feat: add balance bounds exercise

README.md

@@ -58,3 +58,4 @@ You might also be interested in reading more in the [TigerBeetle docs](https://d
 - Two-phase transfers (pending, posting, voiding, timeouts, and partial amounts)
 - Balancing transfers (used when closing accounts)
 - Balance-conditional transfers
+- Balance bounds

exercises/029_balance_bounds.sh

@@ -0,0 +1,46 @@
+#!/bin/bash
+source ./tools/tb_function.sh
+
+# This is an advanced exercise that shows how you can keep an account's balance
+# between an upper and a lower bound.
+
+# We're going to set up 4 accounts:
+CONTROL_ACCOUNT=2900
+OPERATOR_ACCOUNT=2901
+SOURCE_ACCOUNT=2910
+DESTINATION_ACCOUNT=2911
+
+tb "create_accounts id=$CONTROL_ACCOUNT code=10 ledger=290 flags=credits_must_not_exceed_debits,
+                    id=$OPERATOR_ACCOUNT code=10 ledger=290,
+                    id=$SOURCE_ACCOUNT code=10 ledger=290 flags=debits_must_not_exceed_credits,
+                    id=$DESTINATION_ACCOUNT code=10 ledger=290 flags=debits_must_not_exceed_credits;"
+
+# Let's say we wanted to ensure that a certain account's balance stays between 0 and 100.
+# In this case, we're applying this to the DESTINATION_ACCOUNT.
+
+# The DESTINATION_ACCOUNT has the flag `debits_must_not_exceed_credits`, which means that
+# it must maintain a positive credit balance. Let's see how we implement an upper bound
+# on that account's credit balance.
+
+# We'll start off by putting some funds in the SOURCE_ACCOUNT:
+tb "create_transfers id=29000 debit_account_id=$OPERATOR_ACCOUNT credit_account_id=$SOURCE_ACCOUNT amount=1000 ledger=290 code=10;"
+
+# Here's where it gets a little crazy.
+LIMIT_AMOUNT=100
+TRANSFER_AMOUNT=200
+tb "create_transfers id=29001 debit_account_id=$SOURCE_ACCOUNT      credit_account_id=$DESTINATION_ACCOUNT amount=$TRANSFER_AMOUNT ledger=290 code=10 flags=linked,
+                     id=29002 debit_account_id=$CONTROL_ACCOUNT     credit_account_id=$OPERATOR_ACCOUNT    amount=$LIMIT_AMOUNT    ledger=290 code=10 flags=linked,
+                     id=29003 debit_account_id=$DESTINATION_ACCOUNT credit_account_id=$CONTROL_ACCOUNT     amount=0                ledger=290 code=10 flags=balancing_debit | pending | linked,
+                     id=29004 pending_id=29003 flags=void_pending_transfer | linked,
+                     id=29005 debit_account_id=$OPERATOR_ACCOUNT    credit_account_id=$CONTROL_ACCOUNT     amount=$LIMIT_AMOUNT    ledger=290 code=10;"
+
+# This is a set of linked transfers, so either all of them will succeed or none of them will.
+# The first transfer is the one we actually want to create.
+# In the second transfer, we debit the CONTROL_ACCOUNT by the LIMIT_AMOUNT.
+# (The CONTROL_ACCOUNT has the `credits_must_not_exceed_debits` flag so the credits cannot exceed the LIMIT_AMOUNT.)
+# The third transfer pretends to do a `balancing_debit` transfer to send the DESTINATION_ACCOUNT's *net credit balance* to the CONTROL_ACCOUNT.
+# The third transfer will only succeed if the DESTINATION_ACCOUNT's credit balance is within bounds after the first transfer.
+# The fourth transfer voids the third transfer (that's why we said it "pretends" to do a `balancing_debit`).
+# Finally, the fifth transfer resets the CONTROL_ACCOUNT's net balance to zero.
+
+# Can you fix the TRANSFER_AMOUNT so that our batch of transfers succeeds? (Hint: it should be less than the LIMIT_AMOUNT.)

patches/029_balance_bounds.patch

@@ -0,0 +1,13 @@
+diff --git a/exercises/029_balance_bounds.sh b/exercises/029_balance_bounds.sh
+index 6547401..da8182d 100755
+--- a/exercises/029_balance_bounds.sh
++++ b/exercises/029_balance_bounds.sh
+@@ -27,7 +27,7 @@ tb "create_transfers id=29000 debit_account_id=$OPERATOR_ACCOUNT credit_account_
+
+ # Here's where it gets a little crazy.
+ LIMIT_AMOUNT=100
+-TRANSFER_AMOUNT=200
++TRANSFER_AMOUNT=50
+ tb "create_transfers id=29001 debit_account_id=$SOURCE_ACCOUNT      credit_account_id=$DESTINATION_ACCOUNT amount=$TRANSFER_AMOUNT ledger=290 code=10 flags=linked,
+                      id=29002 debit_account_id=$CONTROL_ACCOUNT     credit_account_id=$OPERATOR_ACCOUNT    amount=$LIMIT_AMOUNT    ledger=290 code=10 flags=linked,
+                      id=29003 debit_account_id=$DESTINATION_ACCOUNT credit_account_id=$CONTROL_ACCOUNT     amount=0                ledger=290 code=10 flags=balancing_debit | pending | linked,