Re-use Html escaping code to implement JSON escaping

Cargo.toml

@@ -6,6 +6,6 @@ members = [
     "testing",
     "testing-alloc",
     "testing-no-std",
-    "testing-renamed"
+    "testing-renamed",
 ]
 resolver = "2"

rinja/Cargo.toml

@@ -57,7 +57,7 @@ blocks = ["rinja_derive?/blocks"]
 code-in-doc = ["rinja_derive?/code-in-doc"]
 config = ["rinja_derive?/config"]
 derive = ["rinja_derive"]
-serde_json = ["rinja_derive?/serde_json", "dep:serde", "dep:serde_json"]
+serde_json = ["std", "rinja_derive?/serde_json", "dep:serde", "dep:serde_json"]
 std = [
     "alloc",
     "rinja_derive?/std",

rinja/src/ascii_str.rs

@@ -114,6 +114,19 @@ impl AsciiChar {
             Self::new(ALPHABET[d as usize % ALPHABET.len()]),
         ]
     }
+
+    #[inline]
+    pub const fn two_hex_digits(d: u32) -> [Self; 2] {
+        const ALPHABET: &[u8; 16] = b"0123456789abcdef";
+
+        if d >= ALPHABET.len().pow(2) as u32 {
+            panic!();
+        }
+        [
+            Self::new(ALPHABET[d as usize / ALPHABET.len()]),
+            Self::new(ALPHABET[d as usize % ALPHABET.len()]),
+        ]
+    }
 }
 
 mod _ascii_char {

rinja/src/filters/json.rs

@@ -4,9 +4,10 @@ use std::pin::Pin;
 use std::{fmt, io, str};
 
 use serde::Serialize;
-use serde_json::ser::{PrettyFormatter, Serializer, to_writer};
+use serde_json::ser::{CompactFormatter, PrettyFormatter, Serializer};
 
 use super::FastWritable;
+use crate::ascii_str::{AsciiChar, AsciiStr};
 
 /// Serialize to JSON (requires `json` feature)
 ///
@@ -187,9 +188,8 @@ where
 }
 
 impl<S: Serialize> FastWritable for ToJson<S> {
-    #[inline]
     fn write_into<W: fmt::Write + ?Sized>(&self, f: &mut W) -> crate::Result<()> {
-        fmt_json(f, &self.value)
+        serialize(f, &self.value, CompactFormatter)
     }
 }
 
@@ -201,9 +201,12 @@ impl<S: Serialize> fmt::Display for ToJson<S> {
 }
 
 impl<S: Serialize, I: AsIndent> FastWritable for ToJsonPretty<S, I> {
-    #[inline]
     fn write_into<W: fmt::Write + ?Sized>(&self, f: &mut W) -> crate::Result<()> {
-        fmt_json_pretty(f, &self.value, self.indent.as_indent())
+        serialize(
+            f,
+            &self.value,
+            PrettyFormatter::with_indent(self.indent.as_indent().as_bytes()),
+        )
     }
 }
 
@@ -214,59 +217,152 @@ impl<S: Serialize, I: AsIndent> fmt::Display for ToJsonPretty<S, I> {
     }
 }
 
-fn fmt_json<S: Serialize, W: fmt::Write + ?Sized>(dest: &mut W, value: &S) -> crate::Result<()> {
-    Ok(to_writer(JsonWriter(dest), value)?)
-}
+#[inline]
+fn serialize<S, W, F>(dest: &mut W, value: &S, formatter: F) -> Result<(), crate::Error>
+where
+    S: Serialize + ?Sized,
+    W: fmt::Write + ?Sized,
+    F: serde_json::ser::Formatter,
+{
+    /// The struct must only ever be used with the output of `serde_json`.
+    /// `serde_json` only produces UTF-8 strings in its `io::Write::write()` calls,
+    /// and `<JsonWriter as io::Write>` depends on this invariant.
+    struct JsonWriter<'a, W: fmt::Write + ?Sized>(&'a mut W);
 
-fn fmt_json_pretty<S: Serialize, W: fmt::Write + ?Sized>(
-    dest: &mut W,
-    value: &S,
-    indent: &str,
-) -> crate::Result<()> {
-    let formatter = PrettyFormatter::with_indent(indent.as_bytes());
-    let mut serializer = Serializer::with_formatter(JsonWriter(dest), formatter);
-    Ok(value.serialize(&mut serializer)?)
-}
-
-struct JsonWriter<'a, W: fmt::Write + ?Sized>(&'a mut W);
-
-impl<W: fmt::Write + ?Sized> io::Write for JsonWriter<'_, W> {
+    impl<W: fmt::Write + ?Sized> io::Write for JsonWriter<'_, W> {
+        /// Invariant: must be passed valid UTF-8 slices
         #[inline]
         fn write(&mut self, bytes: &[u8]) -> io::Result<usize> {
             self.write_all(bytes)?;
             Ok(bytes.len())
         }
 
-    #[inline]
+        /// Invariant: must be passed valid UTF-8 slices
         fn write_all(&mut self, bytes: &[u8]) -> io::Result<()> {
-        write(self.0, bytes).map_err(|err| io::Error::new(io::ErrorKind::InvalidData, err))
+            // SAFETY: `serde_json` only writes valid strings
+            let string = unsafe { std::str::from_utf8_unchecked(bytes) };
+            write_escaped_str(&mut *self.0, string)
+                .map_err(|err| io::Error::new(io::ErrorKind::InvalidData, err))
         }
 
         #[inline]
         fn flush(&mut self) -> io::Result<()> {
             Ok(())
         }
-}
+    }
 
-fn write<W: fmt::Write + ?Sized>(f: &mut W, bytes: &[u8]) -> fmt::Result {
+    /// Invariant: no character that needs escaping is multi-byte character when encoded in UTF-8;
+    /// that is true for characters in ASCII range.
+    #[inline]
+    fn write_escaped_str(dest: &mut (impl fmt::Write + ?Sized), src: &str) -> fmt::Result {
+        // This implementation reads one byte after another.
+        // It's not very fast, but should work well enough until portable SIMD gets stabilized.
+
+        let mut escaped_buf = ESCAPED_BUF_INIT;
         let mut last = 0;
-    for (index, byte) in bytes.iter().enumerate() {
-        let escaped = match byte {
-            b'&' => Some(br"\u0026"),
-            b'\'' => Some(br"\u0027"),
-            b'<' => Some(br"\u003c"),
-            b'>' => Some(br"\u003e"),
-            _ => None,
-        };
-        if let Some(escaped) = escaped {
-            f.write_str(unsafe { str::from_utf8_unchecked(&bytes[last..index]) })?;
-            f.write_str(unsafe { str::from_utf8_unchecked(escaped) })?;
+
+        for (index, byte) in src.bytes().enumerate() {
+            if let Some(escaped) = get_escaped(byte) {
+                [escaped_buf[4], escaped_buf[5]] = escaped;
+                write_str_if_nonempty(dest, &src[last..index])?;
+                dest.write_str(AsciiStr::from_slice(&escaped_buf[..ESCAPED_BUF_LEN]))?;
                 last = index + 1;
             }
         }
-    f.write_str(unsafe { str::from_utf8_unchecked(&bytes[last..]) })
+        write_str_if_nonempty(dest, &src[last..])
+    }
+
+    let mut serializer = Serializer::with_formatter(JsonWriter(dest), formatter);
+    Ok(value.serialize(&mut serializer)?)
 }
 
+/// Returns the decimal representation of the codepoint if the character needs HTML escaping.
+#[inline]
+fn get_escaped(byte: u8) -> Option<[AsciiChar; 2]> {
+    const _: () = assert!(CHAR_RANGE < 32);
+
+    if let MIN_CHAR..=MAX_CHAR = byte {
+        if (1u32 << (byte - MIN_CHAR)) & BITS != 0 {
+            return Some(TABLE.0[(byte - MIN_CHAR) as usize]);
+        }
+    }
+    None
+}
+
+#[inline(always)]
+fn write_str_if_nonempty(output: &mut (impl fmt::Write + ?Sized), input: &str) -> fmt::Result {
+    if !input.is_empty() {
+        output.write_str(input)
+    } else {
+        Ok(())
+    }
+}
+
+/// List of characters that need HTML escaping, not necessarily in ordinal order.
+const CHARS: &[u8] = br#"&'<>"#;
+
+/// The character with the lowest codepoint that needs HTML escaping.
+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
+};
+
+/// The character with the highest codepoint that needs HTML escaping.
+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
+};
+
+const BITS: u32 = {
+    let mut bits = 0;
+    let mut i = 0;
+    while i < CHARS.len() {
+        bits |= 1 << (CHARS[i] - MIN_CHAR);
+        i += 1;
+    }
+    bits
+};
+
+/// Number of codepoints between the lowest and highest character that needs escaping, incl.
+const CHAR_RANGE: usize = (MAX_CHAR - MIN_CHAR + 1) as usize;
+
+#[repr(align(64))]
+struct Table([[AsciiChar; 2]; CHAR_RANGE]);
+
+/// For characters that need HTML escaping, the codepoint is formatted as decimal digits,
+/// otherwise `b"\0\0"`. Starting at [`MIN_CHAR`].
+const TABLE: &Table = &{
+    let mut table = Table([UNESCAPED; CHAR_RANGE]);
+    let mut i = 0;
+    while i < CHARS.len() {
+        let c = CHARS[i];
+        table.0[c as u32 as usize - MIN_CHAR as usize] = AsciiChar::two_hex_digits(c as u32);
+        i += 1;
+    }
+    table
+};
+
+const UNESCAPED: [AsciiChar; 2] = AsciiStr::new_sized("");
+
+const ESCAPED_BUF_INIT_UNPADDED: &str = "\\u00__";
+// RATIONALE: llvm generates better code if the buffer is register sized
+const ESCAPED_BUF_INIT: [AsciiChar; 8] = AsciiStr::new_sized(ESCAPED_BUF_INIT_UNPADDED);
+const ESCAPED_BUF_LEN: usize = ESCAPED_BUF_INIT_UNPADDED.len();
+
 #[cfg(all(test, feature = "alloc"))]
 mod tests {
     use alloc::string::ToString;