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BaseMapCodec.decode(): fix O(N²) complexity #110

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BaseMapCodec.decode(): fix O(N²) complexity #110

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15e45ac
BaseMapCodec decode fix
nekotxs Aug 1, 2026
63646b5
replace hashmap with linked hashmap on fallback
nekotxs Aug 18, 2026
c33db6b
import clean
nekotxs Aug 18, 2026
bacc416
comment update
nekotxs Aug 18, 2026
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38 changes: 35 additions & 3 deletions src/main/java/com/mojang/serialization/codecs/BaseMapCodec.java
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Original file line number Diff line number Diff line change
Expand Up @@ -11,9 +11,10 @@
import com.mojang.serialization.Lifecycle;
import com.mojang.serialization.MapLike;
import com.mojang.serialization.RecordBuilder;
import it.unimi.dsi.fastutil.objects.Object2ObjectArrayMap;
import it.unimi.dsi.fastutil.objects.Object2ObjectLinkedOpenHashMap;
import it.unimi.dsi.fastutil.objects.Object2ObjectMap;

import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.stream.Stream;
Expand All @@ -24,10 +25,41 @@ public interface BaseMapCodec<K, V> {
Codec<V> elementCodec();

default <T> DataResult<Map<K, V>> decode(final DynamicOps<T> ops, final MapLike<T> input) {
final Object2ObjectMap<K, V> read = new Object2ObjectArrayMap<>();
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final List<Pair<T, T>> pairs = input.entries().toList();

final ImmutableMap.Builder<K, V> builder = ImmutableMap.builder();
boolean anyDecodeFailure = false;
for (final Pair<T, T> pair : pairs) {
final Optional<K> k = keyCodec().parse(ops, pair.getFirst()).result();
final Optional<V> v = elementCodec().parse(ops, pair.getSecond()).result();
if (k.isPresent() && v.isPresent()) {
builder.put(k.get(), v.get());
} else {
anyDecodeFailure = true;
break;
}
}

// failures and duplicates are rare, so it is okay to fall back to linked HashMap
// no failures: best performance
// failure: acceptable performance
if (!anyDecodeFailure) {
try {
return DataResult.success(builder.buildOrThrow());
} catch (IllegalArgumentException duplicateKey) {
return fallbackDecode(ops, pairs);
}
}

return fallbackDecode(ops, pairs);
}

// HashMap fallback to check for failures and duplicates
private <T> DataResult<Map<K, V>> fallbackDecode(final DynamicOps<T> ops, final List<Pair<T, T>> pairs) {
final Object2ObjectMap<K, V> read = new Object2ObjectLinkedOpenHashMap<>();
final Stream.Builder<Pair<T, T>> failed = Stream.builder();

final DataResult<Unit> result = input.entries().reduce(
final DataResult<Unit> result = pairs.stream().reduce(
DataResult.success(Unit.INSTANCE, Lifecycle.stable()),
(r, pair) -> {
final DataResult<K> key = keyCodec().parse(ops, pair.getFirst());
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