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use std::mem;

/// A type that represents an object that consumes a memory slice on demand.
/// The consumer lives as long as the slice is alive.
pub struct ByteConsumer<'b> {
    /// The memory slice that get's consumed.
    buffer: &'b [u8],
}

impl<'b> ByteConsumer<'b> {
    /// Creates a new ByteConsumer from a stream of bytes.
    pub fn new(stream: &'b [u8]) -> ByteConsumer {
        ByteConsumer { buffer: stream }
    }

    /// Consumes `nr_bytes` from the original slice and returns an `Option` that holds a reference
    /// to a memory slice which is `nr_bytes` long. If `nr_bytes` is greater than the remaining
    /// bytes of the consumer's slice then `None` is returned.
    /// 
    /// # Arguments
    /// 
    /// * `nr_bytes` - A number representing the number of bytes to consume
    /// 
    /// *Note*: The memory slice returned is not copied, it's a reference to the initial slice
    /// 
    /// # Examples
    /// 
    /// ```
    /// let mut consumer = ByteConsumer::new(&[10, 20, 30, 40]);
    /// let first_two = consumer.consume(2);
    /// let second_two = consumer.consume(2);
    /// 
    /// assert!(first_two.is_some());
    /// assert_eq!(first_two.unwrap(), [10, 20]);
    /// 
    /// assert!(second_two.is_some());
    /// assert_eq!(second_two.unwrap(), [30, 40]);
    /// 
    /// assert_eq!(consumer.consume(1), None);
    /// ```
    /// 
    #[inline]
    pub fn consume(&mut self, nr_bytes: usize) -> Option<&'b [u8]> {
        if nr_bytes > self.buffer.len() || nr_bytes == 0 {
            None
        } else {
            let (res, remaining) = self.buffer.split_at(nr_bytes);
            self.buffer = remaining;
            Some(res)
        }
    }

    /// Consumes `nr_bytes` from the original slice and returns a reference to a memory slice which is `nr_bytes` long.
    /// 
    /// # Arguments
    /// 
    /// * `nr_bytes` - A number representing the number of bytes to consume
    /// 
    /// # Panics
    /// 
    /// If `nr_bytes` is greater than the remaining bytes in the consumer's slice then
    /// then it panics with a custom message.
    /// 
    /// *Note*: The memory slice returned is not copied, it's a reference to the initial slice
    ///
    /// # Examples
    /// 
    /// ```
    /// let mut consumer = ByteConsumer::new(&[10, 20, 30, 40]);
    /// let first_two = consumer.consume_unchecked(2);
    /// let second_two = consumer.consume_unchecked(2);
    /// 
    /// assert_eq!(first_two, [10, 20]);
    /// assert_eq!(second_two, [30, 40]);
    /// 
    /// // let _next = consumer.consume_unchecked(1) <-- This should panic.
    /// ``` 
    #[inline]
    pub fn consume_unchecked(&mut self, nr_bytes: usize) -> &'b [u8] {
        self.consume(nr_bytes).expect(format!(
            "[ByteConsumer]: The number of bytes requested ({}) is greater that the number of bytes available ({}).",
            nr_bytes,
            self.buffer.len()
        ).as_str())
    }

    
    #[inline]
    pub fn consume_as<T: Copy>(&mut self) -> Option<T> {
        let bytes = self.consume(mem::size_of::<T>());
        match bytes {
            Some(bytes) => unsafe { Some(*(bytes.as_ptr() as *const T)) },
            None => None,
        }
    }

    #[inline]
    pub fn consume_as_unchecked<T: Copy>(&mut self) -> T {
        self.consume_as::<T>().unwrap()
    }
}

#[cfg(test)]
mod tests {
    #[test]
    fn it_works() {
        assert_eq!(2 + 2, 4);
    }
}