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use std::collections::{BTreeMap, HashSet};
use crate::{
error::Error,
prelude::{Heading, Relation, Tuple},
types::AttributeName,
};
impl Heading {
pub(crate) fn project(&self, attributes: &[AttributeName]) -> Result<Heading, Error> {
let mut seen = HashSet::with_capacity(attributes.len());
let mut heading_attributes = BTreeMap::new();
for attribute in attributes {
let Some(ty) = self.get(attribute) else {
return Err(Error::AttributeNotFound {
name: attribute.clone(),
});
};
if !seen.insert(attribute.clone()) {
return Err(Error::AttributeAlreadyExists {
name: attribute.clone(),
});
}
heading_attributes.insert(attribute.clone(), *ty);
}
Ok(Heading::new(heading_attributes))
}
}
impl Tuple {
pub(crate) fn project(&self, attributes: &[AttributeName]) -> Result<Tuple, Error> {
let mut new_tuple = Vec::with_capacity(attributes.len());
for attribute in attributes {
let Some(value) = self.get(attribute) else {
return Err(Error::AttributeNotFound {
name: attribute.clone(),
});
};
new_tuple.push((attribute.clone(), value.clone()));
}
Tuple::try_from(new_tuple)
}
}
impl Relation {
/// Projects a relation onto a subset of attributes (`π`).
///
/// `people`
///
/// | `id` | `age` |
/// | --- | --- |
/// | `1` | `19` |
/// | `2` | `24` |
///
/// Output
///
/// | `id` |
/// | --- |
/// | `1` |
/// | `2` |
///
/// # Errors
///
/// Returns [`Error::AttributeNotFound`] if one of the requested attributes
/// does not exist in the relation heading.
/// Returns [`Error::AttributeAlreadyExists`] if the projection list repeats
/// the same attribute name more than once.
///
/// # Example
///
/// ```rust
/// use darwen::prelude::{
/// AttributeName, Heading, Relation, Scalar, ScalarType, Tuple,
/// };
///
/// let people = Relation::new_from_iter(
/// Heading::try_from(vec![
/// (AttributeName::from("id"), ScalarType::Integer),
/// (AttributeName::from("age"), ScalarType::Integer),
/// ])?,
/// vec![
/// Tuple::try_from(vec![
/// (AttributeName::from("id"), Scalar::Integer(1)),
/// (AttributeName::from("age"), Scalar::Integer(19)),
/// ])?,
/// Tuple::try_from(vec![
/// (AttributeName::from("id"), Scalar::Integer(2)),
/// (AttributeName::from("age"), Scalar::Integer(24)),
/// ])?,
/// ],
/// )?;
///
/// let ids = people.project(&[AttributeName::from("id")])?;
///
/// assert_eq!(
/// ids,
/// Relation::new_from_iter(
/// Heading::try_from(vec![(AttributeName::from("id"), ScalarType::Integer)])?,
/// vec![
/// Tuple::try_from(vec![(AttributeName::from("id"), Scalar::Integer(1))])?,
/// Tuple::try_from(vec![(AttributeName::from("id"), Scalar::Integer(2))])?,
/// ],
/// )?
/// );
/// # Ok::<(), darwen::prelude::Error>(())
/// ```
pub fn project(&self, attributes: &[AttributeName]) -> Result<Relation, Error> {
let heading = self.heading.project(attributes)?;
let mut relation = Relation::new(heading);
for tuple in &self.body {
relation.insert(tuple.project(attributes)?)?;
}
Ok(relation)
}
}
#[cfg(test)]
mod tests {
use crate::prelude::{Heading, Scalar, ScalarType, Tuple};
use super::*;
#[test]
fn test_project() {
let relation = Relation::new_from_iter(
Heading::try_from(vec![
(AttributeName::from("foo"), ScalarType::Integer),
(AttributeName::from("bar"), ScalarType::Boolean),
])
.unwrap(),
vec![
Tuple::try_from(vec![
(AttributeName::from("foo"), Scalar::Integer(1)),
(AttributeName::from("bar"), Scalar::Boolean(true)),
])
.unwrap(),
Tuple::try_from(vec![
(AttributeName::from("foo"), Scalar::Integer(2)),
(AttributeName::from("bar"), Scalar::Boolean(true)),
])
.unwrap(),
Tuple::try_from(vec![
(AttributeName::from("foo"), Scalar::Integer(2)),
(AttributeName::from("bar"), Scalar::Boolean(false)),
])
.unwrap(),
Tuple::try_from(vec![
(AttributeName::from("foo"), Scalar::Integer(3)),
(AttributeName::from("bar"), Scalar::Boolean(true)),
])
.unwrap(),
Tuple::try_from(vec![
(AttributeName::from("foo"), Scalar::Integer(3)),
(AttributeName::from("bar"), Scalar::Boolean(false)),
])
.unwrap(),
],
)
.unwrap();
assert_eq!(
relation.project(&vec![AttributeName::from("foo")]).unwrap(),
Relation::new_from_iter(
Heading::try_from(vec![(AttributeName::from("foo"), ScalarType::Integer),])
.unwrap(),
vec![
Tuple::try_from(vec![(AttributeName::from("foo"), Scalar::Integer(1))])
.unwrap(),
Tuple::try_from(vec![(AttributeName::from("foo"), Scalar::Integer(2))])
.unwrap(),
Tuple::try_from(vec![(AttributeName::from("foo"), Scalar::Integer(3))])
.unwrap(),
],
)
.unwrap()
);
}
#[test]
fn test_project_all_attributes_is_identity() -> Result<(), Error> {
let relation = Relation::new_from_iter(
Heading::try_from(vec![
(AttributeName::from("foo"), ScalarType::Integer),
(AttributeName::from("bar"), ScalarType::Boolean),
])
.unwrap(),
vec![Tuple::try_from(vec![
(AttributeName::from("foo"), Scalar::Integer(1)),
(AttributeName::from("bar"), Scalar::Boolean(true)),
])
.unwrap()],
)?;
assert_eq!(
relation.project(&[AttributeName::from("foo"), AttributeName::from("bar")])?,
relation
);
Ok(())
}
#[test]
fn test_project_rejects_unknown_attribute() -> Result<(), Error> {
let relation = Relation::new_from_iter(
Heading::try_from(vec![(AttributeName::from("foo"), ScalarType::Integer)]).unwrap(),
vec![Tuple::try_from(vec![(AttributeName::from("foo"), Scalar::Integer(1))]).unwrap()],
)?;
assert_eq!(
relation.project(&[AttributeName::from("bar")]),
Err(Error::AttributeNotFound { name: "bar".into() })
);
Ok(())
}
#[test]
fn test_project_rejects_duplicate_attributes_in_projection_list() -> Result<(), Error> {
let relation = Relation::new_from_iter(
Heading::try_from(vec![(AttributeName::from("foo"), ScalarType::Integer)]).unwrap(),
vec![Tuple::try_from(vec![(AttributeName::from("foo"), Scalar::Integer(1))]).unwrap()],
)?;
assert_eq!(
relation.project(&[AttributeName::from("foo"), AttributeName::from("foo")]),
Err(Error::AttributeAlreadyExists { name: "foo".into() })
);
Ok(())
}
#[test]
fn test_tuple_project_rejects_missing_attribute() {
let tuple =
Tuple::try_from(vec![(AttributeName::from("foo"), Scalar::Integer(1))]).unwrap();
assert_eq!(
tuple.project(&[AttributeName::from("bar")]),
Err(Error::AttributeNotFound { name: "bar".into() })
);
}
}