Implement constraining as per DEVICE-ADAPT § 6

Spec: http://dev.w3.org/csswg/css-device-adapt/#constraining

components/style/Cargo.toml

@@ -38,3 +38,4 @@ url = "0.2.16"
 mod_path = "0.1"
 bitflags = "*"
 cssparser = "0.3.1"
+num = "0.1.24"

components/style/lib.rs

@@ -34,6 +34,7 @@ extern crate selectors;
 #[macro_use]
 extern crate lazy_static;
 
+extern crate num;
 extern crate util;
 
 

components/style/viewport.rs

@@ -3,8 +3,12 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use cssparser::{Parser, DeclarationListParser, AtRuleParser, DeclarationParser, ToCss, parse_important};
+use geom::size::{Size2D, TypedSize2D};
+use geom::scale_factor::ScaleFactor;
 use parser::{ParserContext, log_css_error};
+use properties::longhands;
 use stylesheets::Origin;
+use util::geometry::{Au, PagePx, ViewportPx};
 use values::specified::{AllowedNumericType, Length, LengthOrPercentageOrAuto};
 
 use std::ascii::AsciiExt;
@@ -300,3 +304,206 @@ impl<'a, I> ViewportDescriptorDeclarationCascade for I
         cascade(self)
     }
 }
+
+#[derive(Debug, PartialEq)]
+pub struct ViewportConstraints {
+    pub size: TypedSize2D<ViewportPx, f32>,
+
+    pub initial_zoom: ScaleFactor<PagePx, ViewportPx, f32>,
+    pub min_zoom: Option<ScaleFactor<PagePx, ViewportPx, f32>>,
+    pub max_zoom: Option<ScaleFactor<PagePx, ViewportPx, f32>>,
+
+    pub user_zoom: UserZoom,
+    pub orientation: Orientation
+}
+
+impl ToCss for ViewportConstraints {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+        where W: fmt::Write
+    {
+        try!(write!(dest, "@viewport {{"));
+        try!(write!(dest, " width: {}px;", self.size.width.get()));
+        try!(write!(dest, " height: {}px;", self.size.height.get()));
+        try!(write!(dest, " zoom: {};", self.initial_zoom.get()));
+        if let Some(min_zoom) = self.min_zoom {
+            try!(write!(dest, " min-zoom: {};", min_zoom.get()));
+        }
+        if let Some(max_zoom) = self.max_zoom {
+            try!(write!(dest, " max-zoom: {};", max_zoom.get()));
+        }
+        try!(write!(dest, " user-zoom: ")); try!(self.user_zoom.to_css(dest));
+        try!(write!(dest, "; orientation: ")); try!(self.orientation.to_css(dest));
+        write!(dest, "; }}")
+    }
+}
+
+impl ViewportConstraints {
+    pub fn maybe_new(initial_viewport: TypedSize2D<ViewportPx, f32>,
+                     rule: &ViewportRule)
+                     -> Option<ViewportConstraints>
+    {
+        use std::cmp;
+        use num::{Float, ToPrimitive};
+
+        if rule.declarations.is_empty() {
+            return None
+        }
+
+        let mut min_width = None;
+        let mut max_width = None;
+
+        let mut min_height = None;
+        let mut max_height = None;
+
+        let mut initial_zoom = None;
+        let mut min_zoom = None;
+        let mut max_zoom = None;
+
+        let mut user_zoom = UserZoom::Zoom;
+        let mut orientation = Orientation::Auto;
+
+        // collapse the list of declarations into descriptor values
+        for declaration in rule.declarations.iter() {
+            match declaration.descriptor {
+                ViewportDescriptor::MinWidth(value) => min_width = Some(value),
+                ViewportDescriptor::MaxWidth(value) => max_width = Some(value),
+
+                ViewportDescriptor::MinHeight(value) => min_height = Some(value),
+                ViewportDescriptor::MaxHeight(value) => max_height = Some(value),
+
+                ViewportDescriptor::Zoom(value) => initial_zoom = value.to_f32(),
+                ViewportDescriptor::MinZoom(value) => min_zoom = value.to_f32(),
+                ViewportDescriptor::MaxZoom(value) => max_zoom = value.to_f32(),
+
+                ViewportDescriptor::UserZoom(value) => user_zoom = value,
+                ViewportDescriptor::Orientation(value) => orientation = value
+            }
+        }
+
+        // TODO: return `None` if all descriptors are either absent or initial value
+
+        macro_rules! choose {
+            ($op:ident, $opta:expr, $optb:expr) => {
+                match ($opta, $optb) {
+                    (None, None) => None,
+                    (a, None) => a.clone(),
+                    (None, b) => b.clone(),
+                    (a, b) => Some(a.clone().unwrap().$op(b.clone().unwrap())),
+                }
+            }
+        }
+        macro_rules! min {
+            ($opta:expr, $optb:expr) => {
+                choose!(min, $opta, $optb)
+            }
+        }
+        macro_rules! max {
+            ($opta:expr, $optb:expr) => {
+                choose!(max, $opta, $optb)
+            }
+        }
+
+        // DEVICE-ADAPT § 6.2.1 Resolve min-zoom and max-zoom values
+        if min_zoom.is_some() && max_zoom.is_some() {
+            max_zoom = Some(min_zoom.clone().unwrap().max(max_zoom.unwrap()))
+        }
+
+        // DEVICE-ADAPT § 6.2.2 Constrain zoom value to the [min-zoom, max-zoom] range
+        if initial_zoom.is_some() {
+            initial_zoom = max!(min_zoom, min!(max_zoom, initial_zoom));
+        }
+
+        // DEVICE-ADAPT § 6.2.3 Resolve non-auto lengths to pixel lengths
+        //
+        // Note: DEVICE-ADAPT § 5. states that relative length values are
+        // resolved against initial values
+        let initial_viewport = Size2D(Au::from_f32_px(initial_viewport.width.get()),
+                                      Au::from_f32_px(initial_viewport.height.get()));
+
+        macro_rules! to_pixel_length {
+            ($value:ident, $dimension:ident) => {
+                if let Some($value) = $value {
+                    match $value {
+                        LengthOrPercentageOrAuto::Length(ref value) => Some(match value {
+                            &Length::Absolute(length) => length,
+                            &Length::FontRelative(length) => {
+                                let initial_font_size = longhands::font_size::get_initial_value();
+                                length.to_computed_value(initial_font_size, initial_font_size)
+                            }
+                            &Length::ViewportPercentage(length) =>
+                                length.to_computed_value(initial_viewport),
+                            _ => unreachable!()
+                        }),
+                        LengthOrPercentageOrAuto::Percentage(value) => Some(initial_viewport.$dimension.scale_by(value)),
+                        LengthOrPercentageOrAuto::Auto => None,
+                    }
+                } else {
+                    None
+                }
+            }
+        }
+
+        let min_width = to_pixel_length!(min_width, width);
+        let max_width = to_pixel_length!(max_width, width);
+        let min_height = to_pixel_length!(min_height, height);
+        let max_height = to_pixel_length!(max_height, height);
+
+        // DEVICE-ADAPT § 6.2.4 Resolve initial width and height from min/max descriptors
+        macro_rules! resolve {
+            ($min:ident, $max:ident, $initial:expr) => {
+                if $min.is_some() || $max.is_some() {
+                    let max = match $max {
+                        Some(max) => cmp::min(max, $initial),
+                        None => $initial
+                    };
+
+                    Some(match $min {
+                        Some(min) => cmp::max(min, max),
+                        None => max
+                    })
+                } else {
+                    None
+                };
+            }
+        }
+
+        let width = resolve!(min_width, max_width, initial_viewport.width);
+        let height = resolve!(min_height, max_height, initial_viewport.height);
+
+        // DEVICE-ADAPT § 6.2.5 Resolve width value
+        let width = if width.is_none() && height.is_none() {
+             Some(initial_viewport.width)
+        } else {
+            width
+        };
+
+        let width = width.unwrap_or_else(|| match initial_viewport.height {
+            Au(0) => initial_viewport.width,
+            initial_height => {
+                let ratio = initial_viewport.width.to_f32_px() / initial_height.to_f32_px();
+                Au::from_f32_px(height.clone().unwrap().to_f32_px() * ratio)
+            }
+        });
+
+        // DEVICE-ADAPT § 6.2.6 Resolve height value
+        let height = height.unwrap_or_else(|| match initial_viewport.width {
+            Au(0) => initial_viewport.height,
+            initial_width => {
+                let ratio = initial_viewport.height.to_f32_px() / initial_width.to_f32_px();
+                Au::from_f32_px(width.to_f32_px() * ratio)
+            }
+        });
+
+        Some(ViewportConstraints {
+            size: TypedSize2D(width.to_f32_px(), height.to_f32_px()),
+
+            // TODO: compute a zoom factor for 'auto' as suggested by DEVICE-ADAPT § 10.
+            initial_zoom: ScaleFactor::new(initial_zoom.unwrap_or(1.)),
+            min_zoom: min_zoom.map(ScaleFactor::new),
+            max_zoom: max_zoom.map(ScaleFactor::new),
+
+            user_zoom: user_zoom,
+            orientation: orientation
+        })
+    }
+}