Use the same stable seed logic for SRF generation as HF

workflow/scripts/hf_sim.py

@@ -247,32 +247,6 @@ def hf_simulate_station(
         return station_name, epicentre_distance, station_waveform
 
 
-def stable_hash(station: str) -> int:
-    """Compute stable hashes for station names.
-
-    The HF binary expects seeds. We want the provided seed to be
-    independent of the order of stations in the stations lists. This
-    is so setting HF seed reproduces the same outputs, even for
-    different orders or subsets of the original station file. To do
-    that, we generate stable hashes based on the station name.
-
-
-    Parameters
-    ----------
-    station : str
-        The station name.
-
-    Returns
-    -------
-    int
-        A hash of the station name. This is guaranteed to be in the
-        range of a signed 32-bit integer.
-    """
-    return int.from_bytes(
-        hashlib.blake2b(station.encode("utf-8"), digest_size=4).digest(), signed=True
-    )
-
-
 def station_seeds(seed: int, stations: Iterable[str]) -> npt.NDArray[np.int32]:
     """Create a list of per-station seeds in an order-invariant fashion with a root seed.
 
@@ -289,7 +263,9 @@ def station_seeds(seed: int, stations: Iterable[str]) -> npt.NDArray[np.int32]:
     npt.NDArray[np.int32]
         A list of station seeds.
     """
-    station_hashes = np.array([stable_hash(name) for name in stations], dtype=np.int32)
+    station_hashes = np.array(
+        [utils.stable_hash(name) for name in stations], dtype=np.int32
+    )
     # Rather than add (which could overflow and cause annoying numpy
     # warnings), we just xor the hf seed with the station hashes.
     # Since this is invertible, we ensure that the same hf seed gives

workflow/scripts/realisation_to_srf.py

@@ -546,7 +546,10 @@ def generate_fault_srf(
         gsf_file_path=gsf_file_path,
         nx=nx,
         ny=ny,
-        seed=environment.seeds.genslip_seed,
+        # NOTE: This stable hash trick is also used in hf-sim, and is
+        # designed to give order invariant stable hashes for segments
+        # based on their names.
+        seed=environment.seeds.genslip_seed ^ utils.stable_hash(name),
         velocity_model_path=environment.velocity_model_path,
         shypo=genslip_hypocentre_coords[0],
         dhypo=genslip_hypocentre_coords[1],

workflow/utils.py

@@ -1,5 +1,6 @@
 """Miscellaneous workflow utilities that couldn't go anywhere else."""
 
+import hashlib
 import os
 import tempfile
 import urllib.request
@@ -174,3 +175,24 @@ def dict_zip(*dicts: Mapping[K, Any], strict: bool = True) -> dict[K, tuple[Any,
 
     result = {key: tuple(d[key] for d in dicts) for key in list(keys)}
     return result
+
+
+def stable_hash(value: str, size: int = 4) -> int:
+    """Compute stable hashes for strings.
+
+    Parameters
+    ----------
+    value : str
+        String to hash.
+    size : int
+        Size of the hash value.
+
+    Returns
+    -------
+    int
+        A hash of the value. Guaranteed to be an integer within
+        the signed integer bounds prescribed by ``size``.
+    """
+    return int.from_bytes(
+        hashlib.blake2b(value.encode("utf-8"), digest_size=size).digest(), signed=True
+    )