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pub trait InputStream {
    fn read(&mut self, length: usize) -> &[u8];
    fn len(&self) -> usize;
}

pub struct ByteArrayInputStream<'a> {
    index: usize,
    bytes: &'a [u8],
}

impl ByteArrayInputStream<'_> {
    pub fn new(b: &[u8]) -> ByteArrayInputStream {
        ByteArrayInputStream {
            index: 0,
            bytes: b,
        }
    }
}

impl InputStream for ByteArrayInputStream<'_> {
    fn read(&mut self, length: usize) -> &[u8] {
        let i = self.index;
        self.index = self.bytes.len().min(self.index + length);
        return &self.bytes[i..self.index];
    }

    fn len(&self) -> usize {
        return self.bytes.len() - self.index;
    }
}