Add expression interpreter

pyproject.toml

@@ -24,7 +24,7 @@ requires-python = ">=3.10"
 dependencies = [
     "acres >=0.5",
     "attrs >=24.1",
-    "bidsschematools >=1.1",
+    "bidsschematools[expressions]>=1.1",
     "nibabel>=5.3.0",
     "orjson>=3.11.3",
     "universal_pathlib >=0.2.1",
@@ -102,6 +102,10 @@ exclude = ".*"
 # Ruff configured in ruff.toml
 
 [dependency-groups]
+demo = [
+    "jupyterlab>=4.5.5",
+    "jupytext>=1.19.1",
+]
 dev = [
     "ipython>=8.37.0",
     "ruff>=0.15.5",

src/bids_validator/context.py

@@ -481,6 +481,37 @@ def sidecar(self) -> Namespace | None:
 
         return Namespace(sidecar)
 
+    def exists(self, arg: str| list[str] | None, rule: str | None = "dataset") -> int:
+        if arg is None:
+            return 0
+
+        prefix = UPath()
+        fileTree = self.file.parent if rule == "file" else self.dataset.tree
+
+        if rule == "stimuli":
+            prefix /= "stimuli"
+        elif rule == "subject":
+            if "sub" not in self.entities:
+                return 0
+            prefix /= f"sub-{self.entities["sub"]}"
+
+        if isinstance(arg, str):
+            arg = [arg]
+
+        if rule == "bids-uri":
+            valid_uris = (uri for uri in arg if uri.startswith("bids:") and uri.count(":") == 2)
+            uri_parts = (uri.split(":") for uri in valid_uris)
+            # Find number of files in this dataset that exist 
+            dataset_files = self.exists([part[2] for part in uri_parts if part[1] == ""])
+
+            # Still need to account for bids-uris where middle section is not blank
+            # Find number of files in other dataset that exist 
+            other_files = 0 
+
+            return dataset_files + other_files
+        else:
+            return sum((prefix / file) in fileTree for file in arg)
+
 
 class Sessions:
     """Collections of sessions in subject."""