# 20-matching-MN-to-MxN

## 电信业务场景

CDR详细清单M:N→M×N。多个合同×多个通话明细的笛卡尔积组合，输出完整的CDR详细清单(M×N件)。

**M:N matching — Cartesian product output (M x N records per matching key).**

Tests the scenario where both input files may contain duplicate keys and
every combination of matching master and detail records must be output
(the full Cartesian product).

## Test data

| File        | Records | Keys                                     |
|-------------|---------|------------------------------------------|
| master.dat  | 5       | KEY00001 (x2), KEY00002 (x1), KEY00003 (x2) |
| detail.dat  | 7       | KEY00001 (x3), KEY00003 (x2), KEY00004 (x2) |

- **KEY00001:** 2 masters x 3 details = 6 output records
- **KEY00003:** 2 masters x 2 details = 4 output records
- **Total output:** 10 records
- **Unmatched:** KEY00002 (master only), KEY00004 (detail only)

## Algorithm

1. Read both files synchronously (both assumed sorted by key).
2. When keys match: read all master records for that key into a table,
   read all detail records for that key into a table, then write M x N
   output records covering every combination.
3. Unmatched groups are skipped via the standard key-comparison logic.

## Expected output

- **output.dat:** 10 records x 45 bytes = 450 bytes
- Program displays: `20-matching-MN-to-MxN: PASS`
- Script displays: `20-matching-MN-to-MxN: ALL TESTS PASSED`