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/// Creates a view from a fixed length input (the compiler can generate
/// specialized code for this)
use std::convert::TryInto;

pub fn make_view_inner<const LEN: usize>(data: &[u8]) -> u128 {
    let mut view_buffer = [0; 16];
    view_buffer[0..4].copy_from_slice(&(LEN as u32).to_le_bytes());
    view_buffer[4..4 + LEN].copy_from_slice(&data[..LEN]);
    u128::from_le_bytes(view_buffer)
}

#[derive(Debug, Copy, Clone, Default)]
#[repr(C)]
pub struct ByteView {
    /// The length of the string/bytes.
    pub length: u32,
    /// First 4 bytes of string/bytes data.
    pub prefix: u32,
    /// The buffer index.
    pub buffer_index: u32,
    /// The offset into the buffer.
    pub offset: u32,
}

impl ByteView {
    #[inline(always)]
    pub fn as_u128(self) -> u128 {
        (self.length as u128)
            | ((self.prefix as u128) << 32)
            | ((self.buffer_index as u128) << 64)
            | ((self.offset as u128) << 96)
    }
}

#[no_mangle]
pub fn make_view(data: &[u8], block_id: u32, offset: u32) -> u128 {
    let len = data.len();
    // Generate specialized code for each potential small string length
    // to improve performance
    match len {
        0 => make_view_inner::<0>(data),
        1 => make_view_inner::<1>(data),
        2 => make_view_inner::<2>(data),
        3 => make_view_inner::<3>(data),
        4 => make_view_inner::<4>(data),
        5 => make_view_inner::<5>(data),
        6 => make_view_inner::<6>(data),
        7 => make_view_inner::<7>(data),
        8 => make_view_inner::<8>(data),
        9 => make_view_inner::<9>(data),
        10 => make_view_inner::<10>(data),
        11 => make_view_inner::<11>(data),
        12 => make_view_inner::<12>(data),
        _ => {
            let view = ByteView {
                length: len as u32,
                prefix: u32::from_le_bytes(data[0..4].try_into().unwrap()),
                buffer_index: block_id,
                offset,
            };
            view.as_u128()
        }
    }
}