Add a generic PositionValue struct

components/style/values/generics/mod.rs

@@ -14,6 +14,7 @@ use super::computed::{Context, ToComputedValue};
 pub use self::basic_shape::serialize_radius_values;
 
 pub mod basic_shape;
+pub mod position;
 
 #[derive(Clone, PartialEq, Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]

components/style/values/generics/position.rs

@@ -0,0 +1,114 @@
+/* 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/. */
+
+//! Generic types for CSS handling of specified and computed values of
+//! [`position`](https://drafts.csswg.org/css-backgrounds-3/#position)
+
+use cssparser::Parser;
+use parser::{Parse, ParserContext};
+use std::fmt;
+use style_traits::ToCss;
+use values::HasViewportPercentage;
+use values::specified::{LengthOrPercentage, Percentage};
+
+define_css_keyword_enum!{ Keyword:
+    "center" => Center,
+    "left" => Left,
+    "right" => Right,
+    "top" => Top,
+    "bottom" => Bottom,
+    "x-start" => XStart,
+    "x-end" => XEnd,
+    "y-start" => YStart,
+    "y-end" => YEnd
+}
+
+impl From<Keyword> for LengthOrPercentage {
+    fn from(val: Keyword) -> LengthOrPercentage {
+        match val {
+            Keyword::Center => LengthOrPercentage::Percentage(Percentage(0.5)),
+            Keyword::Left | Keyword::Top => LengthOrPercentage::Percentage(Percentage(0.0)),
+            Keyword::Right | Keyword::Bottom => LengthOrPercentage::Percentage(Percentage(1.0)),
+            // FIXME(canaltinova): Support logical keywords
+            Keyword::XStart | Keyword::YStart => LengthOrPercentage::Percentage(Percentage(0.0)),
+            Keyword::XEnd | Keyword::YEnd => LengthOrPercentage::Percentage(Percentage(1.0)),
+        }
+    }
+}
+
+#[derive(Debug, Clone, PartialEq)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+/// A generic type for representing horizontal or vertical `<position>` value.
+pub struct PositionValue<L> {
+    /// Even though this is generic, it's always a `<length-percentage>` value.
+    pub position: Option<L>,
+    /// A position keyword.
+    pub keyword: Option<Keyword>,
+}
+
+impl<L: HasViewportPercentage> HasViewportPercentage for PositionValue<L> {
+    #[inline]
+    fn has_viewport_percentage(&self) -> bool {
+        self.position.as_ref().map_or(false, |pos| pos.has_viewport_percentage())
+    }
+}
+
+impl<L: Parse> PositionValue<L> {
+    /// Internal parsing function which (after parsing) checks the keyword with the
+    /// given function.
+    pub fn parse_internal<F>(context: &ParserContext, input: &mut Parser,
+                             mut is_allowed_keyword: F) -> Result<PositionValue<L>, ()>
+        where F: FnMut(Keyword) -> bool
+    {
+        let (mut pos, mut keyword) = (None, None);
+        for _ in 0..2 {
+            if let Ok(l) = input.try(|i| L::parse(context, i)) {
+                if pos.is_some() {
+                    return Err(())
+                }
+
+                pos = Some(l);
+            }
+
+            if let Ok(k) = input.try(Keyword::parse) {
+                if keyword.is_some() || !is_allowed_keyword(k) {
+                    return Err(())
+                }
+
+                keyword = Some(k);
+            }
+        }
+
+        if pos.is_some() {
+            if let Some(Keyword::Center) = keyword {
+                return Err(())      // "center" and <length> is not allowed
+            }
+        } else if keyword.is_none() {
+            return Err(())      // at least one value is necessary
+        }
+
+        Ok(PositionValue {
+            position: pos,
+            keyword: keyword,
+        })
+    }
+}
+
+impl<L: ToCss> ToCss for PositionValue<L> {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
+        if let Some(keyword) = self.keyword {
+            keyword.to_css(dest)?;
+        }
+
+        if let Some(ref position) = self.position {
+            if self.keyword.is_some() {
+                dest.write_str(" ")?;
+            }
+
+            position.to_css(dest)?;
+        }
+
+        Ok(())
+    }
+}