1brc.java

import java.io.*;
import java.util.*;

public class BRC {
    public static void main(String[] args) throws FileNotFoundException {
        long millis = System.currentTimeMillis();
        System.out.println(calculateMinMeanMaxPerStation(readCSV()));
        System.out.println("Time taken = " + (System.currentTimeMillis() - millis));
    }

    static Map<String, TemperatureReslt> calculateMinMeanMaxPerStation(List<CsvRecord> records) {
        final Map<String, TemperatureReslt> result = new TreeMap<>();
        for (CsvRecord record : records) {
            if (!result.containsKey(record.city())) {
                result.put(record.city(), new TemperatureReslt(record.temperature(), record.temperature(), record.temperature()));
                continue;
            }
            TemperatureReslt tr = result.get(record.city());
            double min = Math.min(tr.min(), record.temperature());
            double mean = (tr.mean() + record.temperature()) / 2.0;
            double max = Math.max(tr.max(), record.temperature());
            result.put(record.city(), new TemperatureReslt(min, mean, max));
        }
        return result;
    }

    static List<CsvRecord> readCSV() throws FileNotFoundException {
        List<CsvRecord> records = new ArrayList<>(1999999999);
        try (BufferedReader br = new BufferedReader(new FileReader("temperature.csv"))) {
            String line;
            while ((line = br.readLine()) != null) {
                String[] values = line.split(";");
                if (values.length != 2) continue;
                records.add(new CsvRecord(values[0], Double.parseDouble(values[1])));
            }
        } catch (Exception e) {
            System.err.println(e);
        }
        return records;
    }
}

record CsvRecord(String city, double temperature) {

}

record TemperatureReslt(double min, double mean, double max) {
    @Override
    public String toString() {
        return "<" + min + "," + mean + "," + max + ">";
    }
}

1brcTest.java

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;

import java.util.*;

class BRCTest {
    private static final String[] stations = new String[]{"Hamburg", "Bulawayo", "Palembang", "St", "Cracow", "Bridgetown", "Istanbul", "Roseau", "Hamburg", "Bulawayo", "Palembang", "St", "Cracow", "Bridgetown", "Istanbul", "Roseau"};
    private static final double[] temperatures = new double[]{12.0, 8.9, 38.8, 15.2, 12.6, 56.9, 6.2, 34.4, 10.0, 7.9, 6.8, 2.2, 2.6, 6.9, 22.2, 4.4};
    private static final TemperatureResult[] results = new TemperatureResult[]{new TemperatureResult(6.9,31.9,56.9),new TemperatureResult(7.9,8.4,8.9),new TemperatureResult(2.6,7.6,12.6),new TemperatureResult(10,11,12),new TemperatureResult(6.2,14.2,22.2),new TemperatureResult(6.8,22.8,38.8),new TemperatureResult(4.4,19.4,34.4),new TemperatureResult(2.2,8.7,15.2)};

    @Test
    void calculateMinMeanMaxPerStation() {
        TreeMap<String, TemperatureResult> result = BRC.calculateMinMeanMaxPerStation(getCsvRecord());
        var uniqueSortedStations = Arrays.stream(stations).distinct().sorted().toArray();
        Assertions.assertEquals(uniqueSortedStations.length, result.size()); // Assert the Size as early check

        int counter = 0;
        for (Map.Entry<String, TemperatureResult> entry : result.entrySet()) {
            String key = entry.getKey();
            TemperatureResult value = entry.getValue();
            
            Assertions.assertEquals(key, uniqueSortedStations[counter]); // Assert the Order of Elements
            Assertions.assertEquals(results[counter], value); // Assert the min, mean and max values
            
            counter++; // TODO: Better way than this?
        }


    }


    private List<CsvRecord> getCsvRecord() {
        final List<CsvRecord> records = new ArrayList<>(stations.length);
        
        for (int i = 0; i < stations.length; i++) {
            records.add(new CsvRecord(stations[i], temperatures[i]));
        }
        return records;
    }
}