feat(#256): add roll-bases lint

src/main/resources/org/eolang/lints/misc/roll-bases.xsl

@@ -0,0 +1,37 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+* SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+* SPDX-License-Identifier: MIT
+-->
+<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:eo="https://www.eolang.org" version="2.0" id="roll-bases">
+  <xsl:import href="/org/eolang/parser/_funcs.xsl"/>
+  <xsl:import href="/org/eolang/funcs/lineno.xsl"/>
+  <xsl:import href="/org/eolang/funcs/escape.xsl"/>
+  <xsl:import href="/org/eolang/funcs/defect-context.xsl"/>
+  <xsl:output encoding="UTF-8" method="xml"/>
+  <xsl:template match="/">
+    <defects>
+      <xsl:for-each select="//o[starts-with(@base, '.') and o[1][@base and not(starts-with(@base, '.')) and not(@name) and not(o) and not(eo:has-data(.))]]">
+        <defect>
+          <xsl:variable name="line" select="eo:lineno(@line)"/>
+          <xsl:attribute name="line">
+            <xsl:value-of select="$line"/>
+          </xsl:attribute>
+          <xsl:if test="$line = '0'">
+            <xsl:attribute name="context">
+              <xsl:value-of select="eo:defect-context(.)"/>
+            </xsl:attribute>
+          </xsl:if>
+          <xsl:attribute name="severity">warning</xsl:attribute>
+          <xsl:text>The base "</xsl:text>
+          <xsl:value-of select="eo:escape(concat(o[1]/@base, @base))"/>
+          <xsl:text>" must be written instead of the nested bases "</xsl:text>
+          <xsl:value-of select="eo:escape(@base)"/>
+          <xsl:text>" and "</xsl:text>
+          <xsl:value-of select="eo:escape(o[1]/@base)"/>
+          <xsl:text>"</xsl:text>
+        </defect>
+      </xsl:for-each>
+    </defects>
+  </xsl:template>
+</xsl:stylesheet>

src/main/resources/org/eolang/motives/misc/roll-bases.md

@@ -0,0 +1,24 @@
+# Roll bases
+
+A composite `@base` attribute, written as nested objects, must be
+collapsed into a single short form when possible. For example:
+
+```xml
+<o base=".foo">
+  <o base=".bar">
+    <o base="x"/>
+  </o>
+</o>
+```
+
+must be written as:
+
+```xml
+<o base="x.bar.foo"/>
+```
+
+The collapse is possible when the outer object has a `@base` starting
+with `.`, and its single child has a `@base` that does not start with
+`.`, has no inner objects, no data, and no name. Such unrolled bases
+are usually a result of hand-written XMIR or of XMIR produced by tools
+other than `eo-parser`, which already rolls bases during parsing.

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

@@ -404,6 +404,76 @@ void validatesEoPacksForErrors() throws IOException {
         );
     }
 
+    @Test
+    void catchesUnrolledBases() throws ImpossibleModificationException {
+        MatcherAssert.assertThat(
+            "An unrolled composite base must be reported",
+            new LtByXsl("misc/roll-bases").defects(
+                new XMLDocument(
+                    new Xembler(
+                        new Directives().add("object")
+                            .add("o").attr("base", ".foo")
+                            .add("o").attr("base", "x").up()
+                            .up().up()
+                    ).xml()
+                )
+            ),
+            Matchers.iterableWithSize(1)
+        );
+    }
+
+    @Test
+    void allowsRolledBase() throws ImpossibleModificationException {
+        MatcherAssert.assertThat(
+            "A rolled composite base must not be reported",
+            new LtByXsl("misc/roll-bases").defects(
+                new XMLDocument(
+                    new Xembler(
+                        new Directives().add("object")
+                            .add("o").attr("base", "x.foo").up().up()
+                    ).xml()
+                )
+            ),
+            Matchers.emptyIterable()
+        );
+    }
+
+    @Test
+    void allowsNestedBaseWithData() throws ImpossibleModificationException {
+        MatcherAssert.assertThat(
+            "A child with data must not be rolled",
+            new LtByXsl("misc/roll-bases").defects(
+                new XMLDocument(
+                    new Xembler(
+                        new Directives().add("object")
+                            .add("o").attr("base", ".foo")
+                            .add("o").attr("base", "x").set("data").up()
+                            .up().up()
+                    ).xml()
+                )
+            ),
+            Matchers.emptyIterable()
+        );
+    }
+
+    @Test
+    void allowsDoubleDotBases() throws ImpossibleModificationException {
+        MatcherAssert.assertThat(
+            "Two dot-prefixed bases must not be rolled",
+            new LtByXsl("misc/roll-bases").defects(
+                new XMLDocument(
+                    new Xembler(
+                        new Directives().add("object")
+                            .add("o").attr("base", ".foo")
+                            .add("o").attr("base", ".bar").up()
+                            .up().up()
+                    ).xml()
+                )
+            ),
+            Matchers.emptyIterable()
+        );
+    }
+
     /**
      * Whether a pack is eligible for validation?
      * @param pack Pack