Add completion contributor for import statements.

src/main/java/org/antlr/intellij/plugin/ANTLRv4ASTFactory.java

@@ -29,6 +29,7 @@ public class ANTLRv4ASTFactory extends ASTFactory {
 		ruleElementTypeToPsiFactory.put(ANTLRv4TokenTypes.RULE_ELEMENT_TYPES.get(ANTLRv4Parser.RULE_modeSpec), ModeSpecNode.Factory.INSTANCE);
 		ruleElementTypeToPsiFactory.put(ANTLRv4TokenTypes.RULE_ELEMENT_TYPES.get(ANTLRv4Parser.RULE_action), AtAction.Factory.INSTANCE);
 		ruleElementTypeToPsiFactory.put(ANTLRv4TokenTypes.RULE_ELEMENT_TYPES.get(ANTLRv4Parser.RULE_identifier), TokenSpecNode.Factory.INSTANCE);
+		ruleElementTypeToPsiFactory.put(ANTLRv4TokenTypes.RULE_ELEMENT_TYPES.get(ANTLRv4Parser.RULE_delegateGrammar), DelegateGrammarNode.Factory.INSTANCE);
 	}
 
 	/** Create a FileElement for root or a parse tree CompositeElement (not

src/main/java/org/antlr/intellij/plugin/completion/ANTLRv4CompletionContributor.java

@@ -0,0 +1,28 @@
+package org.antlr.intellij.plugin.completion;
+
+import com.intellij.codeInsight.completion.CompletionContributor;
+import com.intellij.codeInsight.completion.CompletionType;
+import com.intellij.patterns.PlatformPatterns;
+import com.intellij.patterns.PsiElementPattern;
+import org.antlr.intellij.plugin.psi.DelegateGrammarNode;
+import org.antlr.intellij.plugin.psi.LexerRuleRefNode;
+import org.antlr.intellij.plugin.psi.ParserRuleRefNode;
+
+public class ANTLRv4CompletionContributor extends CompletionContributor {
+    // a user can type in an upper case letter
+    private final static PsiElementPattern.Capture<LexerRuleRefNode> LEXER_RULE_GRAMMAR_IMPORT_PATTERN =
+            PlatformPatterns
+                    .psiElement(LexerRuleRefNode.class)
+                    .withSuperParent(2, DelegateGrammarNode.class);
+
+    // a user can type in a lower case letter
+    private final static PsiElementPattern.Capture<ParserRuleRefNode> PARSE_RULE_GRAMMAR_IMPORT_PATTERN =
+            PlatformPatterns
+                    .psiElement(ParserRuleRefNode.class)
+                    .withSuperParent(2, DelegateGrammarNode.class);
+
+    public ANTLRv4CompletionContributor() {
+        extend(CompletionType.BASIC, LEXER_RULE_GRAMMAR_IMPORT_PATTERN, new ANTLRv4ImportCompletionProvider());
+        extend(CompletionType.BASIC, PARSE_RULE_GRAMMAR_IMPORT_PATTERN, new ANTLRv4ImportCompletionProvider());
+    }
+}

src/main/java/org/antlr/intellij/plugin/completion/ANTLRv4ImportCompletionProvider.java

@@ -0,0 +1,47 @@
+package org.antlr.intellij.plugin.completion;
+
+import com.intellij.codeInsight.completion.CompletionParameters;
+import com.intellij.codeInsight.completion.CompletionProvider;
+import com.intellij.codeInsight.completion.CompletionResultSet;
+import com.intellij.codeInsight.lookup.LookupElementBuilder;
+import com.intellij.psi.PsiDirectory;
+import com.intellij.psi.PsiElement;
+import com.intellij.psi.PsiFile;
+import com.intellij.psi.impl.source.PsiFileImpl;
+import com.intellij.psi.util.PsiTreeUtil;
+import com.intellij.util.ProcessingContext;
+import org.antlr.intellij.plugin.ANTLRv4FileRoot;
+import org.antlr.intellij.plugin.psi.DelegateGrammarNode;
+import org.jetbrains.annotations.NotNull;
+
+import java.util.Arrays;
+import java.util.Objects;
+
+public class ANTLRv4ImportCompletionProvider extends CompletionProvider<CompletionParameters> {
+    @Override
+    protected void addCompletions(@NotNull CompletionParameters completionParameters, @NotNull ProcessingContext processingContext, @NotNull CompletionResultSet completionResultSet) {
+        PsiElement position = completionParameters.getPosition();
+        DelegateGrammarNode delegateGrammarNode = PsiTreeUtil.getParentOfType(position, DelegateGrammarNode.class);
+        if (delegateGrammarNode == null) return;
+
+        PsiFile containingFile = completionParameters.getOriginalFile();
+        PsiDirectory containingDirectory = containingFile.getContainingDirectory();
+        if (containingDirectory == null) return; // scratch file case
+
+        CompletionResultSet resultSet = completionResultSet.caseInsensitive();
+        Arrays
+                .stream(containingDirectory.getChildren())
+                .map(psiElement -> {
+                    if (psiElement instanceof ANTLRv4FileRoot) {
+                        return (ANTLRv4FileRoot) psiElement;
+                    } else {
+                        return null;
+                    }
+                })
+                .filter(Objects::nonNull)
+                .map(PsiFileImpl::getName)
+                .filter(name -> !name.equals(containingFile.getName()))
+                .map(name -> name.replace(".g4", ""))
+                .forEach(name -> resultSet.addElement(LookupElementBuilder.create(name)));
+    }
+}

src/main/java/org/antlr/intellij/plugin/psi/DelegateGrammarNode.java

@@ -0,0 +1,22 @@
+package org.antlr.intellij.plugin.psi;
+
+import com.intellij.extapi.psi.ASTWrapperPsiElement;
+import com.intellij.lang.ASTNode;
+import com.intellij.psi.PsiElement;
+import org.antlr.intellij.adaptor.parser.PsiElementFactory;
+import org.jetbrains.annotations.NotNull;
+
+public class DelegateGrammarNode extends ASTWrapperPsiElement {
+	public DelegateGrammarNode(@NotNull ASTNode node) {
+		super(node);
+	}
+
+	public static class Factory implements PsiElementFactory {
+		public static Factory INSTANCE = new Factory();
+
+		@Override
+		public PsiElement createElement(ASTNode node) {
+			return new DelegateGrammarNode(node);
+		}
+	}
+}

src/main/resources/META-INF/plugin.xml

@@ -173,5 +173,7 @@ For really big files and slow grammars, there is an appreciable delay when displ
                            instance="org.antlr.intellij.plugin.configdialogs.ANTLRv4ProjectSettings"/>
       <projectService serviceImplementation="org.antlr.intellij.plugin.configdialogs.ANTLRv4GrammarPropertiesComponent"/>
       <lang.refactoringSupport language="ANTLRv4" implementationClass="org.antlr.intellij.plugin.refactor.ANTLRv4RefactoringSupport"/>
+      <completion.contributor language="ANTLRv4"
+                              implementationClass="org.antlr.intellij.plugin.completion.ANTLRv4CompletionContributor"/>
   </extensions>
 </idea-plugin>

src/test/java/org/antlr/intellij/plugin/completion/ANTLRv4CompletionContributorTest.java

@@ -0,0 +1,67 @@
+package org.antlr.intellij.plugin.completion;
+
+import com.intellij.codeInsight.lookup.LookupElement;
+import com.intellij.testFramework.fixtures.BasePlatformTestCase;
+import org.antlr.intellij.plugin.ANTLRv4FileType;
+import org.antlr.intellij.plugin.TestUtils;
+
+import java.util.Arrays;
+import java.util.List;
+
+public class ANTLRv4CompletionContributorTest extends BasePlatformTestCase {
+    public void test_import_completion() {
+        myFixture.copyDirectoryToProject("/import_parser_rules", "");
+        myFixture.configureByText(
+                ANTLRv4FileType.INSTANCE,
+                """
+                        parser grammar ANTLRv4Parser;
+                        import <caret>                
+                        """);
+        List<String> completions = Arrays.stream(myFixture.completeBasic()).map(LookupElement::getLookupString).toList();
+        assertNotNull(completions);
+        assertSize(3, completions);
+        assertTrue(completions.contains("Grammar"));
+        assertTrue(completions.contains("GrammarExpr"));
+        assertTrue(completions.contains("CommonLexerRules"));
+
+    }
+
+    public void test_import_completion_is_case_insensitive_lower_case() {
+        myFixture.copyDirectoryToProject("/import_parser_rules", "");
+        myFixture.configureByText(
+                ANTLRv4FileType.INSTANCE,
+                """
+                        parser grammar ANTLRv4Parser;
+                        import g<caret>               
+                        """);
+        List<String> completions = Arrays.stream(myFixture.completeBasic()).map(LookupElement::getLookupString).toList();
+        assertNotNull(completions);
+        assertSize(2, completions);
+        assertTrue(completions.contains("Grammar"));
+        assertTrue(completions.contains("GrammarExpr"));
+    }
+
+    public void test_import_completion_is_case_insensitive_upper_case() {
+        myFixture.copyDirectoryToProject("/import_parser_rules", "");
+        myFixture.configureByText(
+                ANTLRv4FileType.INSTANCE,
+                """
+                        parser grammar ANTLRv4Parser;
+                        import G<caret>               
+                        """);
+        List<String> completions = Arrays.stream(myFixture.completeBasic()).map(LookupElement::getLookupString).toList();
+        assertNotNull(completions);
+        assertSize(2, completions);
+        assertTrue(completions.contains("Grammar"));
+        assertTrue(completions.contains("GrammarExpr"));
+    }
+
+    protected String getTestDataPath() {
+        return "src/test/resources/completion";
+    }
+
+    @Override
+    protected void tearDown() throws Exception {
+        TestUtils.tearDownIgnoringObjectNotDisposedException(() -> super.tearDown());
+    }
+}