Start working on IntermediaryProvider

src/main/java/net/legacyfabric/multifilament/provider/FabricProvider.java

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+package net.legacyfabric.multifilament.provider;
+
+public class FabricProvider implements IntermediaryProvider {
+    final FabricLikeMetadata metadata = new FabricLikeMetadata("https://meta.fabricmc.net/v2/versions/game");
+
+    @Override
+    public String getIntermediaryURL(String mcVersion) {
+        return "https://github.com/FabricMC/intermediary/raw/master/mappings/" + mcVersion + ".tiny";
+    }
+
+    @Override
+    public boolean supportVersion(String mcVersion) {
+        return metadata.contains(mcVersion);
+    }
+}

src/main/java/net/legacyfabric/multifilament/provider/IntermediaryProvider.java

@@ -0,0 +1,60 @@
+package net.legacyfabric.multifilament.provider;
+
+import org.gradle.internal.impldep.com.google.gson.Gson;
+import org.gradle.internal.impldep.com.google.gson.JsonArray;
+import org.gradle.internal.impldep.com.google.gson.JsonElement;
+import org.gradle.internal.impldep.com.google.gson.JsonObject;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.io.Reader;
+import java.net.MalformedURLException;
+import java.net.URL;
+import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
+import java.util.List;
+
+public interface IntermediaryProvider {
+    String getIntermediaryURL(String mcVersion);
+    boolean supportVersion(String mcVersion);
+
+    class FabricLikeMetadata {
+        private final String url;
+        private final List<String> versions = new ArrayList<>();
+        private static final Gson gson = new Gson();
+
+        public FabricLikeMetadata(String url) {
+            this.url = url;
+        }
+
+        private void computeData() {
+            try {
+                URL url1 = new URL(url);
+                try (InputStream stream = url1.openStream()) {
+                    try (Reader reader = new InputStreamReader(stream, StandardCharsets.UTF_8)) {
+                        JsonArray array = gson.fromJson(reader, JsonArray.class);
+
+                        for (JsonElement element : array) {
+                            if (element instanceof JsonObject object) {
+                                if (object.has("version")) {
+                                    String version = object.get("version").getAsString();
+
+                                    if (!versions.contains(version)) versions.add(version);
+                                }
+                            }
+                        }
+                    }
+                }
+            } catch (IOException e) {
+                e.printStackTrace();
+            }
+        }
+
+        public boolean contains(String version) {
+            if (versions.isEmpty()) this.computeData();
+
+            return versions.contains(version);
+        }
+    }
+}

src/main/java/net/legacyfabric/multifilament/provider/LegacyFabricProvider.java

@@ -0,0 +1,31 @@
+package net.legacyfabric.multifilament.provider;
+
+public class LegacyFabricProvider implements IntermediaryProvider {
+    private final int revision;
+    final FabricLikeMetadata metadata = new FabricLikeMetadata("https://meta.legacyfabric.net/v2/versions/game");
+
+    public LegacyFabricProvider(int revision) {
+        this.revision = revision;
+    }
+
+    @Override
+    public String getIntermediaryURL(String mcVersion) {
+        return "https://github.com/Legacy-Fabric/Legacy-Intermediaries/raw/v2" + getIntermediaryFolder() + "/mappings/" + mcVersion + ".tiny";
+    }
+
+    private String getIntermediaryFolder() {
+        switch (revision) {
+            case 1:
+                return "legacy/";
+            case 0:
+                return "legacy_old/";
+            default:
+                return "";
+        }
+    }
+
+    @Override
+    public boolean supportVersion(String mcVersion) {
+        return metadata.contains(mcVersion);
+    }
+}