fix: map IntValue to unsigned repr when serializing (#2413)

to match Rust definition
Closes #2409

Author: ss2165

hugr-py/src/hugr/std/int.py

@@ -52,16 +52,39 @@ def _int_tv(index: int) -> tys.ExtType:
 INT_T = int_t(5)
 
 
+def _to_unsigned(val: int, bits: int) -> int:
+    """Convert a signed integer to its unsigned representation
+    in twos-complement form.
+
+    Positive integers are unchanged, while negative integers
+    are converted by adding 2^bits to the value.
+
+    Raises ValueError if the value is out of range for the given bit width
+    (valid range is [-2^(bits-1), 2^(bits-1)-1]).
+    """
+    half_max = 1 << (bits - 1)
+    min_val = -half_max
+    max_val = half_max - 1
+    if val < min_val or val > max_val:
+        msg = f"Value {val} out of range for {bits}-bit signed integer."
+        raise ValueError(msg)  #
+
+    if val < 0:
+        return (1 << bits) + val
+    return val
+
+
 @dataclass
 class IntVal(val.ExtensionValue):
-    """Custom value for an integer."""
+    """Custom value for a signed integer."""
 
     v: int
     width: int = field(default=5)
 
     def to_value(self) -> val.Extension:
         name = "ConstInt"
-        payload = {"log_width": self.width, "value": self.v}
+        unsigned = _to_unsigned(self.v, 1 << self.width)
+        payload = {"log_width": self.width, "value": unsigned}
         return val.Extension(
             name,
             typ=int_t(self.width),
@@ -72,8 +95,9 @@ def __str__(self) -> str:
         return f"{self.v}"
 
     def to_model(self) -> model.Term:
+        unsigned = _to_unsigned(self.v, 1 << self.width)
         return model.Apply(
-            "arithmetic.int.const", [model.Literal(self.width), model.Literal(self.v)]
+            "arithmetic.int.const", [model.Literal(self.width), model.Literal(unsigned)]
         )
 
 

hugr-py/tests/test_prelude.py

@@ -1,4 +1,7 @@
+import pytest
+
 from hugr.build.dfg import Dfg
+from hugr.std.int import IntVal, int_t
 from hugr.std.prelude import STRING_T, StringVal
 
 from .conftest import validate
@@ -16,3 +19,38 @@ def test_string_val():
     dfg.set_outputs(v)
 
     validate(dfg.hugr)
+
+
+@pytest.mark.parametrize(
+    ("log_width", "v", "unsigned"),
+    [
+        (5, 1, 1),
+        (4, 0, 0),
+        (6, 42, 42),
+        (2, -1, 15),
+        (1, -2, 2),
+        (3, -23, 233),
+        (3, -256, None),
+        (2, 16, None),
+    ],
+)
+def test_int_val(log_width: int, v: int, unsigned: int | None):
+    val = IntVal(v, log_width)
+    if unsigned is None:
+        with pytest.raises(
+            ValueError,
+            match=f"Value {v} out of range for {1<<log_width}-bit signed integer.",
+        ):
+            val.to_value()
+        return
+    ext_val = val.to_value()
+
+    assert ext_val.name == "ConstInt"
+    assert ext_val.typ == int_t(log_width)
+    assert ext_val.val == {"log_width": log_width, "value": unsigned}
+
+    dfg = Dfg()
+    o = dfg.load(val)
+    dfg.set_outputs(o)
+
+    validate(dfg.hugr)