From c24aa2f59a77c15794db2d81c2574a5dcec177e5 Mon Sep 17 00:00:00 2001
From: dasm <maegereg@gmail.com>
Date: Fri, 31 Oct 2025 17:14:10 -0700
Subject: [PATCH 1/2] Add hash function and re-enable hash tests.

---
 src/flint/test/test_arb.py | 64 ++++++++++++++++++--------------------
 src/flint/types/arb.pyx    |  4 +++
 2 files changed, 35 insertions(+), 33 deletions(-)

diff --git a/src/flint/test/test_arb.py b/src/flint/test/test_arb.py
index 785b0ef2..7863b348 100644
--- a/src/flint/test/test_arb.py
+++ b/src/flint/test/test_arb.py
@@ -110,37 +110,36 @@ def test_contains():
     ]:
         assert (x in y) == expected
 
-# TODO: Re-enable this if we ever add the ability to hash arbs.
-# def test_hash():
-#     """`x` and `y` hash to the same value if they have the same midpoint and radius.
-
-#     Args:
-#         x: An arb.
-#         y: An arb.
-#         expected: Whether `x` and `y` should hash to the same value.
-#     """
-#     def arb_pi(prec):
-#         """Helper to calculate arb to a given precision."""
-#         with ctx.workprec(prec):
-#             return arb.pi()
-#     for x, y, expected in [
-#         (arb(10), arb(10), True),
-#         (arb(10), arb(11), False),
-#         (arb(10.0), arb(10), True),
-#         (
-#             arb(mid=10, rad=2),
-#             arb(mid=10, rad=2),
-#             True,
-#         ),
-#         (
-#             arb(mid=10, rad=2),
-#             arb(mid=10, rad=3),
-#             False,
-#         ),
-#         (arb_pi(100), arb_pi(100), True),
-#         (arb_pi(100), arb_pi(1000), False),
-#     ]:
-#         assert (hash(x) == hash(y)) == expected
+def test_hash():
+    """`x` and `y` hash to the same value if they have the same midpoint and radius.
+
+    Args:
+        x: An arb.
+        y: An arb.
+        expected: Whether `x` and `y` should hash to the same value.
+    """
+    def arb_pi(prec):
+        """Helper to calculate arb to a given precision."""
+        with ctx.workprec(prec):
+            return arb.pi()
+    for x, y, expected in [
+        (arb(10), arb(10), True),
+        (arb(10), arb(11), False),
+        (arb(10.0), arb(10), True),
+        (
+            arb(mid=10, rad=2),
+            arb(mid=10, rad=2),
+            True,
+        ),
+        (
+            arb(mid=10, rad=2),
+            arb(mid=10, rad=3),
+            False,
+        ),
+        (arb_pi(100), arb_pi(100), True),
+        (arb_pi(100), arb_pi(1000), False),
+    ]:
+        assert (hash(x) == hash(y)) == expected
 
 
 
@@ -338,8 +337,7 @@ def test_no_tests_missing():
     test_lower,
     test_upper,
     test_contains,
-    # TODO: Re-enable this if we ever add the ability to hash arbs.
-    # test_hash,
+    test_hash,
     test_arb_sub,
     test_arb_add,
     test_arb_mul,
diff --git a/src/flint/types/arb.pyx b/src/flint/types/arb.pyx
index d552dc7a..643e1dab 100644
--- a/src/flint/types/arb.pyx
+++ b/src/flint/types/arb.pyx
@@ -527,6 +527,10 @@ cdef class arb(flint_scalar):
             arb_clear(tval)
         return res
 
+    def __hash__(self):
+        """Hash."""
+        return hash((self.mid().man_exp(), self.rad().man_exp()))
+
     def __contains__(self, other):
         other = any_as_arb(other)
         return arb_contains(self.val, (<arb>other).val)

From 621b0add5af81f0a20f423b97f5e314d8981f9de Mon Sep 17 00:00:00 2001
From: dasm <maegereg@gmail.com>
Date: Tue, 4 Nov 2025 15:15:32 -0800
Subject: [PATCH 2/2] Make hashing fail for inexact arbs.

---
 src/flint/test/test_arb.py | 23 +++++++++++------------
 src/flint/types/arb.pyx    |  4 +++-
 2 files changed, 14 insertions(+), 13 deletions(-)

diff --git a/src/flint/test/test_arb.py b/src/flint/test/test_arb.py
index 7863b348..66fce5f2 100644
--- a/src/flint/test/test_arb.py
+++ b/src/flint/test/test_arb.py
@@ -126,21 +126,20 @@ def arb_pi(prec):
         (arb(10), arb(10), True),
         (arb(10), arb(11), False),
         (arb(10.0), arb(10), True),
-        (
-            arb(mid=10, rad=2),
-            arb(mid=10, rad=2),
-            True,
-        ),
-        (
-            arb(mid=10, rad=2),
-            arb(mid=10, rad=3),
-            False,
-        ),
-        (arb_pi(100), arb_pi(100), True),
-        (arb_pi(100), arb_pi(1000), False),
     ]:
         assert (hash(x) == hash(y)) == expected
 
+    for x in [
+        arb(mid=10, rad=2),
+        arb_pi(100),
+    ]:
+        try:
+            hash(x)
+        except ValueError:
+            pass
+        else:
+            assert False, f"Expected {x} to raise an error if hashed, but succeeded."
+
 
 
 # Tests for arithmetic functions in `flint.arb`.
diff --git a/src/flint/types/arb.pyx b/src/flint/types/arb.pyx
index 643e1dab..dbb5c066 100644
--- a/src/flint/types/arb.pyx
+++ b/src/flint/types/arb.pyx
@@ -529,7 +529,9 @@ cdef class arb(flint_scalar):
 
     def __hash__(self):
         """Hash."""
-        return hash((self.mid().man_exp(), self.rad().man_exp()))
+        if self.is_exact():
+            return hash((self.mid().man_exp(), self.rad().man_exp()))
+        raise ValueError(f"Cannot hash non-exact arb: {self}. See pull/341 for details.")
 
     def __contains__(self, other):
         other = any_as_arb(other)