grib-reader 0.1.3

//! Data Representation Section (Section 5) and Data Section (Section 7) decoding.

use crate::error::{Error, Result};
use crate::util::grib_i16;

/// Data representation template number and parameters.
#[derive(Debug, Clone, PartialEq)]
pub enum DataRepresentation {
    /// Template 5.0: Simple packing.
    SimplePacking(SimplePackingParams),
    /// Template 5.2/5.3: Complex packing with optional spatial differencing.
    ComplexPacking(ComplexPackingParams),
    /// Unsupported template.
    Unsupported(u16),
}

/// Parameters for simple packing (Template 5.0).
#[derive(Debug, Clone, PartialEq)]
pub struct SimplePackingParams {
    pub encoded_values: usize,
    pub reference_value: f32,
    pub binary_scale: i16,
    pub decimal_scale: i16,
    pub bits_per_value: u8,
    pub original_field_type: u8,
}

/// Parameters for complex packing (Templates 5.2 and 5.3).
#[derive(Debug, Clone, PartialEq)]
pub struct ComplexPackingParams {
    pub encoded_values: usize,
    pub reference_value: f32,
    pub binary_scale: i16,
    pub decimal_scale: i16,
    pub group_reference_bits: u8,
    pub original_field_type: u8,
    pub group_splitting_method: u8,
    pub missing_value_management: u8,
    pub primary_missing_substitute: u32,
    pub secondary_missing_substitute: u32,
    pub num_groups: usize,
    pub group_width_reference: u8,
    pub group_width_bits: u8,
    pub group_length_reference: u32,
    pub group_length_increment: u8,
    pub true_length_last_group: u32,
    pub scaled_group_length_bits: u8,
    pub spatial_differencing: Option<SpatialDifferencingParams>,
}

/// Parameters specific to template 5.3 spatial differencing.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct SpatialDifferencingParams {
    pub order: u8,
    pub descriptor_octets: u8,
}

impl DataRepresentation {
    pub fn parse(section_bytes: &[u8]) -> Result<Self> {
        if section_bytes.len() < 11 {
            return Err(Error::InvalidSection {
                section: 5,
                reason: format!("expected at least 11 bytes, got {}", section_bytes.len()),
            });
        }
        if section_bytes[4] != 5 {
            return Err(Error::InvalidSection {
                section: section_bytes[4],
                reason: "not a data representation section".into(),
            });
        }

        let template = u16::from_be_bytes(section_bytes[9..11].try_into().unwrap());
        match template {
            0 => parse_simple_packing(section_bytes),
            2 => parse_complex_packing(section_bytes, false),
            3 => parse_complex_packing(section_bytes, true),
            _ => Ok(Self::Unsupported(template)),
        }
    }

    pub fn encoded_values(&self) -> Option<usize> {
        match self {
            Self::SimplePacking(params) => Some(params.encoded_values),
            Self::ComplexPacking(params) => Some(params.encoded_values),
            Self::Unsupported(_) => None,
        }
    }
}

/// Decode Section 7 payload into field values, applying Section 6 bitmap when present.
pub fn decode_field(
    data_section: &[u8],
    representation: &DataRepresentation,
    bitmap_section: Option<&[u8]>,
    num_grid_points: usize,
) -> Result<Vec<f64>> {
    if data_section.len() < 5 || data_section[4] != 7 {
        return Err(Error::InvalidSection {
            section: data_section.get(4).copied().unwrap_or(7),
            reason: "not a data section".into(),
        });
    }

    decode_payload(
        &data_section[5..],
        representation,
        bitmap_section,
        num_grid_points,
    )
}

pub(crate) fn decode_payload(
    payload: &[u8],
    representation: &DataRepresentation,
    bitmap_section: Option<&[u8]>,
    num_grid_points: usize,
) -> Result<Vec<f64>> {
    match representation {
        DataRepresentation::SimplePacking(params) => {
            let unpacked = unpack_simple(payload, params, params.encoded_values)?;
            match bitmap_section {
                Some(bitmap) => apply_bitmap(bitmap, unpacked, num_grid_points),
                None => {
                    if params.encoded_values != num_grid_points {
                        return Err(Error::DataLengthMismatch {
                            expected: num_grid_points,
                            actual: params.encoded_values,
                        });
                    }
                    Ok(unpacked)
                }
            }
        }
        DataRepresentation::ComplexPacking(params) => {
            let unpacked = unpack_complex(payload, params)?;
            match bitmap_section {
                Some(bitmap) => apply_bitmap(bitmap, unpacked, num_grid_points),
                None => {
                    if params.encoded_values != num_grid_points {
                        return Err(Error::DataLengthMismatch {
                            expected: num_grid_points,
                            actual: params.encoded_values,
                        });
                    }
                    Ok(unpacked)
                }
            }
        }
        DataRepresentation::Unsupported(template) => Err(Error::UnsupportedDataTemplate(*template)),
    }
}

/// Parse bitmap presence from Section 6.
pub fn bitmap_payload(section_bytes: &[u8]) -> Result<Option<&[u8]>> {
    if section_bytes.len() < 6 {
        return Err(Error::InvalidSection {
            section: 6,
            reason: format!("expected at least 6 bytes, got {}", section_bytes.len()),
        });
    }
    if section_bytes[4] != 6 {
        return Err(Error::InvalidSection {
            section: section_bytes[4],
            reason: "not a bitmap section".into(),
        });
    }

    match section_bytes[5] {
        255 => Ok(None),
        0 => Ok(Some(&section_bytes[6..])),
        indicator => Err(Error::UnsupportedBitmapIndicator(indicator)),
    }
}

fn parse_simple_packing(data: &[u8]) -> Result<DataRepresentation> {
    if data.len() < 21 {
        return Err(Error::InvalidSection {
            section: 5,
            reason: format!("template 5.0 requires 21 bytes, got {}", data.len()),
        });
    }

    let encoded_values = u32::from_be_bytes(data[5..9].try_into().unwrap()) as usize;
    let reference_value = f32::from_be_bytes(data[11..15].try_into().unwrap());
    let binary_scale = grib_i16(&data[15..17]).unwrap();
    let decimal_scale = grib_i16(&data[17..19]).unwrap();
    let bits_per_value = data[19];
    let original_field_type = data[20];

    Ok(DataRepresentation::SimplePacking(SimplePackingParams {
        encoded_values,
        reference_value,
        binary_scale,
        decimal_scale,
        bits_per_value,
        original_field_type,
    }))
}

fn parse_complex_packing(
    data: &[u8],
    with_spatial_differencing: bool,
) -> Result<DataRepresentation> {
    let required = if with_spatial_differencing { 49 } else { 47 };
    if data.len() < required {
        return Err(Error::InvalidSection {
            section: 5,
            reason: format!(
                "template 5.{} requires {required} bytes, got {}",
                if with_spatial_differencing { 3 } else { 2 },
                data.len()
            ),
        });
    }

    let group_splitting_method = data[21];
    if group_splitting_method != 1 {
        return Err(Error::UnsupportedGroupSplittingMethod(
            group_splitting_method,
        ));
    }

    let missing_value_management = data[22];
    if missing_value_management > 2 {
        return Err(Error::UnsupportedMissingValueManagement(
            missing_value_management,
        ));
    }

    let spatial_differencing = if with_spatial_differencing {
        let order = data[47];
        if !matches!(order, 1 | 2) {
            return Err(Error::UnsupportedSpatialDifferencingOrder(order));
        }
        Some(SpatialDifferencingParams {
            order,
            descriptor_octets: data[48],
        })
    } else {
        None
    };

    Ok(DataRepresentation::ComplexPacking(ComplexPackingParams {
        encoded_values: u32::from_be_bytes(data[5..9].try_into().unwrap()) as usize,
        reference_value: f32::from_be_bytes(data[11..15].try_into().unwrap()),
        binary_scale: grib_i16(&data[15..17]).unwrap(),
        decimal_scale: grib_i16(&data[17..19]).unwrap(),
        group_reference_bits: data[19],
        original_field_type: data[20],
        group_splitting_method,
        missing_value_management,
        primary_missing_substitute: u32::from_be_bytes(data[23..27].try_into().unwrap()),
        secondary_missing_substitute: u32::from_be_bytes(data[27..31].try_into().unwrap()),
        num_groups: u32::from_be_bytes(data[31..35].try_into().unwrap()) as usize,
        group_width_reference: data[35],
        group_width_bits: data[36],
        group_length_reference: u32::from_be_bytes(data[37..41].try_into().unwrap()),
        group_length_increment: data[41],
        true_length_last_group: u32::from_be_bytes(data[42..46].try_into().unwrap()),
        scaled_group_length_bits: data[46],
        spatial_differencing,
    }))
}

/// Unpack simple-packed values.
pub fn unpack_simple(
    data_bytes: &[u8],
    params: &SimplePackingParams,
    num_values: usize,
) -> Result<Vec<f64>> {
    let bits = params.bits_per_value as usize;
    if bits == 0 {
        return Ok(vec![params.reference_value as f64; num_values]);
    }
    if bits > u64::BITS as usize {
        return Err(Error::UnsupportedPackingWidth(params.bits_per_value));
    }

    let required_bits = bits
        .checked_mul(num_values)
        .ok_or_else(|| Error::Other("bit count overflow during unpacking".into()))?;
    let required_bytes = required_bits.div_ceil(8);
    if data_bytes.len() < required_bytes {
        return Err(Error::Truncated {
            offset: data_bytes.len() as u64,
        });
    }

    let binary_factor = 2.0_f64.powi(params.binary_scale as i32);
    let decimal_factor = 10.0_f64.powi(-(params.decimal_scale as i32));
    let reference = params.reference_value as f64;
    let mut reader = BitReader::new(data_bytes);
    let mut values = Vec::with_capacity(num_values);

    for _ in 0..num_values {
        let packed = reader.read(bits)?;
        values.push(reference + (packed as f64) * binary_factor * decimal_factor);
    }

    Ok(values)
}

fn unpack_complex(data_bytes: &[u8], params: &ComplexPackingParams) -> Result<Vec<f64>> {
    let restored = unpack_complex_scaled_values(data_bytes, params)?;

    let binary_factor = 2.0_f64.powi(params.binary_scale as i32);
    let decimal_factor = 10.0_f64.powi(-(params.decimal_scale as i32));
    let reference = params.reference_value as f64;

    Ok(restored
        .into_iter()
        .map(|value| match value {
            Some(value) => reference + (value as f64) * binary_factor * decimal_factor,
            None => f64::NAN,
        })
        .collect())
}

fn unpack_complex_scaled_values(
    data_bytes: &[u8],
    params: &ComplexPackingParams,
) -> Result<Vec<Option<i64>>> {
    if params.num_groups == 0 {
        return Err(Error::InvalidSection {
            section: 5,
            reason: "complex packing requires at least one group".into(),
        });
    }

    let mut reader = BitReader::new(data_bytes);
    let spatial_descriptors = params
        .spatial_differencing
        .map(|spatial| read_spatial_descriptors(&mut reader, spatial))
        .transpose()?;

    let group_references = read_bits_array(
        &mut reader,
        params.num_groups,
        params.group_reference_bits as usize,
    )?;
    reader.align_to_byte();

    let group_width_deltas = read_bits_array(
        &mut reader,
        params.num_groups,
        params.group_width_bits as usize,
    )?;
    reader.align_to_byte();

    let group_lengths = build_group_lengths(&mut reader, params)?;
    reader.align_to_byte();

    let mut decoded = Vec::with_capacity(params.encoded_values);
    for ((group_reference, group_width_delta), group_length) in group_references
        .into_iter()
        .zip(group_width_deltas)
        .zip(group_lengths)
    {
        let group_width = usize::from(params.group_width_reference)
            .checked_add(group_width_delta as usize)
            .ok_or_else(|| Error::Other("group width overflow".into()))?;

        if group_width == 0 {
            let value = decode_constant_group_value(
                group_reference,
                params.group_reference_bits as usize,
                params.missing_value_management,
            )?;
            decoded.resize(decoded.len() + group_length, value);
            continue;
        }

        if group_width > u64::BITS as usize {
            return Err(Error::UnsupportedPackingWidth(group_width as u8));
        }

        let group_reference = i64::try_from(group_reference)
            .map_err(|_| Error::Other("group reference exceeds i64 range".into()))?;
        for _ in 0..group_length {
            let packed = reader.read(group_width)?;
            let value = decode_group_value(
                group_reference,
                packed,
                group_width,
                params.missing_value_management,
            )?;
            decoded.push(value);
        }
    }

    if decoded.len() != params.encoded_values {
        return Err(Error::DataLengthMismatch {
            expected: params.encoded_values,
            actual: decoded.len(),
        });
    }

    if let Some(spatial) = spatial_descriptors {
        apply_spatial_descriptors(decoded, spatial)
    } else {
        Ok(decoded)
    }
}

fn read_spatial_descriptors(
    reader: &mut BitReader<'_>,
    params: SpatialDifferencingParams,
) -> Result<SpatialDescriptors> {
    if params.descriptor_octets == 0 {
        return Err(Error::InvalidSection {
            section: 5,
            reason: "spatial differencing requires at least one descriptor octet".into(),
        });
    }

    let bit_count = usize::from(params.descriptor_octets) * 8;
    if bit_count > u64::BITS as usize {
        return Err(Error::Other(
            "spatial differencing descriptors wider than 8 octets are not supported".into(),
        ));
    }

    let mut initial_values = Vec::with_capacity(params.order as usize);
    for _ in 0..params.order {
        initial_values.push(reader.read_signed(bit_count)?);
    }
    let overall_minimum = reader.read_signed(bit_count)?;

    Ok(SpatialDescriptors {
        order: params.order,
        initial_values,
        overall_minimum,
    })
}

fn build_group_lengths(
    reader: &mut BitReader<'_>,
    params: &ComplexPackingParams,
) -> Result<Vec<usize>> {
    let scaled_lengths = read_bits_array(
        reader,
        params.num_groups,
        params.scaled_group_length_bits as usize,
    )?;

    let mut lengths = Vec::with_capacity(params.num_groups);
    for (index, scaled) in scaled_lengths.into_iter().enumerate() {
        let length = if index + 1 == params.num_groups {
            params.true_length_last_group as usize
        } else {
            let scaled = scaled
                .checked_mul(u64::from(params.group_length_increment))
                .ok_or_else(|| Error::Other("group length overflow".into()))?;
            let length = u64::from(params.group_length_reference)
                .checked_add(scaled)
                .ok_or_else(|| Error::Other("group length overflow".into()))?;
            usize::try_from(length).map_err(|_| Error::Other("group length overflow".into()))?
        };
        lengths.push(length);
    }

    let actual_total = lengths.iter().sum::<usize>();
    if actual_total != params.encoded_values {
        return Err(Error::DataLengthMismatch {
            expected: params.encoded_values,
            actual: actual_total,
        });
    }

    Ok(lengths)
}

fn read_bits_array(reader: &mut BitReader<'_>, count: usize, bit_count: usize) -> Result<Vec<u64>> {
    if bit_count > u64::BITS as usize {
        return Err(Error::UnsupportedPackingWidth(bit_count as u8));
    }

    let mut values = Vec::with_capacity(count);
    for _ in 0..count {
        values.push(reader.read(bit_count)?);
    }
    Ok(values)
}

fn decode_constant_group_value(
    group_reference: u64,
    group_reference_bits: usize,
    missing_value_management: u8,
) -> Result<Option<i64>> {
    if is_missing_code(
        group_reference,
        group_reference_bits,
        missing_value_management,
        MissingKind::Primary,
    )? || is_missing_code(
        group_reference,
        group_reference_bits,
        missing_value_management,
        MissingKind::Secondary,
    )? {
        return Ok(None);
    }

    let value = i64::try_from(group_reference)
        .map_err(|_| Error::Other("group reference exceeds i64 range".into()))?;
    Ok(Some(value))
}

fn decode_group_value(
    group_reference: i64,
    packed: u64,
    group_width: usize,
    missing_value_management: u8,
) -> Result<Option<i64>> {
    if is_missing_code(
        packed,
        group_width,
        missing_value_management,
        MissingKind::Primary,
    )? || is_missing_code(
        packed,
        group_width,
        missing_value_management,
        MissingKind::Secondary,
    )? {
        return Ok(None);
    }

    let packed =
        i64::try_from(packed).map_err(|_| Error::Other("packed value exceeds i64 range".into()))?;
    let value = group_reference
        .checked_add(packed)
        .ok_or_else(|| Error::Other("decoded complex packing value overflow".into()))?;
    Ok(Some(value))
}

fn apply_spatial_descriptors(
    values: Vec<Option<i64>>,
    descriptors: SpatialDescriptors,
) -> Result<Vec<Option<i64>>> {
    match descriptors.order {
        1 => restore_first_order_spatial_differencing(values, descriptors),
        2 => restore_second_order_spatial_differencing(values, descriptors),
        other => Err(Error::UnsupportedSpatialDifferencingOrder(other)),
    }
}

fn restore_first_order_spatial_differencing(
    values: Vec<Option<i64>>,
    descriptors: SpatialDescriptors,
) -> Result<Vec<Option<i64>>> {
    let Some(&first_value) = descriptors.initial_values.first() else {
        return Err(Error::InvalidSection {
            section: 5,
            reason: "missing first-order spatial differencing descriptor".into(),
        });
    };

    let mut restored = Vec::with_capacity(values.len());
    let mut previous = None;
    let mut non_missing_seen = 0usize;

    for value in values {
        match value {
            Some(value) => {
                let restored_value = if non_missing_seen == 0 {
                    first_value
                } else {
                    let delta = value
                        .checked_add(descriptors.overall_minimum)
                        .ok_or_else(|| Error::Other("spatial differencing overflow".into()))?;
                    previous
                        .and_then(|previous: i64| previous.checked_add(delta))
                        .ok_or_else(|| Error::Other("spatial differencing overflow".into()))?
                };
                previous = Some(restored_value);
                non_missing_seen += 1;
                restored.push(Some(restored_value));
            }
            None => restored.push(None),
        }
    }

    if non_missing_seen == 0 {
        return Err(Error::DataLengthMismatch {
            expected: 1,
            actual: 0,
        });
    }

    Ok(restored)
}

fn restore_second_order_spatial_differencing(
    values: Vec<Option<i64>>,
    descriptors: SpatialDescriptors,
) -> Result<Vec<Option<i64>>> {
    if descriptors.initial_values.len() < 2 {
        return Err(Error::InvalidSection {
            section: 5,
            reason: "missing second-order spatial differencing descriptors".into(),
        });
    }

    let first_value = descriptors.initial_values[0];
    let second_value = descriptors.initial_values[1];
    let mut restored = Vec::with_capacity(values.len());
    let mut previous: Option<i64> = None;
    let mut previous_difference: Option<i64> = None;
    let mut non_missing_seen = 0usize;

    for value in values {
        match value {
            Some(value) => {
                let restored_value = match non_missing_seen {
                    0 => first_value,
                    1 => {
                        previous_difference = second_value.checked_sub(first_value);
                        second_value
                    }
                    _ => {
                        let second_difference = value
                            .checked_add(descriptors.overall_minimum)
                            .ok_or_else(|| Error::Other("spatial differencing overflow".into()))?;
                        let difference = previous_difference
                            .and_then(|previous_difference| {
                                previous_difference.checked_add(second_difference)
                            })
                            .ok_or_else(|| Error::Other("spatial differencing overflow".into()))?;
                        let next = previous
                            .and_then(|previous: i64| previous.checked_add(difference))
                            .ok_or_else(|| Error::Other("spatial differencing overflow".into()))?;
                        previous_difference = Some(difference);
                        next
                    }
                };

                previous = Some(restored_value);
                non_missing_seen += 1;
                restored.push(Some(restored_value));
            }
            None => restored.push(None),
        }
    }

    if non_missing_seen < 2 {
        return Err(Error::DataLengthMismatch {
            expected: 2,
            actual: non_missing_seen,
        });
    }

    Ok(restored)
}

fn is_missing_code(
    value: u64,
    bit_width: usize,
    missing_value_management: u8,
    kind: MissingKind,
) -> Result<bool> {
    let required_mode = match kind {
        MissingKind::Primary => 1,
        MissingKind::Secondary => 2,
    };
    if missing_value_management < required_mode {
        return Ok(false);
    }

    let Some(code) = missing_code(bit_width, kind)? else {
        return Ok(false);
    };
    Ok(value == code)
}

fn missing_code(bit_width: usize, kind: MissingKind) -> Result<Option<u64>> {
    if bit_width == 0 {
        return Ok(None);
    }
    if bit_width > u64::BITS as usize {
        return Err(Error::UnsupportedPackingWidth(bit_width as u8));
    }

    let max_value = if bit_width == u64::BITS as usize {
        u64::MAX
    } else {
        (1u64 << bit_width) - 1
    };

    let code = match kind {
        MissingKind::Primary => max_value,
        MissingKind::Secondary => max_value.saturating_sub(1),
    };
    Ok(Some(code))
}

fn apply_bitmap(
    bitmap_payload: &[u8],
    packed_values: Vec<f64>,
    num_grid_points: usize,
) -> Result<Vec<f64>> {
    let mut decoded = Vec::with_capacity(num_grid_points);
    let mut packed_iter = packed_values.into_iter();
    let mut present_points = 0usize;

    for bit_index in 0..num_grid_points {
        if bitmap_bit(bitmap_payload, bit_index)? {
            present_points += 1;
            decoded.push(packed_iter.next().ok_or(Error::MissingBitmap)?);
        } else {
            decoded.push(f64::NAN);
        }
    }

    let extra_values = packed_iter.count();
    if extra_values > 0 {
        return Err(Error::DataLengthMismatch {
            expected: present_points,
            actual: present_points + extra_values,
        });
    }

    Ok(decoded)
}

fn bitmap_bit(bitmap_payload: &[u8], index: usize) -> Result<bool> {
    let byte_index = index / 8;
    let bit_index = index % 8;
    let byte = bitmap_payload
        .get(byte_index)
        .copied()
        .ok_or(Error::MissingBitmap)?;
    Ok(((byte >> (7 - bit_index)) & 1) != 0)
}

struct BitReader<'a> {
    data: &'a [u8],
    bit_offset: usize,
}

impl<'a> BitReader<'a> {
    fn new(data: &'a [u8]) -> Self {
        Self {
            data,
            bit_offset: 0,
        }
    }

    fn read(&mut self, bit_count: usize) -> Result<u64> {
        if bit_count == 0 {
            return Ok(0);
        }

        let mut remaining = bit_count;
        let mut value = 0u64;

        while remaining > 0 {
            let byte_index = self.bit_offset / 8;
            let bit_index = self.bit_offset % 8;
            let byte = *self.data.get(byte_index).ok_or(Error::Truncated {
                offset: byte_index as u64,
            })?;
            let available = 8 - bit_index;
            let take = remaining.min(available);
            let mask = ((1u16 << take) - 1) as u8;
            let shift = available - take;
            let bits = (byte >> shift) & mask;

            value = (value << take) | bits as u64;
            self.bit_offset += take;
            remaining -= take;
        }

        Ok(value)
    }

    fn read_signed(&mut self, bit_count: usize) -> Result<i64> {
        let value = self.read(bit_count)?;
        if bit_count == 0 {
            return Ok(0);
        }

        let sign_mask = 1u64 << (bit_count - 1);
        if value & sign_mask == 0 {
            return i64::try_from(value)
                .map_err(|_| Error::Other("signed value exceeds i64 range".into()));
        }

        let magnitude_mask = sign_mask - 1;
        let magnitude = value & magnitude_mask;
        let magnitude = i64::try_from(magnitude)
            .map_err(|_| Error::Other("signed value exceeds i64 range".into()))?;
        Ok(-magnitude)
    }

    fn align_to_byte(&mut self) {
        self.bit_offset = self.bit_offset.next_multiple_of(8);
    }
}

#[derive(Debug, Clone)]
struct SpatialDescriptors {
    order: u8,
    initial_values: Vec<i64>,
    overall_minimum: i64,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum MissingKind {
    Primary,
    Secondary,
}

#[cfg(test)]
mod tests {
    use super::{
        bitmap_payload, decode_field, unpack_complex, unpack_simple, ComplexPackingParams,
        DataRepresentation, SimplePackingParams, SpatialDifferencingParams,
    };
    use crate::error::Error;

    #[test]
    fn unpack_simple_constant() {
        let params = SimplePackingParams {
            encoded_values: 5,
            reference_value: 42.0,
            binary_scale: 0,
            decimal_scale: 0,
            bits_per_value: 0,
            original_field_type: 0,
        };
        let values = unpack_simple(&[], &params, 5).unwrap();
        assert_eq!(values, vec![42.0; 5]);
    }

    #[test]
    fn unpack_simple_basic() {
        let params = SimplePackingParams {
            encoded_values: 5,
            reference_value: 0.0,
            binary_scale: 0,
            decimal_scale: 0,
            bits_per_value: 8,
            original_field_type: 0,
        };
        let values = unpack_simple(&[0, 1, 2, 3, 4], &params, 5).unwrap();
        assert_eq!(values, vec![0.0, 1.0, 2.0, 3.0, 4.0]);
    }

    #[test]
    fn decodes_bitmap_masked_field() {
        let data_section = [0, 0, 0, 8, 7, 10, 20, 30];
        let bitmap_section = [0, 0, 0, 7, 6, 0, 0b1011_0000];
        let representation = DataRepresentation::SimplePacking(SimplePackingParams {
            encoded_values: 3,
            reference_value: 0.0,
            binary_scale: 0,
            decimal_scale: 0,
            bits_per_value: 8,
            original_field_type: 0,
        });

        let bitmap = bitmap_payload(&bitmap_section).unwrap();
        let decoded = decode_field(&data_section, &representation, bitmap, 4).unwrap();
        assert_eq!(decoded[0], 10.0);
        assert!(decoded[1].is_nan());
        assert_eq!(decoded[2], 20.0);
        assert_eq!(decoded[3], 30.0);
    }

    #[test]
    fn rejects_simple_packing_wider_than_u64() {
        let params = SimplePackingParams {
            encoded_values: 1,
            reference_value: 0.0,
            binary_scale: 0,
            decimal_scale: 0,
            bits_per_value: 65,
            original_field_type: 0,
        };
        let err = unpack_simple(&[0; 9], &params, 1).unwrap_err();
        assert!(matches!(err, Error::UnsupportedPackingWidth(65)));
    }

    #[test]
    fn rejects_encoded_value_count_mismatch_without_bitmap() {
        let data_section = [0, 0, 0, 8, 7, 10, 20, 30];
        let representation = DataRepresentation::SimplePacking(SimplePackingParams {
            encoded_values: 3,
            reference_value: 0.0,
            binary_scale: 0,
            decimal_scale: 0,
            bits_per_value: 8,
            original_field_type: 0,
        });

        let err = decode_field(&data_section, &representation, None, 4).unwrap_err();
        assert!(matches!(
            err,
            Error::DataLengthMismatch {
                expected: 4,
                actual: 3,
            }
        ));
    }

    #[test]
    fn unpacks_complex_packing_with_explicit_missing() {
        let params = ComplexPackingParams {
            encoded_values: 4,
            reference_value: 0.0,
            binary_scale: 0,
            decimal_scale: 0,
            group_reference_bits: 4,
            original_field_type: 0,
            group_splitting_method: 1,
            missing_value_management: 1,
            primary_missing_substitute: u32::MAX,
            secondary_missing_substitute: u32::MAX,
            num_groups: 2,
            group_width_reference: 0,
            group_width_bits: 2,
            group_length_reference: 2,
            group_length_increment: 1,
            true_length_last_group: 2,
            scaled_group_length_bits: 0,
            spatial_differencing: None,
        };

        let values = unpack_complex(&[0x79, 0x90, 0x34], &params).unwrap();
        assert_eq!(values[0], 7.0);
        assert!(values[1].is_nan());
        assert_eq!(values[2], 9.0);
        assert!(values[3].is_nan());
    }

    #[test]
    fn unpacks_complex_packing_with_second_order_spatial_differencing() {
        let params = ComplexPackingParams {
            encoded_values: 4,
            reference_value: 0.0,
            binary_scale: 0,
            decimal_scale: 0,
            group_reference_bits: 1,
            original_field_type: 0,
            group_splitting_method: 1,
            missing_value_management: 0,
            primary_missing_substitute: u32::MAX,
            secondary_missing_substitute: u32::MAX,
            num_groups: 1,
            group_width_reference: 1,
            group_width_bits: 0,
            group_length_reference: 4,
            group_length_increment: 1,
            true_length_last_group: 4,
            scaled_group_length_bits: 0,
            spatial_differencing: Some(SpatialDifferencingParams {
                order: 2,
                descriptor_octets: 2,
            }),
        };

        let values =
            unpack_complex(&[0x00, 0x0A, 0x00, 0x0D, 0x00, 0x03, 0x00, 0x10], &params).unwrap();
        assert_eq!(values, vec![10.0, 13.0, 19.0, 29.0]);
    }

    #[test]
    fn unpacks_complex_packing_with_spatial_differencing_and_missing_values() {
        let params = ComplexPackingParams {
            encoded_values: 4,
            reference_value: 0.0,
            binary_scale: 0,
            decimal_scale: 0,
            group_reference_bits: 1,
            original_field_type: 0,
            group_splitting_method: 1,
            missing_value_management: 1,
            primary_missing_substitute: u32::MAX,
            secondary_missing_substitute: u32::MAX,
            num_groups: 1,
            group_width_reference: 2,
            group_width_bits: 0,
            group_length_reference: 4,
            group_length_increment: 1,
            true_length_last_group: 4,
            scaled_group_length_bits: 0,
            spatial_differencing: Some(SpatialDifferencingParams {
                order: 1,
                descriptor_octets: 2,
            }),
        };

        let values = unpack_complex(&[0x00, 0x0A, 0x00, 0x03, 0x00, 0x32], &params).unwrap();
        assert_eq!(values[0], 10.0);
        assert!(values[1].is_nan());
        assert_eq!(values[2], 13.0);
        assert_eq!(values[3], 18.0);
    }
}