left_side, right_side), cmp(top_side, bottom_side))
+}
+
+fn position_to_offset(position: LengthOrPercentage, total_length: Au) -> f32 {
+    if total_length == Au(0) {
+        return 0.0;
+    }
+    match position {
+        LengthOrPercentage::Length(l) => l.to_i32_au() as f32 / total_length.0 as f32,
+        LengthOrPercentage::Percentage(percentage) => percentage.0 as f32,
+        LengthOrPercentage::Calc(calc) => {
+            calc.to_used_value(Some(total_length)).unwrap().0 as f32 / total_length.0 as f32
+        },
+    }
+}