/// This module implements Nix attribute sets. They have flexible
/// backing implementations, as they are used in very versatile
/// use-cases that are all exposed the same way in the language
/// surface.
///
/// Due to this, construction and management of attribute sets has
/// some peculiarities that are encapsulated within this module.
use std::collections::BTreeMap;
use std::fmt::Display;
use std::rc::Rc;
use crate::errors::{Error, EvalResult};
use super::string::NixString;
use super::Value;
#[cfg(test)]
mod tests;
#[derive(Clone, Debug)]
pub enum NixAttrs {
Empty,
Map(BTreeMap<NixString, Value>),
KV { name: Value, value: Value },
}
impl Display for NixAttrs {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
f.write_str("{ ")?;
match self {
NixAttrs::KV { name, value } => {
f.write_fmt(format_args!("name = \"{}\"; ", name))?;
f.write_fmt(format_args!("value = {}; ", value))?;
f.write_str("/* optimised pair! */")?;
}
NixAttrs::Map(map) => {
for (name, value) in map {
f.write_fmt(format_args!("{} = {}; ", name.ident_str(), value))?;
}
}
NixAttrs::Empty => {
/* no values to print! */
f.write_str("/* optimised empty set! */")?;
}
}
f.write_str("}")
}
}
impl PartialEq for NixAttrs {
fn eq(&self, _other: &Self) -> bool {
todo!("attrset equality")
}
}
impl NixAttrs {
/// Retrieve reference to a mutable map inside of an attrs,
/// optionally changing the representation if required.
fn map_mut(&mut self) -> &mut BTreeMap<NixString, Value> {
match self {
NixAttrs::Map(m) => m,
NixAttrs::Empty => {
*self = NixAttrs::Map(BTreeMap::new());
self.map_mut()
}
NixAttrs::KV { name, value } => {
*self = NixAttrs::Map(BTreeMap::from([
("name".into(), std::mem::replace(name, Value::Blackhole)),
("value".into(), std::mem::replace(value, Value::Blackhole)),
]));
self.map_mut()
}
}
}
/// Implement construction logic of an attribute set, to encapsulate
/// logic about attribute set optimisations inside of this module.
pub fn construct(count: usize, mut stack_slice: Vec<Value>) -> EvalResult<Self> {
debug_assert!(
stack_slice.len() == count * 2,
"construct_attrs called with count == {}, but slice.len() == {}",
count,
stack_slice.len(),
);
// Optimisation: Empty attribute set
if count == 0 {
return Ok(NixAttrs::Empty);
}
// Optimisation: KV pattern
if count == 2 {
if let Some(kv) = attempt_optimise_kv(&mut stack_slice) {
return Ok(kv);
}
}
// TODO(tazjin): extend_reserve(count) (rust#72631)
let mut attrs = NixAttrs::Map(BTreeMap::new());
for _ in 0..count {
let value = stack_slice.pop().unwrap();
// It is at this point that nested attribute sets need to
// be constructed (if they exist).
//
let key = stack_slice.pop().unwrap();
match key {
Value::String(ks) => set_attr(&mut attrs, ks, value)?,
Value::AttrPath(mut path) => {
set_nested_attr(
&mut attrs,
path.pop().expect("AttrPath is never empty"),
path,
value,
)?;
}
other => {
return Err(Error::InvalidKeyType {
given: other.type_of(),
})
}
}
}
Ok(attrs)
}
}
// In Nix, name/value attribute pairs are frequently constructed from
// literals. This particular case should avoid allocation of a map,
// additional heap values etc. and use the optimised `KV` variant
// instead.
//
// `slice` is the top of the stack from which the attrset is being
// constructed, e.g.
//
// slice: [ "value" 5 "name" "foo" ]
// index: 0 1 2 3
// stack: 3 2 1 0
fn attempt_optimise_kv(slice: &mut [Value]) -> Option<NixAttrs> {
let (name_idx, value_idx) = {
match (&slice[2], &slice[0]) {
(Value::String(s1), Value::String(s2))
if (*s1 == NixString::NAME && *s2 == NixString::VALUE) =>
{
(3, 1)
}
(Value::String(s1), Value::String(s2))
if (*s1 == NixString::VALUE && *s2 == NixString::NAME) =>
{
(1, 3)
}
// Technically this branch lets type errors pass,
// but they will be caught during normal attribute
// set construction instead.
_ => return None,
}
};
Some(NixAttrs::KV {
name: std::mem::replace(&mut slice[name_idx], Value::Blackhole),
value: std::mem::replace(&mut slice[value_idx], Value::Blackhole),
})
}
// Set an attribute on an in-construction attribute set, while
// checking against duplicate keys.
fn set_attr(attrs: &mut NixAttrs, key: NixString, value: Value) -> EvalResult<()> {
let attrs = attrs.map_mut();
let entry = attrs.entry(key);
match entry {
std::collections::btree_map::Entry::Occupied(entry) => {
return Err(Error::DuplicateAttrsKey {
key: entry.key().as_str().to_string(),
})
}
std::collections::btree_map::Entry::Vacant(entry) => {
entry.insert(value);
return Ok(());
}
};
}
// Set a nested attribute inside of an attribute set, throwing a
// duplicate key error if a non-hashmap entry already exists on the
// path.
//
// There is some optimisation potential for this simple implementation
// if it becomes a problem.
fn set_nested_attr(
attrs: &mut NixAttrs,
key: NixString,
mut path: Vec<NixString>,
value: Value,
) -> EvalResult<()> {
// If there is no next key we are at the point where we
// should insert the value itself.
if path.is_empty() {
return set_attr(attrs, key, value);
}
let attrs = attrs.map_mut();
let entry = attrs.entry(key);
// If there is not we go one step further down, in which case we
// need to ensure that there either is no entry, or the existing
// entry is a hashmap into which to insert the next value.
//
// If a value of a different type exists, the user specified a
// duplicate key.
match entry {
// Vacant entry -> new attribute set is needed.
std::collections::btree_map::Entry::Vacant(entry) => {
let mut map = NixAttrs::Map(BTreeMap::new());
// TODO(tazjin): technically recursing further is not
// required, we can create the whole hierarchy here, but
// it's noisy.
set_nested_attr(&mut map, path.pop().expect("next key exists"), path, value)?;
entry.insert(Value::Attrs(Rc::new(map)));
}
// Occupied entry: Either error out if there is something
// other than attrs, or insert the next value.
std::collections::btree_map::Entry::Occupied(mut entry) => match entry.get_mut() {
Value::Attrs(attrs) => {
set_nested_attr(
Rc::make_mut(attrs),
path.pop().expect("next key exists"),
path,
value,
)?;
}
_ => {
return Err(Error::DuplicateAttrsKey {
key: entry.key().as_str().to_string(),
})
}
},
}
Ok(())
}
