Implement conversion between hex-string and Color32

Closes emilk#3466

crates/ecolor/src/hex_color_runtime.rs

@@ -0,0 +1,224 @@
+//! Convert colors to and from the hex-color string format at runtime
+//!
+//! Supports the 3, 4, 6, and 8-digit formats, according to the specification in
+//! <https://drafts.csswg.org/css-color-4/#hex-color>
+
+use std::{fmt::Display, str::FromStr};
+
+use crate::Color32;
+
+#[repr(C)]
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
+/// A wrapper around Color32 that converts to and from a hex-color string
+///
+/// Implements [`Display`] and [`FromStr`] to convert to and from the hex string.
+pub enum HexColor {
+    /// 3 hexadecimal digits, one for each of the r, g, b channels
+    Hex3(Color32),
+
+    /// 4 hexadecimal digits, one for each of the r, g, b, a channels
+    Hex4(Color32),
+
+    /// 6 hexadecimal digits, two for each of the r, g, b channels
+    Hex6(Color32),
+
+    /// 8 hexadecimal digits, one for each of the r, g, b, a channels
+    Hex8(Color32),
+}
+
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub enum ParseHexColorError {
+    MissingHash,
+    InvalidLength,
+    InvalidInt(std::num::ParseIntError),
+}
+
+impl FromStr for HexColor {
+    type Err = ParseHexColorError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        s.strip_prefix('#')
+            .ok_or(ParseHexColorError::MissingHash)
+            .and_then(Self::from_str_without_hash)
+    }
+}
+
+impl Display for HexColor {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            HexColor::Hex3(color) => {
+                let [r, g, b, _] = color.to_array().map(|u| u >> 4);
+                f.write_fmt(format_args!("#{r:x}{g:x}{b:x}"))
+            }
+            HexColor::Hex4(color) => {
+                let [r, g, b, a] = color.to_array().map(|u| u >> 4);
+                f.write_fmt(format_args!("#{r:x}{g:x}{b:x}{a:x}"))
+            }
+            HexColor::Hex6(color) => {
+                let [r, g, b, _] = color.to_array();
+                let u = u32::from_be_bytes([0, r, g, b]);
+                f.write_fmt(format_args!("#{u:06x}"))
+            }
+            HexColor::Hex8(color) => {
+                let [r, g, b, a] = color.to_array();
+                let u = u32::from_be_bytes([r, g, b, a]);
+                f.write_fmt(format_args!("#{u:08x}"))
+            }
+        }
+    }
+}
+
+impl HexColor {
+    /// Retrieves the inner [`Color32`]
+    #[inline]
+    pub fn color(&self) -> Color32 {
+        match self {
+            HexColor::Hex3(color)
+            | HexColor::Hex4(color)
+            | HexColor::Hex6(color)
+            | HexColor::Hex8(color) => *color,
+        }
+    }
+
+    /// Parses a string as a hex color without the leading `#` character
+    ///
+    /// # Errors
+    /// Returns an error if the length of the string does not correspond to one of the standard
+    /// formats (3, 4, 6, or 8), or if it contains non-hex characters.
+    #[inline]
+    pub fn from_str_without_hash(s: &str) -> Result<Self, ParseHexColorError> {
+        match s.len() {
+            3 => {
+                let [r, gb] = u16::from_str_radix(s, 16)
+                    .map_err(ParseHexColorError::InvalidInt)?
+                    .to_be_bytes();
+                let [r, g, b] = [r, gb >> 4, gb & 0x0f].map(|u| u << 4 | u);
+                Ok(HexColor::Hex3(Color32::from_rgb(r, g, b)))
+            }
+            4 => {
+                let [rg, ba] = u16::from_str_radix(s, 16)
+                    .map_err(ParseHexColorError::InvalidInt)?
+                    .to_be_bytes();
+                let [r, g, b, a] = [rg >> 4, rg & 0x0f, ba >> 4, ba & 0x0f].map(|u| u << 4 | u);
+                Ok(HexColor::Hex4(Color32::from_rgba_premultiplied(r, g, b, a)))
+            }
+            6 => {
+                let [_, r, g, b] = u32::from_str_radix(s, 16)
+                    .map_err(ParseHexColorError::InvalidInt)?
+                    .to_be_bytes();
+                Ok(HexColor::Hex6(Color32::from_rgb(r, g, b)))
+            }
+            8 => {
+                let [r, g, b, a] = u32::from_str_radix(s, 16)
+                    .map_err(ParseHexColorError::InvalidInt)?
+                    .to_be_bytes();
+                Ok(HexColor::Hex8(Color32::from_rgba_premultiplied(r, g, b, a)))
+            }
+            _ => Err(ParseHexColorError::InvalidLength)?,
+        }
+    }
+}
+
+impl Color32 {
+    /// Parses a color from a hex string
+    ///
+    /// Supports the 3, 4, 6, and 8-digit formats.
+    ///
+    /// # Example
+    /// ```rust
+    /// use ecolor::Color32;
+    /// assert_eq!(Ok(Color32::RED), Color32::from_hex_string("#ff0000"));
+    /// assert_eq!(Ok(Color32::GREEN), Color32::from_hex_string("#00ff00ff"));
+    /// assert_eq!(Ok(Color32::BLUE), Color32::from_hex_string("#00f"));
+    /// assert_eq!(Ok(Color32::TRANSPARENT), Color32::from_hex_string("#0000"));
+    /// ```
+    ///
+    /// # Errors
+    /// Returns an error if the string doesn't start with the hash `#` character, if the remaining
+    /// length does not correspond to one of the standard formats (3, 4, 6, or 8), if it contains
+    /// non-hex characters.
+    pub fn from_hex_string(hex: &str) -> Result<Self, ParseHexColorError> {
+        HexColor::from_str(hex).map(|h| h.color())
+    }
+
+    /// Formats the color as a hex string
+    ///
+    /// # Example
+    /// ```rust
+    /// use ecolor::Color32;
+    /// assert_eq!(Color32::RED.to_hex_string(), "#ff0000ff");
+    /// assert_eq!(Color32::GREEN.to_hex_string(), "#00ff00ff");
+    /// assert_eq!(Color32::BLUE.to_hex_string(), "#0000ffff");
+    /// assert_eq!(Color32::TRANSPARENT.to_hex_string(), "#00000000");
+    /// ```
+    ///
+    /// Uses the 8-digit format, as that is the only format that is lossless.
+    /// For other formats, see [`HexColor`].
+    #[inline]
+    pub fn to_hex_string(&self) -> String {
+        HexColor::Hex8(*self).to_string()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn hex_string_formats() {
+        use Color32 as C;
+        use HexColor as H;
+        let cases = [
+            (H::Hex3(C::RED), "#f00"),
+            (H::Hex4(C::RED), "#f00f"),
+            (H::Hex6(C::RED), "#ff0000"),
+            (H::Hex8(C::RED), "#ff0000ff"),
+            (H::Hex3(C::GREEN), "#0f0"),
+            (H::Hex4(C::GREEN), "#0f0f"),
+            (H::Hex6(C::GREEN), "#00ff00"),
+            (H::Hex8(C::GREEN), "#00ff00ff"),
+            (H::Hex3(C::BLUE), "#00f"),
+            (H::Hex4(C::BLUE), "#00ff"),
+            (H::Hex6(C::BLUE), "#0000ff"),
+            (H::Hex8(C::BLUE), "#0000ffff"),
+            (H::Hex3(C::WHITE), "#fff"),
+            (H::Hex4(C::WHITE), "#ffff"),
+            (H::Hex6(C::WHITE), "#ffffff"),
+            (H::Hex8(C::WHITE), "#ffffffff"),
+            (H::Hex3(C::BLACK), "#000"),
+            (H::Hex4(C::BLACK), "#000f"),
+            (H::Hex6(C::BLACK), "#000000"),
+            (H::Hex8(C::BLACK), "#000000ff"),
+            (H::Hex4(C::TRANSPARENT), "#0000"),
+            (H::Hex8(C::TRANSPARENT), "#00000000"),
+        ];
+        for (color, string) in cases {
+            assert_eq!(color.to_string(), string, "{color:?} <=> {string}");
+            assert_eq!(
+                H::from_str(string).unwrap(),
+                color,
+                "{color:?} <=> {string}"
+            );
+        }
+    }
+
+    #[test]
+    fn hex_string_round_trip() {
+        use Color32 as C;
+        let cases = [
+            C::from_rgba_premultiplied(10, 20, 30, 0),
+            C::from_rgba_premultiplied(10, 20, 30, 40),
+            C::from_rgba_premultiplied(10, 20, 30, 255),
+            C::from_rgba_premultiplied(0, 20, 30, 0),
+            C::from_rgba_premultiplied(10, 0, 30, 40),
+            C::from_rgba_premultiplied(10, 20, 0, 255),
+        ];
+        for color in cases {
+            assert_eq!(
+                C::from_hex_string(color.to_hex_string().as_str()),
+                Ok(color)
+            );
+        }
+    }
+}

crates/ecolor/src/lib.rs

@@ -30,6 +30,9 @@ mod hex_color_macro;
 mod rgba;
 pub use rgba::*;
 
+mod hex_color_runtime;
+pub use hex_color_runtime::*;
+
 // ----------------------------------------------------------------------------
 // Color conversion: