The Kraaij-Pohlmann stemming algorithm

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The Kraaij-Pohlmann stemming algorithm is an ANSI C program for stemming in Dutch. Although advertised as an algorithm, it is in fact a program without an accompanying algorithmic description. It is possible to produce a fairly clean Snowball version, but only by sacrificing exact functional equivalence. But that does not matter too much, since in the demonstration vocabulary only 32 words out of over 45,000 stem differently. Here they are:

source ANSI C stemmer Snowball stemmer
airways airways airway
algerije algerije alrije
assays assays assay
bruys bruys bruy
cleanaways cleanaways cleanaway
creys creys crey
croyden croyd croy
edele edel edeel
essays essays essay
gedijen gedij dij
geoff of off
gevrey gevrey vrey
geysels ysel gey
grootmeesteres grootmee grootmeest
gròotmeesteres gròotmee gròotmeest
hectares hectaar hect
huys huys huy
kayen kayen kaay
lagerwey lagerwey larwey
mayen mayen maay
meesteres meester meest
oppasseres oppasser oppas
pays pays pay
royale royale royaal
schilderes schilder schild
summerhayes summerhayes summerhaye
tyumen tyuum tyum
verheyen verheyen verheey
verleideres verleider verleid
ytsen yts ytsen
yves yve yves
zangeres zanger zang

The Kraaij-Pohmann stemmer can make fairly drastic reductions to a word. For example, infixed ge is removed, so geluidgevoelige stems to luidvoel. Often, therefore, the original word cannot be easily guessed from the stemmed form.

Here then is the Snowball equivalent of the Kraaij-Pohlmann algorithm.

strings ( ch )
integers ( x p1 p2 )
booleans ( Y_found 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 AOU AIOU )

stringescapes {}

stringdef '   hex '27'  // yuk

define v        'aeiouy'
define v_WX     v + 'wx'
define AOU      'aou'
define AIOU     'aiou'

backwardmode (

    define R1 as (setmark x $x >= p1)
    define R2 as (setmark x $x >= p2)

    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 [ (AOU] test (non-v or atlimit)) or
                   ('e'] test (non-v or atlimit
                               not AIOU
                               not (next AIOU non-v)))
        ->ch insert ch
    )

    define Step_1 as
    (
        [among ( (])

            '{'}s' (delete)
            's'    (R1 not ('t' R1) C delete)
            'ies'  (R1 <-'ie')
            'es'
                   (('ar' R1 C ] delete lengthen_V) or
                    ('er' R1 C ] delete) or
                    (R1 C <-'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
    (
        [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
    (
        [among ( (])
            'atie'  (R1 <-'eer')
            'iteit' (R1 delete lengthen_V)
            'heid'
            'sel'
            'ster'  (R1 delete)
            'rder'  (<-'r')
            'ing'
            'isme'
            'erij'  (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
    (
        (   [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
        (   [among ( (])
                'iger'
                'igst'
                'ig'      (R1 C delete lengthen_V)
            )
        )
    )

    define Step_7 as
    (
        [among ( (])
            'kt'   (<-'k')
            'ft'   (<-'f')
            'pt'   (<-'p')
        )
    )

    define Step_6 as
    (
        [among ( (])
            'bb'   (<-'b')
            'cc'   (<-'c')
            'dd'   (<-'d')
            'ff'   (<-'f')
            'gg'   (<-'g')
            'hh'   (<-'h')
            'jj'   (<-'j')
            'kk'   (<-'k')
            'll'   (<-'l')
            'mm'   (<-'m')
            'nn'   (<-'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
    (
        [among ( (] R1 C)
            'd' (not ('n' R1) delete)
            't' (not ('h' R1) delete)
        )
    )
)

define Lose_prefix as (
    ['ge'] test hop 3 (goto v goto non-v)
    set GE_removed
    delete
)

define Lose_infix as (
    next
    gopast (['ge']) test hop 3 (goto v goto non-v)
    set GE_removed
    delete
)

define measure as (
    do (
        tolimit
        setmark p1
        setmark p2
    )
    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 Y_found
    unset stemmed
    do ( ['y'] <-'Y' set Y_found )
    do repeat(goto (v  ['y'])<-'Y' set Y_found )

    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 Step_1c)
        )
    unset GE_removed
    do (Lose_infix and measure)
    backwards (
            do (GE_removed Step_1c)
        )
    backwards (
            do (Step_7 set stemmed )
            do (stemmed or GE_removed Step_6)
        )
    do(Y_found  repeat(goto (['Y']) <-'y'))
)