lib: add 'Transaction' variants

Represent each enum variant with its own type to allow stronger typing
when processing only a singular type of transactions.

src/lib.rs

@@ -1,2 +1,5 @@
 pub mod core;
 pub use crate::core::*;
+
+pub mod transaction;
+pub use crate::transaction::*;

src/transaction.rs

@@ -0,0 +1,57 @@
+//! Define all supported transactions.
+use crate::core::{ClientId, TxAmount, TxId};
+
+/// A generic [Transaction].
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub enum Transaction {
+    Deposit(Deposit),
+    Withdrawal(Withdrawal),
+    Dispute(Dispute),
+    Resolve(Resolve),
+    Chargeback(Chargeback),
+}
+
+/// Deposit funds into an account, i.e: increase its balance by the amount given.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Deposit {
+    pub client: ClientId,
+    pub tx: TxId,
+    pub amount: TxAmount,
+}
+
+/// Withdraw funds from an account, i.e: the opposite of a [Deposit]. It is not allowed to withdraw
+/// more than is available on the given account, and should result in a no-op.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Withdrawal {
+    pub client: ClientId,
+    pub tx: TxId,
+    pub amount: TxAmount,
+}
+
+/// Hold funds for an erroneous transaction that should be reversed. Extract the amount of funds
+/// corresponding to the given transaction into a held funds envelop by transfering it from their
+/// available funds. If the given transaction does not exist, this results in a no-op.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Dispute {
+    pub client: ClientId,
+    pub tx: TxId,
+}
+
+/// Resolve a [Dispute] in favor of the client: move the held funds for the diputed transaction
+/// back to the available funds. If either the given transaction does not exist, or is not
+/// disputed, this results in a no-op.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Resolve {
+    pub client: ClientId,
+    pub tx: TxId,
+}
+
+/// Resolve [Dispute] by withdrawing held funds. The held funds are decreased by the amount of the
+/// transaction. An account which succesffully executed a chargeback is subsequently frozen. If
+/// either the transaction does not exist, or is not disputed, this results in a no-op and the
+/// account is *not* frozen.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Chargeback {
+    pub client: ClientId,
+    pub tx: TxId,
+}