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A universal transaction manager that achieves database-agnostic transactions and distributed transactions that span multiple databases

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package com.scalar.database.storage.cassandra;

import static com.google.common.base.Preconditions.checkNotNull;

import com.datastax.driver.core.ResultSet;
import com.datastax.driver.core.Row;
import com.datastax.driver.core.TableMetadata;
import com.scalar.database.api.Result;
import com.scalar.database.api.Scanner;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Optional;
import javax.annotation.Nonnull;
import javax.annotation.concurrent.NotThreadSafe;

@NotThreadSafe
public final class ScannerImpl implements Scanner {
  private final ResultSet resultSet;
  private final TableMetadata metadata;

  public ScannerImpl(ResultSet resultSet, TableMetadata metadata) {
    this.resultSet = checkNotNull(resultSet);
    this.metadata = checkNotNull(metadata);
  }

  @Override
  @Nonnull
  public Optional one() {
    Row row = resultSet.one();
    if (row == null) {
      return Optional.empty();
    }
    return Optional.of(new ResultImpl(row, metadata));
  }

  @Override
  @Nonnull
  public List all() {
    List results = new ArrayList<>();
    resultSet.forEach(r -> results.add(new ResultImpl(r, metadata)));
    return results;
  }

  @Override
  public Iterator iterator() {
    return new ScannerIterator(resultSet, metadata);
  }
}