Inner join of two CSV datasets

Two systems export CSV data that you need to combine. Implement:

def join_datasets(customer_file: list[str], processor_file: list[str], field: str) -> list[str]

Each dataset is a list of lines. The first line is a header of comma-separated column names. Every other line is a data row with one value per column. Both headers contain the column field, but it can be at a different position in each file. Values never contain commas or quotes.

Return an inner join on field, as a list of CSV lines:

• The first line is the customer header followed by the processor header.
• Then, for each customer row (in input order), add one line for every processor row whose field value is equal (in processor order). Each line holds all customer values followed by all processor values.
• Customer rows with no match, and processor rows with no match, do not appear.
• A key can repeat in either file. Every matching pair produces a line.

Constraints: up to 10^5 rows per file and up to 20 columns. Comparisons are exact string matches.

Example

customer_file  = ["cust_id,name,country", "c1,Ada,US", "c2,Lin,SG", "c3,Ola,NG"]
processor_file = ["amount,cust_id", "500,c1", "120,c3", "75,c1", "900,c9"]
field = "cust_id"

Output:

["cust_id,name,country,amount,cust_id",
 "c1,Ada,US,500,c1",
 "c1,Ada,US,75,c1",
 "c3,Ola,NG,120,c3"]

c2 has no charge and c9 has no customer, so neither appears. c1 has two charges, so it appears twice.

Later parts may ask for a left join that keeps unmatched customer rows, and for options that change which rows are kept.