Here is a sample of vocabulary, with the stemmed forms that will be generated by this algorithm:
| word | stem | word | stem | |||||||||
|
lichaamsziek lichamelijk lichamelijke lichamelijkheden lichamen lichere licht lichtbeeld lichtbruin lichtdoorlatende lichte lichten lichtende lichtenvoorde lichter lichtere lichters lichtgevoeligheid lichtgewicht lichtgrijs lichthoeveelheid lichtintensiteit lichtje lichtjes lichtkranten lichtkring lichtkringen lichtregelsystemen lichtste lichtstromende lichtte lichtten lichttoetreding lichtverontreinigde lichtzinnige lid lidia lidmaatschap lidstaten lidvereniging |
⇒ |
lichaamsziek lichamelijk lichamelijk lichamelijk lichaam licher licht lichtbeeld lichtbruin lichtdoorlaat licht licht licht lichtenvoor lichter lichter lichter lichtvoel lichtwicht lichtgrijs lichthoeveel lichtintens licht licht lichtkrant lichtkr lichtkr lichtrelsysteem licht lichtstroom licht licht lichttoetreed lichtverontrein lichtzin lid lidia lidmaatschap lidstaat lidvereen |
opgingen opglanzing opglanzingen opglimlachten opglimpen opglimpende opglimping opglimpingen opgraven opgrijnzen opgrijzende opgroeien opgroeiende opgroeiplaats ophaal ophaaldienst ophaalkosten ophaalsystemen ophaalt ophaaltruck ophalen ophalend ophalers ophef opheffen opheffende opheffing opheldering ophemelde ophemelen opheusden ophief ophield ophieven ophoepelt ophoog ophoogzand ophopen ophoping ophouden |
⇒ |
opg opglans opglans opglimlacht opglimp opglimp opglimp opglimp opgraaf opgrijns opgrijs opgroei opgroeiend opgroeiplaats ophaal ophaaldienst ophaalkost ophaalsysteem ophaalt ophaaltruck ophaal ophaal ophaler ophef ophef ophef ophef opheldeer ophemel ophemeel opheus ophief ophield ophief ophoepelt ophoog ophoogzand ophoop ophoop ophoud |
The original Kraaij-Pohlmann Dutch stemming algorithm is effectively only specified by its implementation in ANSI C. There is a partial description of the algorithm in the two papers linked to above, but these only show a few examples of the suffix removal rules, and even these don't fully match up with the rules in the C implementation!
For example both papers say the first step has a rule to remove -e and that the second step has rules to remove -etj and -tj, but the C code lacks the -e rule and the second step instead has rules to remove -etje and -tje, presumably compensating for not having already removed -e.
The C code also has steps 1c and 7 which the paper doesn't mention. Step 7 gets applied before step 6, which suggests it was a later addition. The papers are dated 1994 and 1995, while the C code states a copyright date of 1995. All of this seems to indicate that the C code is a newer version of the algorithm than that described in the papers.
Martin Porter implemented a Snowball version of the Kraaij-Pohlmann Dutch stemmer following the rules in the C implementation, but his aim was mainly to demonstrate that the then-new Snowball language was flexible enough to implement existing stemmers, and he didn't aim for exact functional equivalence. His original conclusion from testing was that only 32 words in a list of over 45,000 Dutch words stemmed differently. However that comparison appears to have been done by feeding a Dutch wordlist encoded in DOS codepage 850 into the C implementation, which expects iso-8859-1. These character sets are also respectively known as "DOS Latin 1" and "ISO Latin 1" which likely explains the mix-up. Repeating the comparison using the correct encoding there were are actually 220 differences.
Based on feedback from Dutch users, in Snowball 3.0.0 we switched the default Dutch stemmer to use the Kraaij-Pohlmann algorithm. As part of doing this we resolved all differences in output between the Snowball and C implementations of the algorithm (tested with our Dutch vocabulary list, but also a much larger list extracted from Dutch wikipedia, and additionally vocabulary lists for other languages to check handling of foreign words).
We then identified cases where Martin Porter's Dutch stemmer did better than Kraaij-Pohlmann and adjusted our Kraaij-Pohlmann implementation to address these, resulting in the following intentional differences from the C version:
Step 3: Extra condition on -erij removal to avoid conflating schilderij ("painting") and schild ("shield"):
if preceded by 'ild' replace with 'er' instead of normal action
Step 4: Extra condition on -iger/-igst/-ig removal to avoid conflating innig ("intimate") and in ("in").
do not replace if preceded by exactly 'inn'
Step 6: Extra condition on undoubling nn to avoid conflating geïnd ("collected"/"cashed") and in ("in"). Instead we conflate geïnd with innen.
do not replace if preceded by exactly 'i'
Step 1c: extra condition on -d removal to avoid conflating geïnd ("collected"/"cashed") and in ("in"). Instead we conflate geïnd with innen.
if preceded by exactly 'in' replace with 'n' instead of normal action
Step 1c: extra condition on -t removal to avoid conflating geënt ("grafted") and en ("and").
do not replace if preceded by 'en'
Lose_prefix: extra conditions of removing ge- prefix:
do not remove if followed by one of: 'eft' // Avoid conflating `geeft` and `effen`/`effende`\`geeffende`. 'val' (unless it's 'vali') // Avoid conflating `gevallen`/`geval` and `vallen`; still remove from e.g. gevalideerd 'vaa' or 'vare' // Avoid conflating `gevaren`/`gevaar` (danger), `gevaarten` (huge objects) and `varen` (to sail)
The Kraaij-Pohmann stemmer can make fairly drastic reductions to a word. Both prefixed and infixed ge are removed, so for example geluidgevoelige stems to luidvoel. In such cases it may not be obvious what the original word was, but in practice this isn't really a problem as stems are best thought of as opaque tokens (albeit opaque tokens that happen to look a lot like words in the language being stemmed for convenience of algorithm design).
Here then is the Snowball equivalent of the Kraaij-Pohlmann algorithm.
// Dutch stemming algorithm developed by Wessel Kraaij and Renée Pohlmann
strings ( ch )
integers ( p1 p2 )
booleans ( stemmed GE_removed )
routines (
R1 R2
C V VX
lengthen_V
Step_1 Step_2 Step_3 Step_4 Step_7
Step_6 Step_1c
Lose_prefix
Lose_infix
measure
)
externals ( stem )
groupings ( v v_WX A AEIOU AIOU E I O U )
stringescapes {}
/* special characters */
stringdef a` '{U+00E0}'
stringdef a' '{U+00E1}'
stringdef a^ '{U+00E2}'
stringdef a" '{U+00E4}'
stringdef e` '{U+00E8}'
stringdef e' '{U+00E9}'
stringdef e^ '{U+00EA}'
stringdef e" '{U+00EB}'
stringdef i` '{U+00EC}'
stringdef i' '{U+00ED}'
stringdef i^ '{U+00EE}'
stringdef i" '{U+00EF}'
stringdef o` '{U+00F2}'
stringdef o' '{U+00F3}'
stringdef o^ '{U+00F4}'
stringdef o" '{U+00F6}'
stringdef u` '{U+00F9}'
stringdef u' '{U+00FA}'
stringdef u^ '{U+00FB}'
stringdef u" '{U+00FC}'
define A 'a{a"}{a'}{a`}{a^}'
define E 'e{e"}{e'}{e`}{e^}'
define I 'i{i"}{i'}{i`}{i^}'
define O 'o{o"}{o'}{o`}{o^}'
define U 'u{u"}{u'}{u`}{u^}'
define AIOU A + I + O + U
define AEIOU A + E + I + O + U
define v AEIOU + 'y'
define v_WX v + 'wx'
backwardmode (
define R1 as ($p1 <= cursor)
define R2 as ($p2 <= cursor)
define V as test (v or 'ij')
define VX as test (next v or 'ij')
define C as test (not 'ij' non-v)
define lengthen_V as do (
non-v_WX
[substring] among (
'a' '{a"}' '{a'}' '{a`}' '{a^}'
'o' '{o"}' '{o'}' '{o`}' '{o^}'
'u' '{u"}' '{u'}' '{u`}' '{u^}'
(test (non-AEIOU or atlimit)
->ch insert ch)
'e' '{e'}' '{e`}' '{e^}'
(test (non-AEIOU or atlimit
not (AIOU or (E atlimit))
not (next AIOU non-AEIOU))
->ch insert ch)
'e{e"}'
(<-'e{e"}e')
'i{e"}'
(<-'iee')
)
)
define Step_1 as
(
[substring] among (
'{'}s' (delete)
's' (R1 not ('t' R1) C delete)
'ies' (R1 <-'ie')
'es'
((test ('ar' R1 C) delete lengthen_V) or
(test ('er' R1 C) delete) or
(R1 C <-'e'))
'{e'}s'
(R1 <-'{e'}')
'aus' (R1 V <-'au')
'en' (('hed' R1 ] <-'heid') or
('nd' delete) or
('d' R1 C ] delete) or
('i' or 'j' V delete) or
(R1 C delete lengthen_V))
'nde' (<-'nd')
)
)
define Step_2 as
(
[substring] among (
'je' (('{'}t' ] delete) or
('et' ] R1 C delete) or
('rnt' ] <-'rn') or
('t' ] R1 VX delete) or
('ink' ] <-'ing') or
('mp' ] <-'m') or
('{'}' ] R1 delete) or
(] R1 C delete))
'ge' (R1 <-'g')
'lijke'(R1 <-'lijk')
'ische'(R1 <-'isch')
'de' (R1 C delete)
'te' (R1 <-'t')
'se' (R1 <-'s')
're' (R1 <-'r')
'le' (R1 delete attach 'l' lengthen_V)
'ene' (R1 C delete attach 'en' lengthen_V)
'ieve' (R1 C <-'ief')
)
)
define Step_3 as
(
[substring] among (
'atie' (R1 <-'eer')
'iteit' (R1 delete lengthen_V)
'heid'
'sel'
'ster' (R1 delete)
'rder' (<-'r')
'ing'
'isme'
'erij' (// Exception added to avoid conflating
// `schilderij` (painting) and `schild` (shield).
('ild' <- 'er')
or
(R1 delete lengthen_V))
'arij' (R1 C <-'aar')
'fie' (R2 delete attach 'f' lengthen_V)
'gie' (R2 delete attach 'g' lengthen_V)
'tst' (R1 C <-'t')
'dst' (R1 C <-'d')
)
)
define Step_4 as
(
( [substring] among (
'ioneel' (R1 <-'ie')
'atief' (R1 <-'eer')
'baar' (R1 delete)
'naar' (R1 V <-'n')
'laar' (R1 V <-'l')
'raar' (R1 V <-'r')
'tant' (R1 <-'teer')
'lijker'
'lijkst' (R1 <-'lijk')
'achtig'
'achtiger'
'achtigst'(R1 delete)
'eriger'
'erigst'
'erig'
'end' (R1 C delete lengthen_V)
)
)
or
( [substring] among (
'iger'
'igst'
'ig' (R1
// Exception added to avoid conflating
// `innig` (intimate) and `in` (in).
not ('inn' atlimit)
C delete lengthen_V)
)
)
)
define Step_7 as
(
[substring] among (
'kt' (<-'k')
'ft' (<-'f')
'pt' (<-'p')
)
)
define Step_6 as
(
[substring] among (
'bb' (<-'b')
'cc' (<-'c')
'dd' (<-'d')
'ff' (<-'f')
'gg' (<-'g')
'hh' (<-'h')
'jj' (<-'j')
'kk' (<-'k')
'll' (<-'l')
'mm' (<-'m')
'nn' (// Exception added to avoid conflating
// `innen` (to collect/cash) and `in` (in).
not ('i' atlimit)
<-'n')
'pp' (<-'p')
'qq' (<-'q')
'rr' (<-'r')
'ss' (<-'s')
'tt' (<-'t')
'vv' (<-'v')
'ww' (<-'w')
'xx' (<-'x')
'zz' (<-'z')
'v' (<-'f')
'z' (<-'s')
)
)
define Step_1c as
(
[substring] R1 C among (
'd' (not ('n' R1)
// Exception added to avoid conflating
// `geïnd` (collected/cashed) and `in` (in).
// Instead we conflate `geïnd` with `innen`.
('in' atlimit <-'n') or
delete)
't' (not ('h' R1)
// Exception added to avoid conflating
// `geënt` (grafted) and `en` (and).
not ('en' atlimit)
delete
)
)
)
)
define Lose_prefix as (
['ge'] test hop 3 test (gopast ('ij' or v) repeat ('ij' or v) not atlimit)
// Exceptions added:
among (
// Avoid conflating `geeft` and `effen`/`effende`\`geeffende`.
'eft' (false)
// Avoid conflating `gevallen`/`geval` and `vallen`.
'val' (false)
'vali' (true)
// Avoid conflating `gevaren`/`gevaar` (danger), `gevaarten` (huge
// objects) and `varen` (to sail)
'vaa' 'vare' (false)
'' (true)
)
set GE_removed
delete
do ( [substring] among (
'{e"}' (<-'e')
'{i"}' (<-'i')
)
)
)
define Lose_infix as (
next
gopast (['ge']) test hop 3 test (gopast ('ij' or v) repeat ('ij' or v) not atlimit)
set GE_removed
delete
do ( [substring] among (
'{e"}' (<-'e')
'{i"}' (<-'i')
)
)
)
define measure as (
$p1 = limit
$p2 = limit
do(
repeat non-v atleast 1 ('ij' or v) non-v setmark p1
repeat non-v atleast 1 ('ij' or v) non-v setmark p2
)
)
define stem as (
unset stemmed
measure
backwards (
do (Step_1 set stemmed )
do (Step_2 set stemmed )
do (Step_3 set stemmed )
do (Step_4 set stemmed )
)
unset GE_removed
do (Lose_prefix and measure)
backwards (
do (GE_removed set stemmed Step_1c)
)
unset GE_removed
do (Lose_infix and measure)
backwards (
do (GE_removed set stemmed Step_1c)
)
backwards (
do (Step_7 set stemmed )
do (stemmed Step_6)
)
)