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der 0.4.0

Pure Rust embedded-friendly implementation of the Distinguished Encoding Rules (DER) for Abstract Syntax Notation One (ASN.1) as described in ITU X.690 with full support for heapless no_std targets
Documentation 
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//! ASN.1 `BIT STRING` support.

use crate::{ByteSlice, Encodable, Encoder, ErrorKind, Header, Length, Result, Tag, Tagged};

/// ASN.1 `BIT STRING` type.
#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
pub struct BitString<'a> {
    /// Inner value
    pub(crate) inner: ByteSlice<'a>,

    /// Length after encoding (with leading `0`0 byte)
    pub(crate) encoded_len: Length,
}

impl<'a> BitString<'a> {
    /// Create a new ASN.1 `BIT STRING` from a byte slice.
    pub fn new(bytes: &'a [u8]) -> Result<Self> {
        let inner = ByteSlice::new(bytes).map_err(|_| ErrorKind::Length { tag: Self::TAG })?;
        let encoded_len = (inner.len() + 1u8).map_err(|_| ErrorKind::Length { tag: Self::TAG })?;
        Ok(Self { inner, encoded_len })
    }

    /// Borrow the inner byte slice.
    pub fn as_bytes(&self) -> &'a [u8] {
        self.inner.as_bytes()
    }

    /// Get the length of the inner byte slice (sans leading `0` byte).
    pub fn len(&self) -> Length {
        self.inner.len()
    }

    /// Is the inner byte slice empty?
    pub fn is_empty(&self) -> bool {
        self.inner.is_empty()
    }
}

impl AsRef<[u8]> for BitString<'_> {
    fn as_ref(&self) -> &[u8] {
        self.as_bytes()
    }
}

impl<'a> From<&BitString<'a>> for BitString<'a> {
    fn from(value: &BitString<'a>) -> BitString<'a> {
        *value
    }
}

impl<'a> From<BitString<'a>> for &'a [u8] {
    fn from(bit_string: BitString<'a>) -> &'a [u8] {
        bit_string.as_bytes()
    }
}

impl<'a> Encodable for BitString<'a> {
    fn encoded_len(&self) -> Result<Length> {
        self.encoded_len.for_tlv()
    }

    fn encode(&self, encoder: &mut Encoder<'_>) -> Result<()> {
        Header::new(Self::TAG, (Length::ONE + self.inner.len())?)?.encode(encoder)?;
        encoder.byte(0)?;
        encoder.bytes(self.as_bytes())
    }
}

impl<'a> Tagged for BitString<'a> {
    const TAG: Tag = Tag::BitString;
}

#[cfg(test)]
mod tests {
    use super::{BitString, Result, Tag};
    use crate::asn1::Any;
    use core::convert::TryInto;

    /// Parse a `BitString` from an ASN.1 `Any` value to test decoding behaviors.
    fn parse_bitstring_from_any(bytes: &[u8]) -> Result<BitString<'_>> {
        Any::new(Tag::BitString, bytes)?.try_into()
    }

    #[test]
    fn decode_empty_bitstring() {
        let bs = parse_bitstring_from_any(&[]).unwrap();
        assert_eq!(bs.as_ref(), &[]);
    }

    #[test]
    fn decode_non_empty_bitstring() {
        let bs = parse_bitstring_from_any(&[1, 2, 3]).unwrap();
        assert_eq!(bs.as_ref(), &[1, 2, 3]);
    }
}