ledger: introduce 'Ledger' type

Unfortunately, rust-csv does not support serializing maps [1], so
instead of trying to write a `Deserialize` impl, the `dump_csv` method
takes care of formatting the output as expected.

[1]: https://github.com/BurntSushi/rust-csv/issues/98

src/ledger.rs

@@ -0,0 +1,62 @@
+//! A ledger implementation to track all transactions.
+
+use crate::{ClientId, TxAmount, TxId};
+
+/// A ledger of accounts, which processes transactions one at a time.
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
+pub struct Ledger {
+    accounts: std::collections::HashMap<ClientId, AccountInfo>,
+    reversible_transactions: std::collections::HashMap<TxId, (ClientId, TxAmount)>,
+}
+
+#[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord)]
+pub struct AccountInfo {
+    available_funds: TxAmount,
+    held_funds: TxAmount,
+    locked: bool,
+}
+
+impl Ledger {
+    pub fn new() -> Self {
+        Default::default()
+    }
+
+    /// Serialize a [Ledger] to CSV.
+    pub fn dump_csv<W: std::io::Write>(&self, writer: &mut csv::Writer<W>) -> csv::Result<()> {
+        // Keep list of accounts ordered for easier diffs
+        let ordered_accounts: std::collections::BTreeMap<_, _> = self.accounts.iter().collect();
+        writer.write_record(&["client", "available", "held", "total", "locked"])?;
+        for (id, info) in ordered_accounts.into_iter() {
+            writer.write_record(&[
+                id.0.to_string(),
+                info.available_funds().0.to_string(),
+                info.held_funds().0.to_string(),
+                info.total_funds().0.to_string(),
+                info.is_locked().to_string(),
+            ])?
+        }
+        Ok(())
+    }
+}
+
+impl AccountInfo {
+    /// Whether or not an account has been locked.
+    pub fn is_locked(&self) -> bool {
+        self.locked
+    }
+
+    /// The funds that are usable on this account.
+    pub fn available_funds(&self) -> TxAmount {
+        self.available_funds
+    }
+
+    /// The funds that have been locked pending resolution of dispute.
+    pub fn held_funds(&self) -> TxAmount {
+        self.held_funds
+    }
+
+    /// The totals funds on an account, i.e: available funds and held funds.
+    pub fn total_funds(&self) -> TxAmount {
+        self.available_funds + self.held_funds
+    }
+}

src/lib.rs

@@ -4,5 +4,8 @@ pub use crate::core::*;
 pub mod error;
 pub use crate::error::*;
 
+pub mod ledger;
+pub use crate::ledger::*;
+
 pub mod transaction;
 pub use crate::transaction::*;