10 · Batch files

Validate an entire SBF-80 payment batch

Validate an invented 80-character fixed-width payment file and reject the whole batch on any error.

Pythonmedium

What the hardened grader checks

CRLF and LF files parse identically
The trailer's count, control total and account hash total are all checked
One bad detail line rejects the entire batch
Every error is reported with its line number
Rendering and re-parsing a random valid batch round-trips; one changed digit is caught

Solutions

Reference

reference

Slices fixed columns, then checks counts and totals in a second pass.

Naive: accepted (3/3)Hardened: accepted (39/39)

Alternative

alt_valid

Regular-expression record decoding, a regex line splitter and a single streaming pass with incremental totals, versus column slicing and a two-pass validation and control phase.

Naive: accepted (3/3)Hardened: accepted (39/39)

`wrong_crlf_slicing`

Validates physical line width before normalizing CRLF.

Naive: accepted (3/3)Hardened: rejected (34/39)

`wrong_ignores_hash_total`

Does not check the trailer account hash total.

Naive: accepted (3/3)Hardened: rejected (31/39)

`wrong_partial_accept`

Accepts valid details from a batch containing invalid detail records.

Naive: accepted (3/3)Hardened: rejected (38/39)

Held-out solutions

zero_amount_acceptedExpect: rejectObserved: rejected
Accepts zero detail amounts, violating the positive minor-unit amount field.
blank_reference_acceptedExpect: rejectObserved: rejected
Accepts blank references, violating the nonblank detail reference field.
header_currency_any_alphaExpect: rejectObserved: rejected
Accepts arbitrary alphabetic currencies, violating the enumerated header currency set.
lowercase_batch_idExpect: rejectObserved: rejected
Accepts lowercase batch ids, violating uppercase ASCII letters/digits.
bare_cr_is_terminatorExpect: rejectObserved: rejected
Treats bare CR as a terminator, violating invalid_encoding for bare CR.
blank_lines_skippedExpect: rejectObserved: rejected
Skips interior blank lines, violating their treatment as invalid records.
missing_trailer_after_lastExpect: rejectObserved: rejected
Reports a missing trailer beyond the last line, violating last-line error positioning.
reference_strip_bothExpect: rejectObserved: rejected
Strips leading reference spaces, violating right-padding-only removal.
crlf_all_or_nothingExpect: rejectObserved: rejected
Rejects mixed LF/CRLF records, violating acceptance of mixed terminators.
alt_cleanroomExpect: acceptObserved: accepted
Uses an independently written rule-table parser with separate record and structure validation.

Tests × solutions

Each row groups a test’s parameterized cases; any failing case marks the row as Fail. * marks a wrong solution’s declared must_fail test.

Hardened · 39 recorded cases · 13 test rows
Test	Starter	Reference	Alternative	crlf slicing	ignores hash total	partial accept
Crlf	Fail	Pass	Pass	Fail *	Pass	Pass
Hash total	Fail	Pass	Pass	Pass	Fail *	Pass
Count mismatch	Fail	Pass	Pass	Pass	Pass	Pass
Atomic batch rejection	Fail	Pass	Pass	Pass	Fail	Fail *
Multiple errors and line numbers	Fail	Pass	Pass	Pass	Fail	Pass
Invalid records	Fail	Pass	Pass	Pass	Pass	Pass
Missing and order	Fail	Pass	Pass	Pass	Pass	Pass
Field formats	Fail	Pass	Pass	Pass	Fail	Pass
Supported currencies preserve minor units	Pass	Pass	Pass	Pass	Pass	Pass
Mixed terminators and optional final	Pass	Pass	Pass	Fail	Pass	Pass
Terminator blank and missing trailer errors	Fail	Pass	Pass	Pass	Pass	Pass
Reference preserves leading spaces	Pass	Pass	Pass	Pass	Pass	Pass
Roundtrip and single digit corruption	Fail	Pass	Pass	Fail	Pass	Pass

Naive grader

Naive · 3 recorded cases · 3 test rows
Test	Starter	Reference	Alternative	crlf slicing	ignores hash total	partial accept
Valid lf	Pass	Pass	Pass	Pass	Pass	Pass
Ticket worked example	Pass	Pass	Pass	Pass	Pass	Pass
Count mismatch	Fail	Pass	Pass	Pass	Pass	Pass

The ticket

Ticket: validate invented SBF-80 batch files atomically

batch.parser.parse(data:bytes)->BatchResult returns frozen dataclasses exported from that module: Payment(payee_account:str,amount_minor:int,reference:str), BatchError(line_no:int,code:str), BatchResult(accepted:bool,payments:list[Payment],errors:list[BatchError]).

SBF-80 is invented for this synthetic task. Each record is exactly 80 printable ASCII characters, followed by LF or CRLF. A final terminator is optional; mixed LF/CRLF is accepted. Strip the terminator before checking width and slicing. Interior blank lines are records and invalid. Bare CR, non-ASCII or control bytes are invalid_encoding. All positions below are 1-based inclusive.

Record	Columns	Field
H	1	H
H	2–13	batch id: 12 uppercase ASCII letters/digits
H	14–21	creation date YYYYMMDD, valid calendar date
H	22–24	currency CAD, USD, EUR, GBP, JPY, KWD or BHD
H	25–80	spaces
D	1	D
D	2–13	12 ASCII digits, payee account (retain leading zeros)
D	14–25	12 ASCII digits, positive zero-padded integer minor units
D	26–45	20 printable chars, nonblank reference, right-space padded
D	46–80	spaces
T	1	T
T	2–7	detail count, 6 ASCII digits
T	8–22	control total of amounts, 15 ASCII digits
T	23–32	hash total, 10 ASCII digits
T	33–80	spaces

Exactly one header first, then one or more details, then exactly one trailer last. The count is the number of D records, including malformed D records. Control total and hash total use only fully valid detail records. Hash = sum(int(last 8 digits of each payee account)) mod 10**10. Money is in header-currency minor units: CAD/USD/EUR/GBP=2 dp, JPY=0, KWD/BHD=3. Do not convert. Returned references remove right padding only. Preserve detail order and account strings.

Any error rejects the whole batch: accepted=False and payments=[]. Report all detectable errors, deduplicated by (line_no,code), sorted by line_no then code. Continue checking other records. For a wrong-width or invalid-encoding record, report only that local error (no field errors), but structural/order and trailer comparisons can still be reported. Field errors are one code per record even if multiple fields are invalid. Trailer totals compare independently when its fields are valid, even if other records failed.

Codes: invalid_encoding, line_length, record_type, record_order, invalid_header, invalid_detail, invalid_trailer, count_mismatch, control_total_mismatch, hash_total_mismatch, missing_header, missing_trailer. H outside first position, T outside last, or D at first/last -> record_order. Unknown type -> record_type. A missing first H -> missing_header at line1; missing final T -> missing_trailer at the last line (line1 for empty input). An empty detail section -> count_mismatch on the trailer. Header/trailer field errors include invalid reserved spaces. No other codes.

Worked example (each record is padded with spaces to 80 characters; trailing spaces are not shown):

HSYNTHBATCH0120260115CAD
D000000012345000000010000INV-2026-001
D000000067890000000002550INV-2026-002
T0000020000000000125500000080235

The batch is accepted with two payments: Payment("000000012345", 10000, "INV-2026-001") and Payment("000000067890", 2550, "INV-2026-002"). Count 2; control total 10000 + 2550 = 12550; hash total 12345 + 67890 = 80235. Changing the trailer count to 000003 rejects the whole batch with BatchError(4, "count_mismatch") and payments == [].

Run from starter: python -m pytest tests -q. No real batch format, institution or customer is used.

Source

Ticket starter/solutions/grader/naive/grader/final/task.toml