Modernize legacy code
without losing the logic inside it.

       IDENTIFICATION DIVISION.
       PROGRAM-ID. POST-DEBIT.
       DATA DIVISION.
       WORKING-STORAGE SECTION.
       01  WS-BALANCE     PIC S9(9)V99 COMP-3.
       01  WS-AMOUNT      PIC S9(9)V99 COMP-3.
       01  WS-OVERDRAFT   PIC S9(9)V99 COMP-3 VALUE 500.00.
       01  WS-STATUS      PIC X(3).
       PROCEDURE DIVISION.
       POST-DEBIT-PARA.
           IF WS-AMOUNT > WS-BALANCE + WS-OVERDRAFT
               MOVE 'NSF' TO WS-STATUS
           ELSE
               SUBTRACT WS-AMOUNT FROM WS-BALANCE
               MOVE 'OK ' TO WS-STATUS
           END-IF.
           GOBACK.

• R1
  Overdraft limit. A debit is rejected with status NSF when the amount exceeds the balance plus a fixed overdraft of 500.00. Source: lines 7, 11. Review: the limit is hard-coded. Should it be configurable per account?
• R2
  Exact money. Amounts are signed fixed-point with 2 decimals (S9(9)V99, max 999,999,999.99). They must not become floating-point. Source: lines 5–7.
• R3
  Status contract. WS-STATUS is 3 characters: 'NSF' or 'OK ' (with a trailing space). Downstream consumers may compare the padded value. Source: lines 8, 12, 15. Review: confirm the downstream consumers before changing the format.

package ledger

import "errors"

// Cents is signed fixed-point money with 2 implied decimals,
// matching PIC S9(9)V99 in POST-DEBIT. (R2)
type Cents int64

// OverdraftLimit mirrors WS-OVERDRAFT. Flagged for review:
// candidate for per-account configuration. (R1)
const OverdraftLimit Cents = 500_00

// ErrNSF maps to legacy status 'NSF'. The API adapter emits
// the padded 3-char codes 'NSF' / 'OK '. (R3)
var ErrNSF = errors.New("NSF")

// PostDebit mirrors POST-DEBIT-PARA. (R1, R2)
func PostDebit(balance, amount Cents) (Cents, error) {
	if amount > balance+OverdraftLimit {
		return balance, ErrNSF
	}
	return balance - amount, nil
}

// Characterization cases: each one was run against the legacy
// program and its observed output is recorded here as the oracle.
func TestPostDebit_MatchesLegacy(t *testing.T) {
	cases := []struct {
		name            string
		balance, amount Cents
		want            Cents
		wantErr         error
	}{
		{"within balance",     1000_00, 250_00, 750_00,  nil},
		{"exactly at limit",   100_00,  600_00, -500_00, nil},
		{"one cent over limit", 100_00,  600_01, 100_00,  ErrNSF},
	}
	for _, tc := range cases {
		got, err := PostDebit(tc.balance, tc.amount)
		if got != tc.want || !errors.Is(err, tc.wantErr) {
			t.Errorf("%s: got (%d, %v), want (%d, %v)",
				tc.name, got, err, tc.want, tc.wantErr)
		}
	}
}