style: Make resolutions more like the rest of the CSS values.

Bug: 1460655
Reviewed-by: xidorn
MozReview-Commit-ID: 3Gt8VX1KhjC

components/style/values/specified/resolution.rs

@@ -0,0 +1,73 @@
+/* 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/. */
+
+//! Resolution values:
+//!
+//! https://drafts.csswg.org/css-values/#resolution
+
+use cssparser::{Parser, Token};
+use parser::{Parse, ParserContext};
+use style_traits::{ParseError, StyleParseErrorKind};
+use values::CSSFloat;
+
+/// A specified resolution.
+#[derive(Clone, Debug, PartialEq, ToCss)]
+pub enum Resolution {
+    /// Dots per inch.
+    #[css(dimension)]
+    Dpi(CSSFloat),
+    /// Dots per pixel.
+    #[css(dimension)]
+    Dppx(CSSFloat),
+    /// Dots per centimeter.
+    #[css(dimension)]
+    Dpcm(CSSFloat),
+}
+
+impl Resolution {
+    /// Convert this resolution value to dppx units.
+    pub fn to_dppx(&self) -> CSSFloat {
+        match *self {
+            Resolution::Dppx(f) => f,
+            _ => self.to_dpi() / 96.0,
+        }
+    }
+
+    /// Convert this resolution value to dpi units.
+    pub fn to_dpi(&self) -> CSSFloat {
+        match *self {
+            Resolution::Dpi(f) => f,
+            Resolution::Dppx(f) => f * 96.0,
+            Resolution::Dpcm(f) => f * 2.54,
+        }
+    }
+}
+
+impl Parse for Resolution {
+    fn parse<'i, 't>(
+        _: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        let location = input.current_source_location();
+        let (value, unit) = match *input.next()? {
+            Token::Dimension {
+                value, ref unit, ..
+            } => (value, unit),
+            ref t => return Err(location.new_unexpected_token_error(t.clone())),
+        };
+
+        if value <= 0. {
+            return Err(location.new_custom_error(StyleParseErrorKind::UnspecifiedError));
+        }
+
+        match_ignore_ascii_case! { &unit,
+            "dpi" => Ok(Resolution::Dpi(value)),
+            "dppx" => Ok(Resolution::Dppx(value)),
+            "dpcm" => Ok(Resolution::Dpcm(value)),
+            _ => Err(location.new_custom_error(
+                StyleParseErrorKind::UnexpectedDimension(unit.clone())
+            )),
+        }
+    }
+}