fix gen2 intermediary provider for pre-beta versions

src/main/java/net/ornithemc/ploceus/mappings/CalamusGen2Provider.java

@@ -1,13 +1,75 @@
 package net.ornithemc.ploceus.mappings;
 
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.Map;
+
+import org.gradle.api.Project;
 import org.gradle.api.provider.Property;
 
+import net.fabricmc.loom.api.mappings.layered.MappingsNamespace;
 import net.fabricmc.loom.configuration.providers.mappings.IntermediaryMappingsProvider;
+import net.fabricmc.mappingio.MappingReader;
+import net.fabricmc.mappingio.MappingWriter;
+import net.fabricmc.mappingio.adapter.MappingNsCompleter;
+import net.fabricmc.mappingio.adapter.MappingNsRenamer;
+import net.fabricmc.mappingio.adapter.MappingSourceNsSwitch;
+import net.fabricmc.mappingio.format.MappingFormat;
+import net.fabricmc.mappingio.tree.MemoryMappingTree;
 
 public abstract class CalamusGen2Provider extends IntermediaryMappingsProvider {
 
 	public abstract Property<Integer> getGeneration();
 
+	@Override
+	public void provide(Path tinyMappings, Project project) throws IOException {
+		if (Files.exists(tinyMappings) && !getRefreshDeps().get()) {
+			return;
+		}
+
+		super.provide(tinyMappings, project);
+
+		if (getIsLegacyMinecraft().get()) {
+			/*
+			 * Loom expects intermediary files for all pre-1.3 versions to be in
+			 * intermediary -> [clientOfficial, serverOfficial] format, but gen2
+			 * Calamus only does this for >=b1.0 <1.3, so for Alpha and older we
+			 * need to rewrite the mapping file and switch out the namespaces.
+			 * All these versions are either client-only or server-only, which
+			 * means filling both the clientOfficial and serverOfficial namespaces
+			 * will make it work for either.
+			 */
+
+			MemoryMappingTree mappings = new MemoryMappingTree();
+			MappingReader.read(tinyMappings, mappings);
+
+			if (mappings.getSrcNamespace().equals(MappingsNamespace.OFFICIAL.toString())) {
+				try (MappingWriter writer = MappingWriter.create(tinyMappings, MappingFormat.TINY_2_FILE)) {
+					mappings.accept(
+						new MappingSourceNsSwitch(
+							new MappingNsRenamer(
+								new MappingNsCompleter(
+									writer,
+									Map.of(
+										MappingsNamespace.SERVER_OFFICIAL.toString(),
+										MappingsNamespace.CLIENT_OFFICIAL.toString()
+									),
+									true
+								),
+								Map.of(
+									MappingsNamespace.OFFICIAL.toString(),
+									MappingsNamespace.CLIENT_OFFICIAL.toString()
+								)
+							),
+							MappingsNamespace.INTERMEDIARY.toString()
+						)
+					);
+				}
+			}
+		}
+	}
+
 	@Override
 	public String getName() {
 		return "calamus-gen" + getGeneration().get() + "-" + NAME;