rust/flexbuffers/src/bitwidth.rs

// Copyright 2019 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use crate::bitwidth::BitWidth::*;
use std::slice::Iter;

/// Represents the size of Flexbuffers data.
///
/// Flexbuffers automatically compresses numbers to the smallest possible width
/// (`250u64` is stored as `250u8`).
#[derive(
    Debug,
    Clone,
    Copy,
    PartialEq,
    Eq,
    PartialOrd,
    Serialize,
    Deserialize,
    Ord,
    num_enum::TryFromPrimitive,
)]
#[repr(u8)]
pub enum BitWidth {
    W8 = 0,
    W16 = 1,
    W32 = 2,
    W64 = 3,
}
impl BitWidth {
    pub(crate) fn iter() -> Iter<'static, Self> {
        [W8, W16, W32, W64].iter()
    }
    pub fn n_bytes(self) -> usize {
        1 << self as usize
    }
    pub fn from_nbytes(n: impl std::convert::Into<usize>) -> Option<Self> {
        match n.into() {
            1 => Some(W8),
            2 => Some(W16),
            4 => Some(W32),
            8 => Some(W64),
            _ => None,
        }
    }
}

impl Default for BitWidth {
    fn default() -> Self {
        W8
    }
}

// TODO(cneo): Overloading with `from` is probably not the most readable idea in hindsight.
macro_rules! impl_bitwidth_from {
    ($from: ident, $w64: ident, $w32: ident, $w16: ident, $w8: ident) => {
        impl From<$from> for BitWidth {
            fn from(x: $from) -> BitWidth {
                let x = x as $w64;
                if x >= $w8::min_value() as $w64 && x <= $w8::max_value() as $w64 {
                    return W8;
                }
                if x >= $w16::min_value() as $w64 && x <= $w16::max_value() as $w64 {
                    return W16;
                }
                if x >= $w32::min_value() as $w64 && x <= $w32::max_value() as $w64 {
                    return W32;
                }
                W64
            }
        }
    };
}
impl_bitwidth_from!(u64, u64, u32, u16, u8);
impl_bitwidth_from!(usize, u64, u32, u16, u8);
impl_bitwidth_from!(i64, i64, i32, i16, i8);

#[allow(clippy::float_cmp)]
impl From<f64> for BitWidth {
    fn from(x: f64) -> BitWidth {
        if x != x as f32 as f64 {
            W64
        } else {
            W32
        }
    }
}
impl From<f32> for BitWidth {
    fn from(_: f32) -> BitWidth {
        W32
    }
}

/// Zero pad `v` until `T` will be byte aligned when pushed.
pub fn align(buffer: &mut Vec<u8>, width: BitWidth) {
    let bytes = 1 << width as u8;
    let alignment = (bytes - buffer.len() % bytes) % bytes;
    // Profiling reveals the loop is faster than Vec::resize.
    for _ in 0..alignment as usize {
        buffer.push(0);
    }
}