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Parse \uXXXX escapes faster #1172

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merged 1 commit into from
Aug 12, 2024
Merged

Parse \uXXXX escapes faster #1172

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106 changes: 54 additions & 52 deletions src/read.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -361,16 +361,14 @@ where
}

fn decode_hex_escape(&mut self) -> Result<u16> {
let mut n = 0;
for _ in 0..4 {
match decode_hex_val(tri!(next_or_eof(self))) {
None => return error(self, ErrorCode::InvalidEscape),
Some(val) => {
n = (n << 4) + val;
}
}
let a = tri!(next_or_eof(self));
let b = tri!(next_or_eof(self));
let c = tri!(next_or_eof(self));
let d = tri!(next_or_eof(self));
match decode_four_hex_digits(a, b, c, d) {
Some(val) => Ok(val),
None => error(self, ErrorCode::InvalidEscape),
}
Ok(n)
}

#[cfg(feature = "raw_value")]
Expand Down Expand Up @@ -609,24 +607,21 @@ impl<'a> Read<'a> for SliceRead<'a> {
}
}

#[inline]
fn decode_hex_escape(&mut self) -> Result<u16> {
if self.index + 4 > self.slice.len() {
self.index = self.slice.len();
return error(self, ErrorCode::EofWhileParsingString);
}

let mut n = 0;
for _ in 0..4 {
let ch = decode_hex_val(self.slice[self.index]);
self.index += 1;
match ch {
None => return error(self, ErrorCode::InvalidEscape),
Some(val) => {
n = (n << 4) + val;
match self.slice[self.index..] {
[a, b, c, d, ..] => {
self.index += 4;
match decode_four_hex_digits(a, b, c, d) {
Some(val) => Ok(val),
None => error(self, ErrorCode::InvalidEscape),
}
}
_ => {
self.index = self.slice.len();
error(self, ErrorCode::EofWhileParsingString)
}
}
Ok(n)
}

#[cfg(feature = "raw_value")]
Expand Down Expand Up @@ -993,34 +988,41 @@ where
Ok(())
}

static HEX: [u8; 256] = {
const __: u8 = 255; // not a hex digit
[
// 1 2 3 4 5 6 7 8 9 A B C D E F
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 0
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 1
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 2
00, 01, 02, 03, 04, 05, 06, 07, 08, 09, __, __, __, __, __, __, // 3
__, 10, 11, 12, 13, 14, 15, __, __, __, __, __, __, __, __, __, // 4
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 5
__, 10, 11, 12, 13, 14, 15, __, __, __, __, __, __, __, __, __, // 6
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 7
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 8
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 9
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // A
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // B
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // C
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // D
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // E
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // F
]
};

fn decode_hex_val(val: u8) -> Option<u16> {
let n = HEX[val as usize] as u16;
if n == 255 {
None
} else {
Some(n)
const fn decode_hex_val_slow(val: u8) -> Option<u8> {
match val {
b'0'..=b'9' => Some(val - b'0'),
b'A'..=b'F' => Some(val - b'A' + 10),
b'a'..=b'f' => Some(val - b'a' + 10),
_ => None,
}
}

const fn build_hex_table(shift: usize) -> [i16; 256] {
let mut table = [0; 256];
let mut ch = 0;
while ch < 256 {
table[ch] = match decode_hex_val_slow(ch as u8) {
Some(val) => (val as i16) << shift,
None => -1,
};
ch += 1;
}
table
}

static HEX0: [i16; 256] = build_hex_table(0);
static HEX1: [i16; 256] = build_hex_table(4);

fn decode_four_hex_digits(a: u8, b: u8, c: u8, d: u8) -> Option<u16> {
let a = HEX1[a as usize];
let b = HEX0[b as usize];
let c = HEX1[c as usize];
let d = HEX0[d as usize];

// A single sign bit check.
if (a | b | c | d) < 0 {
return None;
}

Some((((a | b) << 8) | c | d) as u16)
}
6 changes: 3 additions & 3 deletions tests/test.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1091,15 +1091,15 @@ fn test_parse_string() {
),
(
&[b'"', b'\\', b'u', 250, 48, 51, 48, b'"'],
"invalid escape at line 1 column 4",
"invalid escape at line 1 column 7",
),
(
&[b'"', b'\\', b'u', 48, 250, 51, 48, b'"'],
"invalid escape at line 1 column 5",
"invalid escape at line 1 column 7",
),
(
&[b'"', b'\\', b'u', 48, 48, 250, 48, b'"'],
"invalid escape at line 1 column 6",
"invalid escape at line 1 column 7",
),
(
&[b'"', b'\\', b'u', 48, 48, 51, 250, b'"'],
Expand Down