1#[derive(Debug, Default)]
24pub struct VLQDecoder {
25 in_progress: u64,
27 shift: u32,
28}
29
30impl VLQDecoder {
31 pub fn long(&mut self, buf: &mut &[u8]) -> Option<i64> {
33 while let Some(byte) = buf.first().copied() {
34 *buf = &buf[1..];
35 self.in_progress |= ((byte & 0x7F) as u64) << self.shift;
36 self.shift += 7;
37 if byte & 0x80 == 0 {
38 let val = self.in_progress;
39 self.in_progress = 0;
40 self.shift = 0;
41 return Some((val >> 1) as i64 ^ -((val & 1) as i64));
42 }
43 }
44 None
45 }
46}
47
48#[inline]
50pub(crate) fn read_varint(buf: &[u8]) -> Option<(u64, usize)> {
51 let first = *buf.first()?;
52 if first < 0x80 {
53 return Some((first as u64, 1));
54 }
55
56 if let Some(array) = buf.get(..10) {
57 return read_varint_array(array.try_into().unwrap());
58 }
59
60 read_varint_slow(buf)
61}
62
63#[inline]
68fn read_varint_array(buf: [u8; 10]) -> Option<(u64, usize)> {
69 let mut in_progress = 0_u64;
70 for (idx, b) in buf.into_iter().take(9).enumerate() {
71 in_progress += (b as u64) << (7 * idx);
72 if b < 0x80 {
73 return Some((in_progress, idx + 1));
74 }
75 in_progress -= 0x80 << (7 * idx);
76 }
77
78 let b = buf[9] as u64;
79 in_progress += b << (7 * 9);
80 (b < 0x02).then_some((in_progress, 10))
81}
82
83#[inline(never)]
84#[cold]
85fn read_varint_slow(buf: &[u8]) -> Option<(u64, usize)> {
86 let mut value = 0;
87 for (count, byte) in buf.iter().take(10).enumerate() {
88 let byte = buf[count];
89 value |= u64::from(byte & 0x7F) << (count * 7);
90 if byte <= 0x7F {
91 return (count != 9 || byte < 2).then_some((value, count + 1));
94 }
95 }
96
97 None
98}
99
100#[cfg(test)]
101mod tests {
102 use super::*;
103
104 fn encode_var(mut n: u64, dst: &mut [u8]) -> usize {
105 let mut i = 0;
106
107 while n >= 0x80 {
108 dst[i] = 0x80 | (n as u8);
109 i += 1;
110 n >>= 7;
111 }
112
113 dst[i] = n as u8;
114 i + 1
115 }
116
117 fn varint_test(a: u64) {
118 let mut buf = [0_u8; 10];
119 let len = encode_var(a, &mut buf);
120 assert_eq!(read_varint(&buf[..len]).unwrap(), (a, len));
121 assert_eq!(read_varint(&buf).unwrap(), (a, len));
122 }
123
124 #[test]
125 fn test_varint() {
126 varint_test(0);
127 varint_test(4395932);
128 varint_test(u64::MAX);
129
130 for _ in 0..1000 {
131 varint_test(rand::random());
132 }
133 }
134}