Add JAR manifest header folding

java/manifest/src/META-INF/ManifestSupport.xml

@@ -4,6 +4,7 @@
   </extensionPoints>
   <extensions defaultExtensionNs="com.intellij">
     <fileType name="Manifest" implementationClass="org.jetbrains.lang.manifest.ManifestFileType" fileNamesCaseInsensitive="manifest.mf" fieldName="INSTANCE" language="Manifest"/>
+    <lang.foldingBuilder language="Manifest" implementationClass="org.jetbrains.lang.manifest.folding.ManifestFoldingBuilder" />
     <lang.parserDefinition language="Manifest" implementationClass="org.jetbrains.lang.manifest.parser.ManifestParserDefinition"/>
     <lang.syntaxHighlighterFactory language="Manifest" implementationClass="org.jetbrains.lang.manifest.highlighting.ManifestSyntaxHighlighterFactory"/>
     <annotator language="Manifest" implementationClass="org.jetbrains.lang.manifest.highlighting.HeaderAnnotator"/>

java/manifest/src/org/jetbrains/lang/manifest/folding/ManifestFoldingBuilder.java

@@ -0,0 +1,78 @@
+// Copyright 2000-2024 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
+package org.jetbrains.lang.manifest.folding;
+
+import com.intellij.lang.ASTNode;
+import com.intellij.lang.folding.FoldingBuilderEx;
+import com.intellij.lang.folding.FoldingDescriptor;
+import com.intellij.openapi.editor.Document;
+import com.intellij.openapi.project.DumbAware;
+import com.intellij.openapi.util.TextRange;
+import com.intellij.psi.PsiElement;
+import com.intellij.psi.PsiRecursiveElementVisitor;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+import org.jetbrains.lang.manifest.psi.Header;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class ManifestFoldingBuilder extends FoldingBuilderEx implements DumbAware {
+
+  /**
+   * The <a href="https://docs.oracle.com/en/java/javase/22/docs/specs/jar/jar.html#notes-on-manifest-and-signature-files">JAR file
+   * specification</a> states each line should be no longer than 72 bytes
+   */
+  private static final int MAX_BYTES_PER_LINE = 72;
+  private static final String ELLIPSIS = "...";
+
+  @Override
+  public FoldingDescriptor @NotNull [] buildFoldRegions(@NotNull PsiElement root, @NotNull Document document, boolean quick) {
+    List<FoldingDescriptor> descriptors = new ArrayList<>();
+
+    root.accept(new PsiRecursiveElementVisitor() {
+      @Override
+      public void visitElement(@NotNull PsiElement element) {
+        super.visitElement(element);
+
+        if (element instanceof Header header) {
+          // Prevent collapsing the new lines too
+          var textRange = TextRange.from(header.getTextRange().getStartOffset(), getTextLengthWithoutFinalNewline(header));
+          descriptors.add(new FoldingDescriptor(element, textRange));
+        }
+      }
+    });
+
+    return descriptors.toArray(FoldingDescriptor.EMPTY_ARRAY);
+  }
+
+  @Nullable
+  @Override
+  public String getPlaceholderText(@NotNull ASTNode node) {
+    if (node.getPsi() instanceof Header) {
+      String usefulText = node.getText().trim();
+      if (usefulText.length() <= MAX_BYTES_PER_LINE) {
+        return node.getText();
+      }
+      int endIndex = Math.min(MAX_BYTES_PER_LINE - ELLIPSIS.length(), usefulText.length());
+      return node.getText().substring(0, endIndex) + ELLIPSIS;
+    }
+
+    return null;
+  }
+
+  @Override
+  public boolean isCollapsedByDefault(@NotNull ASTNode node) {
+    return false; // no change in existing behaviour
+  }
+
+  private static int getTextLengthWithoutFinalNewline(PsiElement psiElement) {
+    int lengthOfTextWithoutFinalNewline = psiElement.getTextLength();
+    if (psiElement.getText().endsWith("\n") || psiElement.getText().endsWith("\r")) {
+      lengthOfTextWithoutFinalNewline -= 1;
+    }
+    else if (psiElement.getText().endsWith("\r\n")) {
+      lengthOfTextWithoutFinalNewline -= 2;
+    }
+    return lengthOfTextWithoutFinalNewline;
+  }
+}