Fix issue with leading decimal places

src/pantab/writer.cpp

@@ -360,9 +360,21 @@ class DecimalInsertHelper : public InsertHelper {
     std::string str{reinterpret_cast<const char *>(buffer.data),
                     static_cast<size_t>(buffer.size_bytes)};
     // The Hyper API wants the string to include the decimal place, which
-    // nanoarrow does not provide
-    if (scale_ > 0)
-      str = str.insert(str.size() - scale_, 1, '.');
+    // nanoarrow does not provide.
+    if (scale_ > 0) {
+      // nanoarrow strips leading zeros
+      const auto insert_pos = static_cast<int64_t>(str.size()) - scale_;
+      if (insert_pos < 0) {
+        std::string newstr{};
+        newstr.reserve(str.size() - insert_pos + 1);
+        newstr.append(1, '.');
+        newstr.append(-insert_pos, '0');
+        newstr.append(str, str.size());
+        str = std::move(newstr);
+      } else {
+        str = str.insert(str.size() - scale_, 1, '.');
+      }
+    }
 
     constexpr auto PrecisionLimit = 39; // of-by-one error in solution?
     if (precision_ >= PrecisionLimit) {

tests/test_writer.py

@@ -1,4 +1,5 @@
 import datetime
+import decimal
 import re
 
 import narwhals as nw
@@ -395,3 +396,19 @@ def test_writer_invalid_process_params_raises(tmp_hyper):
     msg = r"No internal setting named 'not_a_real_parameter'"
     with pytest.raises(RuntimeError, match=msg):
         pt.frame_to_hyper(frame, tmp_hyper, table="test", process_params=params)
+
+
+@pytest.mark.parametrize(
+    "value,precision,scale",
+    [
+        ("0.00", 3, 2),
+        ("0E-10", 3, 2),
+        ("100", 3, 0),
+        ("1.00", 3, 2),
+        (".001", 3, 3),
+    ],
+)
+def test_write_decimal_values(tmp_hyper, value, precision, scale):
+    arr = pa.array([decimal.Decimal(value)], type=pa.decimal128(precision, scale))
+    tbl = pa.Table.from_arrays([arr], names=["col"])
+    pt.frame_to_hyper(tbl, tmp_hyper, table="test")