fix: Improve binning in Series.hist with bin_count when all value…

…s are the same (#20034)

crates/polars-ops/src/chunked_array/hist.rs

@@ -69,8 +69,14 @@ where
                 // Determine outer bin edges from the data itself
                 let min_value = ca.min().unwrap().to_f64().unwrap();
                 let max_value = ca.max().unwrap().to_f64().unwrap();
-                pad_lower = true;
-                (min_value, (max_value - min_value) / bin_count as f64)
+
+                // All data points are identical--use unit interval.
+                if min_value == max_value {
+                    (min_value - 0.5, 1.0 / bin_count as f64)
+                } else {
+                    pad_lower = true;
+                    (min_value, (max_value - min_value) / bin_count as f64)
+                }
             };
             let out = (0..bin_count + 1)
                 .map(|x| (x as f64 * width) + offset)

py-polars/tests/unit/operations/test_hist.py

@@ -370,6 +370,8 @@ def test_hist_all_null() -> None:
 def test_hist_rand(n_values: int, n_null: int) -> None:
     s_rand = pl.Series([None] * n_null, dtype=pl.Int64)
     s_values = pl.Series(np.random.randint(0, 100, n_values), dtype=pl.Int64)
+    if s_values.n_unique() == 1:
+        pytest.skip("Identical values not tested.")
     s = pl.concat((s_rand, s_values))
     out = s.hist(bin_count=10)
 
@@ -424,3 +426,15 @@ def test_hist_max_boundary_19998() -> None:
     )
     result = s.hist(bin_count=50)
     assert result["count"].sum() == 4
+
+
+def test_hist_same_values_20030() -> None:
+    out = pl.Series([1, 1]).hist(bin_count=2)
+    expected = pl.DataFrame(
+        {
+            "breakpoint": pl.Series([1.0, 1.5], dtype=pl.Float64),
+            "category": pl.Series(["(0.5, 1.0]", "(1.0, 1.5]"], dtype=pl.Categorical),
+            "count": pl.Series([2, 0], dtype=pl.get_index_type()),
+        }
+    )
+    assert_frame_equal(out, expected)