architecturekit 0.22.0

use std::{any, error::Error, fmt};

use eventsourcingdb::error::{ClientError, code};
use serde_json::Value;

use crate::{Categorized, Category, Failure, Permanent, Revision, Subject};

#[cfg(test)]
mod tests;

/// A command, which knows the subject it acts on, and the conditions under
/// which its events may be written.
///
/// The preconditions are where optimistic concurrency, idempotency and
/// uniqueness live. Every command declares at least one, and the kit adds none
/// of its own:
///
/// ```
/// use architecturekit::{Command, Precondition, Subject};
///
/// #[derive(Clone)]
/// struct AcquireBook {
///     book: Subject,
///     title: String,
/// }
///
/// impl Command for AcquireBook {
///     fn subject(&self) -> Subject {
///         self.book.clone()
///     }
///
///     fn preconditions(&self) -> Vec<Precondition> {
///         vec![Precondition::on_state_read()]
///     }
/// }
/// ```
pub trait Command {
    /// Returns the subject the command acts on.
    fn subject(&self) -> Subject;

    /// Returns the conditions under which the events of the command may be
    /// written, at least one of them.
    fn preconditions(&self) -> Vec<Precondition>;
}

/// A condition that must hold for the events of a command to be written.
///
/// Every command declares at least one, so that writing without any check is
/// always a decision, never an oversight. Create one with
/// [`on_pristine_subject`](Precondition::on_pristine_subject),
/// [`on_populated_subject`](Precondition::on_populated_subject),
/// [`on_event_id`](Precondition::on_event_id),
/// [`on_state_read`](Precondition::on_state_read), or
/// [`unconditionally`](Precondition::unconditionally), or with
/// [`require`](Precondition::require) for any other precondition of the client
/// SDK, such as an EventQL query.
pub struct Precondition {
    kind: Kind,
}

enum Kind {
    Required(eventsourcingdb::Precondition),
    OnEventId(eventsourcingdb::Precondition),
    OnStateRead,
    Unconditionally,
}

impl Precondition {
    /// Makes a precondition of the client SDK one of the command, for one that
    /// the kit has no function of its own for, such as an EventQL query:
    ///
    /// ```
    /// use architecturekit::Precondition;
    ///
    /// let precondition = Precondition::require(eventsourcingdb::Precondition::is_eventql_query_true(
    ///     "FROM e IN events WHERE e.type == 'io.eventsourcingdb.library.book-acquired' PROJECT INTO COUNT() < 1000",
    /// ));
    /// ```
    #[must_use]
    pub fn require(precondition: eventsourcingdb::Precondition) -> Self {
        Self {
            kind: Kind::Required(precondition),
        }
    }

    /// Lets the events be written only if the subject has no events yet, which
    /// is what a command that creates something usually declares. It is the
    /// same as [`require`](Precondition::require) with
    /// `eventsourcingdb::Precondition::is_subject_pristine`.
    #[must_use]
    pub fn on_pristine_subject(subject: Subject) -> Self {
        Self::require(eventsourcingdb::Precondition::is_subject_pristine(
            subject, None,
        ))
    }

    /// Lets the events be written only if the subject has events already, for
    /// a command that acts on something that has to exist. It is the same as
    /// [`require`](Precondition::require) with
    /// `eventsourcingdb::Precondition::is_subject_populated`.
    ///
    /// It needs EventSourcingDB 1.2 or later, which added this precondition.
    /// An older database refuses the write, which then fails with a permanent
    /// failure.
    #[must_use]
    pub fn on_populated_subject(subject: Subject) -> Self {
        Self::require(eventsourcingdb::Precondition::is_subject_populated(
            subject, None,
        ))
    }

    /// Lets the events be written only if the last event of the subject is the
    /// one with the given ID, for a command that was decided on a revision the
    /// caller hands over.
    ///
    /// The ID usually comes from the caller, who reads it as a [`Revision`]
    /// first, such as with `parse`, so that a value that is not one is refused
    /// before anything is read. For the revision of the state that the command
    /// was decided on, use [`on_state_read`](Precondition::on_state_read)
    /// instead.
    #[must_use]
    pub fn on_event_id(subject: Subject, event_id: Revision) -> Self {
        Self {
            kind: Kind::OnEventId(eventsourcingdb::Precondition::is_subject_on_event_id(
                subject,
                event_id.to_string(),
                None,
            )),
        }
    }

    /// Lets the events be written only if nothing has been written to the
    /// subject since the state the command was decided on was read. For a
    /// subject without events, it requires the subject to still be pristine.
    ///
    /// Use it when the command decides on the state that is read for it,
    /// rather than on a revision the caller hands over.
    #[must_use]
    pub fn on_state_read() -> Self {
        Self {
            kind: Kind::OnStateRead,
        }
    }

    /// Writes the events whatever has been written to the subject in the
    /// meantime. It can not be combined with other preconditions.
    #[must_use]
    pub fn unconditionally() -> Self {
        Self {
            kind: Kind::Unconditionally,
        }
    }

    /// Returns the precondition of the client SDK that the database checks for
    /// this one: the one [`require`](Precondition::require) made it from, which
    /// is how [`on_pristine_subject`](Precondition::on_pristine_subject) and
    /// [`on_populated_subject`](Precondition::on_populated_subject) make
    /// theirs, or the revision check of
    /// [`on_event_id`](Precondition::on_event_id). For any other, it returns
    /// `None`.
    #[must_use]
    pub fn database(&self) -> Option<&eventsourcingdb::Precondition> {
        match &self.kind {
            Kind::Required(database) | Kind::OnEventId(database) => Some(database),
            Kind::OnStateRead | Kind::Unconditionally => None,
        }
    }

    /// Tells whether this precondition was made with
    /// [`on_state_read`](Precondition::on_state_read).
    #[must_use]
    pub fn is_on_state_read(&self) -> bool {
        matches!(self.kind, Kind::OnStateRead)
    }

    /// Tells whether this precondition was made with
    /// [`unconditionally`](Precondition::unconditionally).
    #[must_use]
    pub fn is_unconditional(&self) -> bool {
        matches!(self.kind, Kind::Unconditionally)
    }
}

/// Two preconditions are equal if they check the same in the same way: if they
/// are of the same kind, and the database checks the same for them. A check of
/// a subject with options that leave it to the subject alone is the same as one
/// without options, which is what they mean to the database.
/// [`on_event_id`](Precondition::on_event_id) and
/// [`require`](Precondition::require) with the same revision check are not
/// equal, since they are made by different functions, which their `Debug`
/// shows as well.
impl PartialEq for Precondition {
    fn eq(&self, other: &Self) -> bool {
        match (&self.kind, &other.kind) {
            (Kind::Required(left), Kind::Required(right))
            | (Kind::OnEventId(left), Kind::OnEventId(right)) => {
                checked_alike(left) == checked_alike(right)
            }
            (Kind::OnStateRead, Kind::OnStateRead)
            | (Kind::Unconditionally, Kind::Unconditionally) => true,
            _ => false,
        }
    }
}

impl Eq for Precondition {}

/// Shows a precondition as the function that made it, with the JSON of the
/// precondition of the client SDK that the database checks for it, such as
/// `Precondition::require({"type":"isSubjectPristine",…})`.
impl fmt::Debug for Precondition {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        let (made_by, database) = match &self.kind {
            Kind::Required(database) => ("require", Some(database)),
            Kind::OnEventId(database) => ("on_event_id", Some(database)),
            Kind::OnStateRead => ("on_state_read", None),
            Kind::Unconditionally => ("unconditionally", None),
        };

        match database {
            Some(database) => write!(
                formatter,
                "Precondition::{made_by}({})",
                checked_alike(database)
            ),
            None => write!(formatter, "Precondition::{made_by}()"),
        }
    }
}

/// Returns what the database checks for a precondition of the client SDK, as
/// the JSON the client sends for it, without options that change nothing.
fn checked_alike(precondition: &eventsourcingdb::Precondition) -> Value {
    let mut checked =
        serde_json::to_value(precondition).expect("a precondition of the client SDK is JSON");
    let unchanged = serde_json::to_value(eventsourcingdb::PreconditionOptions::default())
        .expect("the options of a precondition are JSON");

    if let Some(payload) = checked.get_mut("payload").and_then(Value::as_object_mut)
        && payload.get("options") == Some(&unchanged)
    {
        let _ = payload.remove("options");
    }

    checked
}

/// Checks that a command declares its preconditions in a way they can be
/// honored, before anything is read, as executing a command does.
///
/// # Errors
///
/// Fails with a permanent failure if the command declares none at all, or
/// [`Precondition::unconditionally`] together with others.
pub fn check_preconditions<C: Command + ?Sized>(command: &C) -> Result<(), PreconditionsError> {
    checked_preconditions(command).map(|_| ())
}

/// Returns the preconditions a command declares, once they are checked as
/// [`check_preconditions`] checks them.
pub(crate) fn checked_preconditions<C: Command + ?Sized>(
    command: &C,
) -> Result<Vec<Precondition>, PreconditionsError> {
    let declared = command.preconditions();
    check(&declared, any::type_name::<C>())?;

    Ok(declared)
}

/// Returns the declared precondition that the database names as the one that
/// did not hold, given the failure of a conflict.
///
/// A database that offers the v2 API names it in the details of its answer, by
/// its index among the preconditions it checked, which is the index of the
/// declared one (see [`resolve_preconditions`]). A database that offers just
/// the v1 API names none, nor does an answer with another code, or details
/// without an index that fits the declared preconditions, so then it returns
/// `None`.
pub(crate) fn violated_precondition<'a>(
    conflict: &Failure,
    declared: &'a [Precondition],
) -> Option<&'a Precondition> {
    let Some(ClientError::DbApiError(answer)) = conflict.cause()?.downcast_ref::<ClientError>()
    else {
        return None;
    };
    if answer.code != code::PRECONDITION_VIOLATED {
        return None;
    }

    let index = answer.details.get("precondition")?.as_u64()?;
    declared.get(usize::try_from(index).ok()?)
}

/// Turns the declared preconditions of a command into the ones the database
/// checks, given the ID of the last event the state was read up to, which is
/// `None` for a subject without events.
///
/// It turns every declared precondition into exactly one, in the order they
/// are declared, and adds none of its own, so that the index by which the
/// database names a precondition that did not hold is the index of the
/// declared one. Only [`Precondition::unconditionally`] turns into none, but a
/// command declares it alone, so then nothing is checked at all.
pub(crate) fn resolve_preconditions(
    subject: &Subject,
    declared: Vec<Precondition>,
    last_event_id: Option<Revision>,
) -> Vec<eventsourcingdb::Precondition> {
    declared
        .into_iter()
        .filter_map(|precondition| match precondition.kind {
            Kind::Required(database) | Kind::OnEventId(database) => Some(database),
            Kind::OnStateRead => Some(match last_event_id {
                Some(event_id) => eventsourcingdb::Precondition::is_subject_on_event_id(
                    subject.as_str(),
                    event_id.to_string(),
                    None,
                ),
                None => eventsourcingdb::Precondition::is_subject_pristine(subject.as_str(), None),
            }),
            Kind::Unconditionally => None,
        })
        .collect()
}

/// Checks the preconditions of a write the way [`check_preconditions`] checks
/// those of a command, except that [`Precondition::on_state_read`] has nothing
/// to refer to, and turns them into the ones the database checks, in their
/// order, so that the index by which the database names a precondition that
/// did not hold is the index of the given one.
pub(crate) fn write_preconditions(
    declared: Vec<Precondition>,
) -> Result<Vec<eventsourcingdb::Precondition>, PreconditionsError> {
    let refuse = |message: &str| {
        Err(PreconditionsError::Permanent(Permanent::Other(
            Failure::new(message, None),
        )))
    };

    if declared.is_empty() {
        return refuse(
            "a write declares no preconditions, use Precondition::unconditionally to write without any",
        );
    }
    if declared.iter().any(Precondition::is_on_state_read) {
        return refuse(
            "a write reads no state, so Precondition::on_state_read has nothing to guard, \
             use on_pristine_subject, on_populated_subject, on_event_id, or require instead",
        );
    }
    if declared.len() > 1 && declared.iter().any(Precondition::is_unconditional) {
        return refuse("a write combines Precondition::unconditionally with other preconditions");
    }

    Ok(declared
        .into_iter()
        .filter_map(|precondition| match precondition.kind {
            Kind::Required(database) | Kind::OnEventId(database) => Some(database),
            Kind::OnStateRead | Kind::Unconditionally => None,
        })
        .collect())
}

fn check(declared: &[Precondition], command: &str) -> Result<(), PreconditionsError> {
    let refuse = |message: String| {
        Err(PreconditionsError::Permanent(Permanent::Other(
            Failure::new(message, None),
        )))
    };

    if declared.is_empty() {
        return refuse(format!(
            "{command} declares no preconditions, use Precondition::unconditionally to write without any",
        ));
    }
    if declared.len() > 1 && declared.iter().any(Precondition::is_unconditional) {
        return refuse(format!(
            "{command} combines Precondition::unconditionally with other preconditions",
        ));
    }

    Ok(())
}

/// Why a command declares its preconditions in a way they can not be honored
/// (see [`check_preconditions`]).
#[derive(Debug)]
pub enum PreconditionsError {
    /// The command declares none, or
    /// [`Precondition::unconditionally`] together with others, which is a
    /// mistake in the code.
    Permanent(Permanent),
}

impl fmt::Display for PreconditionsError {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::Permanent(failure) => failure.fmt(formatter),
        }
    }
}

impl Error for PreconditionsError {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        match self {
            Self::Permanent(failure) => failure.source(),
        }
    }
}

impl Categorized for PreconditionsError {
    fn category(&self) -> Option<Category> {
        match self {
            Self::Permanent(failure) => failure.category(),
        }
    }
}