My go at Cologne Phonetics in PostgreSQL

cologne-phonetics.sql

CREATE FUNCTION cologne_phonetics_preprocess(txt varchar) RETURNS varchar AS $$
    BEGIN
        RETURN replace(replace(replace(replace(upper(txt), 'Ä', 'A'), 'Ö', 'O'), 'Ü', 'U'), 'ß', 'SS');
    END;
$$ LANGUAGE plpgsql
IMMUTABLE
RETURNS NULL ON NULL INPUT;

CREATE FUNCTION cologne_phonetics_encode_except_cx(ch varchar, next_ch varchar) RETURNS varchar AS $$
    DECLARE
        result varchar;
    BEGIN
        IF ch = 'A' OR ch = 'E' OR ch = 'I' OR ch = 'J' OR ch = 'O' OR ch = 'U' OR ch = 'Y' THEN
            result := '0';
        /* ELSIF ch = 'H' THEN
            result := '-'; */
        ELSIF ch = 'B' THEN
            result := '1';
        ELSIF ch = 'F' OR ch = 'V' OR ch = 'W' THEN
            result := '3';
        ELSIF ch = 'G' OR ch = 'K' OR ch = 'Q' THEN
            result := '4';
        ELSIF ch = 'L' THEN
            result := '5';
        ELSIF ch = 'M' OR ch = 'N' THEN
            result := '6';
        ELSIF ch = 'R' THEN
            result := '7';
        ELSIF ch = 'S' OR ch = 'Z' THEN
            result := '8';
        ELSIF ch = 'P' THEN
            IF next_ch IS NULL OR next_ch != 'H' THEN
                result := '1';
            ELSE
                result := '3';
            END IF;
        ELSIF ch = 'D' OR ch = 'T' THEN
            IF next_ch IS NULL OR (next_ch != 'C' AND next_ch != 'S' AND next_ch != 'Z') THEN
                result := '2';
            ELSE
                result := '8';
            END IF;
        ELSE
            result := '';
        END IF;

        RETURN result;
    END;
$$ LANGUAGE plpgsql
IMMUTABLE
CALLED ON NULL INPUT;

CREATE FUNCTION cologne_phonetics_encode_only_c(prev_ch varchar, next_ch varchar) RETURNS varchar AS $$
    DECLARE
        result varchar;
    BEGIN
        IF prev_ch IS NULL THEN
            IF next_ch IS NOT NULL AND (next_ch = 'A' OR next_ch = 'H' OR next_ch = 'K' OR next_ch = 'L'
                    OR next_ch = 'O' OR next_ch = 'Q' OR next_ch = 'R' OR next_ch = 'U' OR next_ch = 'X') THEN
                result := '4';
            ELSE
                result := '8';
            END IF;
        ELSIF prev_ch = 'S' OR prev_ch = 'Z' THEN
            result := '8';
        ELSIF next_ch IS NOT NULL AND (next_ch = 'A' OR next_ch = 'H' OR next_ch = 'K' OR next_ch = 'O'
                OR next_ch = 'Q' OR next_ch = 'U' OR next_ch = 'X') THEN
            result := '4';
        ELSE
            result := '8';
        END IF;

        RETURN result;
    END;
$$ LANGUAGE plpgsql
IMMUTABLE
CALLED ON NULL INPUT;

CREATE FUNCTION cologne_phonetics_encode_only_x(prev_ch varchar) RETURNS varchar AS $$
    DECLARE
        result varchar;
    BEGIN
        IF prev_ch IS NOT NULL AND (prev_ch = 'C' OR prev_ch = 'K' OR prev_ch = 'Q') THEN
            result := '48';
        ELSE
            result := '8';
        END IF;

        RETURN result;
    END;
$$ LANGUAGE plpgsql
IMMUTABLE
CALLED ON NULL INPUT;

CREATE FUNCTION cologne_phonetics_encode(prepped varchar) RETURNS varchar AS $$
    DECLARE
        result varchar;

        prepped_len int;
        prepped_array varchar[];

        i int;
        prev_ch varchar;
        ch varchar;
        next_ch varchar;
    BEGIN
        prepped_array := regexp_split_to_array(prepped, '');
        prepped_len := array_length(prepped_array, 1);

        result := '';

        i := 1;
        ch := prepped_array[1];
        WHILE i <= prepped_len LOOP
            IF (i + 1) <= prepped_len THEN
                next_ch := prepped_array[i + 1];
            ELSE
                next_ch := NULL;
            END IF;

            IF ch = 'C' THEN
                result := result || cologne_phonetics_encode_only_c(prev_ch, next_ch);
            ELSIF ch = 'X' THEN
                result := result || cologne_phonetics_encode_only_x(prev_ch);
            ELSE
                result := result || cologne_phonetics_encode_except_cx(ch, next_ch);
            END IF;

            prev_ch := ch;
            ch := next_ch;
            i := i + 1;
        END LOOP;

        RETURN result;
    END;
$$ LANGUAGE plpgsql
IMMUTABLE
RETURNS NULL ON NULL INPUT;

CREATE FUNCTION cologne_phonetics_finalize(encoded varchar) RETURNS varchar AS $$
    DECLARE
        encoded_array varchar[];
        result varchar;
        ch varchar;
        prev_ch varchar;
    BEGIN
        encoded_array := regexp_split_to_array(encoded, '');

        FOREACH ch IN ARRAY encoded_array
        LOOP
            IF prev_ch IS NULL THEN
                result := ch;
                prev_ch := ch;
            ELSIF ch != prev_ch AND ch != '0' THEN
                result := result || ch;
                prev_ch := ch;
            END IF;
        END LOOP;

        RETURN result;
    END;
$$ LANGUAGE plpgsql
IMMUTABLE
RETURNS NULL ON NULL INPUT;

CREATE FUNCTION cologne_phonetics(txt varchar) RETURNS varchar AS $$
    DECLARE
        prepped varchar;
        encoded varchar;
        result varchar;
    BEGIN
        prepped := cologne_phonetics_preprocess(txt);

        encoded := cologne_phonetics_encode(prepped);

        result := cologne_phonetics_finalize(encoded);

        RETURN result;
    END;
$$ LANGUAGE plpgsql
IMMUTABLE
RETURNS NULL ON NULL INPUT;