tinybufr 0.1.3

A decoder for BUFR meteorological data format
Documentation 
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//! Descriptors (FXY)

use std::fmt::Debug;
use std::io::Read;

use byteorder::{BigEndian, ReadBytesExt};

use crate::{
    Error,
    tables::{TableBEntry, TableDEntry, Tables},
};

/// Descriptor (FXY).
#[derive(Hash, Copy, Clone, Eq, PartialEq)]
#[cfg_attr(feature = "serde", derive(serde::Serialize))]
pub struct Descriptor {
    pub f: u8,
    pub x: u8,
    pub y: u8,
}

impl Descriptor {
    pub fn read<R: Read>(reader: &mut R) -> Result<Self, Error> {
        let val = reader.read_u16::<BigEndian>()?;
        Ok(Descriptor {
            f: (val >> 14) as u8,
            x: ((val >> 8) & 0x3f) as u8,
            y: (val & 0xff) as u8,
        })
    }
}

impl Debug for Descriptor {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "Descriptor {0:1}{1:02}{2:03}", self.f, self.x, self.y)
    }
}

impl Descriptor {
    pub fn xy(&self) -> XY {
        XY {
            x: self.x,
            y: self.y,
        }
    }
}

/// X and Y parts of a descriptor.
#[derive(Hash, Debug, Clone, Copy, Eq, PartialEq)]
pub struct XY {
    pub x: u8,
    pub y: u8,
}

/// A descriptor that has been resolved with table lookups.
#[derive(Debug)]
pub enum ResolvedDescriptor<'a> {
    Data(&'a TableBEntry),
    Replication {
        y: u8,
        delayed_bits: u8,
        descriptors: Vec<ResolvedDescriptor<'a>>,
    },
    Operator(XY),
    Sequence(&'a TableDEntry, Vec<ResolvedDescriptor<'a>>),
}

impl<'a> ResolvedDescriptor<'a> {
    pub fn from_descriptor(desc: &Descriptor, tables: &Tables) -> Result<Self, Error> {
        Ok(match desc.f {
            0 => {
                let Some(b) = tables.table_b.get(&desc.xy()) else {
                    return Err(Error::Table(format!(
                        "Table B entry not found for xy: {:?}",
                        desc.xy()
                    )));
                };
                ResolvedDescriptor::Data(b)
            }
            1 => unreachable!(),
            2 => ResolvedDescriptor::Operator(desc.xy()),
            3 => {
                let Some(d) = tables.table_d.get(&desc.xy()) else {
                    return Err(Error::Table(format!(
                        "Table D entry not found for xy: {:?}",
                        desc.xy()
                    )));
                };
                let resolved_elements = resolve_descriptors(tables, d.elements)?;
                ResolvedDescriptor::Sequence(d, resolved_elements)
            }
            _ => {
                return Err(Error::Table(format!(
                    "Table B entry not found for xy: {:?}",
                    desc.xy()
                )));
            }
        })
    }
}

pub(crate) fn resolve_descriptors<'a>(
    tables: &Tables,
    descriptors: &'a [Descriptor],
) -> Result<Vec<ResolvedDescriptor<'a>>, Error> {
    let mut resolved = vec![];
    let mut pos = 0;
    while pos < descriptors.len() {
        match &descriptors[pos] {
            &Descriptor { f: 1, x, y } => {
                let delayed_bits = match y {
                    // delayed replication when YYY = 0
                    0 => {
                        pos += 1;
                        match descriptors[pos] {
                            Descriptor { f: 0, x: 31, y: 0 } => 1,
                            Descriptor { f: 0, x: 31, y: 1 } => 8,
                            Descriptor { f: 0, x: 31, y: 2 } => 16,
                            Descriptor { f: 0, x: 31, y: 3 } => 8, // Note: JMA-local?
                            desc => {
                                return Err(Error::NotSupported(format!(
                                    "Unsupported delayed descriptor replication factor: {desc:#?}",
                                )));
                            }
                        }
                    }
                    _ => 0,
                };
                pos += 1;
                if pos + x as usize > descriptors.len() {
                    return Err(Error::Invalid(
                        "Replication range out of bounds".to_string(),
                    ));
                }
                resolved.push(ResolvedDescriptor::Replication {
                    y,
                    descriptors: resolve_descriptors(tables, &descriptors[pos..pos + x as usize])?,
                    delayed_bits,
                });
                pos += x as usize;
            }
            desc => {
                resolved.push(ResolvedDescriptor::from_descriptor(desc, tables)?);
                pos += 1;
            }
        }
    }

    Ok(resolved)
}