Improve the either codec error message

Author: ErrorCraft

src/main/java/net/errorcraft/codecium/mixin/mojang/serialization/codecs/EitherCodecExtender.java

@@ -0,0 +1,30 @@
+package net.errorcraft.codecium.mixin.mojang.serialization.codecs;
+
+import com.llamalad7.mixinextras.sugar.Local;
+import com.mojang.datafixers.util.Either;
+import com.mojang.datafixers.util.Pair;
+import com.mojang.serialization.DataResult;
+import com.mojang.serialization.codecs.EitherCodec;
+import net.errorcraft.codecium.util.StringUtil;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.ModifyArg;
+
+import java.util.function.Supplier;
+
+@Mixin(value = EitherCodec.class, remap = false)
+public class EitherCodecExtender<F, S> {
+    @ModifyArg(
+        method = "decode",
+        at = @At(
+            value = "INVOKE",
+            target = "Lcom/mojang/serialization/DataResult;error(Ljava/util/function/Supplier;)Lcom/mojang/serialization/DataResult;"
+        )
+    )
+    private <T> Supplier<String> useBetterErrorMessage(Supplier<String> message, @Local(name = "firstRead") final DataResult<Pair<Either<F, S>, T>> firstRead, @Local(name = "secondRead") final DataResult<Pair<Either<F, S>, T>> secondRead) {
+        return () -> "Failed to decode either:\n" + StringUtil.indent(
+            "1: " + firstRead.error().orElseThrow().message() + "\n" +
+            "2: " + secondRead.error().orElseThrow().message()
+        );
+    }
+}

src/main/java/net/errorcraft/codecium/util/StringUtil.java

@@ -0,0 +1,34 @@
+package net.errorcraft.codecium.util;
+
+import java.util.Objects;
+import java.util.stream.Stream;
+
+public class StringUtil {
+    private static final String DEFAULT_INDENTATION = "  ";
+
+    private StringUtil() {}
+
+    public static String indent(String value) {
+        return indent(value, DEFAULT_INDENTATION);
+    }
+
+    public static String indent(String value, String indentation) {
+        return indent(value.lines(), indentation);
+    }
+
+    private static String indent(Stream<String> lines, String indentation) {
+        Objects.requireNonNull(lines);
+        Objects.requireNonNull(indentation);
+        StringBuilder builder = new StringBuilder();
+        lines.forEach(line -> {
+            builder.append(indentation);
+            builder.append(line);
+            builder.append('\n');
+        });
+        if (builder.isEmpty()) {
+            return "";
+        }
+        builder.setLength(builder.length() - 1);
+        return builder.toString();
+    }
+}

src/main/resources/codecium.mixins.json

@@ -3,7 +3,8 @@
   "package": "net.errorcraft.codecium.mixin",
   "compatibilityLevel": "JAVA_21",
   "mixins": [
-    "mojang.serialization.CodecExtender"
+    "mojang.serialization.CodecExtender",
+    "mojang.serialization.codecs.EitherCodecExtender"
   ],
   "injectors": {
     "defaultRequire": 1