/
varint.rs
95 lines (82 loc) · 2.36 KB
/
varint.rs
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pub const fn varint_max<T: Sized>() -> usize {
const BITS_PER_BYTE: usize = 8;
const BITS_PER_VARINT_BYTE: usize = 7;
// How many data bits do we need for this type?
let bits = core::mem::size_of::<T>() * BITS_PER_BYTE;
// We add (BITS_PER_BYTE - 1), to ensure any integer divisions
// with a remainder will always add exactly one full byte, but
// an evenly divided number of bits will be the same
let roundup_bits = bits + (BITS_PER_BYTE - 1);
// Apply division, using normal "round down" integer division
roundup_bits / BITS_PER_VARINT_BYTE
}
#[inline]
pub fn varint_usize(n: usize, out: &mut [u8; varint_max::<usize>()]) -> &mut [u8] {
let mut value = n;
for i in 0..varint_max::<usize>() {
out[i] = value.to_le_bytes()[0];
if value < 128 {
return &mut out[..=i];
}
out[i] |= 0x80;
value >>= 7;
}
debug_assert_eq!(value, 0);
&mut out[..]
}
#[inline]
pub fn varint_u16(n: u16, out: &mut [u8; varint_max::<u16>()]) -> &mut [u8] {
let mut value = n;
for i in 0..varint_max::<u16>() {
out[i] = value.to_le_bytes()[0];
if value < 128 {
return &mut out[..=i];
}
out[i] |= 0x80;
value >>= 7;
}
debug_assert_eq!(value, 0);
&mut out[..]
}
#[inline]
pub fn varint_u32(n: u32, out: &mut [u8; varint_max::<u32>()]) -> &mut [u8] {
let mut value = n;
for i in 0..varint_max::<u32>() {
out[i] = value.to_le_bytes()[0];
if value < 128 {
return &mut out[..=i];
}
out[i] |= 0x80;
value >>= 7;
}
debug_assert_eq!(value, 0);
&mut out[..]
}
#[inline]
pub fn varint_u64(n: u64, out: &mut [u8; varint_max::<u64>()]) -> &mut [u8] {
let mut value = n;
for i in 0..varint_max::<u64>() {
out[i] = value.to_le_bytes()[0];
if value < 128 {
return &mut out[..=i];
}
out[i] |= 0x80;
value >>= 7;
}
debug_assert_eq!(value, 0);
&mut out[..]
}
#[inline]
pub fn varint_u128(n: u128, out: &mut [u8; varint_max::<u128>()]) -> &mut [u8] {
let mut value = n;
for i in 0..varint_max::<u128>() {
out[i] = value.to_le_bytes()[0];
if value < 128 {
return &mut out[..=i];
}
out[i] |= 0x80;
value >>= 7;
}
debug_assert_eq!(value, 0);
&mut out[..]
}