Struct VarintEncoding

pub struct VarintEncoding;

Variable-size integer encoding (excepting [ui]8).

Encoding an unsigned integer v (of any type excepting u8) works as follows:

1. If u < 251, encode it as a single byte with that value.
2. If 251 <= u < 2**16, encode it as a literal byte 251, followed by a u16 with value u.
3. If 2**16 <= u < 2**32, encode it as a literal byte 252, followed by a u32 with value u.
4. If 2**32 <= u < 2**64, encode it as a literal byte 253, followed by a u64 with value u.
5. If 2**64 <= u < 2**128, encode it as a literal byte 254, followed by a u128 with value u.

Then, for signed integers, we first convert to unsigned using the zigzag algorithm, and then encode them as we do for unsigned integers generally. The reason we use this algorithm is that it encodes those values which are close to zero in less bytes; the obvious algorithm, where we encode the cast values, gives a very large encoding for all negative values.

The zigzag algorithm is defined as follows:

fn zigzag(v: Signed) -> Unsigned {
    match v {
        0 => 0,
        v if v < 0 => |v| * 2 - 1
        v if v > 0 => v * 2
    }
}

And works such that:

assert_eq!(zigzag(0), 0);
assert_eq!(zigzag(-1), 1);
assert_eq!(zigzag(1), 2);
assert_eq!(zigzag(-2), 3);
assert_eq!(zigzag(2), 4);
assert_eq!(zigzag(i64::min_value()), u64::max_value());

Note that u256 and the like are unsupported by this format; if and when they are added to the language, they may be supported via the extension point given by the 255 byte.

Trait Implementations

impl Clone for VarintEncoding

fn clone(&self) -> VarintEncoding

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for VarintEncoding