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|
/// Returns the length of the longest LEB128 encoding for `T`, assuming `T` is an integer type
pub const fn max_leb128_len<T>() -> usize {
// The longest LEB128 encoding for an integer uses 7 bits per byte.
(std::mem::size_of::<T>() * 8 + 6) / 7
}
/// Returns the length of the longest LEB128 encoding of all supported integer types.
pub const fn largest_max_leb128_len() -> usize {
max_leb128_len::<u128>()
}
macro_rules! impl_write_unsigned_leb128 {
($fn_name:ident, $int_ty:ty) => {
#[inline]
pub fn $fn_name(
out: &mut [::std::mem::MaybeUninit<u8>; max_leb128_len::<$int_ty>()],
mut value: $int_ty,
) -> &[u8] {
let mut i = 0;
loop {
if value < 0x80 {
unsafe {
*out.get_unchecked_mut(i).as_mut_ptr() = value as u8;
}
i += 1;
break;
} else {
unsafe {
*out.get_unchecked_mut(i).as_mut_ptr() = ((value & 0x7f) | 0x80) as u8;
}
value >>= 7;
i += 1;
}
}
unsafe { ::std::mem::MaybeUninit::slice_assume_init_ref(&out.get_unchecked(..i)) }
}
};
}
impl_write_unsigned_leb128!(write_u16_leb128, u16);
impl_write_unsigned_leb128!(write_u32_leb128, u32);
impl_write_unsigned_leb128!(write_u64_leb128, u64);
impl_write_unsigned_leb128!(write_u128_leb128, u128);
impl_write_unsigned_leb128!(write_usize_leb128, usize);
macro_rules! impl_read_unsigned_leb128 {
($fn_name:ident, $int_ty:ty) => {
#[inline]
pub fn $fn_name(slice: &[u8], position: &mut usize) -> $int_ty {
// The first iteration of this loop is unpeeled. This is a
// performance win because this code is hot and integer values less
// than 128 are very common, typically occurring 50-80% or more of
// the time, even for u64 and u128.
let byte = slice[*position];
*position += 1;
if (byte & 0x80) == 0 {
return byte as $int_ty;
}
let mut result = (byte & 0x7F) as $int_ty;
let mut shift = 7;
loop {
let byte = slice[*position];
*position += 1;
if (byte & 0x80) == 0 {
result |= (byte as $int_ty) << shift;
return result;
} else {
result |= ((byte & 0x7F) as $int_ty) << shift;
}
shift += 7;
}
}
};
}
impl_read_unsigned_leb128!(read_u16_leb128, u16);
impl_read_unsigned_leb128!(read_u32_leb128, u32);
impl_read_unsigned_leb128!(read_u64_leb128, u64);
impl_read_unsigned_leb128!(read_u128_leb128, u128);
impl_read_unsigned_leb128!(read_usize_leb128, usize);
macro_rules! impl_write_signed_leb128 {
($fn_name:ident, $int_ty:ty) => {
#[inline]
pub fn $fn_name(
out: &mut [::std::mem::MaybeUninit<u8>; max_leb128_len::<$int_ty>()],
mut value: $int_ty,
) -> &[u8] {
let mut i = 0;
loop {
let mut byte = (value as u8) & 0x7f;
value >>= 7;
let more = !(((value == 0) && ((byte & 0x40) == 0))
|| ((value == -1) && ((byte & 0x40) != 0)));
if more {
byte |= 0x80; // Mark this byte to show that more bytes will follow.
}
unsafe {
*out.get_unchecked_mut(i).as_mut_ptr() = byte;
}
i += 1;
if !more {
break;
}
}
unsafe { ::std::mem::MaybeUninit::slice_assume_init_ref(&out.get_unchecked(..i)) }
}
};
}
impl_write_signed_leb128!(write_i16_leb128, i16);
impl_write_signed_leb128!(write_i32_leb128, i32);
impl_write_signed_leb128!(write_i64_leb128, i64);
impl_write_signed_leb128!(write_i128_leb128, i128);
impl_write_signed_leb128!(write_isize_leb128, isize);
macro_rules! impl_read_signed_leb128 {
($fn_name:ident, $int_ty:ty) => {
#[inline]
pub fn $fn_name(slice: &[u8], position: &mut usize) -> $int_ty {
let mut result = 0;
let mut shift = 0;
let mut byte;
loop {
byte = slice[*position];
*position += 1;
result |= <$int_ty>::from(byte & 0x7F) << shift;
shift += 7;
if (byte & 0x80) == 0 {
break;
}
}
if (shift < <$int_ty>::BITS) && ((byte & 0x40) != 0) {
// sign extend
result |= (!0 << shift);
}
result
}
};
}
impl_read_signed_leb128!(read_i16_leb128, i16);
impl_read_signed_leb128!(read_i32_leb128, i32);
impl_read_signed_leb128!(read_i64_leb128, i64);
impl_read_signed_leb128!(read_i128_leb128, i128);
impl_read_signed_leb128!(read_isize_leb128, isize);
|
