From 586f21a66f76adee46fbbf15717a2e82f14056bc Mon Sep 17 00:00:00 2001
From: Cursor Agent <cursoragent@cursor.com>
Date: Thu, 1 Oct 2026 12:18:58 +0000
Subject: [PATCH] [qec] Share Stim QUBIT_COORDS emission in the emitter module

Both builders call stim_emit::emit_qubit_coords. Lattice surgery still walks blocks in workload order. Measure lines and closing detectors stay in the builders. Checked-in Stim gold stays byte-identical.

Refs #473

Co-authored-by: Arnab Ghosh <ag2583@cornell.edu>
---
 quon_qec/src/experiment.rs           |  8 ++----
 quon_qec/src/experiment/stim_emit.rs | 37 +++++++++++++++++++++++-----
 2 files changed, 33 insertions(+), 12 deletions(-)

diff --git a/quon_qec/src/experiment.rs b/quon_qec/src/experiment.rs
index c9e93cd..255e6e0 100644
--- a/quon_qec/src/experiment.rs
+++ b/quon_qec/src/experiment.rs
@@ -486,9 +486,7 @@ fn emit_stim_single_block_memory(expanded: &ExpandedWorkload) -> Result<String,
         measure_basis.as_str(),
     );
 
-    for (atom, &(x, y)) in block.atoms.iter().zip(block.coords.iter()) {
-        out.push_str(&format!("QUBIT_COORDS({x}, {y}) {}\n", atom.0));
-    }
+    stim_emit::emit_qubit_coords(&mut out, &block.atoms, &block.coords);
 
     // Prepare |0…0⟩ then optional data H for X-init / X-memory (|+⟩^n).
     let mut reset_ids = Vec::with_capacity(block.atoms.len());
@@ -647,9 +645,7 @@ fn emit_stim_lattice_surgery_cx(expanded: &ExpandedWorkload) -> Result<String, E
     stim_emit::emit_lattice_surgery_header(&mut out, distance, expanded.blocks.len());
 
     for block in &expanded.blocks {
-        for (atom, &(x, y)) in block.atoms.iter().zip(block.coords.iter()) {
-            out.push_str(&format!("QUBIT_COORDS({x}, {y}) {}\n", atom.0));
-        }
+        stim_emit::emit_qubit_coords(&mut out, &block.atoms, &block.coords);
     }
 
     let mut all_atoms: Vec<u32> = expanded
diff --git a/quon_qec/src/experiment/stim_emit.rs b/quon_qec/src/experiment/stim_emit.rs
index 2d8166a..b3e69f0 100644
--- a/quon_qec/src/experiment/stim_emit.rs
+++ b/quon_qec/src/experiment/stim_emit.rs
@@ -10,16 +10,17 @@
 //! kind is a new impl plus one arm in [`lattice_round_emitter`].
 //!
 //! Both builders share [`emit_reset_tick`] for the prepare-`R` / `TICK` line,
-//! [`emit_single_block_header`] / [`emit_lattice_surgery_header`] for the
-//! comment preamble, and [`emit_observable_include`] for `OBSERVABLE_INCLUDE`
-//! record offsets. `QUBIT_COORDS` stay in the builders. Lattice surgery still
-//! appends frame-byproduct records after the shared observable line.
+//! [`emit_qubit_coords`] for `QUBIT_COORDS`, [`emit_single_block_header`] /
+//! [`emit_lattice_surgery_header`] for the comment preamble, and
+//! [`emit_observable_include`] for `OBSERVABLE_INCLUDE` record offsets.
+//! Measure lines and closing detectors stay in the builders. Lattice surgery
+//! still appends frame-byproduct records after the shared observable line.
 
 use std::collections::HashMap;
 
 use crate::expand::{
-    ExpandedBlock, ExpandedWorkload, MergeBoundary, PauliFrameUpdate, PhysicalRound, RoundKind,
-    RoundTerminal,
+    ExpandedBlock, ExpandedWorkload, MergeBoundary, PauliFrameUpdate, PhysicalAtomId,
+    PhysicalRound, RoundKind, RoundTerminal,
 };
 use crate::workload::LogicalBasis;
 
@@ -85,6 +86,16 @@ pub(crate) fn emit_single_block_header(
     ));
 }
 
+/// Write `QUBIT_COORDS(x, y) id` in caller order.
+///
+/// Single-block memory passes one block. Lattice surgery calls this once per
+/// block, in workload order. Pairs stop at the shorter slice, matching `zip`.
+pub(crate) fn emit_qubit_coords(out: &mut String, atoms: &[PhysicalAtomId], coords: &[(i32, i32)]) {
+    for (atom, &(x, y)) in atoms.iter().zip(coords.iter()) {
+        out.push_str(&format!("QUBIT_COORDS({x}, {y}) {}\n", atom.0));
+    }
+}
+
 /// Write the lattice-surgery CX comment preamble.
 pub(crate) fn emit_lattice_surgery_header(out: &mut String, distance: u32, blocks: usize) {
     out.push_str(&format!(
@@ -794,6 +805,20 @@ mod tests {
         );
     }
 
+    #[test]
+    fn qubit_coords_keep_caller_order() {
+        let mut out = String::new();
+        emit_qubit_coords(
+            &mut out,
+            &[PhysicalAtomId(4), PhysicalAtomId(0)],
+            &[(1, 2), (3, 4)],
+        );
+        assert_eq!(out, "QUBIT_COORDS(1, 2) 4\nQUBIT_COORDS(3, 4) 0\n");
+        let mut short = String::new();
+        emit_qubit_coords(&mut short, &[PhysicalAtomId(1)], &[(0, 0), (9, 9)]);
+        assert_eq!(short, "QUBIT_COORDS(0, 0) 1\n");
+    }
+
     #[test]
     fn lattice_surgery_header_matches_gold_preamble() {
         let mut out = String::new();