Struct bincode::config::VarintEncoding

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pub struct VarintEncoding;
Expand description

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§

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impl Clone for VarintEncoding

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fn clone(&self) -> VarintEncoding

Returns a copy of the value. Read more
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fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source. Read more
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impl Copy for VarintEncoding

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impl<T> Any for T
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fn borrow_mut(&mut self) -> &mut T

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impl<T> CloneToUninit for T
where T: Clone,

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default unsafe fn clone_to_uninit(&self, dst: *mut T)

🔬This is a nightly-only experimental API. (clone_to_uninit)
Performs copy-assignment from self to dst. Read more
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impl<T> CloneToUninit for T
where T: Copy,

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unsafe fn clone_to_uninit(&self, dst: *mut T)

🔬This is a nightly-only experimental API. (clone_to_uninit)
Performs copy-assignment from self to dst. Read more
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impl<T> From<T> for T

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Returns the argument unchanged.

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impl<T, U> Into<U> for T
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fn into(self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

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where T: Clone,

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type Owned = T

The resulting type after obtaining ownership.
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fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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impl<T, U> TryFrom<U> for T
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type Error = Infallible

The type returned in the event of a conversion error.
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fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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type Error = <U as TryFrom<T>>::Error

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fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.