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Simplify Relabel logic #2085

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Jan 9, 2025
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Simplify Relabel logic #2085

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64 changes: 19 additions & 45 deletions compiler/qsc_eval/src/intrinsic.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -377,56 +377,30 @@ pub fn qubit_relabel(
return Err(Error::RelabelingMismatch(arg_span));
}

let mut map = FxHashMap::default();
map.reserve(left.len());
// Start with a mapping of each qubit to itself.
let mut mappings: FxHashMap<usize, usize> =
left.iter().copied().zip(left.iter().copied()).collect();
for (l, r) in left.into_iter().zip(right.into_iter()) {
// Trivial case where the qubit is already mapped to itself in the relabel, which can be short circuited.
if l == r {
continue;
}
match (map.contains_key(&l), map.contains_key(&r)) {
(false, false) => {
// Neither qubit has been relabeled yet.
swap(l, r);
map.insert(l, r);
map.insert(r, l);
}
(false, true) => {
// The right qubit has been relabeled, so we need to swap the left qubit with the
// new label for the right qubit.
let label = *map
.keys()
.find(|k| map[*k] == r)
.expect("mapped qubit should be present as both key and value");
swap(l, label);
map.insert(l, r);
map.insert(label, l);
}
(true, false) => {
// The left qubit has been relabeled, so we swap the qubits as normal but
// remember the new mapping of the right qubit.
let mapped = *map.get(&l).expect("mapped qubit should be present");
swap(l, r);
map.insert(l, r);
map.insert(r, mapped);
}
(true, true) => {
// Both qubits have been relabeled, so we need to swap new label for the right qubit with
// the left qubit and remember the new mapping of both qubits.
// This is effectively a combination of the second and third cases above.
let label_r = *map
.keys()
.find(|k| map[*k] == r)
.expect("mapped qubit should be present as both key and value");
let mapped_l = *map.get(&l).expect("mapped qubit should be present");
let mapped_r = *map.get(&r).expect("mapped qubit should be present");

// This swap is only necessary if the labels don't already point to each other.
if mapped_l != r && mapped_r != l {
swap(label_r, l);
map.insert(label_r, mapped_l);
map.insert(l, mapped_r);
}
}
// Check what each label currently maps to.
let mapped_l = *mappings.get(&l).expect("mapped qubit should be present");
let mapped_r = *mappings.get(&r).expect("mapped qubit should be present");

// We only need to swap if the label is not pointing to the correct qubit.
if mapped_l != r && mapped_r != l {
// Do a reverse lookup to find which label is currently mapped to the desired right qubit.
// This tells us which label to use in the swap, which we will use in the update of the mappings too.
let label_r = *mappings
.keys()
.find(|k| mappings[*k] == r)
.expect("mapped qubit should be present as both key and value");
swap(l, label_r);
mappings.insert(label_r, mapped_l);
mappings.insert(l, mapped_r);
}
}

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