Fix missing violations

When base or patch reports contain violations, that are duplicated in
terms of eql? (same rule, same line, same message), then these were
deduplicated when creating the union of all violations for a file.
This resulted in odd numbers in the diff, e.g. less than
all violations being removed when no violation were actually being
left over.

Author: adangel

lib/pmdtester/report_diff.rb

@@ -211,7 +211,7 @@ def build_diffs(counters, &getter)
       # Values are lists of violations/errors
       diffs = base_hash.to_h.merge(patch_hash.to_h) do |_key, base_value, patch_value|
         # make the difference of values
-        (base_value | patch_value) - (base_value & patch_value)
+        (base_value + patch_value) - (base_value & patch_value)
       end
 
       diffs.delete_if do |_key, value|

test/resources/diff_builder/test_violation_diffs_rule_message_change_base.xml

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<pmd xmlns="http://pmd.sourceforge.net/report/2.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://pmd.sourceforge.net/report/2.0.0 https://pmd.github.io/schema/report_2_0_0.xsd"
+    version="6.3.0-SNAPSHOT" timestamp="2018-04-16T22:41:45.065">
+<file name="File1.java">
+<violation beginline="7" endline="8" begincolumn="1" endcolumn="2" rule="GenericNaming" ruleset="Codestyle" class="Null" method="Null" externalInfoUrl="http://pmd.sourceforge.net/snapshot/pmd_rules_java_codestyle.html#genericnaming" priority="3">
+  Same message for the two violations on the same line.
+</violation>
+<violation beginline="7" endline="8" begincolumn="1" endcolumn="2" rule="GenericNaming" ruleset="Codestyle" class="Null" method="Null" externalInfoUrl="http://pmd.sourceforge.net/snapshot/pmd_rules_java_codestyle.html#genericnaming" priority="3">
+  Same message for the two violations on the same line.
+</violation>
+</file>
+</pmd>

test/resources/diff_builder/test_violation_diffs_rule_message_change_patch.xml

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<pmd xmlns="http://pmd.sourceforge.net/report/2.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://pmd.sourceforge.net/report/2.0.0 https://pmd.github.io/schema/report_2_0_0.xsd"
+    version="6.3.0-SNAPSHOT" timestamp="2018-04-16T22:41:45.065">
+<file name="File1.java">
+<violation beginline="7" endline="8" begincolumn="1" endcolumn="2" rule="TypeParameterNamingConvention" ruleset="Codestyle" class="Null" method="Null" externalInfoUrl="http://pmd.sourceforge.net/snapshot/pmd_rules_java_codestyle.html#typeparameternamingconvention" priority="3">
+  Message for first type name.
+</violation>
+<violation beginline="7" endline="8" begincolumn="1" endcolumn="2" rule="TypeParameterNamingConvention" ruleset="Codestyle" class="Null" method="Null" externalInfoUrl="http://pmd.sourceforge.net/snapshot/pmd_rules_java_codestyle.html#typeparameternamingconvention" priority="3">
+  Message for second type name.
+</violation>
+</file>
+</pmd>

test/test_diff_builder.rb

@@ -37,6 +37,24 @@ def test_violation_diffs
     # assert_equal('00:00:56', diffs_report.diff_execution_time)
   end
 
+  def test_violation_diffs_rule_message_change
+    base_report_path = 'test/resources/diff_builder/test_violation_diffs_rule_message_change_base.xml'
+    patch_report_path = 'test/resources/diff_builder/test_violation_diffs_rule_message_change_patch.xml'
+    diffs_report = build_report_diff(base_report_path, patch_report_path,
+                                     BASE_REPORT_INFO_PATH, PATCH_REPORT_INFO_PATH)
+    violation_diffs = diffs_report.violation_diffs_by_file
+    keys = violation_diffs.keys
+
+    assert_counters_empty(diffs_report.error_counts)
+    assert_counters_eq(diffs_report.violation_counts,
+                       base_total: 2, patch_total: 2, changed_total: 4)
+    assert_changes_eq(diffs_report.violation_counts,
+                      removed: 2, added: 2, changed: 0)
+
+    assert_equal('File1.java', keys[0])
+    assert_equal(4, violation_diffs[keys[0]].size)
+  end
+
   def test_violation_diffs_with_filter
     base_report_path = 'test/resources/diff_builder/test_violation_diffs_base.xml'
     patch_report_path = 'test/resources/diff_builder/test_violation_diffs_patch.xml'