grib/encoder/

simple.rs

use crate::{
    WriteToBuffer,
    def::grib2::template::param_set::SimplePacking,
    encoder::{Encode, WriteGrib2DataSections, bitmap::Bitmap, helpers::BitsRequired, writer},
};

/// Strategies applied when performing simple packing on numerical sequences.
/// Simple packing is a method for discretizing continuous numerical values as
/// integers, and various approaches can be taken during this process.
#[derive(Debug, PartialEq, Eq, Clone)]
#[non_exhaustive]
pub enum SimplePackingStrategy {
    /// A strategy specifying how many decimal places to consider valid for the
    /// numbers. This strategy is effective for various types of data, such as
    /// observation data obtained from specific observation instruments, where
    /// precision is clearly defined.
    Decimal(i16),
}

pub(crate) struct Encoder<'a> {
    data: &'a [f64],
    strategy: SimplePackingStrategy,
}

impl<'a> Encoder<'a> {
    pub(crate) fn new(data: &'a [f64], strategy: SimplePackingStrategy) -> Self {
        Self { data, strategy }
    }
}

impl<'a> Encode for Encoder<'a> {
    type Output = Encoded;

    fn encode(&self) -> Self::Output {
        match self.strategy {
            SimplePackingStrategy::Decimal(dec) => {
                let (params, scaled, bitmap) = determine_simple_packing_params(self.data, dec);
                let coded = if params.num_bits == 0 {
                    CodedValues::Unique(scaled.len())
                } else {
                    let exp = 2_f64.powf(params.exp as f64);
                    let coded = scaled
                        .iter()
                        .map(|value| ((value - params.ref_val as f64) / exp).round() as u32)
                        .collect::<Vec<_>>();
                    CodedValues::NonUnique(coded)
                };
                Encoded::new(params, coded, bitmap)
            }
        }
    }
}

#[derive(Debug)]
pub(crate) struct Encoded {
    params: SimplePacking,
    coded: CodedValues,
    bitmap: Bitmap,
}

impl Encoded {
    fn new(params: SimplePacking, coded: CodedValues, bitmap: Bitmap) -> Self {
        Self {
            params,
            coded,
            bitmap,
        }
    }

    pub(crate) fn params(&self) -> &SimplePacking {
        &self.params
    }
}

impl WriteGrib2DataSections for Encoded {
    fn section5_len(&self) -> usize {
        21
    }

    fn write_section5(&self, buf: &mut [u8]) -> Result<usize, &'static str> {
        let len = self.section5_len();
        if buf.len() < len {
            return Err("destination buffer is too small");
        }

        let mut pos = 0;
        pos += super::write_section_header(len as u32, 5, &mut buf[pos..])?;
        pos += (self.coded.num_values() as u32).write_to_buffer(&mut buf[pos..])?; // payload.num_encoded_points
        pos += 0_u16.write_to_buffer(&mut buf[pos..])?; // payload.template_num
        pos += self.params.write_to_buffer(&mut buf[pos..])?;
        pos += 0_u8.write_to_buffer(&mut buf[pos..])?; // payload.template.orig_field_type

        Ok(pos)
    }

    fn section6_len(&self) -> usize {
        let bitmap_size = if self.bitmap.has_nan() {
            self.bitmap.num_bytes_required()
        } else {
            0
        };
        6 + bitmap_size
    }

    fn write_section6(&self, buf: &mut [u8]) -> Result<usize, &'static str> {
        let len = self.section6_len();
        if buf.len() < len {
            return Err("destination buffer is too small");
        }

        let mut pos = 0;
        pos += super::write_section_header(len as u32, 6, &mut buf[pos..])?;
        if self.bitmap.has_nan() {
            pos += 0_u8.write_to_buffer(&mut buf[pos..])?;
            pos += self.bitmap.write_to_buffer(&mut buf[pos..])?;
        } else {
            pos += 255_u8.write_to_buffer(&mut buf[pos..])?;
        }

        Ok(pos)
    }

    fn section7_len(&self) -> usize {
        let len = match &self.coded {
            CodedValues::NonUnique(vec) => {
                let nbitwise = writer::NBitwise::new(&vec, self.params.num_bits as usize);
                nbitwise.num_bytes_required()
            }
            CodedValues::Unique(_) => 0,
        };
        5 + len
    }

    fn write_section7(&self, buf: &mut [u8]) -> Result<usize, &'static str> {
        let len = self.section7_len();
        if buf.len() < len {
            return Err("destination buffer is too small");
        }

        let mut pos = 0;
        pos += super::write_section_header(len as u32, 7, &mut buf[pos..])?;
        match &self.coded {
            CodedValues::NonUnique(vec) => {
                let nbitwise = writer::NBitwise::new(&vec, self.params.num_bits as usize);
                pos += nbitwise.write_to_buffer(&mut buf[pos..])?;
            }
            CodedValues::Unique(_) => {}
        }

        Ok(pos)
    }
}

pub(crate) fn determine_simple_packing_params(
    values: &[f64],
    dec: i16,
) -> (SimplePacking, Vec<f64>, Bitmap) {
    let mut min = f64::MAX;
    let mut max = f64::MIN;
    let mut bitmap = Bitmap::for_values(values);
    let scaled = values
        .iter()
        .filter_map(|value| {
            if value.is_nan() {
                bitmap.push(false);
                None
            } else {
                bitmap.push(true);
                let scaled = value * 10_f64.powf(dec as f64);
                (min, max) = (scaled.min(min), scaled.max(max));
                Some(scaled)
            }
        })
        .collect::<Vec<_>>();
    let ref_val = min as f32;
    let params = if min == max {
        SimplePacking {
            ref_val,
            exp: 0,
            dec,
            num_bits: 0,
        }
    } else {
        let range = max - min;
        let exp = 0;
        let num_bits = range.bits_required();
        // TODO: if `num_bits` is too large, increase `exp` to reduce `num_bits`.
        SimplePacking {
            ref_val,
            exp,
            dec,
            num_bits,
        }
    };
    (params, scaled, bitmap)
}

#[derive(Debug)]
enum CodedValues {
    NonUnique(Vec<u32>),
    Unique(usize),
}

impl CodedValues {
    pub(crate) fn num_values(&self) -> usize {
        match self {
            Self::NonUnique(vec) => vec.len(),
            Self::Unique(size) => *size,
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    macro_rules! test_decimal_strategy {
        ($((
            $name:ident,
            $input:expr,
            $decimal:expr,
            $expected_params:expr,
        ),)*) => ($(
            #[test]
            fn $name() {
                let values = $input;
                let encoder = Encoder::new(&values, SimplePackingStrategy::Decimal($decimal));
                let encoded = encoder.encode();
                let actual_params = encoded.params();
                let expected_params = $expected_params;
                assert_eq!(actual_params, &expected_params);
                let actual_num_values = encoded.coded.num_values();
                assert_eq!(actual_num_values, values.len());
            }
        )*);
    }

    test_decimal_strategy! {
        (
            decimal_strategy_with_decimal_0,
            (2..11).map(|val| val as f64).collect::<Vec<_>>(),
            0,
            SimplePacking {
                ref_val: 2.,
                exp: 0,
                dec: 0,
                num_bits: 4,
            },
        ),
        (
            decimal_strategy_with_decimal_1,
            (2..11).map(|val| val as f64).collect::<Vec<_>>(),
            1,
            SimplePacking {
                ref_val: 20.,
                exp: 0,
                dec: 1,
                num_bits: 7,
            },
        ),
        (
            decimal_strategy_with_decimal_0_for_unique_values,
            vec![10.0_f64; 256],
            0,
            SimplePacking {
                ref_val: 10.0,
                exp: 0,
                dec: 0,
                num_bits: 0,
            },
        ),
    }

    macro_rules! grib2_coded_values_roundtrip_tests {
        ($(($name:ident, $input:expr, $decimal:expr),)*) => ($(
            #[test]
            fn $name() -> Result<(), Box<dyn std::error::Error>> {
                let values = $input;
                let encoder = Encoder::new(&values, SimplePackingStrategy::Decimal($decimal));
                let encoded = encoder.encode();
                let mut sect5 = vec![0; encoded.section5_len()];
                let pos = encoded.write_section5(&mut sect5)?;
                assert_eq!(pos, sect5.len());
                let mut sect6 = vec![0; encoded.section6_len()];
                let pos = encoded.write_section6(&mut sect6)?;
                assert_eq!(pos, sect6.len());
                let mut sect7 = vec![0; encoded.section7_len()];
                let pos = encoded.write_section7(&mut sect7)?;
                assert_eq!(pos, sect7.len());
                let decoder = crate::Grib2SubmessageDecoder::new(values.len(), sect5, sect6, sect7)?;
                let actual = decoder.dispatch()?.collect::<Vec<_>>();
                let expected = values.iter().map(|val| *val as f32).collect::<Vec<_>>();
                assert_eq!(actual.len(), expected.len());
                actual
                    .iter()
                    .zip(expected.iter())
                    .all(|(a, b)| (a.is_nan() && b.is_nan()) || (a == b));
                Ok(())
            }
        )*);
    }

    grib2_coded_values_roundtrip_tests! {
        (
            grib2_coded_values_roundtrip_test_with_decimal_0_and_nonunique_values,
            (2..11).map(|val| val as f64).collect::<Vec<_>>(),
            0
        ),
        (
            grib2_coded_values_roundtrip_test_with_decimal_0_and_unique_values,
            vec![10.0_f64; 256],
            0
        ),
        (
            grib2_coded_values_roundtrip_test_with_data_containing_nan_values,
            [f64::NAN; 32]
                .into_iter()
                .chain([1., 2., 3.].into_iter())
                .chain([f64::NAN; 32].into_iter())
                .chain([4., 5., 6., 7.].into_iter())
                .chain([f64::NAN; 32].into_iter())
                .chain([8., 9., 10., 11.].into_iter())
                .collect::<Vec<_>>(),
            0
        ),
    }
}