stringescapes {}

/* the 33 Cyrillic letters represented in ASCII characters following the
 * conventions of the standard Library of Congress transliteration: */

stringdef a    '{U+0430}'
stringdef b    '{U+0431}'
stringdef v    '{U+0432}'
stringdef g    '{U+0433}'
stringdef d    '{U+0434}'
stringdef e    '{U+0435}'
stringdef e"   '{U+0451}'
stringdef zh   '{U+0436}'
stringdef z    '{U+0437}'
stringdef i    '{U+0438}'
stringdef i`   '{U+0439}'
stringdef k    '{U+043A}'
stringdef l    '{U+043B}'
stringdef m    '{U+043C}'
stringdef n    '{U+043D}'
stringdef o    '{U+043E}'
stringdef p    '{U+043F}'
stringdef r    '{U+0440}'
stringdef s    '{U+0441}'
stringdef t    '{U+0442}'
stringdef u    '{U+0443}'
stringdef f    '{U+0444}'
stringdef kh   '{U+0445}'
stringdef ts   '{U+0446}'
stringdef ch   '{U+0447}'
stringdef sh   '{U+0448}'
stringdef shch '{U+0449}'
stringdef "    '{U+044A}'
stringdef y    '{U+044B}'
stringdef '    '{U+044C}'
stringdef e`   '{U+044D}'
stringdef iu   '{U+044E}'
stringdef ia   '{U+044F}'

routines ( mark_regions R2
           perfective_gerund
           adjective
           adjectival
           reflexive
           verb
           noun
           derivational
           tidy_up
)

externals ( stem )

integers ( pV p2 )

groupings ( v )

define v '{a}{e}{i}{o}{u}{y}{e`}{iu}{ia}'

define mark_regions as (

    $pV = limit
    $p2 = limit
    do (
        gopast v  setmark pV  gopast non-v
        gopast v  gopast non-v  setmark p2
       )
)

backwardmode (

    define R2 as $p2 <= cursor

    define perfective_gerund as (
        [substring] among (
            '{v}'
            '{v}{sh}{i}'
            '{v}{sh}{i}{s}{'}'
                ('{a}' or '{ia}' delete)
            '{i}{v}'
            '{i}{v}{sh}{i}'
            '{i}{v}{sh}{i}{s}{'}'
            '{y}{v}'
            '{y}{v}{sh}{i}'
            '{y}{v}{sh}{i}{s}{'}'
                (delete)
        )
    )

    define adjective as (
        [substring] among (
            '{e}{e}' '{i}{e}' '{y}{e}' '{o}{e}' '{i}{m}{i}' '{y}{m}{i}'
            '{e}{i`}' '{i}{i`}' '{y}{i`}' '{o}{i`}' '{e}{m}' '{i}{m}'
            '{y}{m}' '{o}{m}' '{e}{g}{o}' '{o}{g}{o}' '{e}{m}{u}'
            '{o}{m}{u}' '{i}{kh}' '{y}{kh}' '{u}{iu}' '{iu}{iu}' '{a}{ia}'
            '{ia}{ia}'
                        // and -
            '{o}{iu}'   // - which is somewhat archaic
            '{e}{iu}'   // - soft form of {o}{iu}
                (delete)
        )
    )

    define adjectival as (
        adjective

        /* of the participle forms, em, vsh, ivsh, yvsh are readily removable.
           nn, {iu}shch, shch, u{iu}shch can be removed, with a small proportion of
           errors. Removing im, uem, enn creates too many errors.
        */

        try (
            [substring] among (
                '{e}{m}'                  // present passive participle
                '{n}{n}'                  // adjective from past passive participle
                '{v}{sh}'                 // past active participle
                '{iu}{shch}' '{shch}'     // present active participle
                    ('{a}' or '{ia}' delete)

     //but not  '{i}{m}' '{u}{e}{m}'      // present passive participle
     //or       '{e}{n}{n}'               // adjective from past passive participle

                '{i}{v}{sh}' '{y}{v}{sh}'// past active participle
                '{u}{iu}{shch}'          // present active participle
                    (delete)
            )
        )

    )

    define reflexive as (
        [substring] among (
            '{s}{ia}'
            '{s}{'}'
                (delete)
        )
    )

    define verb as (
        [substring] among (
            '{l}{a}' '{n}{a}' '{e}{t}{e}' '{i`}{t}{e}' '{l}{i}' '{i`}'
            '{l}' '{e}{m}' '{n}' '{l}{o}' '{n}{o}' '{e}{t}' '{iu}{t}'
            '{n}{y}' '{t}{'}' '{e}{sh}{'}'

            '{n}{n}{o}'
                ('{a}' or '{ia}' delete)

            '{i}{l}{a}' '{y}{l}{a}' '{e}{n}{a}' '{e}{i`}{t}{e}'
            '{u}{i`}{t}{e}' '{i}{t}{e}' '{i}{l}{i}' '{y}{l}{i}' '{e}{i`}'
            '{u}{i`}' '{i}{l}' '{y}{l}' '{i}{m}' '{y}{m}' '{e}{n}'
            '{i}{l}{o}' '{y}{l}{o}' '{e}{n}{o}' '{ia}{t}' '{u}{e}{t}'
            '{u}{iu}{t}' '{i}{t}' '{y}{t}' '{e}{n}{y}' '{i}{t}{'}'
            '{y}{t}{'}' '{i}{sh}{'}' '{u}{iu}' '{iu}'
                (delete)
            /* note the short passive participle tests:
               '{n}{a}' '{n}' '{n}{o}' '{n}{y}'
               '{e}{n}{a}' '{e}{n}' '{e}{n}{o}' '{e}{n}{y}'
            */
        )
    )

    define noun as (
        [substring] among (
            '{a}' '{e}{v}' '{o}{v}' '{i}{e}' '{'}{e}' '{e}'
            '{i}{ia}{m}{i}' '{ia}{m}{i}' '{a}{m}{i}' '{e}{i}' '{i}{i}'
            '{i}' '{i}{e}{i`}' '{e}{i`}' '{o}{i`}' '{i}{i`}' '{i`}'
            '{i}{ia}{m}' '{ia}{m}' '{i}{e}{m}' '{e}{m}' '{a}{m}' '{o}{m}'
            '{o}' '{u}' '{a}{kh}' '{i}{ia}{kh}' '{ia}{kh}' '{y}' '{'}'
            '{i}{iu}' '{'}{iu}' '{iu}' '{i}{ia}' '{'}{ia}' '{ia}'
                (delete)
            /* the small class of neuter forms '{e}{n}{i}' '{e}{n}{e}{m}'
               '{e}{n}{a}' '{e}{n}' '{e}{n}{a}{m}' '{e}{n}{a}{m}{i}' '{e}{n}{a}{x}'
               omitted - they only occur on 12 words.
            */
        )
    )

    define derivational as (
        [substring] R2 among (
            '{o}{s}{t}'
            '{o}{s}{t}{'}'
                (delete)
        )
    )

    define tidy_up as (
        [substring] among (

            '{e}{i`}{sh}'
            '{e}{i`}{sh}{e}'  // superlative forms
               (delete
                ['{n}'] '{n}' delete
               )
            '{n}'
               ('{n}' delete) // e.g. -nno endings
            '{'}'
               (delete)  // with some slight false conflations
        )
    )
)

define stem as (

    // Normalise {e"} to {e}.  The documentation has long suggested the user
    // should do this before calling the stemmer - we now do it for them.
    do repeat ( goto (['{e"}']) <- '{e}' )

    do mark_regions
    backwards setlimit tomark pV for (
        do (
             perfective_gerund or
             ( try reflexive
               adjectival or verb or noun
             )
        )
        try([ '{i}' ] delete)
        // because noun ending -i{iu} is being treated as verb ending -{iu}

        do derivational
        do tidy_up
    )
)