core/
escape.rs

1//! Helper code for character escaping.
2
3use crate::ascii;
4use crate::num::NonZero;
5use crate::ops::Range;
6
7const HEX_DIGITS: [ascii::Char; 16] = *b"0123456789abcdef".as_ascii().unwrap();
8
9#[inline]
10const fn backslash<const N: usize>(a: ascii::Char) -> ([ascii::Char; N], Range<u8>) {
11    const { assert!(N >= 2) };
12
13    let mut output = [ascii::Char::Null; N];
14
15    output[0] = ascii::Char::ReverseSolidus;
16    output[1] = a;
17
18    (output, 0..2)
19}
20
21#[inline]
22const fn hex_escape<const N: usize>(byte: u8) -> ([ascii::Char; N], Range<u8>) {
23    const { assert!(N >= 4) };
24
25    let mut output = [ascii::Char::Null; N];
26
27    let hi = HEX_DIGITS[(byte >> 4) as usize];
28    let lo = HEX_DIGITS[(byte & 0xf) as usize];
29
30    output[0] = ascii::Char::ReverseSolidus;
31    output[1] = ascii::Char::SmallX;
32    output[2] = hi;
33    output[3] = lo;
34
35    (output, 0..4)
36}
37
38#[inline]
39const fn verbatim<const N: usize>(a: ascii::Char) -> ([ascii::Char; N], Range<u8>) {
40    const { assert!(N >= 1) };
41
42    let mut output = [ascii::Char::Null; N];
43
44    output[0] = a;
45
46    (output, 0..1)
47}
48
49/// Escapes an ASCII character.
50///
51/// Returns a buffer and the length of the escaped representation.
52const fn escape_ascii<const N: usize>(byte: u8) -> ([ascii::Char; N], Range<u8>) {
53    const { assert!(N >= 4) };
54
55    #[cfg(feature = "optimize_for_size")]
56    {
57        match byte {
58            b'\t' => backslash(ascii::Char::SmallT),
59            b'\r' => backslash(ascii::Char::SmallR),
60            b'\n' => backslash(ascii::Char::SmallN),
61            b'\\' => backslash(ascii::Char::ReverseSolidus),
62            b'\'' => backslash(ascii::Char::Apostrophe),
63            b'"' => backslash(ascii::Char::QuotationMark),
64            0x00..=0x1F | 0x7F => hex_escape(byte),
65            _ => match ascii::Char::from_u8(byte) {
66                Some(a) => verbatim(a),
67                None => hex_escape(byte),
68            },
69        }
70    }
71
72    #[cfg(not(feature = "optimize_for_size"))]
73    {
74        /// Lookup table helps us determine how to display character.
75        ///
76        /// Since ASCII characters will always be 7 bits, we can exploit this to store the 8th bit to
77        /// indicate whether the result is escaped or unescaped.
78        ///
79        /// We additionally use 0x80 (escaped NUL character) to indicate hex-escaped bytes, since
80        /// escaped NUL will not occur.
81        const LOOKUP: [u8; 256] = {
82            let mut arr = [0; 256];
83            let mut idx = 0;
84            while idx <= 255 {
85                arr[idx] = match idx as u8 {
86                    // use 8th bit to indicate escaped
87                    b'\t' => 0x80 | b't',
88                    b'\r' => 0x80 | b'r',
89                    b'\n' => 0x80 | b'n',
90                    b'\\' => 0x80 | b'\\',
91                    b'\'' => 0x80 | b'\'',
92                    b'"' => 0x80 | b'"',
93
94                    // use NUL to indicate hex-escaped
95                    0x00..=0x1F | 0x7F..=0xFF => 0x80 | b'\0',
96
97                    idx => idx,
98                };
99                idx += 1;
100            }
101            arr
102        };
103
104        let lookup = LOOKUP[byte as usize];
105
106        // 8th bit indicates escape
107        let lookup_escaped = lookup & 0x80 != 0;
108
109        // SAFETY: We explicitly mask out the eighth bit to get a 7-bit ASCII character.
110        let lookup_ascii = unsafe { ascii::Char::from_u8_unchecked(lookup & 0x7F) };
111
112        if lookup_escaped {
113            // NUL indicates hex-escaped
114            if matches!(lookup_ascii, ascii::Char::Null) {
115                hex_escape(byte)
116            } else {
117                backslash(lookup_ascii)
118            }
119        } else {
120            verbatim(lookup_ascii)
121        }
122    }
123}
124
125/// Escapes a character `\u{NNNN}` representation.
126///
127/// Returns a buffer and the length of the escaped representation.
128const fn escape_unicode<const N: usize>(c: char) -> ([ascii::Char; N], Range<u8>) {
129    const { assert!(N >= 10 && N < u8::MAX as usize) };
130
131    let c = c as u32;
132
133    // OR-ing `1` ensures that for `c == 0` the code computes that
134    // one digit should be printed.
135    let start = (c | 1).leading_zeros() as usize / 4 - 2;
136
137    let mut output = [ascii::Char::Null; N];
138    output[3] = HEX_DIGITS[((c >> 20) & 15) as usize];
139    output[4] = HEX_DIGITS[((c >> 16) & 15) as usize];
140    output[5] = HEX_DIGITS[((c >> 12) & 15) as usize];
141    output[6] = HEX_DIGITS[((c >> 8) & 15) as usize];
142    output[7] = HEX_DIGITS[((c >> 4) & 15) as usize];
143    output[8] = HEX_DIGITS[((c >> 0) & 15) as usize];
144    output[9] = ascii::Char::RightCurlyBracket;
145    output[start + 0] = ascii::Char::ReverseSolidus;
146    output[start + 1] = ascii::Char::SmallU;
147    output[start + 2] = ascii::Char::LeftCurlyBracket;
148
149    (output, (start as u8)..(N as u8))
150}
151
152/// An iterator over an fixed-size array.
153///
154/// This is essentially equivalent to array’s IntoIter except that indexes are
155/// limited to u8 to reduce size of the structure.
156#[derive(Clone, Debug)]
157pub(crate) struct EscapeIterInner<const N: usize> {
158    // The element type ensures this is always ASCII, and thus also valid UTF-8.
159    data: [ascii::Char; N],
160
161    // Invariant: `alive.start <= alive.end <= N`
162    alive: Range<u8>,
163}
164
165impl<const N: usize> EscapeIterInner<N> {
166    pub(crate) const fn backslash(c: ascii::Char) -> Self {
167        let (data, range) = backslash(c);
168        Self { data, alive: range }
169    }
170
171    pub(crate) const fn ascii(c: u8) -> Self {
172        let (data, range) = escape_ascii(c);
173        Self { data, alive: range }
174    }
175
176    pub(crate) const fn unicode(c: char) -> Self {
177        let (data, range) = escape_unicode(c);
178        Self { data, alive: range }
179    }
180
181    #[inline]
182    pub(crate) const fn empty() -> Self {
183        Self { data: [ascii::Char::Null; N], alive: 0..0 }
184    }
185
186    #[inline]
187    pub(crate) fn as_ascii(&self) -> &[ascii::Char] {
188        // SAFETY: `self.alive` is guaranteed to be a valid range for indexing `self.data`.
189        unsafe {
190            self.data.get_unchecked(usize::from(self.alive.start)..usize::from(self.alive.end))
191        }
192    }
193
194    #[inline]
195    pub(crate) fn as_str(&self) -> &str {
196        self.as_ascii().as_str()
197    }
198
199    #[inline]
200    pub(crate) fn len(&self) -> usize {
201        usize::from(self.alive.end - self.alive.start)
202    }
203
204    pub(crate) fn next(&mut self) -> Option<u8> {
205        let i = self.alive.next()?;
206
207        // SAFETY: `i` is guaranteed to be a valid index for `self.data`.
208        unsafe { Some(self.data.get_unchecked(usize::from(i)).to_u8()) }
209    }
210
211    pub(crate) fn next_back(&mut self) -> Option<u8> {
212        let i = self.alive.next_back()?;
213
214        // SAFETY: `i` is guaranteed to be a valid index for `self.data`.
215        unsafe { Some(self.data.get_unchecked(usize::from(i)).to_u8()) }
216    }
217
218    pub(crate) fn advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
219        self.alive.advance_by(n)
220    }
221
222    pub(crate) fn advance_back_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
223        self.alive.advance_back_by(n)
224    }
225}