LUCENE-9699: Support German-like compound words (#2248)

lucene/analysis/common/src/java/org/apache/lucene/analysis/hunspell/Dictionary.java

@@ -141,8 +141,9 @@ public class Dictionary {
   char keepcase;
   char needaffix;
   char forbiddenword;
-  char onlyincompound;
-  int compoundMin = 3;
+  char onlyincompound, compoundBegin, compoundMiddle, compoundEnd, compoundPermit;
+  boolean checkCompoundCase;
+  int compoundMin = 3, compoundMax = Integer.MAX_VALUE;
   List<CompoundRule> compoundRules; // nullable
 
   // ignored characters (dictionary, affix, inputs)
@@ -377,8 +378,20 @@ public class Dictionary {
         forbiddenword = flagParsingStrategy.parseFlag(singleArgument(reader, line));
       } else if ("COMPOUNDMIN".equals(firstWord)) {
         compoundMin = Math.max(1, Integer.parseInt(singleArgument(reader, line)));
+      } else if ("COMPOUNDWORDMAX".equals(firstWord)) {
+        compoundMax = Math.max(1, Integer.parseInt(singleArgument(reader, line)));
       } else if ("COMPOUNDRULE".equals(firstWord)) {
         compoundRules = parseCompoundRules(reader, Integer.parseInt(singleArgument(reader, line)));
+      } else if ("COMPOUNDBEGIN".equals(firstWord)) {
+        compoundBegin = flagParsingStrategy.parseFlag(singleArgument(reader, line));
+      } else if ("COMPOUNDMIDDLE".equals(firstWord)) {
+        compoundMiddle = flagParsingStrategy.parseFlag(singleArgument(reader, line));
+      } else if ("COMPOUNDEND".equals(firstWord)) {
+        compoundEnd = flagParsingStrategy.parseFlag(singleArgument(reader, line));
+      } else if ("COMPOUNDPERMITFLAG".equals(firstWord)) {
+        compoundPermit = flagParsingStrategy.parseFlag(singleArgument(reader, line));
+      } else if ("CHECKCOMPOUNDCASE".equals(firstWord)) {
+        checkCompoundCase = true;
       }
     }
 
@@ -1303,10 +1316,6 @@ public class Dictionary {
     }
   }
 
-  boolean hasCompounding() {
-    return compoundRules != null;
-  }
-
   boolean hasFlag(int entryId, char flag, BytesRef scratch) {
     return flag != FLAG_UNSET && hasFlag(decodeFlags(entryId, scratch), flag);
   }

lucene/analysis/common/src/java/org/apache/lucene/analysis/hunspell/SpellChecker.java

@@ -87,18 +87,54 @@ public class SpellChecker {
       return false;
     }
 
-    if (!stemmer.doStem(wordChars, length, caseVariant).isEmpty()) {
+    if (hasStems(wordChars, 0, length, caseVariant, WordContext.SIMPLE_WORD)) {
       return true;
     }
 
-    if (dictionary.hasCompounding()) {
-      return checkCompounds(wordChars, 0, length, new ArrayList<>());
+    if (dictionary.compoundRules != null
+        && checkCompoundRules(wordChars, 0, length, new ArrayList<>())) {
+      return true;
+    }
+
+    return dictionary.compoundBegin > 0 && checkCompounds(wordChars, 0, length, caseVariant, 0);
+  }
+
+  private boolean hasStems(
+      char[] chars, int offset, int length, boolean caseVariant, WordContext context) {
+    return !stemmer.doStem(chars, offset, length, caseVariant, context).isEmpty();
+  }
+
+  private boolean checkCompounds(
+      char[] chars, int offset, int length, boolean caseVariant, int depth) {
+    if (depth > dictionary.compoundMax - 2) return false;
+
+    int limit = length - dictionary.compoundMin + 1;
+    for (int breakPos = dictionary.compoundMin; breakPos < limit; breakPos++) {
+      WordContext context = depth == 0 ? WordContext.COMPOUND_BEGIN : WordContext.COMPOUND_MIDDLE;
+      int breakOffset = offset + breakPos;
+      if (checkCompoundCase(chars, breakOffset)
+          && hasStems(chars, offset, breakPos, caseVariant, context)) {
+        int remainingLength = length - breakPos;
+        if (hasStems(chars, breakOffset, remainingLength, caseVariant, WordContext.COMPOUND_END)) {
+          return true;
+        }
+
+        if (checkCompounds(chars, breakOffset, remainingLength, caseVariant, depth + 1)) {
+          return true;
+        }
+      }
     }
 
     return false;
   }
 
-  private boolean checkCompounds(char[] wordChars, int offset, int length, List<IntsRef> words) {
+  private boolean checkCompoundCase(char[] chars, int breakPos) {
+    if (!dictionary.checkCompoundCase) return true;
+    return Character.isUpperCase(chars[breakPos - 1]) == Character.isUpperCase(chars[breakPos]);
+  }
+
+  private boolean checkCompoundRules(
+      char[] wordChars, int offset, int length, List<IntsRef> words) {
     if (words.size() >= 100) return false;
 
     int limit = length - dictionary.compoundMin + 1;
@@ -113,7 +149,7 @@ public class SpellChecker {
             return true;
           }
 
-          if (checkCompounds(wordChars, offset + breakPos, length - breakPos, words)) {
+          if (checkCompoundRules(wordChars, offset + breakPos, length - breakPos, words)) {
             return true;
           }
         }
@@ -132,8 +168,7 @@ public class SpellChecker {
 
     words.add(forms);
     boolean result =
-        dictionary.compoundRules != null
-            && dictionary.compoundRules.stream().anyMatch(r -> r.fullyMatches(words, scratch));
+        dictionary.compoundRules.stream().anyMatch(r -> r.fullyMatches(words, scratch));
     words.remove(words.size() - 1);
     return result;
   }

lucene/analysis/common/src/java/org/apache/lucene/analysis/hunspell/Stemmer.java

@@ -94,18 +94,18 @@ final class Stemmer {
     }
 
     WordCase wordCase = caseOf(word, length);
-    List<CharsRef> list = doStem(word, length, false);
+    List<CharsRef> list = doStem(word, 0, length, false, WordContext.SIMPLE_WORD);
     if (wordCase == WordCase.UPPER) {
       caseFoldTitle(word, length);
       char[] aposCase = capitalizeAfterApostrophe(titleBuffer, length);
       if (aposCase != null) {
-        list.addAll(doStem(aposCase, length, true));
+        list.addAll(doStem(aposCase, 0, length, true, WordContext.SIMPLE_WORD));
       }
-      list.addAll(doStem(titleBuffer, length, true));
+      list.addAll(doStem(titleBuffer, 0, length, true, WordContext.SIMPLE_WORD));
     }
     if (wordCase == WordCase.UPPER || wordCase == WordCase.TITLE) {
       caseFoldLower(wordCase == WordCase.UPPER ? titleBuffer : word, length);
-      list.addAll(doStem(lowerBuffer, length, true));
+      list.addAll(doStem(lowerBuffer, 0, length, true, WordContext.SIMPLE_WORD));
     }
     return list;
   }
@@ -158,9 +158,10 @@ final class Stemmer {
     return null;
   }
 
-  List<CharsRef> doStem(char[] word, int length, boolean caseVariant) {
+  List<CharsRef> doStem(
+      char[] word, int offset, int length, boolean caseVariant, WordContext context) {
     List<CharsRef> stems = new ArrayList<>();
-    IntsRef forms = dictionary.lookupWord(word, 0, length);
+    IntsRef forms = dictionary.lookupWord(word, offset, length);
     if (forms != null) {
       for (int i = 0; i < forms.length; i += formStep) {
         char[] wordFlags = dictionary.decodeFlags(forms.ints[forms.offset + i], scratch);
@@ -172,15 +173,32 @@ final class Stemmer {
           continue;
         }
         // we can't add this form, it only belongs inside a compound word
-        if (Dictionary.hasFlag(wordFlags, dictionary.onlyincompound)) {
+        if (!context.isCompound() && Dictionary.hasFlag(wordFlags, dictionary.onlyincompound)) {
           continue;
         }
-        stems.add(newStem(word, 0, length, forms, i));
+        if (context.isCompound()
+            && !Dictionary.hasFlag(wordFlags, context.requiredFlag(dictionary))) {
+          continue;
+        }
+        stems.add(newStem(word, offset, length, forms, i));
       }
     }
     try {
       stems.addAll(
-          stem(word, 0, length, -1, (char) 0, -1, 0, true, true, false, false, caseVariant));
+          stem(
+              word,
+              offset,
+              length,
+              context,
+              -1,
+              (char) 0,
+              -1,
+              0,
+              true,
+              true,
+              false,
+              false,
+              caseVariant));
     } catch (IOException bogus) {
       throw new RuntimeException(bogus);
     }
@@ -287,6 +305,7 @@ final class Stemmer {
       char[] word,
       int offset,
       int length,
+      WordContext context,
       int previous,
       char prevFlag,
       int prefixId,
@@ -328,7 +347,7 @@ final class Stemmer {
             continue;
           }
 
-          if (isAffixCompatible(prefix, prevFlag, recursionDepth, false)) {
+          if (isAffixCompatible(prefix, prevFlag, recursionDepth, true, false, context)) {
             char[] strippedWord = stripAffix(word, offset, length, i, prefix, true);
             if (strippedWord == null) {
               continue;
@@ -340,6 +359,7 @@ final class Stemmer {
                     strippedWord,
                     pureAffix ? offset + i : 0,
                     pureAffix ? length - i : strippedWord.length,
+                    context,
                     prefix,
                     -1,
                     recursionDepth,
@@ -378,7 +398,8 @@ final class Stemmer {
             continue;
           }
 
-          if (isAffixCompatible(suffix, prevFlag, recursionDepth, previousWasPrefix)) {
+          if (isAffixCompatible(
+              suffix, prevFlag, recursionDepth, false, previousWasPrefix, context)) {
             char[] strippedWord = stripAffix(word, offset, length, length - i, suffix, false);
             if (strippedWord == null) {
               continue;
@@ -390,6 +411,7 @@ final class Stemmer {
                     strippedWord,
                     pureAffix ? offset : 0,
                     pureAffix ? i : strippedWord.length,
+                    context,
                     suffix,
                     prefixId,
                     recursionDepth,
@@ -442,18 +464,31 @@ final class Stemmer {
   }
 
   private boolean isAffixCompatible(
-      int affix, char prevFlag, int recursionDepth, boolean previousWasPrefix) {
+      int affix,
+      char prevFlag,
+      int recursionDepth,
+      boolean isPrefix,
+      boolean previousWasPrefix,
+      WordContext context) {
     int append = dictionary.affixData(affix, Dictionary.AFFIX_APPEND);
 
+    if (context.isCompound() && dictionary.compoundPermit > 0) {
+      WordContext allowed = isPrefix ? WordContext.COMPOUND_BEGIN : WordContext.COMPOUND_END;
+      if (context != allowed && !dictionary.hasFlag(append, dictionary.compoundPermit, scratch)) {
+        return false;
+      }
+    }
+
     if (recursionDepth == 0) {
       // check if affix is allowed in a non-compound word
-      return !dictionary.hasFlag(append, dictionary.onlyincompound, scratch);
+      return context.isCompound()
+          || !dictionary.hasFlag(append, dictionary.onlyincompound, scratch);
     }
 
     if (isCrossProduct(affix)) {
       // cross check incoming continuation class (flag of previous affix) against list.
       char[] appendFlags = dictionary.decodeFlags(append, scratch);
-      if (!Dictionary.hasFlag(appendFlags, dictionary.onlyincompound)) {
+      if (context.isCompound() || !Dictionary.hasFlag(appendFlags, dictionary.onlyincompound)) {
         return previousWasPrefix || Dictionary.hasFlag(appendFlags, prevFlag);
       }
     }
@@ -491,8 +526,9 @@ final class Stemmer {
   /**
    * Applies the affix rule to the given word, producing a list of stems if any are found
    *
-   * @param strippedWord Word the affix has been removed and the strip added
-   * @param length valid length of stripped word
+   * @param strippedWord Char array containing the word with the affix removed and the strip added
+   * @param offset where the word actually starts in the array
+   * @param length the length of the stripped word
    * @param affix HunspellAffix representing the affix rule itself
    * @param prefixId when we already stripped a prefix, we can't simply recurse and check the
    *     suffix, unless both are compatible so we must check dictionary form against both to add it
@@ -505,6 +541,7 @@ final class Stemmer {
       char[] strippedWord,
       int offset,
       int length,
+      WordContext context,
       int affix,
       int prefixId,
       int recursionDepth,
@@ -546,10 +583,15 @@ final class Stemmer {
           if (!acceptCase(caseVariant, wordFlags)) {
             continue;
           }
-          // we aren't decompounding (yet)
-          if (Dictionary.hasFlag(wordFlags, dictionary.onlyincompound)) {
+          if (!context.isCompound() && Dictionary.hasFlag(wordFlags, dictionary.onlyincompound)) {
             continue;
           }
+          if (context.isCompound()) {
+            char cFlag = context.requiredFlag(dictionary);
+            if (!Dictionary.hasFlag(wordFlags, cFlag) && !isFlagAppendedByAffix(affix, cFlag)) {
+              continue;
+            }
+          }
           stems.add(newStem(strippedWord, offset, length, forms, i));
         }
       }
@@ -594,6 +636,7 @@ final class Stemmer {
               strippedWord,
               offset,
               length,
+              context,
               affix,
               flag,
               prefixId,

lucene/analysis/common/src/java/org/apache/lucene/analysis/hunspell/WordContext.java

@@ -0,0 +1,41 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.lucene.analysis.hunspell;
+
+enum WordContext {
+  SIMPLE_WORD,
+  COMPOUND_BEGIN,
+  COMPOUND_MIDDLE,
+  COMPOUND_END;
+
+  boolean isCompound() {
+    return this != SIMPLE_WORD;
+  }
+
+  char requiredFlag(Dictionary dictionary) {
+    switch (this) {
+      case COMPOUND_BEGIN:
+        return dictionary.compoundBegin;
+      case COMPOUND_MIDDLE:
+        return dictionary.compoundMiddle;
+      case COMPOUND_END:
+        return dictionary.compoundEnd;
+      default:
+        return Dictionary.FLAG_UNSET;
+    }
+  }
+}

lucene/analysis/common/src/test/org/apache/lucene/analysis/hunspell/SpellCheckerTest.java

@@ -80,6 +80,10 @@ public class SpellCheckerTest extends StemmerTestBase {
     doTest("compoundrule8");
   }
 
+  public void testGermanCompounding() throws Exception {
+    doTest("germancompounding");
+  }
+
   protected void doTest(String name) throws Exception {
     InputStream affixStream =
         Objects.requireNonNull(getClass().getResourceAsStream(name + ".aff"), name);

lucene/analysis/common/src/test/org/apache/lucene/analysis/hunspell/germancompounding.aff

@@ -0,0 +1,91 @@
+# German compounding
+
+# handle special casing of German sharp s
+
+CHECKSHARPS
+
+# compound flags
+
+COMPOUNDBEGIN U
+COMPOUNDMIDDLE V
+COMPOUNDEND W
+
+# Prefixes are allowed at the beginning of compounds,
+# suffixes are allowed at the end of compounds by default:
+# (prefix)?(root)+(affix)?
+# Affixes with COMPOUNDPERMITFLAG may be inside of compounds.
+COMPOUNDPERMITFLAG P
+
+# for German fogemorphemes (Fuge-element)
+# Hint: ONLYINCOMPOUND is not required everywhere, but the
+# checking will be a little faster with it.
+
+ONLYINCOMPOUND X
+
+# forbid uppercase characters at compound word bounds
+CHECKCOMPOUNDCASE
+
+# for handling Fuge-elements with dashes (Arbeits-) 
+# dash will be a special word
+
+COMPOUNDMIN 1
+WORDCHARS -
+
+# compound settings and fogemorpheme for `Arbeit'
+
+SFX A Y 3
+SFX A 0 s/UPX .
+SFX A 0 s/VPDX .
+SFX A 0 0/WXD .
+
+SFX B Y 2
+SFX B 0 0/UPX .
+SFX B 0 0/VWXDP .
+
+# a suffix for `Computer'
+
+SFX C Y 1
+SFX C 0 n/WD .
+
+# for forbid exceptions (*Arbeitsnehmer)
+
+FORBIDDENWORD Z
+
+# dash prefix for compounds with dash (Arbeits-Computer)
+
+PFX - Y 1
+PFX - 0 -/P .
+
+# decapitalizing prefix
+# circumfix for positioning in compounds
+
+PFX D Y 29
+PFX D A a/PX A
+PFX D Ä ä/PX Ä
+PFX D B b/PX B
+PFX D C c/PX C
+PFX D D d/PX D
+PFX D E e/PX E
+PFX D F f/PX F
+PFX D G g/PX G
+PFX D H h/PX H
+PFX D I i/PX I
+PFX D J j/PX J
+PFX D K k/PX K
+PFX D L l/PX L
+PFX D M m/PX M
+PFX D N n/PX N
+PFX D O o/PX O
+PFX D Ö ö/PX Ö
+PFX D P p/PX P
+PFX D Q q/PX Q
+PFX D R r/PX R
+PFX D S s/PX S
+PFX D T t/PX T
+PFX D U u/PX U
+PFX D Ü ü/PX Ü
+PFX D V v/PX V
+PFX D W w/PX W
+PFX D X x/PX X
+PFX D Y y/PX Y
+PFX D Z z/PX Z

lucene/analysis/common/src/test/org/apache/lucene/analysis/hunspell/germancompounding.dic

@@ -0,0 +1,5 @@
+4
+Arbeit/A-
+Computer/BC-
+-/W
+Arbeitsnehmer/Z

lucene/analysis/common/src/test/org/apache/lucene/analysis/hunspell/germancompounding.good

@@ -0,0 +1,20 @@
+Computer
+Computern
+Arbeit
+Arbeits-
+Computerarbeit
+Computerarbeits-
+Arbeitscomputer
+Computercomputer
+Computercomputern
+Arbeitscomputern
+Computerarbeitscomputer
+Computerarbeitscomputern
+Arbeitscomputercomputer
+Computercomputerarbeit
+Arbeitscomputerarbeit
+Arbeitsarbeitsarbeit
+Computerarbeitsarbeit
+Computerarbeits-Computer
+Computerarbeits-Computern
+Computer-Arbeit

lucene/analysis/common/src/test/org/apache/lucene/analysis/hunspell/germancompounding.wrong

@@ -0,0 +1,50 @@
+computer
+computern
+arbeit
+Arbeits
+arbeits
+ComputerArbeit
+ComputernArbeit
+Computernarbeit
+ComputerArbeits
+Arbeitcomputer
+Arbeitcomputern
+ArbeitsComputer
+ArbeitsComputern
+Computerarbeitcomputer
+ComputerArbeitcomputer
+ComputerArbeitscomputer
+Computerarbeitcomputern
+ComputerArbeitcomputern
+ComputerArbeitscomputern
+Arbeitscomputerarbeits
+Arbeitscomputernarbeits
+Computerarbeits-computer
+Arbeitsnehmer
+computers
+computern
+computernarbeit
+computernArbeit
+computerArbeit
+computerArbeits
+arbeitcomputer
+arbeitsComputer
+computerarbeitcomputer
+computerArbeitcomputer
+computerArbeitscomputer
+arbeitscomputerarbeits
+computerarbeits-computer
+arbeitsnehmer
+computernarbeit
+computernArbeit
+arbeits-
+computerarbeit
+computerarbeits-
+arbeitscomputer
+arbeitscomputern
+computerarbeitscomputer
+computerarbeitscomputern
+computerarbeitscomputers
+arbeitscomputerarbeit
+computerarbeits-Computer
+computerarbeits-Computern