fix(parquet/metadata): fix default unsigned int statistics (#210)

### Rationale for this change
fixes #209 

### What changes are included in this PR?
Fixing `UpdateSpaced` statistics methods to properly check unsigned
values to ensure proper handling of default min/max values.

### Are these changes tested?
Yes, unit test was added to confirm.

### Are there any user-facing changes?
None, other than fixing the reported bug.

parquet/metadata/statistics_test.go

@@ -28,6 +28,7 @@ import (
 	"github.com/apache/arrow-go/v18/parquet/metadata"
 	"github.com/apache/arrow-go/v18/parquet/schema"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 // NOTE(zeroshade): tests will be added and updated after merging the "file" package
@@ -260,3 +261,32 @@ func TestBooleanStatisticsEncoding(t *testing.T) {
 	assert.Equal(t, []byte{1}, maxEnc)
 	assert.Equal(t, []byte{0}, minEnc)
 }
+
+func TestUnsignedDefaultStats(t *testing.T) {
+	// testing issue github.com/apache/arrow-go/issues/209
+	// stats for unsigned columns should not have invalid min values
+	{
+		n, err := schema.NewPrimitiveNodeLogical("uint16", parquet.Repetitions.Optional,
+			schema.NewIntLogicalType(16, false), parquet.Types.Int32, 4, -1)
+		require.NoError(t, err)
+		descr := schema.NewColumn(n, 1, 0)
+		s := metadata.NewStatistics(descr, nil).(*metadata.Int32Statistics)
+		s.UpdateSpaced([]int32{0}, []byte{1}, 0, 0)
+
+		minv, maxv := s.Min(), s.Max()
+		assert.Zero(t, minv)
+		assert.Zero(t, maxv)
+	}
+	{
+		n, err := schema.NewPrimitiveNodeLogical("uint64", parquet.Repetitions.Optional,
+			schema.NewIntLogicalType(64, false), parquet.Types.Int64, 4, -1)
+		require.NoError(t, err)
+		descr := schema.NewColumn(n, 1, 0)
+		s := metadata.NewStatistics(descr, nil).(*metadata.Int64Statistics)
+		s.UpdateSpaced([]int64{0}, []byte{1}, 0, 0)
+
+		minv, maxv := s.Min(), s.Max()
+		assert.Zero(t, minv)
+		assert.Zero(t, maxv)
+	}
+}

parquet/metadata/statistics_types.gen.go.tmpl

@@ -248,13 +248,26 @@ func (s *{{.Name}}Statistics) getMinMaxSpaced(values []{{.name}}, validBits []by
   max = s.defaultMax()
 
 {{- if or (eq .name "int32") (eq .name "int64")}}
-  var fn func([]{{.name}}) ({{.name}}, {{.name}})
+  var fn func([]{{.name}})
   if s.order == schema.SortSIGNED {
-    fn = shared_utils.GetMinMax{{.Name}}
+    fn = func(v []{{.name}}) {
+      localMin, localMax := shared_utils.GetMinMax{{.Name}}(v)
+      if min > localMin { 
+        min = localMin
+      }
+      if max < localMax {
+        max = localMax
+      }
+    }
   } else {
-    fn = func(v []{{.name}}) ({{.name}}, {{.name}}) {
-      umin, umax := shared_utils.GetMinMaxU{{.name}}(arrow.U{{.name}}Traits.CastFromBytes(arrow.{{.Name}}Traits.CastToBytes(values)))
-      return {{.name}}(umin), {{.name}}(umax)
+    fn = func(v []{{.name}}) {
+      umin, umax := shared_utils.GetMinMaxU{{.name}}(arrow.U{{.name}}Traits.CastFromBytes(arrow.{{.Name}}Traits.CastToBytes(v)))
+      if u{{.name}}(min) > umin {
+        min = {{.name}}(umin)
+      }
+      if u{{.name}}(max) < umax {
+        max = {{.name}}(umax)
+      }      
     }
   }
 {{- end}}
@@ -271,13 +284,7 @@ func (s *{{.Name}}Statistics) getMinMaxSpaced(values []{{.name}}, validBits []by
       break
     }
 {{- if or (eq .name "int32") (eq .name "int64")}}
-    localMin, localMax := fn(values[int(run.Pos):int(run.Pos+run.Length)])
-    if min > localMin {
-      min = localMin
-    }
-    if max < localMax {
-      max = localMax
-    }
+    fn(values[int(run.Pos):int(run.Pos+run.Length)])    
 {{- else}}
     for _, v := range values[int(run.Pos):int(run.Pos+run.Length)] {
 {{- if or (eq .name "float32") (eq .name "float64") (eq .Name "Float16") }}
@@ -385,7 +392,7 @@ func (s *{{.Name}}Statistics) UpdateFromArrow(values arrow.Array, updateCounts b
   )
 
   for i := 0; i < arr.Len(); i++ {
-    nextOffset := arr.ValueOffset64(i + 1)
+    nextOffset := curOffset + int64(arr.ValueLen(i))
     v := data[curOffset:nextOffset]
     curOffset = nextOffset