basekit 0.5.0

Universal library for encoding in various bases
Documentation 
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use core::ptr;

use super::super::config::Base32EncodeConfig;
use super::super::error::Base32Error;

/// Encodes the remaining 1–4 input bytes of a base32 stream.
///
/// Writes padding characters when `config.padding` is set.
#[inline(always)]
#[allow(unsafe_op_in_unsafe_fn)]
pub(crate) unsafe fn encode_tail_into(
    config: &Base32EncodeConfig,
    dst: &mut [u8],
    src: &[u8],
) -> Result<usize, Base32Error> {
    debug_assert!(
        !src.is_empty() && src.len() <= 4,
        "src length must be between 1 and 4"
    );

    let src_len = src.len();
    let output_len_with_padding = 8;
    let output_len_without_padding = match src_len {
        1 => 2,
        2 => 4,
        3 => 5,
        4 => 7,
        _ => unreachable!(),
    };
    let output_len = if config.padding.is_some() {
        output_len_with_padding
    } else {
        output_len_without_padding
    };

    if dst.len() < output_len {
        return Err(Base32Error::DestinationBufferTooSmall {
            needed: output_len,
            provided: dst.len(),
        });
    }

    let alphabet_ptr = config.alphabet.as_ptr();
    let ptr = dst.as_mut_ptr();

    match src_len {
        1 => {
            let b0 = src[0] as u32;
            let c0 = ((b0 >> 3) & 0x1F) as usize;
            let c1 = ((b0 << 2) & 0x1F) as usize;

            ptr.write(ptr::read_unaligned(alphabet_ptr.add(c0)));
            ptr.offset(1)
                .write(ptr::read_unaligned(alphabet_ptr.add(c1)));

            if let Some(padding) = config.padding {
                ptr.offset(2).write(padding);
                ptr.offset(3).write(padding);
                ptr.offset(4).write(padding);
                ptr.offset(5).write(padding);
                ptr.offset(6).write(padding);
                ptr.offset(7).write(padding);
                Ok(output_len)
            } else {
                Ok(output_len_without_padding)
            }
        }
        2 => {
            let b0 = src[0] as u32;
            let b1 = src[1] as u32;
            let c0 = ((b0 >> 3) & 0x1F) as usize;
            let c1 = (((b0 << 2) | (b1 >> 6)) & 0x1F) as usize;
            let c2 = ((b1 >> 1) & 0x1F) as usize;
            let c3 = ((b1 << 4) & 0x1F) as usize;

            ptr.write(ptr::read_unaligned(alphabet_ptr.add(c0)));
            ptr.offset(1)
                .write(ptr::read_unaligned(alphabet_ptr.add(c1)));
            ptr.offset(2)
                .write(ptr::read_unaligned(alphabet_ptr.add(c2)));
            ptr.offset(3)
                .write(ptr::read_unaligned(alphabet_ptr.add(c3)));

            if let Some(padding) = config.padding {
                ptr.offset(4).write(padding);
                ptr.offset(5).write(padding);
                ptr.offset(6).write(padding);
                ptr.offset(7).write(padding);
                Ok(output_len)
            } else {
                Ok(output_len_without_padding)
            }
        }
        3 => {
            let b0 = src[0] as u32;
            let b1 = src[1] as u32;
            let b2 = src[2] as u32;
            let c0 = ((b0 >> 3) & 0x1F) as usize;
            let c1 = (((b0 << 2) | (b1 >> 6)) & 0x1F) as usize;
            let c2 = ((b1 >> 1) & 0x1F) as usize;
            let c3 = (((b1 << 4) | (b2 >> 4)) & 0x1F) as usize;
            let c4 = ((b2 << 1) & 0x1F) as usize;

            ptr.write(ptr::read_unaligned(alphabet_ptr.add(c0)));
            ptr.offset(1)
                .write(ptr::read_unaligned(alphabet_ptr.add(c1)));
            ptr.offset(2)
                .write(ptr::read_unaligned(alphabet_ptr.add(c2)));
            ptr.offset(3)
                .write(ptr::read_unaligned(alphabet_ptr.add(c3)));
            ptr.offset(4)
                .write(ptr::read_unaligned(alphabet_ptr.add(c4)));

            if let Some(padding) = config.padding {
                ptr.offset(5).write(padding);
                ptr.offset(6).write(padding);
                ptr.offset(7).write(padding);
                Ok(output_len)
            } else {
                Ok(output_len_without_padding)
            }
        }
        4 => {
            let b0 = src[0] as u32;
            let b1 = src[1] as u32;
            let b2 = src[2] as u32;
            let b3 = src[3] as u32;
            let c0 = ((b0 >> 3) & 0x1F) as usize;
            let c1 = (((b0 << 2) | (b1 >> 6)) & 0x1F) as usize;
            let c2 = ((b1 >> 1) & 0x1F) as usize;
            let c3 = (((b1 << 4) | (b2 >> 4)) & 0x1F) as usize;
            let c4 = (((b2 << 1) | (b3 >> 7)) & 0x1F) as usize;
            let c5 = ((b3 >> 2) & 0x1F) as usize;
            let c6 = ((b3 << 3) & 0x1F) as usize;

            ptr.write(ptr::read_unaligned(alphabet_ptr.add(c0)));
            ptr.offset(1)
                .write(ptr::read_unaligned(alphabet_ptr.add(c1)));
            ptr.offset(2)
                .write(ptr::read_unaligned(alphabet_ptr.add(c2)));
            ptr.offset(3)
                .write(ptr::read_unaligned(alphabet_ptr.add(c3)));
            ptr.offset(4)
                .write(ptr::read_unaligned(alphabet_ptr.add(c4)));
            ptr.offset(5)
                .write(ptr::read_unaligned(alphabet_ptr.add(c5)));
            ptr.offset(6)
                .write(ptr::read_unaligned(alphabet_ptr.add(c6)));

            if let Some(padding) = config.padding {
                ptr.offset(7).write(padding);
                Ok(output_len)
            } else {
                Ok(output_len_without_padding)
            }
        }
        _ => unreachable!(),
    }
}