private static final String FILE = "./measurements.txt";

private static class MeasurementAggregator {

private int min = Integer.MAX_VALUE;

private int max = Integer.MIN_VALUE;

// final long startTs = System.currentTimeMillis();

private long sum = 0;

private long count = 0;

private double mean = 0;

public void finish() {

double sum = this.sum / 10.0;

mean = sum / (double) count;

}

public void printInto(PrintStream out) {

double min = (double) this.min / 10.0;

double max = (double) this.max / 10.0;

out.printf("%.1f/%.1f/%.1f", min, mean, max);

}

}

public static class StationIdent {

private final byte[] name;

private final int nameHash;

public StationIdent(byte[] name, int nameHash) {

this.name = name;

this.nameHash = nameHash;

}

@Override

public int hashCode() {

return nameHash;

}

@Override

public boolean equals(Object obj) {

var other = (StationIdent) obj;

if (other.name.length != name.length) {

return false;

}

return Arrays.equals(name, other.name);

}

}

public static class ChunkReader implements Runnable {

RandomAccessFile file;

// Start offset of this chunk

private final long startOffset;

// end offset of this chunk

private final long endOffset;

// state

private int chunkSize = 0;

private MappedByteBuffer buffer = null;

private MemorySegment memorySegment = null;

private int ptr = 0;

private HashMap<StationIdent, MeasurementAggregator> workSet;

public ChunkReader(RandomAccessFile file, long startOffset, long endOffset) {

this.file = file;

this.startOffset = startOffset;

this.endOffset = endOffset;

}

// private StationIdent readStationName() {

// int startPtr = ptr;

// int hashCode = 0;

// int hashBytePtr = 0;

// byte c;

// while ((c = buffer.get(ptr++)) != ';') {

// hashCode ^= ((int) c) << (hashBytePtr * 8);

// hashBytePtr = (hashBytePtr + 1) % 4;

// }

// byte[] stationNameBfr = new byte[ptr - startPtr - 1];

// buffer.get(startPtr, stationNameBfr);

// return new StationIdent(stationNameBfr, hashCode);

// }

private StationIdent readStationName() {

final var VECTOR_SPECIES = ByteVector.SPECIES_256;

if (chunkSize - ptr - 100 < VECTOR_SPECIES.length()) { // fallback

int startPtr = ptr;

while (buffer.get(ptr++) != ';') {

}

byte[] stationNameBfr = new byte[ptr - startPtr - 1];

buffer.get(startPtr, stationNameBfr);

return new StationIdent(stationNameBfr, Arrays.hashCode(stationNameBfr) ^ stationNameBfr.length);

}

else { // SIMD

int sepIdx = 0;

while (true) {

ByteVector tmp = ByteVector.fromMemorySegment(VECTOR_SPECIES, memorySegment, ptr + sepIdx, ByteOrder.LITTLE_ENDIAN);

final var cmpResult = tmp.compare(VectorOperators.EQ, ';');

if (cmpResult.anyTrue()) {

sepIdx += cmpResult.firstTrue();

break;

}

else {

sepIdx += tmp.length();

}

}

int endPtr = ptr + sepIdx;

byte[] stationNameBfr = new byte[endPtr - ptr];

buffer.get(ptr, stationNameBfr);

ptr = endPtr + 1;

return new StationIdent(stationNameBfr, Arrays.hashCode(stationNameBfr) ^ stationNameBfr.length);

}

}

private int readTemperature() {

int ret = 0;

byte c = buffer.get(ptr++);

final boolean neg = (c == '-');

if (neg) {

c = buffer.get(ptr++);

}

do {

if (c != '.') {

ret = ret * 10 + c - '0';

}

} while ((c = buffer.get(ptr++)) != '\n');

if (neg)

return -ret;

return ret;

}

@Override

public void run() {

workSet = new HashMap<>();

if (endOffset - startOffset > Integer.MAX_VALUE) {

throw new RuntimeException("Mapping a block larger than 2GB is not possible with Java! Welcome to 2024 :)");

}

chunkSize = (int) (endOffset - startOffset);

try {

buffer = file.getChannel().map(FileChannel.MapMode.READ_ONLY, startOffset, chunkSize);

memorySegment = MemorySegment.ofBuffer(buffer);

while (ptr < chunkSize) {

var station = readStationName();

int temp = readTemperature();

var stationWorkSet = workSet.get(station);

if (stationWorkSet == null) {

stationWorkSet = new MeasurementAggregator();

workSet.put(station, stationWorkSet);

}

stationWorkSet.min = Math.min(temp, stationWorkSet.min);

stationWorkSet.max = Math.max(temp, stationWorkSet.max);

stationWorkSet.sum += temp;

stationWorkSet.count += 1;

}

}

catch (Throwable e) {

e.printStackTrace();

throw new RuntimeException(e);

}

}

}

private static void printWorkSet(TreeMap<String, MeasurementAggregator> result, PrintStream out) {

out.write('{');

final var iterator = result.entrySet().iterator();

while (iterator.hasNext()) {

var entry = iterator.next();

out.print(entry.getKey());

out.write('=');

entry.getValue().printInto(out);

if (iterator.hasNext()) {

out.print(", ");

}

}

out.println('}');

}

private static int createChunks(final RandomAccessFile file, final ChunkReader[] chunks) throws IOException {

final long fileEndPtr = file.length();

final long chunkSize = Math.max(1, fileEndPtr / chunks.length);

int jobCnt = 0;

long chunkStartPtr = 0;

final byte[] tmpBuffer = new byte[128];

while (chunkStartPtr < fileEndPtr) {

long chunkEndPtr = Math.min(chunkStartPtr + chunkSize, fileEndPtr);

// Seek into file at the calculated chunk end ptr, then extend it until the next

// new-line or EOF

if (chunkEndPtr < fileEndPtr) {

file.seek(Math.max(0, chunkEndPtr - 1));

file.read(tmpBuffer);

int offset = 0;

while (tmpBuffer[offset] != '\n') {

offset += 1;

}

chunkEndPtr += offset;

}

chunks[jobCnt] = new ChunkReader(file, chunkStartPtr, chunkEndPtr);

jobCnt += 1;

chunkStartPtr = chunkEndPtr;

}

return jobCnt;

}

public static void main(String[] args) throws IOException, InterruptedException {

final RandomAccessFile file = new RandomAccessFile(FILE, "r");

int jobCnt = Runtime.getRuntime().availableProcessors();

final var chunks = new ChunkReader[jobCnt];

jobCnt = createChunks(file, chunks);

try (final var executor = Executors.newFixedThreadPool(jobCnt)) {

for (int i = 0; i < jobCnt; ++i) {

executor.submit(chunks[i]);

}

executor.shutdown();

final var ignored = executor.awaitTermination(1, TimeUnit.DAYS);

}

// merge chunks

final var result = new TreeMap<String, MeasurementAggregator>();

for (int i = 0; i < jobCnt; ++i) {

chunks[i].workSet.forEach((ident, otherStationWorkSet) -> {

final var identStr = new String(ident.name);

final var stationWorkSet = result.get(identStr);

if (stationWorkSet == null) {

result.put(identStr, otherStationWorkSet);

}

else {

stationWorkSet.min = Math.min(stationWorkSet.min, otherStationWorkSet.min);

stationWorkSet.max = Math.max(stationWorkSet.max, otherStationWorkSet.max);

stationWorkSet.sum += otherStationWorkSet.sum;

stationWorkSet.count += otherStationWorkSet.count;

}

});

}

result.forEach((ignored, meas) -> meas.finish());

// print in required format

printWorkSet(result, System.out);

}