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use std::mem::size_of;
/// Most-significant byte, == 0x80
pub const MSB: u8 = 0b1000_0000;
/// All bits except for the most significant. Can be used as bitmask to drop the most-signficant
/// bit using `&` (binary-and).
const DROP_MSB: u8 = 0b0111_1111;
/// How many bytes an integer uses when being encoded as a VarInt.
#[inline]
fn required_encoded_space_unsigned(mut v: u64) -> usize {
if v == 0 {
return 1;
}
let mut logcounter = 0;
while v > 0 {
logcounter += 1;
v >>= 7;
}
logcounter
}
/// How many bytes an integer uses when being encoded as a VarInt.
#[inline]
fn required_encoded_space_signed(v: i64) -> usize {
required_encoded_space_unsigned(zigzag_encode(v))
}
/// Varint (variable length integer) encoding, as described in
/// https://developers.google.com/protocol-buffers/docs/encoding.
///
/// Uses zigzag encoding (also described there) for signed integer representation.
pub trait VarInt: Sized + Copy {
/// Returns the number of bytes this number needs in its encoded form. Note: This varies
/// depending on the actual number you want to encode.
fn required_space(self) -> usize;
/// Decode a value from the slice. Returns the value and the number of bytes read from the
/// slice (can be used to read several consecutive values from a big slice)
/// return None if all bytes has MSB set.
fn decode_var(src: &[u8]) -> Option<(Self, usize)>;
/// Encode a value into the slice. The slice must be at least `required_space()` bytes long.
/// The number of bytes taken by the encoded integer is returned.
fn encode_var(self, src: &mut [u8]) -> usize;
/// Helper: Encode a value and return the encoded form as Vec. The Vec must be at least
/// `required_space()` bytes long.
fn encode_var_vec(self) -> Vec<u8> {
let mut v = Vec::new();
v.resize(self.required_space(), 0);
self.encode_var(&mut v);
v
}
}
#[inline]
fn zigzag_encode(from: i64) -> u64 {
((from << 1) ^ (from >> 63)) as u64
}
// see: http://stackoverflow.com/a/2211086/56332
// casting required because operations like unary negation
// cannot be performed on unsigned integers
#[inline]
fn zigzag_decode(from: u64) -> i64 {
((from >> 1) ^ (-((from & 1) as i64)) as u64) as i64
}
pub(crate) trait VarIntMaxSize {
fn varint_max_size() -> usize;
}
impl<VI: VarInt> VarIntMaxSize for VI {
fn varint_max_size() -> usize {
(size_of::<VI>() * 8 + 7) / 7
}
}
macro_rules! impl_varint {
($t:ty, unsigned) => {
impl VarInt for $t {
fn required_space(self) -> usize {
required_encoded_space_unsigned(self as u64)
}
fn decode_var(src: &[u8]) -> Option<(Self, usize)> {
let (n, s) = u64::decode_var(src)?;
Some((n as Self, s))
}
fn encode_var(self, dst: &mut [u8]) -> usize {
(self as u64).encode_var(dst)
}
}
};
($t:ty, signed) => {
impl VarInt for $t {
fn required_space(self) -> usize {
required_encoded_space_signed(self as i64)
}
fn decode_var(src: &[u8]) -> Option<(Self, usize)> {
let (n, s) = i64::decode_var(src)?;
Some((n as Self, s))
}
fn encode_var(self, dst: &mut [u8]) -> usize {
(self as i64).encode_var(dst)
}
}
};
}
impl_varint!(usize, unsigned);
impl_varint!(u32, unsigned);
impl_varint!(u16, unsigned);
impl_varint!(u8, unsigned);
impl_varint!(isize, signed);
impl_varint!(i32, signed);
impl_varint!(i16, signed);
impl_varint!(i8, signed);
// Below are the "base implementations" doing the actual encodings; all other integer types are
// first cast to these biggest types before being encoded.
impl VarInt for u64 {
fn required_space(self) -> usize {
required_encoded_space_unsigned(self)
}
#[inline]
fn decode_var(src: &[u8]) -> Option<(Self, usize)> {
let mut result: u64 = 0;
let mut shift = 0;
let mut success = false;
for b in src.iter() {
let msb_dropped = b & DROP_MSB;
result |= (msb_dropped as u64) << shift;
shift += 7;
if b & MSB == 0 || shift > (9 * 7) {
success = b & MSB == 0;
break;
}
}
if success {
Some((result, shift / 7 as usize))
} else {
None
}
}
#[inline]
fn encode_var(self, dst: &mut [u8]) -> usize {
assert!(dst.len() >= self.required_space());
let mut n = self;
let mut i = 0;
while n >= 0x80 {
dst[i] = MSB | (n as u8);
i += 1;
n >>= 7;
}
dst[i] = n as u8;
i + 1
}
}
impl VarInt for i64 {
fn required_space(self) -> usize {
required_encoded_space_signed(self)
}
#[inline]
fn decode_var(src: &[u8]) -> Option<(Self, usize)> {
if let Some((result, size)) = u64::decode_var(src) {
Some((zigzag_decode(result) as Self, size))
} else {
None
}
}
#[inline]
fn encode_var(self, dst: &mut [u8]) -> usize {
assert!(dst.len() >= self.required_space());
let mut n: u64 = zigzag_encode(self as i64);
let mut i = 0;
while n >= 0x80 {
dst[i] = MSB | (n as u8);
i += 1;
n >>= 7;
}
dst[i] = n as u8;
i + 1
}
}