Spark soundex function implementation

[kazantsev-maksim]

Which issue does this PR close?

N/A

Rationale for this change

Add new spark function: https://spark.apache.org/docs/latest/api/sql/index.html#soundex

What changes are included in this PR?

• Implementation
• SLT tests

Are these changes tested?

Yes, tests added as part of this PR.

Are there any user-facing changes?

No, these are new function.

[davidlghellin]

I'd be happy to add the SLT tests for these edge cases if you'd like — I already have them validated against Spark JVM. Just let me know!

[davidlghellin]

Hey! I had actually started working on a Spark soundex implementation too and didn't realize there was already a PR for it. Happy to see this moving forward!

I had put together a battery of edge-case tests validated against Spark JVM that might be useful. The current SLT coverage is a bit thin — there are some tricky Soundex behaviors that are easy to get wrong:

tests = [
    # H/W transparency (must NOT separate same codes)
    ("H/W transparency", "SELECT soundex('Ashcroft') AS result"),
    # Separators (digit, space, vowel MUST separate same codes)
    ("Digit separates same-code", "SELECT soundex('B1B') AS result"),
    ("Space separates same-code", "SELECT soundex('B B') AS result"),
    ("Vowel separates same-code", "SELECT soundex('BAB') AS result"),
    # Non-alpha first character (returns input unchanged)
    ("Non-alpha first char", "SELECT soundex('#hello') AS result"),
    ("Space first char", "SELECT soundex(' hello') AS result"),
    ("Only spaces", "SELECT soundex('   ') AS result"),
    ("Tab prefix", "SELECT soundex('\thello') AS result"),
    ("Emoji prefix", "SELECT soundex('😀hello') AS result"),
    ("Only digits", "SELECT soundex('123') AS result"),
    ("Starts with digit", "SELECT soundex('1abc') AS result"),
    # Basic behavior
    ("Single character", "SELECT soundex('A') AS result"),
    ("All same-code letters", "SELECT soundex('BFPV') AS result"),
    ("Similar names Robert", "SELECT soundex('Robert') AS result"),
    ("Similar names Rupert", "SELECT soundex('Rupert') AS result"),
    ("NULL", "SELECT soundex(NULL) AS result"),
    ("Empty string", "SELECT soundex('') AS result"),
    # Case insensitivity
    ("Lowercase", "SELECT soundex('robert') AS result"),
    ("Mixed case same", "SELECT soundex('rObErT') AS result"),
    # Unicode
    ("Unicode umlaut", "SELECT soundex('Müller') AS result"),
    # Truncation (only first 3 codes after initial)
    ("Long string", "SELECT soundex('Abcdefghijklmnop') AS result"),
    # Extra edge cases
    ("Adjacent same codes collapse", "SELECT soundex('Lloyd') AS result"),
    ("W between same codes", "SELECT soundex('BWB') AS result"),
    ("H between same codes", "SELECT soundex('BHB') AS result"),
    ("Double letters", "SELECT soundex('Tymczak') AS result"),
    ("All vowels after first", "SELECT soundex('Aeiou') AS result"),
    ("First char digit rest alpha", "SELECT soundex('1Robert') AS result"),
    ("Hyphen in name", "SELECT soundex('Smith-Jones') AS result"),
    ("Single non-alpha", "SELECT soundex('#') AS result"),
    ("Newline prefix", "SELECT soundex('\nhello') AS result"),
]

for label, sql in tests:
    r = spark.sql(sql).collect()
    print(f"{label}: {repr(r[0].result)}")

# Multi-row column test
print("\nColumn test:")
spark.sql("""
    SELECT soundex(name) AS result 
    FROM VALUES ('Robert'), ('Rupert'), (NULL), (''), ('123') AS t(name)
""").show()

Spark-3.5

[kazantsev-maksim]

Big thanks to @davidlghellin for the test cases.