der 0.4.2

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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//! Support for encoding negative integers

use super::is_highest_bit_set;
use crate::{asn1::Any, Encodable, Encoder, Header, Length, Result, Tag};
use core::convert::TryFrom;

/// Decode an unsigned integer of the specified size.
///
/// Returns a byte array of the requested size containing a big endian integer.
pub(super) fn decode_array<const N: usize>(any: Any<'_>) -> Result<[u8; N]> {
    any.tag().assert_eq(Tag::Integer)?;
    let mut output = [0xFFu8; N];
    let offset = N.saturating_sub(any.as_bytes().len());
    output[offset..].copy_from_slice(any.as_bytes());
    Ok(output)
}

/// Encode the given big endian bytes representing an integer as ASN.1 DER.
pub(super) fn encode(encoder: &mut Encoder<'_>, bytes: &[u8]) -> Result<()> {
    let bytes = strip_leading_ones(&bytes);
    let len = Length::try_from(bytes.len())?;
    Header::new(Tag::Integer, len)?.encode(encoder)?;
    encoder.bytes(bytes)
}

/// Get the encoded length for the given unsigned integer serialized as bytes.
#[inline]
pub(super) fn encoded_len(bytes: &[u8]) -> Result<Length> {
    Length::try_from(strip_leading_ones(&bytes).len())
}

/// Strip the leading all-ones bytes from the given byte slice.
fn strip_leading_ones(mut bytes: &[u8]) -> &[u8] {
    while let Some((byte, rest)) = bytes.split_first() {
        if *byte == 0xFF && is_highest_bit_set(rest) {
            bytes = rest;
            continue;
        }

        break;
    }

    bytes
}