Improve Color::hex performance (bevyengine#6940)

# Objective

Improve `Color::hex` performance

#### Bench

```bash
running 2 tests
test bench_color_hex_after  ... bench:           4 ns/iter (+/- 0)
test bench_color_hex_before ... bench:          14 ns/iter (+/- 0)
```

## Solution

Use `const fn` decode hex value.

---

## Changelog

Rename

```rust
HexColorError::Hex(FromHexError) -> HexColorError::Char(char)
```

Author: wyhaya

crates/bevy_render/Cargo.toml

@@ -62,7 +62,6 @@ thread_local = "1.1"
 thiserror = "1.0"
 futures-lite = "1.4.0"
 anyhow = "1.0"
-hex = "0.4.2"
 hexasphere = "8.0"
 parking_lot = "0.12.1"
 regex = "1.5"

crates/bevy_render/src/color/mod.rs

@@ -260,37 +260,29 @@ impl Color {
         let hex = hex.as_ref();
         let hex = hex.strip_prefix('#').unwrap_or(hex);
 
-        // RGB
-        if hex.len() == 3 {
-            let mut data = [0; 6];
-            for (i, ch) in hex.chars().enumerate() {
-                data[i * 2] = ch as u8;
-                data[i * 2 + 1] = ch as u8;
+        match *hex.as_bytes() {
+            // RGB
+            [r, g, b] => {
+                let [r, g, b, ..] = decode_hex([r, r, g, g, b, b])?;
+                Ok(Color::rgb_u8(r, g, b))
             }
-            return decode_rgb(&data);
-        }
-
-        // RGBA
-        if hex.len() == 4 {
-            let mut data = [0; 8];
-            for (i, ch) in hex.chars().enumerate() {
-                data[i * 2] = ch as u8;
-                data[i * 2 + 1] = ch as u8;
+            // RGBA
+            [r, g, b, a] => {
+                let [r, g, b, a, ..] = decode_hex([r, r, g, g, b, b, a, a])?;
+                Ok(Color::rgba_u8(r, g, b, a))
             }
-            return decode_rgba(&data);
-        }
-
-        // RRGGBB
-        if hex.len() == 6 {
-            return decode_rgb(hex.as_bytes());
-        }
-
-        // RRGGBBAA
-        if hex.len() == 8 {
-            return decode_rgba(hex.as_bytes());
+            // RRGGBB
+            [r1, r2, g1, g2, b1, b2] => {
+                let [r, g, b, ..] = decode_hex([r1, r2, g1, g2, b1, b2])?;
+                Ok(Color::rgb_u8(r, g, b))
+            }
+            // RRGGBBAA
+            [r1, r2, g1, g2, b1, b2, a1, a2] => {
+                let [r, g, b, a, ..] = decode_hex([r1, r2, g1, g2, b1, b2, a1, a2])?;
+                Ok(Color::rgba_u8(r, g, b, a))
+            }
+            _ => Err(HexColorError::Length),
         }
-
-        Err(HexColorError::Length)
     }
 
     /// New `Color` from sRGB colorspace.
@@ -1337,38 +1329,49 @@ impl encase::private::CreateFrom for Color {
 
 impl encase::ShaderSize for Color {}
 
-#[derive(Debug, Error)]
+#[derive(Debug, Error, PartialEq, Eq)]
 pub enum HexColorError {
     #[error("Unexpected length of hex string")]
     Length,
-    #[error("Error parsing hex value")]
-    Hex(#[from] hex::FromHexError),
+    #[error("Invalid hex char")]
+    Char(char),
 }
 
-fn decode_rgb(data: &[u8]) -> Result<Color, HexColorError> {
-    let mut buf = [0; 3];
-    match hex::decode_to_slice(data, &mut buf) {
-        Ok(_) => {
-            let r = buf[0] as f32 / 255.0;
-            let g = buf[1] as f32 / 255.0;
-            let b = buf[2] as f32 / 255.0;
-            Ok(Color::rgb(r, g, b))
-        }
-        Err(err) => Err(HexColorError::Hex(err)),
+/// Converts hex bytes to an array of RGB\[A\] components
+///
+/// # Example
+/// For RGB: *b"ffffff" -> [255, 255, 255, ..]
+/// For RGBA: *b"E2E2E2FF" -> [226, 226, 226, 255, ..]
+const fn decode_hex<const N: usize>(mut bytes: [u8; N]) -> Result<[u8; N], HexColorError> {
+    let mut i = 0;
+    while i < bytes.len() {
+        // Convert single hex digit to u8
+        let val = match hex_value(bytes[i]) {
+            Ok(val) => val,
+            Err(byte) => return Err(HexColorError::Char(byte as char)),
+        };
+        bytes[i] = val;
+        i += 1;
     }
+    // Modify the original bytes to give an `N / 2` length result
+    i = 0;
+    while i < bytes.len() / 2 {
+        // Convert pairs of u8 to R/G/B/A
+        // e.g `ff` -> [102, 102] -> [15, 15] = 255
+        bytes[i] = bytes[i * 2] * 16 + bytes[i * 2 + 1];
+        i += 1;
+    }
+    Ok(bytes)
 }
 
-fn decode_rgba(data: &[u8]) -> Result<Color, HexColorError> {
-    let mut buf = [0; 4];
-    match hex::decode_to_slice(data, &mut buf) {
-        Ok(_) => {
-            let r = buf[0] as f32 / 255.0;
-            let g = buf[1] as f32 / 255.0;
-            let b = buf[2] as f32 / 255.0;
-            let a = buf[3] as f32 / 255.0;
-            Ok(Color::rgba(r, g, b, a))
-        }
-        Err(err) => Err(HexColorError::Hex(err)),
+/// Parse a single hex digit (a-f/A-F/0-9) as a `u8`
+const fn hex_value(b: u8) -> Result<u8, u8> {
+    match b {
+        b'0'..=b'9' => Ok(b - b'0'),
+        b'A'..=b'F' => Ok(b - b'A' + 10),
+        b'a'..=b'f' => Ok(b - b'a' + 10),
+        // Wrong hex digit
+        _ => Err(b),
     }
 }
 
@@ -1378,29 +1381,21 @@ mod tests {
 
     #[test]
     fn hex_color() {
-        assert_eq!(Color::hex("FFF").unwrap(), Color::rgb(1.0, 1.0, 1.0));
-        assert_eq!(Color::hex("000").unwrap(), Color::rgb(0.0, 0.0, 0.0));
-        assert!(Color::hex("---").is_err());
-
-        assert_eq!(Color::hex("FFFF").unwrap(), Color::rgba(1.0, 1.0, 1.0, 1.0));
-        assert_eq!(Color::hex("0000").unwrap(), Color::rgba(0.0, 0.0, 0.0, 0.0));
-        assert!(Color::hex("----").is_err());
-
-        assert_eq!(Color::hex("FFFFFF").unwrap(), Color::rgb(1.0, 1.0, 1.0));
-        assert_eq!(Color::hex("000000").unwrap(), Color::rgb(0.0, 0.0, 0.0));
-        assert!(Color::hex("------").is_err());
-
-        assert_eq!(
-            Color::hex("FFFFFFFF").unwrap(),
-            Color::rgba(1.0, 1.0, 1.0, 1.0)
-        );
-        assert_eq!(
-            Color::hex("00000000").unwrap(),
-            Color::rgba(0.0, 0.0, 0.0, 0.0)
-        );
-        assert!(Color::hex("--------").is_err());
-
-        assert!(Color::hex("1234567890").is_err());
+        assert_eq!(Color::hex("FFF"), Ok(Color::WHITE));
+        assert_eq!(Color::hex("FFFF"), Ok(Color::WHITE));
+        assert_eq!(Color::hex("FFFFFF"), Ok(Color::WHITE));
+        assert_eq!(Color::hex("FFFFFFFF"), Ok(Color::WHITE));
+        assert_eq!(Color::hex("000"), Ok(Color::BLACK));
+        assert_eq!(Color::hex("000F"), Ok(Color::BLACK));
+        assert_eq!(Color::hex("000000"), Ok(Color::BLACK));
+        assert_eq!(Color::hex("000000FF"), Ok(Color::BLACK));
+        assert_eq!(Color::hex("03a9f4"), Ok(Color::rgb_u8(3, 169, 244)));
+        assert_eq!(Color::hex("yy"), Err(HexColorError::Length));
+        assert_eq!(Color::hex("yyy"), Err(HexColorError::Char('y')));
+        assert_eq!(Color::hex("#f2a"), Ok(Color::rgb_u8(255, 34, 170)));
+        assert_eq!(Color::hex("#e23030"), Ok(Color::rgb_u8(226, 48, 48)));
+        assert_eq!(Color::hex("#ff"), Err(HexColorError::Length));
+        assert_eq!(Color::hex("##fff"), Err(HexColorError::Char('#')));
     }
 
     #[test]