feat(#155): add too-deep-object lint

src/main/resources/org/eolang/lints/design/too-deep-object.xsl

@@ -0,0 +1,34 @@
+<?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="too-deep-object">
+  <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[count(ancestor::o[not(@base='Φ.tuple') and not(@base='Φ.bytes') and not(@base='Φ.number') and not(@base='Φ.string')]) &gt; 12 and not(o[count(ancestor::o[not(@base='Φ.tuple') and not(@base='Φ.bytes') and not(@base='Φ.number') and not(@base='Φ.string')]) &gt; 12])]">
+        <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 object </xsl:text>
+          <xsl:value-of select="eo:escape(@base)"/>
+          <xsl:text> is nested too deep: </xsl:text>
+          <xsl:value-of select="count(ancestor::o)"/>
+          <xsl:text> levels of nesting, while the maximum is 12</xsl:text>
+        </defect>
+      </xsl:for-each>
+    </defects>
+  </xsl:template>
+</xsl:stylesheet>

src/main/resources/org/eolang/motives/design/too-deep-object.md

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+# Too deep object
+
+Objects nested deeper than twelve levels are hard to read and to debug.
+The nesting level of an object is the number of its ancestor objects.
+If it exceeds twelve, a warning is raised.
+
+Incorrect:
+
+```eo
+# Foo.
+[] > foo
+  a > x
+    b
+      c
+        d
+          e
+            f
+              g
+                h
+                  i
+                    j
+                      k
+                        l
+                          m
+```
+
+Here, `m` is nested thirteen levels deep, which is hard to follow.
+It should be refactored by extracting some of the nested objects into
+their own top-level objects.

src/test/resources/org/eolang/lints/packs/single/too-deep-object/allows-normal-nesting.yaml

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+# SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+# SPDX-License-Identifier: MIT
+---
+sheets:
+  - /org/eolang/lints/design/too-deep-object.xsl
+asserts:
+  - /defects[count(defect[@severity='warning'])=0]
+input: |
+  # Foo.
+
+  [] > foo
+    a > x
+      b
+        c
+          d
+            e
+              f
+                g

src/test/resources/org/eolang/lints/packs/single/too-deep-object/catches-too-deep-object.yaml

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+# SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+# SPDX-License-Identifier: MIT
+---
+sheets:
+  - /org/eolang/lints/design/too-deep-object.xsl
+asserts:
+  - /defects[count(defect[@severity='warning'])=1]
+  - /defects/defect[@line='16']
+input: |
+  # Foo.
+
+  [] > foo
+    a > x
+      b
+        c
+          d
+            e
+              f
+                g
+                  h
+                    i
+                      j
+                        k
+                          l
+                            m