# 31-validation-withdup: Field Validation with Duplicate Check

## 电信业务场景

重复CDR检测。对已按CDR-ID排序的明细文件进行WS-PREV-KEY重复检测，首次出现通过，同一ID再次出现判定为重复并拒否。

## Description

Reads a sorted input file and checks for duplicate KEY values using
the WS-PREV-KEY pattern. The first occurrence of a key is written
to FILE-OUT-GOOD. Any subsequent occurrence of the same key is
written to FILE-OUT-BAD with an error code (duplicate detection).

## Record Layout

### Input / Good Output (35 bytes)

| Field  | Type     | Length | Description      |
|--------|----------|--------|------------------|
| KEY    | PIC X    | 5      | Sort key         |
| DATA   | PIC X    | 30     | Payload data     |

### Bad Output (37 bytes)

| Field  | Type     | Length | Description         |
|--------|----------|--------|---------------------|
| KEY    | PIC X    | 5      | Duplicated key      |
| DATA   | PIC X    | 30     | Original data       |
| ERR    | PIC X    | 2      | Error code ('01')   |

## Files

| File                        | Purpose                          |
|-----------------------------|----------------------------------|
| main-31-validation-withdup.cbl | Main COBOL program            |
| data-gen.sh                 | Generate sorted test data        |
| run.sh                      | Compile, run, verify             |
| README.md                   | This file                        |

## Test Data

```
KEY01  FIRST-RECORD         -> PASS (1st unique)
KEY02  SECOND-RECORD        -> PASS (2nd unique)
KEY03  FIRST-DUP-A          -> PASS (1st)
KEY03  SECOND-DUP-A         -> FAIL (dup)
KEY03  THIRD-DUP-A          -> FAIL (dup)
KEY04  UNIQUE-AFTER-DUP     -> PASS (unique after dups)
KEY05  DUP-GROUP-B-1ST      -> PASS (1st)
KEY05  DUP-GROUP-B-2ND      -> FAIL (dup)
KEY06  TRIPLE-GROUP-1ST     -> PASS (1st)
KEY06  TRIPLE-GROUP-2ND     -> FAIL (dup)
KEY06  TRIPLE-GROUP-3RD     -> FAIL (dup)
KEY99  LAST-UNIQUE          -> PASS (final unique)
```

Expected: 7 good records, 5 bad records.

## Usage

```bash
cd 31-validation-withdup
bash data-gen.sh
bash run.sh
```

## Expected Behavior

- The first occurrence of each KEY is always accepted.
- Subsequent occurrences of the same KEY are rejected as duplicates.
- The WS-PREV-KEY is updated only when a record passes validation.
- Duplicate records contain error code '01' in the BAD output.
- Input must be sorted by KEY for correct sequential detection.