Added linter for warning about unnecessary usage of ^ in code

src/main/java/org/eolang/lints/LtRedundantHat.java

@@ -0,0 +1,82 @@
+/*
+ * SPDX-FileCopyrightText: Copyright (c) 2016-2025 Objectionary.com
+ * SPDX-License-Identifier: MIT
+ */
+package org.eolang.lints;
+
+import com.github.lombrozo.xnav.Xnav;
+import com.jcabi.xml.XML;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.List;
+import java.util.stream.Collectors;
+import org.eolang.parser.OnDefault;
+
+/**
+ * Lint that warns if a redundant {@code ^} is used.
+ * @since 0.0.59
+ */
+final class LtRedundantHat implements Lint<XML> {
+    @Override
+    public Collection<Defect> defects(final XML xmir) throws IOException {
+        final Collection<Defect> defects = new ArrayList<>(0);
+        final Xnav xml = new Xnav(xmir.inner());
+        final List<Xnav> objs = xml
+            .path("//o[@base='^']")
+            .collect(Collectors.toList());
+        for (final Xnav obj : objs) {
+            final String name = obj.attribute("name").text().orElse("");
+            final List<Xnav> matches = xml
+                .path(String.format("//o[@name='%s']", name))
+                .collect(Collectors.toList());
+            final List<Xnav> ancestors = obj
+                .path(String.format("ancestor::o[@name='%s']", name))
+                .collect(Collectors.toList());
+            if (matches.size() == 1) {
+                defects.add(
+                    new Defect.Default(
+                        this.name(),
+                        Severity.WARNING,
+                        new OnDefault(xmir).get(),
+                        Integer.parseInt(obj.attribute("line").text().orElse("0")),
+                        String.format(
+                            "Redundant '^' notation: '%s' can be accessed without it",
+                            name
+                        )
+                    )
+                );
+            } else if (!ancestors.isEmpty() && matches.size() > 1) {
+                final Xnav target = ancestors.get(0);
+                for (final Xnav match : matches) {
+                    if (match.equals(target)) {
+                        defects.add(
+                            new Defect.Default(
+                                this.name(),
+                                Severity.WARNING,
+                                new OnDefault(xmir).get(),
+                                Integer.parseInt(obj.attribute("line").text().orElse("0")),
+                                String.format(
+                                    "Redundant '^' notation: '%s' resolves to the same object without it",
+                                    name
+                                )
+                            )
+                        );
+                        break;
+                    }
+                }
+            }
+        }
+        return defects;
+    }
+
+    @Override
+    public String motive() throws IOException {
+        return "The '^' notation is redundant and can be omitted for brevity.";
+    }
+
+    @Override
+    public String name() {
+        return "redundant-hat";
+    }
+}

src/test/java/org/eolang/lints/LtRedundantHatTest.java

@@ -0,0 +1,83 @@
+/*
+ * SPDX-FileCopyrightText: Copyright (c) 2016-2025 Objectionary.com
+ * SPDX-License-Identifier: MIT
+ */
+package org.eolang.lints;
+
+import com.jcabi.xml.XML;
+import com.jcabi.xml.XMLDocument;
+import java.io.IOException;
+import java.util.Collection;
+import org.hamcrest.MatcherAssert;
+import org.hamcrest.Matchers;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Test for {@link LtRedundantHat}.
+ *
+ * @since 0.0.1
+ */
+final class LtRedundantHatTest {
+
+    @Test
+    void reportsNoDefectsForNecessaryHat() throws IOException {
+        final XML xml = new XMLDocument(
+            "<program><object name='foo' line='10'><o name='+test' line='12'></o></object></program>"
+        );
+        final LtRedundantHat lint = new LtRedundantHat();
+        final Collection<Defect> defects = lint.defects(xml);
+        MatcherAssert.assertThat(
+            "Should not report a defect when '^' is necessary to resolve ambiguity",
+            defects,
+            Matchers.empty()
+        );
+    }
+
+    @Test
+    void containsGuidanceInMotive() throws IOException {
+        final LtRedundantHat lint = new LtRedundantHat();
+        MatcherAssert.assertThat(
+            "Motive must explain when to omit redundant '^'",
+            lint.motive(),
+            Matchers.containsString("notation is redundant and can be omitted for brevity")
+        );
+    }
+
+    @Test
+    void namesStableId() {
+        final LtRedundantHat lint = new LtRedundantHat();
+        MatcherAssert.assertThat(
+            "Rule id must be 'redundant-hat'",
+            lint.name(),
+            Matchers.equalTo("redundant-hat")
+        );
+    }
+
+    @Test
+    void doesNotReportDefectWhenCaretIsNecessary() throws IOException {
+        final String xml = "<object><o name='bar' line='13'></o></object>";
+        final XML doc = new XMLDocument(xml);
+        final LtRedundantHat lint = new LtRedundantHat();
+        final Collection<Defect> defects = lint.defects(doc);
+        MatcherAssert.assertThat(
+            "Should not report a defect when '^' is necessary due to ambiguity",
+            defects,
+            Matchers.empty()
+        );
+    }
+
+    @Test
+    void reportsDefectWhenCaretIsRedundantWithMultipleMatches() throws IOException {
+        final String xml =
+            "<object><o name='foo' base='^' line='12'></o></object>";
+        final XML doc = new XMLDocument(xml);
+        final LtRedundantHat lint = new LtRedundantHat();
+        final Collection<Defect> defects = lint.defects(doc);
+        MatcherAssert.assertThat(
+            "Should report a defect when '^' is redundant among multiple matches",
+            defects,
+                Matchers.hasSize(1)
+        );
+    }
+
+}