polyvec 0.1.0

Reuse vector capacity
Documentation 
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//!

use std::mem::size_of;

use raw_parts::RawParts;

pub struct PolyVec {
    pointer: *mut u8,
    capacity: usize,
    elem_size: usize,
}

impl PolyVec {
    pub fn new<T>() -> Self {
        Vec::<T>::new().into()
    }
}

impl<T> From<Vec<T>> for PolyVec {
    fn from(mut vec: Vec<T>) -> Self {
        vec.clear();

        let RawParts { capacity, ptr, .. } = RawParts::from_vec(vec);

        Self {
            pointer: ptr as *mut u8,
            elem_size: std::mem::size_of::<T>(),
            capacity: capacity,
        }
    }
}

#[derive(Debug, thiserror::Error)]
pub enum PolyVecError {
    #[error("Invalid memory alignment")]
    InvalidMemAlign { expected: usize, detected: usize },
}

impl<T> TryFrom<PolyVec> for Vec<T> {
    type Error = PolyVecError;

    fn try_from(value: PolyVec) -> Result<Self, Self::Error> {
        if value.elem_size == size_of::<T>()
            || value.elem_size % size_of::<usize>() == 0 && size_of::<T>() % size_of::<usize>() == 0
        {
            unsafe {
                Ok(Self::from_raw_parts(
                    value.pointer as *mut T,
                    0,
                    value.capacity * value.elem_size / size_of::<T>(),
                ))
            }
        } else {
            Err(PolyVecError::InvalidMemAlign {
                expected: value.elem_size,
                detected: size_of::<T>(),
            })
        }
    }
}