hotfix_message/encoding/

field_access.rs

use std::iter::FusedIterator;
use std::ops::Range;

use crate::encoding::Buffer;

/// Either a field that is missing or has an invalid value.
#[derive(Debug, thiserror::Error)]
pub enum FieldValueError<E> {
    /// No such field was found.
    #[error("Missing field tag")]
    Missing,
    /// The field was found, but can't be parsed.
    #[error("Invalid field value: {0}")]
    Invalid(#[from] E),
}

impl<E> PartialEq<FieldValueError<E>> for FieldValueError<E> {
    fn eq(&self, other: &FieldValueError<E>) -> bool {
        matches!(
            (self, other),
            (FieldValueError::Missing, FieldValueError::Missing)
        )
    }
}

impl<E> From<Option<E>> for FieldValueError<E> {
    fn from(e: Option<E>) -> Self {
        match e {
            Some(e) => FieldValueError::Invalid(e),
            None => FieldValueError::Missing,
        }
    }
}

/// Provides (de)serialization logic for a Rust type as FIX field values.
///
/// See the [`field_types`](crate::field_types) module for more information.
pub trait FieldType<'a>
where
    Self: Sized,
{
    /// The error type that can arise during deserialization.
    type Error;
    /// A type with values that customize the serialization algorithm, e.g.
    /// padding information.
    type SerializeSettings: Default;

    /// Writes `self` to `buffer` using default settings.
    #[inline]
    fn serialize<B>(&self, buffer: &mut B) -> usize
    where
        B: Buffer,
    {
        self.serialize_with(buffer, Self::SerializeSettings::default())
    }

    /// Writes `self` to `buffer` using custom serialization `settings`.
    fn serialize_with<B>(&self, buffer: &mut B, settings: Self::SerializeSettings) -> usize
    where
        B: Buffer;

    /// Parses and deserializes from `data`.
    fn deserialize(data: &'a [u8]) -> Result<Self, Self::Error>;

    /// Like [`FieldType::deserialize`], but it's allowed to skip *some* amount of
    /// input checking. Invalid inputs might not trigger errors and instead be
    /// deserialized as random values.
    ///
    /// # Safety
    ///
    /// This method remains 100% safe even on malformed inputs.
    fn deserialize_lossy(data: &'a [u8]) -> Result<Self, Self::Error> {
        Self::deserialize(data)
    }

    /// Serializes `self` to a [`Vec`] of bytes, allocated on the fly.
    fn to_bytes(&self) -> Vec<u8> {
        let mut buffer = Vec::new();
        self.serialize(&mut buffer);
        buffer
    }

    /// Allocates a [`String`] representation of `self`, using [`FieldType::to_bytes`].
    ///
    /// # Panics
    ///
    /// This function will panic if the underlying byte representation is not
    /// valid UTF-8. As such, you should only *ever* use this function for
    /// [`FieldType`] implementors that are guaranteed to be representable with
    /// valid UTF-8 (like numbers with ASCII digits).
    fn to_string(&self) -> String {
        String::from_utf8(self.to_bytes()).expect("Invalid UTF-8 representation of FIX field.")
    }
}

/// Provides random (i.e. non-sequential) access to FIX fields and groups within
/// messages.
///
/// # Methods
///
/// [`FieldMap`] provides two kinds of methods:
///
/// 1. Group getters: [`FieldMap::group`] and
///    [`FieldMap::group_opt`].
///
/// 2. Field getters: [`FieldMap::get_raw`], [`FieldMap::get`],
///    etc..
///
/// The most basic form of field access is done via
/// [`FieldMap::get_raw`], which performs no deserialization at all: it
/// simply returns the bytes contents associated with a FIX field, if found.
///
/// Building upon [`FieldMap::get_raw`] and [`FieldType`], the other
/// field access methods all provide some utility deserialization logic. These
/// methods all have the `get_` prefix, with the following considerations:
///
/// - `get_lossy` methods perform "lossy" deserialization via
///   [`FieldType::deserialize_lossy`]. Unlike lossless deserialization, these
///   methods may skip some error checking logic and thus prove to be faster.
///   Memory-safety is still guaranteed, but malformed FIX fields won't be
///   detected 100% of the time.
/// - `get_opt` methods work exactly like their non-`_opt` counterparties, but they
///   have a different return type: instead of returning [`Err(None)`] for missing
///   fields, these methods return [`None`] for missing fields and
///   [`Some(Ok(field))`] for existing fields.
///
/// # Type parameters
///
/// This trait is generic over a type `F`, which must univocally identify FIX
/// fields (besides FIX repeating groups, which allow repetitions).
#[allow(dead_code)]
pub trait FieldMap<F> {
    /// The type returned by [`FieldMap::group`] and
    /// [`FieldMap::group_opt`].
    type Group: RepeatingGroup<Entry = Self>;

    /// Looks for a `field` within `self` and then returns its raw byte
    /// contents, if it exists.
    fn get_raw(&self, field: F) -> Option<&[u8]>;

    /// Looks for a group that starts with `field` within `self`.
    fn group(&self, field: F) -> Result<Self::Group, FieldValueError<<usize as FieldType>::Error>>;

    /// Like [`FieldMap::group`], but doesn't return an [`Err`] if the
    /// group is missing.
    #[inline]
    fn group_opt(&self, field: F) -> Result<Option<Self::Group>, <usize as FieldType>::Error> {
        match self.group(field) {
            Ok(group) => Ok(Some(group)),
            Err(FieldValueError::Missing) => Ok(None),
            Err(FieldValueError::Invalid(e)) => Err(e),
        }
    }

    /// Looks for a `field` within `self` and then decodes its raw byte contents
    /// via [`FieldType::deserialize`], if found.
    #[inline]
    fn get<'a, V>(&'a self, field: F) -> Result<V, FieldValueError<V::Error>>
    where
        V: FieldType<'a>,
    {
        self.get_opt(field)
            .map_err(FieldValueError::Invalid)
            .and_then(|opt| opt.ok_or(FieldValueError::Missing))
    }

    /// Like [`FieldMap::get`], but with lossy deserialization.
    #[inline]
    fn get_lossy<'a, V>(&'a self, field: F) -> Result<V, FieldValueError<V::Error>>
    where
        V: FieldType<'a>,
    {
        self.get_lossy_opt(field)
            .map_err(FieldValueError::Invalid)
            .and_then(|opt| opt.ok_or(FieldValueError::Missing))
    }

    /// Like [`FieldMap::get`], but doesn't return an [`Err`] if `field`
    /// is missing.
    #[inline]
    fn get_opt<'a, V>(&'a self, field: F) -> Result<Option<V>, V::Error>
    where
        V: FieldType<'a>,
    {
        self.get_raw(field).map(V::deserialize).transpose()
    }

    /// Like [`FieldMap::get_opt`], but with lossy deserialization.
    #[inline]
    fn get_lossy_opt<'a, V>(&'a self, field: F) -> Result<Option<V>, V::Error>
    where
        V: FieldType<'a>,
    {
        self.get_raw(field).map(V::deserialize_lossy).transpose()
    }
}

/// Provides access to entries within a FIX repeating group.
pub trait RepeatingGroup: Sized {
    /// The type of entries in this FIX repeating group. Must implement
    /// [`FieldMap`].
    type Entry;

    /// Returns the number of FIX group entries in `self`.
    fn len(&self) -> usize;

    /// Returns the `i` -th entry in `self`, if present.
    fn get(&self, i: usize) -> Option<Self::Entry>;

    /// Creates and returns an [`Iterator`] over the entries in `self`.
    /// Iteration MUST be done in sequential order, i.e. in which they appear in
    /// the original FIX message.
    #[allow(dead_code)]
    fn entries(&self) -> GroupEntries<Self> {
        GroupEntries {
            group: self,
            range: 0..self.len(),
        }
    }
}

/// An [`Iterator`] over the entries of a FIX repeating group.
///
/// This `struct` is created by the method [`RepeatingGroup::entries`]. It
/// also implements [`FusedIterator`], [`DoubleEndedIterator`], and
/// [`ExactSizeIterator`].
#[derive(Debug, Clone)]
pub struct GroupEntries<'a, G> {
    group: &'a G,
    range: Range<usize>,
}

impl<'a, G> Iterator for GroupEntries<'a, G>
where
    G: RepeatingGroup,
{
    type Item = G::Entry;

    fn next(&mut self) -> Option<Self::Item> {
        let i = self.range.next()?;
        self.group.get(i)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.range.size_hint()
    }
}

impl<'a, G> FusedIterator for GroupEntries<'a, G> where G: RepeatingGroup {}
impl<'a, G> ExactSizeIterator for GroupEntries<'a, G> where G: RepeatingGroup {}

impl<'a, G> DoubleEndedIterator for GroupEntries<'a, G>
where
    G: RepeatingGroup,
{
    fn next_back(&mut self) -> Option<Self::Item> {
        let i = self.range.next_back()?;
        self.group.get(i)
    }
}