Speed-up html-escaping using jetscii

```text
$ cargo bench --bench escape

Before the PR:           [3.6464 µs 3.6512 µs 3.6564 µs]
Impl. without `jetscii`: [3.4837 µs 3.4899 µs 3.4968 µs]
Impl with `jetscii`:     [2.0264 µs 2.0335 µs 2.0418 µs]
```

Until portable SIMD gets stabilized, I don't think we can do much for
non-X86 platforms. And even after it is stabilized, I guess any
optimizations should be implemented upstream in memchr and/or jetscii.

rinja/Cargo.toml

@@ -42,6 +42,9 @@ percent-encoding = { version = "2.1.0", optional = true }
 serde = { version = "1.0", optional = true }
 serde_json = { version = "1.0", optional = true }
 
+[target.'cfg(any(target_arch = "x86", target_arch = "x86_64"))'.dependencies]
+jetscii = "0.5.3"
+
 [dev-dependencies]
 criterion = "0.5"
 

rinja/src/html.rs

@@ -1,71 +1,203 @@
-use std::fmt;
-use std::num::NonZeroU8;
+use std::{fmt, str};
 
+#[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
 #[allow(unused)]
 pub(crate) fn write_escaped_str(mut fmt: impl fmt::Write, string: &str) -> fmt::Result {
-    let mut escaped_buf = *b"&#__;";
+    // Even though [`jetscii`] ships a generic implementation for unsupported platforms,
+    // it is not well optimized for this case. This implementation should work well enough in
+    // the meantime, until portable SIMD gets stabilized.
+
+    // Instead of testing the platform, we could test the CPU features. But given that the needed
+    // instruction set SSE 4.2 was introduced in 2008, that it has an 99.61 % availability rate
+    // in Steam's June 2024 hardware survey, and is a prerequisite to run Windows 11, I don't
+    // think we need to care.
+
+    let mut escaped_buf = ESCAPED_BUF_INIT;
     let mut last = 0;
 
     for (index, byte) in string.bytes().enumerate() {
         let escaped = match byte {
             MIN_CHAR..=MAX_CHAR => TABLE.lookup[(byte - MIN_CHAR) as usize],
-            _ => None,
+            _ => 0,
         };
-        if let Some(escaped) = escaped {
-            escaped_buf[2] = escaped[0].get();
-            escaped_buf[3] = escaped[1].get();
-            fmt.write_str(&string[last..index])?;
-            fmt.write_str(unsafe { std::str::from_utf8_unchecked(escaped_buf.as_slice()) })?;
+        if escaped != 0 {
+            [escaped_buf[2], escaped_buf[3]] = escaped.to_ne_bytes();
+            write_str_if_nonempty(&mut fmt, &string[last..index])?;
+            // SAFETY: the content of `escaped_buf` is pure ASCII
+            fmt.write_str(unsafe {
+                std::str::from_utf8_unchecked(&escaped_buf[..ESCAPED_BUF_LEN])
+            })?;
             last = index + 1;
         }
     }
-    fmt.write_str(&string[last..])
+    write_str_if_nonempty(&mut fmt, &string[last..])
+}
+
+#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+#[allow(unused)]
+pub(crate) fn write_escaped_str(mut fmt: impl fmt::Write, mut string: &str) -> fmt::Result {
+    let jetscii = jetscii::bytes!(b'"', b'&', b'\'', b'<', b'>');
+
+    let mut escaped_buf = ESCAPED_BUF_INIT;
+    loop {
+        if string.is_empty() {
+            return Ok(());
+        }
+
+        let found = if string.len() >= 16 {
+            // Only strings of at least 16 bytes can be escaped using SSE instructions.
+            match jetscii.find(string.as_bytes()) {
+                Some(index) => {
+                    let escaped = TABLE.lookup[(string.as_bytes()[index] - MIN_CHAR) as usize];
+                    Some((index, escaped))
+                }
+                None => None,
+            }
+        } else {
+            // The small-string fallback of [`jetscii`] is quite slow, so we roll our own
+            // implementation.
+            string.as_bytes().iter().find_map(|byte: &u8| {
+                let escaped = get_escaped(*byte)?;
+                let index = (byte as *const u8 as usize) - (string.as_ptr() as usize);
+                Some((index, escaped))
+            })
+        };
+        let Some((index, escaped)) = found else {
+            return fmt.write_str(string);
+        };
+
+        [escaped_buf[2], escaped_buf[3]] = escaped.to_ne_bytes();
+
+        // SAFETY: index points at an ASCII char in `string`
+        let front;
+        (front, string) = unsafe {
+            (
+                string.get_unchecked(..index),
+                string.get_unchecked(index + 1..),
+            )
+        };
+
+        write_str_if_nonempty(&mut fmt, front)?;
+        // SAFETY: the content of `escaped_buf` is pure ASCII
+        fmt.write_str(unsafe { str::from_utf8_unchecked(&escaped_buf[..ESCAPED_BUF_LEN]) })?;
+    }
 }
 
 #[allow(unused)]
 pub(crate) fn write_escaped_char(mut fmt: impl fmt::Write, c: char) -> fmt::Result {
-    fmt.write_str(match (c.is_ascii(), c as u8) {
-        (true, b'"') => "&#34;",
-        (true, b'&') => "&#38;",
-        (true, b'\'') => "&#39;",
-        (true, b'<') => "&#60;",
-        (true, b'>') => "&#62;",
-        _ => return fmt.write_char(c),
-    })
+    if !c.is_ascii() {
+        fmt.write_char(c)
+    } else if let Some(escaped) = get_escaped(c as u8) {
+        let mut escaped_buf = ESCAPED_BUF_INIT;
+        [escaped_buf[2], escaped_buf[3]] = escaped.to_ne_bytes();
+        // SAFETY: the content of `escaped_buf` is pure ASCII
+        fmt.write_str(unsafe { str::from_utf8_unchecked(&escaped_buf[..ESCAPED_BUF_LEN]) })
+    } else {
+        // RATIONALE: `write_char(c)` gets optimized if it is known that `c.is_ascii()`
+        fmt.write_char(c)
+    }
 }
 
-const MIN_CHAR: u8 = b'"';
-const MAX_CHAR: u8 = b'>';
+#[inline(always)]
+fn get_escaped(byte: u8) -> Option<u16> {
+    let c = byte.wrapping_sub(MIN_CHAR);
+    if (c < u32::BITS as u8) && (BITS & (1 << c as u32) != 0) {
+        Some(TABLE.lookup[c as usize])
+    } else {
+        None
+    }
+}
 
-struct Table {
-    _align: [usize; 0],
-    lookup: [Option<[NonZeroU8; 2]>; (MAX_CHAR - MIN_CHAR + 1) as usize],
+#[inline(always)]
+fn write_str_if_nonempty(output: &mut impl fmt::Write, input: &str) -> fmt::Result {
+    if !input.is_empty() {
+        output.write_str(input)
+    } else {
+        Ok(())
+    }
 }
 
-const TABLE: Table = {
-    const fn n(c: u8) -> Option<[NonZeroU8; 2]> {
-        assert!(MIN_CHAR <= c && c <= MAX_CHAR);
+/// List of characters that need HTML escaping, not necessarily in ordinal order.
+/// Filling the [`TABLE`] and [`BITS`] constants will ensure that the range of lowest to hightest
+/// codepoint wont exceed [`u32::BITS`] (=32) items.
+const CHARS: &[u8] = br#""&'<>"#;
 
-        let n0 = match NonZeroU8::new(c / 10 + b'0') {
-            Some(n) => n,
-            None => panic!(),
-        };
-        let n1 = match NonZeroU8::new(c % 10 + b'0') {
-            Some(n) => n,
-            None => panic!(),
-        };
-        Some([n0, n1])
+/// The character with the smallest codepoint that needs HTML escaping.
+/// Both [`TABLE`] and [`BITS`] start at this value instead of `0`.
+const MIN_CHAR: u8 = {
+    let mut v = u8::MAX;
+    let mut i = 0;
+    while i < CHARS.len() {
+        if v > CHARS[i] {
+            v = CHARS[i];
+        }
+        i += 1;
+    }
+    v
+};
+
+#[allow(unused)]
+const MAX_CHAR: u8 = {
+    let mut v = u8::MIN;
+    let mut i = 0;
+    while i < CHARS.len() {
+        if v < CHARS[i] {
+            v = CHARS[i];
+        }
+        i += 1;
     }
+    v
+};
+
+struct Table {
+    _align: [usize; 0],
+    lookup: [u16; u32::BITS as usize],
+}
 
+/// For characters that need HTML escaping, the codepoint formatted as decimal digits,
+/// otherwise `b"\0\0"`. Starting at [`MIN_CHAR`].
+const TABLE: Table = {
     let mut table = Table {
         _align: [],
-        lookup: [None; (MAX_CHAR - MIN_CHAR + 1) as usize],
+        lookup: [0; u32::BITS as usize],
     };
-
-    table.lookup[(b'"' - MIN_CHAR) as usize] = n(b'"');
-    table.lookup[(b'&' - MIN_CHAR) as usize] = n(b'&');
-    table.lookup[(b'\'' - MIN_CHAR) as usize] = n(b'\'');
-    table.lookup[(b'<' - MIN_CHAR) as usize] = n(b'<');
-    table.lookup[(b'>' - MIN_CHAR) as usize] = n(b'>');
+    let mut i = 0;
+    while i < CHARS.len() {
+        let c = CHARS[i];
+        let h = c / 10 + b'0';
+        let l = c % 10 + b'0';
+        table.lookup[(c - MIN_CHAR) as usize] = u16::from_ne_bytes([h, l]);
+        i += 1;
+    }
     table
 };
+
+/// A bitset of the characters that need escaping, starting at [`MIN_CHAR`]
+const BITS: u32 = {
+    let mut i = 0;
+    let mut bits = 0;
+    while i < CHARS.len() {
+        bits |= 1 << (CHARS[i] - MIN_CHAR) as u32;
+        i += 1;
+    }
+    bits
+};
+
+// RATIONALE: llvm generates better code if the buffer is register sized
+const ESCAPED_BUF_INIT: [u8; 8] = *b"&#__;\0\0\0";
+const ESCAPED_BUF_LEN: usize = b"&#__;".len();
+
+#[test]
+fn simple() {
+    let mut buf = String::new();
+    write_escaped_str(&mut buf, "<script>").unwrap();
+    assert_eq!(buf, "&#60;script&#62;");
+
+    buf.clear();
+    write_escaped_str(&mut buf, "s<crip>t").unwrap();
+    assert_eq!(buf, "s&#60;crip&#62;t");
+
+    buf.clear();
+    write_escaped_str(&mut buf, "s<cripcripcripcripcripcripcripcripcripcrip>t").unwrap();
+    assert_eq!(buf, "s&#60;cripcripcripcripcripcripcripcripcripcrip&#62;t");
+}

rinja_derive/Cargo.toml

@@ -36,6 +36,9 @@ quote = "1"
 serde = { version = "1.0", optional = true, features = ["derive"] }
 syn = "2.0.3"
 
+[target.'cfg(any(target_arch = "x86", target_arch = "x86_64"))'.dependencies]
+jetscii = "0.5.3"
+
 [dev-dependencies]
 console = "0.15.8"
 similar = "2.6.0"

rinja_derive_standalone/Cargo.toml

@@ -35,6 +35,9 @@ quote = "1"
 serde = { version = "1.0", optional = true, features = ["derive"] }
 syn = "2"
 
+[target.'cfg(any(target_arch = "x86", target_arch = "x86_64"))'.dependencies]
+jetscii = "0.5.3"
+
 [dev-dependencies]
 criterion = "0.5"