From 58acc2e9343bb2c780154f6d5ea9f9d011d4f022 Mon Sep 17 00:00:00 2001
From: Pyhroff <pyhroff@gmail.com>
Date: Sat, 24 Oct 2026 12:05:06 +0530
Subject: [PATCH] Fix Add.build_call_graph CNOT count for unequal register
 bitsizes (#1818)

---
 qualtran/bloqs/arithmetic/addition.py      | 10 +++++++++-
 qualtran/bloqs/arithmetic/addition_test.py | 11 +++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/qualtran/bloqs/arithmetic/addition.py b/qualtran/bloqs/arithmetic/addition.py
index 7da0af6e7..d008f6779 100644
--- a/qualtran/bloqs/arithmetic/addition.py
+++ b/qualtran/bloqs/arithmetic/addition.py
@@ -219,7 +219,15 @@ def decompose_from_registers(
 
     def build_call_graph(self, ssa: 'SympySymbolAllocator') -> 'BloqCountDictT':
         n = self.b_dtype.bitsize
-        n_cnot = (n - 2) * 6 + 3
+        m = self.a_dtype.bitsize
+        if not is_symbolic(n) and n == 1:
+            return {CNOT(): 1}
+        if m == n:
+            n_cnot: SymbolicInt = (n - 2) * 6 + 3
+        elif not is_symbolic(m) and not is_symbolic(n):
+            n_cnot = (n - 2) * 3 + 2 + 3 * m - 3
+        else:
+            n_cnot = (n - 2) * 3 + 2 + sympy.Piecewise((3 * n - 5, sympy.Eq(m, n)), (3 * m - 3, True))
         return {And(): n - 1, And().adjoint(): n - 1, CNOT(): n_cnot}
 
     def get_ctrl_system(self, ctrl_spec: 'CtrlSpec') -> Tuple['Bloq', 'AddControlledT']:
diff --git a/qualtran/bloqs/arithmetic/addition_test.py b/qualtran/bloqs/arithmetic/addition_test.py
index 85af0f39e..53ddd4c00 100644
--- a/qualtran/bloqs/arithmetic/addition_test.py
+++ b/qualtran/bloqs/arithmetic/addition_test.py
@@ -205,6 +205,17 @@ def test_addition_gate_counts(n: int):
     qlt_testing.assert_equivalent_bloq_counts(add, ignore_split_join)
 
 
+@pytest.mark.parametrize(
+    "m,n",
+    [(a, b) for b in range(1, 7) for a in range(1, b + 1)],
+)
+def test_addition_gate_counts_unequal_bitsizes(m: int, n: int):
+    add = Add(QUInt(m), QUInt(n))
+    qlt_testing.assert_valid_bloq_decomposition(add)
+    assert add.t_complexity() == add.decompose_bloq().t_complexity()
+    qlt_testing.assert_equivalent_bloq_counts(add, ignore_split_join)
+
+
 @pytest.mark.parametrize('a,b', itertools.product(range(2**3), repeat=2))
 def test_add_tensor_contract(a, b):
     num_bits = 5