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use std::io::{Write, Error};
use bitbit::BitWriter;
use crate::util::range::Range;
use crate::util::source_model::SourceModel;


pub struct ArithmeticEncoder {
    _precision: u64,
    pending_bit_count: u32,
    range: Range,
}

impl ArithmeticEncoder {
    /// # Arguments
   /// `precision` is the [bit precision](https://en.wikipedia.org/wiki/Arithmetic_coding#Precision_and_renormalization)
   /// that the encoder should use. If the
   /// precision is too low than symbols will not be able to be differentiated.
    pub fn new(precision: u64) -> Self {
        Self {
            _precision: precision,
            pending_bit_count: 0,
            range: Range::new(precision),
        }
    }
    pub fn encode<T: Write>(&mut self, symbol: u32,
                            source_model: &SourceModel,
                            output: &mut BitWriter<T>)
                            -> Result<(), Error> {
        let low_high: (u64, u64) = self.range.calculate_range(symbol, &source_model);
        self.range.update_range(low_high);

        while self.range.in_bottom_half() || self.range.in_upper_half() {
            if self.range.in_bottom_half() {
                self.range.scale_bottom_half();
                self.emit(false, output)?;
            } else if self.range.in_upper_half() {
                self.range.scale_upper_half();
                self.emit(true, output)?;
            }
        }

        while self.range.in_middle_half() {
            self.pending_bit_count += 1;
            self.range.scale_middle_half();
        }
        Ok(())
    }
    fn emit<T: Write>(&mut self, bit: bool, output: &mut BitWriter<T>) -> Result<(), Error> {
        output.write_bit(bit)?;
        while self.pending_bit_count > 0 {
            output.write_bit(!bit)?;
            self.pending_bit_count -= 1;
        }
        Ok(())
    }
    pub fn finish_encode<T: Write>(&mut self, output: &mut BitWriter<T>) -> Result<bool, Error> {
        self.pending_bit_count += 1;
        if self.range.in_bottom_quarter() {
            self.emit(false, output)?;
        } else {
            self.emit(true, output)?;
        }
        Ok(true)
    }
}

#[cfg(test)]
mod test {
    use crate::encode::encoder::ArithmeticEncoder;
    use crate::util::source_model::SourceModel;
    use std::io::Cursor;
    use bitbit::BitWriter;

    #[test]
    fn e2e() {
        let mut encoder = ArithmeticEncoder::new(30);
        let mut source_model = SourceModel::new(10, 9);
        let mut output = Cursor::new(vec![]);
        let mut out_writer = BitWriter::new(&mut output);
        let to_encode: [u32; 5] = [7, 2, 2, 2, 7];
        for x in to_encode.iter() {
            encoder.encode(*x, &mut source_model, &mut out_writer).unwrap();
            source_model.update_symbol(*x);
        }
        encoder.encode(source_model.get_eof(), &source_model, &mut out_writer).unwrap();
        encoder.finish_encode(&mut out_writer).unwrap();
        out_writer.pad_to_byte().unwrap();
        assert_eq!(output.get_ref(), &[184, 96, 208]);
    }
}