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//! Parallel projection merging and sequential composition.
use super::merge::{merge_labeled_local_types, LabeledLocalType};
use super::*;
use proc_macro2::Ident;
impl<'a> ProjectionContext<'a> {
pub(super) fn merge_parallel_projections(
&mut self,
projections: Vec<LocalType>,
) -> Result<LocalType, ProjectionError> {
// Remove End projections as they don't contribute
let non_end: Vec<_> = projections
.into_iter()
.filter(|p| p != &LocalType::End)
.collect();
match non_end.len() {
0 => Ok(LocalType::End),
1 => Ok(non_end.into_iter().next().unwrap_or(LocalType::End)),
_ => {
// Check for conflicts
self.check_parallel_conflicts(&non_end)?;
// Multiple non-trivial projections - merge them
// Interleaving is normalized to declaration order.
// The merge creates a deterministic sequential composition.
let mut merged = LocalType::End;
for proj in non_end.into_iter().rev() {
merged = self.sequential_merge(proj, merged);
}
Ok(merged)
}
}
}
/// Check for conflicts between parallel projections
///
/// Returns an error if the projections have incompatible operations
/// that cannot be safely interleaved.
fn check_parallel_conflicts(&self, projections: &[LocalType]) -> Result<(), ProjectionError> {
// Check for conflicting sends
let mut send_targets = Vec::new();
let mut recv_sources = Vec::new();
for proj in projections {
match proj {
LocalType::Send { to, .. } => {
if send_targets.contains(to) {
return Err(ProjectionError::InconsistentParallel);
}
send_targets.push(to.clone());
}
LocalType::Receive { from, .. } => {
if recv_sources.contains(from) {
return Err(ProjectionError::InconsistentParallel);
}
recv_sources.push(from.clone());
}
LocalType::Select { to, .. } => {
if send_targets.contains(to) {
return Err(ProjectionError::InconsistentParallel);
}
send_targets.push(to.clone());
}
LocalType::Branch { from, .. } => {
if recv_sources.contains(from) {
return Err(ProjectionError::InconsistentParallel);
}
recv_sources.push(from.clone());
}
_ => {
// Other types are compatible with parallel composition
}
}
}
Ok(())
}
fn sequential_merge(&self, first: LocalType, second: LocalType) -> LocalType {
// Merge two local types sequentially
match (first, second) {
(LocalType::End, other) | (other, LocalType::End) => other,
(first, second) => {
// Chain them together
self.append_continuation(first, second)
}
}
}
fn append_continuation(&self, local_type: LocalType, continuation: LocalType) -> LocalType {
Self::append_continuation_static(local_type, continuation)
}
fn append_continuation_static(local_type: LocalType, continuation: LocalType) -> LocalType {
match local_type {
LocalType::Send {
to,
message,
continuation: cont,
} => LocalType::Send {
to,
message,
continuation: Box::new(Self::append_continuation_static(
*cont,
continuation.clone(),
)),
},
LocalType::Receive {
from,
message,
continuation: cont,
} => LocalType::Receive {
from,
message,
continuation: Box::new(Self::append_continuation_static(
*cont,
continuation.clone(),
)),
},
LocalType::End => continuation,
// For other types, just return as-is with continuation appended at the end
other => other,
}
}
pub(super) fn project_rec(
&mut self,
label: &proc_macro2::Ident,
body: &Protocol,
) -> Result<LocalType, ProjectionError> {
let body_projection = self.project_protocol(body)?;
// Only include Rec if the body actually uses this role
if body_projection == LocalType::End {
Ok(LocalType::End)
} else {
Ok(LocalType::Rec {
label: label.clone(),
body: Box::new(body_projection),
})
}
}
pub(super) fn project_var(
&mut self,
label: &proc_macro2::Ident,
) -> Result<LocalType, ProjectionError> {
Ok(LocalType::Var(label.clone()))
}
pub(super) fn merge_choice_continuations(
&mut self,
branches: &[Branch],
) -> Result<LocalType, ProjectionError> {
// Project each branch and preserve its label
let mut labeled_projections = Vec::new();
for branch in branches {
let projection = self.project_protocol(&branch.protocol)?;
labeled_projections.push((branch.label.clone(), projection));
}
// Check if all projections are identical (common case)
let projections_only: Vec<_> = labeled_projections.iter().map(|(_, p)| p).collect();
if projections_only.windows(2).all(|w| w[0] == w[1]) {
Ok(labeled_projections
.into_iter()
.next()
.map(|(_, projection)| projection)
.unwrap_or(LocalType::End))
} else {
// Different projections per branch - need to merge with label awareness
self.merge_local_types_labeled(labeled_projections)
}
}
/// Merge multiple labeled local types preserving choice labels.
///
/// This is used for merging choice continuations where we need to preserve
/// the original choice labels. When a receiver gets different messages per branch,
/// the resulting Branch node should use the semantic choice labels, not message names.
pub(super) fn merge_local_types_labeled(
&self,
labeled_projections: Vec<(Ident, LocalType)>,
) -> Result<LocalType, ProjectionError> {
if labeled_projections.is_empty() {
return Ok(LocalType::End);
}
// Filter out End types as they're neutral for merging
let non_end: Vec<_> = labeled_projections
.into_iter()
.filter(|(_, p)| p != &LocalType::End)
.collect();
if non_end.is_empty() {
return Ok(LocalType::End);
}
// Fold the merge operation over all projections, preserving labels
// Safety: non_end is non-empty (checked above)
let mut iter = non_end.into_iter();
let Some((first_label, first_type)) = iter.next() else {
return Ok(LocalType::End);
};
let mut result = LabeledLocalType {
label: first_label,
local_type: first_type,
};
for (label, next) in iter {
result = merge_labeled_local_types(
&result,
&LabeledLocalType {
label,
local_type: next,
},
)?;
}
Ok(result.local_type)
}
}