[hma] Add hash compare endpoint

hasher-matcher-actioner/src/OpenMediaMatch/blueprints/matching.py

@@ -306,6 +306,53 @@ def index_status():
     return status_by_name
 
 
+@bp.route("/compare", methods=["POST"])
+def compare():
+    """
+    Compare pairs of hashes and get the match distance between them.
+    Example input:
+    {
+        "pdq": ["facd8b...", "facd8b..."],
+        "not_pdq": ["bdec19...","bdec19..."]
+    }
+    Example output
+    {
+        "pdq": [
+            true,
+            {
+                "distance": 9
+            }
+        ],
+        "not_pdq": 20
+            true,
+            {
+                "distance": 341
+            }
+        }
+    }
+    """
+    request_data = request.get_json()
+    if type(request_data) != dict:
+        abort(400, "Request input was not a dict")
+    storage = get_storage()
+    results = {}
+    for signal_type_str in request_data.keys():
+        hashes_to_compare = request_data.get(signal_type_str)
+        if type(hashes_to_compare) != list:
+            abort(400, f"Comparison hashes for {signal_type_str} was not a list")
+        if hashes_to_compare.__len__() != 2:
+            abort(400, f"Comparison hash list lenght must be exactly 2")
+        signal_type = _validate_and_transform_signal_type(signal_type_str, storage)
+        try:
+            left = signal_type.validate_signal_str(hashes_to_compare[0])
+            right = signal_type.validate_signal_str(hashes_to_compare[1])
+            comparison = signal_type.compare_hash(left, right)
+            results[signal_type_str] = comparison
+        except Exception as e:
+            abort(400, f"Invalid {signal_type_str} hash: {e}")
+    return results
+
+
 def initiate_index_cache(app: Flask, scheduler: APScheduler | None) -> None:
     assert not hasattr(app, "signal_type_index_cache"), "Aready initialized?"
     storage = get_storage()

hasher-matcher-actioner/src/OpenMediaMatch/tests/test_api.py

@@ -281,3 +281,31 @@ def test_exchange_api_set_auth(app: Flask, client: FlaskClient):
         "supports_authentification": True,
         "has_set_authentification": False,
     }
+
+
+def test_compare_hashes(app: Flask, client: FlaskClient):
+    specimen1 = "facd8bcb2a49bcebdec1985298d5fe84bcd006c187c598c720c3c087b3fdb318"
+    specimen2 = "facd8bcb2a49bcebdec1985228d5ae84bcd006c187c598c720c2b087b3fdb318"
+    # Happy path
+    resp = client.post("/m/compare", json={"pdq": [specimen1, specimen2]})
+    assert resp.json == {"pdq": [True, {"distance": 9}]}
+
+    # Malformed input
+    bad_inputs = [
+        # Not a dict
+        ["banana"],
+        # Dict, but values are not lists
+        {"pdq": "banana"},
+        # List of comparison hashes is empty
+        {"pdq": []},
+        # Hashes are invalid
+        {"pdq": ["banana", "banana"]},
+        # Too many hashes (must be exactly 2)
+        {"pdq": [specimen1, specimen2, specimen1]},
+    ]
+    for bad_input in bad_inputs:
+        resp = client.post(
+            "/m/compare",
+            json=bad_input,
+        )
+        assert resp.status_code == 400