From f9255c608cfc980f4976e0790cb473df4a63b352 Mon Sep 17 00:00:00 2001
From: rochisha0 <rochisha.agarwal2302@gmail.com>
Date: Fri, 25 Jul 2025 13:37:50 +0900
Subject: [PATCH] shift decomposition to gate class

---
 src/qutip_qip/operations/decomposer.py | 49 ++++++++++++++++++++++++++
 src/qutip_qip/operations/gateclass.py  | 18 ++++++++++
 tests/test_dec.py                      | 14 ++++++++
 3 files changed, 81 insertions(+)
 create mode 100644 src/qutip_qip/operations/decomposer.py
 create mode 100644 tests/test_dec.py

diff --git a/src/qutip_qip/operations/decomposer.py b/src/qutip_qip/operations/decomposer.py
new file mode 100644
index 00000000..eace80b6
--- /dev/null
+++ b/src/qutip_qip/operations/decomposer.py
@@ -0,0 +1,49 @@
+from qutip_qip.operations.gateclass import Gate
+
+__all__ = ["resolve_decomposition", "fallback_decomposition"]
+
+def resolve_decomposition(gate: Gate, basis_1q: list[str], basis_2q: list[str]) -> list[Gate]:
+    """
+    Attempt to decompose a gate into the given basis.
+    """
+    try:
+        return gate.decompose_to_basis(basis_1q, basis_2q)
+    except NotImplementedError:
+        return fallback_decomposition(gate, basis_1q, basis_2q)
+
+
+# Optional graph-based fallback resolver
+GATE_DECOMP_GRAPH = {
+    "MS": ["CNOT"],
+    "CNOT": ["ISWAP"],
+    # You can add more conversion steps here
+}
+
+
+def fallback_decomposition(gate: Gate, basis_1q: list[str], basis_2q: list[str]) -> list[Gate]:
+    from collections import deque
+
+    visited = set()
+    queue = deque([(gate.name, [])])
+
+    while queue:
+        current_gate, path = queue.popleft()
+        if current_gate in basis_2q:
+            # Reconstruct intermediate decompositions
+            decomposed = gate
+            for intermediate in path:
+                decomposed = decomposed.decompose_to_basis(basis_1q, [intermediate])
+                decomposed = sum(
+                    (g.decompose_to_basis(basis_1q, basis_2q) for g in decomposed),
+                    start=[]
+                )
+            return decomposed
+
+        for neighbor in GATE_DECOMP_GRAPH.get(current_gate, []):
+            if neighbor not in visited:
+                visited.add(neighbor)
+                queue.append((neighbor, path + [neighbor]))
+
+    raise NotImplementedError(
+        f"No valid decomposition path from {gate.name} to basis {basis_2q}"
+    )
diff --git a/src/qutip_qip/operations/gateclass.py b/src/qutip_qip/operations/gateclass.py
index 317f40da..ce4f82d7 100644
--- a/src/qutip_qip/operations/gateclass.py
+++ b/src/qutip_qip/operations/gateclass.py
@@ -413,6 +413,15 @@ def get_qobj(self, num_qubits=None, dims=None):
             dims=dims,
             targets=all_targets,
         )
+    def decompose_to_basis(self, basis_1q: list[str], basis_2q: list[str]) -> list["Gate"]:
+        """
+        Return a decomposition of this gate into the given basis.
+        This default method raises an error unless overridden in subclasses.
+        """
+        raise NotImplementedError(
+            f"Decomposition not implemented for {self.name} to basis {basis_2q}"
+        )
+
 
 
 class SingleQubitGate(Gate):
@@ -580,6 +589,14 @@ def __init__(self, targets, **kwargs):
 
     def get_compact_qobj(self):
         return snot()
+    
+    def decompose_to_basis(self, basis_1q, basis_2q):
+        if "RX" in basis_1q and "RY" in basis_1q:
+            return [
+                RY(self.targets[0], arg_value=np.pi / 2, arg_label=r"\pi/2"),
+                RX(self.targets[0], arg_value=np.pi, arg_label=r"\pi")
+            ]
+        raise NotImplementedError("Cannot decompose Hadamard to given basis")
 
 
 SNOT = H
@@ -606,6 +623,7 @@ def __init__(self, targets, **kwargs):
 
     def get_compact_qobj(self):
         return sqrtnot()
+    
 
 
 class S(SingleQubitGate):
diff --git a/tests/test_dec.py b/tests/test_dec.py
new file mode 100644
index 00000000..9ebdf5dd
--- /dev/null
+++ b/tests/test_dec.py
@@ -0,0 +1,14 @@
+import numpy as np
+from qutip_qip.operations import H, RX, RY
+from qutip_qip.operations.decomposer import resolve_decomposition
+
+
+def test_hadamard_decomposition():
+    gate = H(0)
+    decomposed = resolve_decomposition(gate, basis_1q=["RX", "RY"], basis_2q=["CNOT"])
+
+    assert len(decomposed) == 2
+    assert isinstance(decomposed[0], RY)
+    assert isinstance(decomposed[1], RX)
+    np.testing.assert_allclose(decomposed[0].arg_value, np.pi / 2)
+    np.testing.assert_allclose(decomposed[1].arg_value, np.pi)