rdblue commented on code in PR #16936:
URL: https://github.com/apache/iceberg/pull/16936#discussion_r4170310637


##########
core/src/main/java/org/apache/iceberg/MapBackedContentStats.java:
##########
@@ -0,0 +1,206 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.iceberg;
+
+import java.nio.ByteBuffer;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Set;
+import org.apache.iceberg.relocated.com.google.common.base.Preconditions;
+import org.apache.iceberg.relocated.com.google.common.collect.Iterables;
+import org.apache.iceberg.relocated.com.google.common.collect.Maps;
+import org.apache.iceberg.types.Conversions;
+import org.apache.iceberg.types.Type;
+import org.apache.iceberg.types.Types;
+
+/** Reusable {@link ContentStats} view over a {@link ContentFile}'s stat maps. 
*/
+class MapBackedContentStats implements ContentStats {
+  private final Types.StructType type;
+  private final Map<Integer, FieldStats<?>> statsById = Maps.newHashMap();
+
+  private Map<Integer, Long> valueCounts;
+  private Map<Integer, Long> nullValueCounts;
+  private Map<Integer, Long> nanValueCounts;
+  private Map<Integer, Integer> avgValueSizes;
+  private Map<Integer, ByteBuffer> lowerBounds;
+  private Map<Integer, ByteBuffer> upperBounds;
+
+  MapBackedContentStats(Schema tableSchema, MetricsConfig metricsConfig) {
+    Preconditions.checkArgument(tableSchema != null, "Invalid table schema: 
null");
+    Preconditions.checkArgument(metricsConfig != null, "Invalid metrics 
config: null");
+    this.type = StatsUtil.statsWriteSchema(tableSchema, metricsConfig);
+  }
+
+  MapBackedContentStats wrap(ContentFile<?> file) {
+    this.valueCounts = file.valueCounts();
+    this.nullValueCounts = file.nullValueCounts();
+    this.nanValueCounts = file.nanValueCounts();
+    this.avgValueSizes = file.avgValueSizes();
+    this.lowerBounds = file.lowerBounds();
+    this.upperBounds = file.upperBounds();
+    return this;
+  }
+
+  @Override
+  public Iterable<FieldStats<?>> fieldStats() {
+    return Iterables.filter(
+        Iterables.transform(type.fields(), field -> 
statsFor(StatsUtil.toFieldId(field.fieldId()))),
+        Objects::nonNull);
+  }
+
+  @Override
+  @SuppressWarnings("unchecked")
+  public <T> FieldStats<T> statsFor(int fieldId) {
+    if (!hasStats(fieldId)) {
+      return null;
+    }
+
+    FieldStats<?> fieldStats = statsById.get(fieldId);
+    if (fieldStats == null) {
+      fieldStats = new MapBackedFieldStats<>(fieldId);
+      statsById.put(fieldId, fieldStats);
+    }
+
+    return (FieldStats<T>) fieldStats;
+  }
+
+  @Override
+  public Types.StructType type() {
+    return type;
+  }
+
+  @Override
+  public ContentStats copy() {
+    throw new UnsupportedOperationException("copy is not implemented");
+  }
+
+  @Override
+  public ContentStats copy(Set<Integer> fieldIds) {
+    throw new UnsupportedOperationException("copy is not implemented");
+  }
+
+  private boolean hasStats(int id) {
+    return containsId(valueCounts, id)
+        || containsId(nullValueCounts, id)
+        || containsId(nanValueCounts, id)
+        || containsId(avgValueSizes, id)
+        || containsId(lowerBounds, id)
+        || containsId(upperBounds, id);
+  }
+
+  private static boolean containsId(Map<Integer, ?> map, int id) {
+    return map != null && map.containsKey(id);
+  }
+
+  /** Reusable {@link FieldStats} view over one field's entries in a {@link 
ContentFile}'s maps. */
+  private class MapBackedFieldStats<T> implements FieldStats<T> {
+    private final int fieldId;
+    private final Types.StructType struct;
+    private final Type boundType;
+
+    MapBackedFieldStats(int fieldId) {
+      Types.NestedField field = type.field(StatsUtil.toBaseId(fieldId));
+      Preconditions.checkArgument(
+          field != null,
+          "Cannot convert stats for field ID %s: unknown, not a scalar, or not 
in metrics config",
+          fieldId);
+      this.fieldId = fieldId;
+      this.struct = field.type().asStructType();
+      this.boundType = struct.fieldType(StatsUtil.LOWER_BOUND_NAME);
+    }
+
+    @Override
+    public int fieldId() {
+      return fieldId;
+    }
+
+    @Override
+    public Types.StructType type() {
+      return struct;
+    }
+
+    @Override
+    public T lowerBound() {
+      return decodeBound(lowerBounds);

Review Comment:
   This doesn't currently work for geo types. `Conversion.fromByteBuffer` will 
produce `GeospatialBound`, which is not the correct value for a lower or upper 
bound because it is not a valid representation in the internal object model 
(`InternalData`).
   
   I think we could fix this by implementing `StructLike` in `GeospatialBound`. 
That class has XYZM values and would just need to implement `StructLike`. We 
can probably do this in a follow up since we are going to return 
`GeospatialBound` either way, but I have a slight preference for fixing it now.



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