1 files changed, 47 insertions, 12 deletions
diff --git a/bsfs/query/ast/filter_.py b/bsfs/query/ast/filter_.py
index b129ded..2f0270c 100644
--- a/bsfs/query/ast/filter_.py
+++ b/bsfs/query/ast/filter_.py
@@ -252,8 +252,7 @@ class Has(FilterExpression):
 
 
 class _Value(FilterExpression):
-    """
-    """
+    """Matches some value."""
 
     # target value.
     value: typing.Any
@@ -277,13 +276,13 @@ class Is(_Value):
 
 class Equals(_Value):
     """Value matches exactly.
-    NOTE: Value format must correspond to literal type; can be a string, a number, or a Node
+    NOTE: Value must correspond to literal type.
     """
 
 
 class Substring(_Value):
     """Value matches a substring
-    NOTE: value format must be a string
+    NOTE: value must be a string.
     """
 
 
@@ -295,9 +294,49 @@ class EndsWith(_Value):
     """Value ends with a given string."""
 
 
+class Distance(FilterExpression):
+    """Distance to a reference is (strictly) below a threshold. Assumes a Feature literal."""
+
+    # FIXME:
+    # (a) pass a node/predicate as anchor instead of a value.
+    #     Then we don't need to materialize the reference.
+    # (b) pass a FilterExpression (_Bounded) instead of a threshold.
+    #     Then, we could also query values greater than a threshold.
+
+    # reference value.
+    reference: typing.Any
+
+    # distance threshold.
+    threshold: float
+
+    # closed (True) or open (False) bound.
+    strict: bool
+
+    def __init__(
+            self,
+            reference: typing.Any,
+            threshold: float,
+            strict: bool = False,
+            ):
+        self.reference = reference
+        self.threshold = float(threshold)
+        self.strict = bool(strict)
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.reference}, {self.threshold}, {self.strict})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), tuple(self.reference), self.threshold, self.strict))
+
+    def __eq__(self, other) -> bool:
+        return super().__eq__(other) \
+           and self.reference == other.reference \
+           and self.threshold == other.threshold \
+           and self.strict == other.strict
+
+
 class _Bounded(FilterExpression):
-    """
-    """
+    """Value is bounded by a threshold. Assumes a Number literal."""
 
     # bound.
     threshold: float
@@ -327,15 +366,11 @@ class _Bounded(FilterExpression):
 
 
 class LessThan(_Bounded):
-    """Value is (strictly) smaller than threshold.
-    NOTE: only on numerical literals
-    """
+    """Value is (strictly) smaller than threshold. Assumes a Number literal."""
 
 
 class GreaterThan(_Bounded):
-    """Value is (strictly) larger than threshold
-    NOTE: only on numerical literals
-    """
+    """Value is (strictly) larger than threshold. Assumes a Number literal."""
 
 
 class Predicate(PredicateExpression):