6 files changed, 516 insertions, 0 deletions
diff --git a/src/types/account.rs b/src/types/account.rs
new file mode 100644
index 0000000..66796a6
--- /dev/null
+++ b/src/types/account.rs
@@ -0,0 +1,46 @@
+use super::ids::AccountId;
+use super::path::MaterializedPath;
+
+#[derive(PartialEq, Clone)]
+pub enum AccountType {
+    Asset,
+    Liability,
+    Equity,
+    Revenue,
+    Expense
+}
+
+#[derive(Clone)]
+pub struct Account {
+    account_id: AccountId,
+    pub account_type: AccountType,
+    pub name: String,
+    pub description: String,
+    pub path: MaterializedPath
+}
+
+impl Account {
+    pub(crate) fn from_parts(account_id: AccountId, account_type: AccountType, name: String, description: String, path: MaterializedPath) -> Self {
+        Account { account_id, account_type, name, description, path }
+    }
+    
+    pub fn id(&self) -> AccountId { self.account_id }
+}
+
+pub struct NewAccount {
+    pub account_type: AccountType,
+    pub name: String,
+    pub description: String,
+    pub parent: Option<AccountId>
+}
+
+impl NewAccount {
+    fn new(account_type: AccountType, name: impl Into<String>, description: impl Into<String>, parent: Option<AccountId>) -> NewAccount {
+        NewAccount {
+            account_type,
+            name: name.into(),
+            description: description.into(),
+            parent
+        }
+    }
+}
diff --git a/src/types/ids.rs b/src/types/ids.rs
new file mode 100644
index 0000000..03ebb59
--- /dev/null
+++ b/src/types/ids.rs
@@ -0,0 +1,43 @@
+use ulid::Ulid;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
+pub struct AccountId(Ulid);
+
+impl AccountId {
+    pub(crate) fn from_ulid(ulid: Ulid) -> Self {
+        AccountId(ulid)
+    }
+    pub fn as_ulid(&self) -> Ulid {
+        self.0
+    }
+}
+
+impl std::fmt::Display for AccountId {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
+pub struct PostingId(Ulid);
+
+impl PostingId {
+    pub(crate) fn from_ulid(ulid: Ulid) -> Self {
+        PostingId(ulid)
+    }
+    pub fn as_ulid(&self) -> Ulid {
+        self.0
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
+pub struct JournalEntryId(Ulid);
+
+impl JournalEntryId {
+    pub(crate) fn from_ulid(ulid: Ulid) -> Self {
+        JournalEntryId(ulid)
+    }
+    pub fn as_ulid(&self) -> Ulid {
+        self.0
+    }
+}
diff --git a/src/types/journal.rs b/src/types/journal.rs
new file mode 100644
index 0000000..b09c50f
--- /dev/null
+++ b/src/types/journal.rs
@@ -0,0 +1,208 @@
+use chrono::{DateTime, Utc};
+
+use super::ids::{AccountId, JournalEntryId, PostingId};
+
+/// Maximum magnitude for a single posting, in KRW.
+/// The won has no minor units, so this is a whole-won ceiling (~700 trillion KRW).
+/// A domain sanity bound: no legitimate single posting approaches it, and it keeps
+/// any realistic entry's sums far clear of overflow. Tune per currency if that changes.
+const MAX_POSTING_AMOUNT: u64 = 1_000_000_000_000_000;
+
+#[derive(Debug, thiserror::Error)]
+pub enum InvalidJournalEntry {
+    #[error("unbalanced")]
+    Unbalanced,
+    #[error("account {0} does not exist")]
+    AccountNotFound(AccountId),
+    #[error("a journal entry needs at least 2 postings, got {0}")]
+    TooFewPostings(usize),
+    #[error("posting amount must be non-zero")]
+    ZeroAmount,
+    #[error("posting amount {0} exceeds the maximum allowed")]
+    AmountTooLarge(u64),
+}
+
+#[derive(PartialEq)]
+pub enum PostingType {
+    Debit,
+    Credit
+}
+
+pub struct Posting {
+    posting_id: PostingId,
+    pub posting_type: PostingType,
+    pub account_id: AccountId,
+    pub amount: u64
+}
+
+pub struct NewPosting {
+    pub posting_type: PostingType,
+    pub account_id: AccountId,
+    pub amount: u64
+}
+
+impl NewPosting {
+    fn new(posting_type: PostingType, account_id: AccountId, amount: u64) -> NewPosting {
+        NewPosting {
+            posting_type,
+            account_id,
+            amount
+        }
+    }
+}
+
+pub struct JournalEntry {
+    journal_entry_id: JournalEntryId,
+    pub date: DateTime<Utc>,
+    pub postings: Vec<Posting>,
+    pub description: String
+}
+
+pub struct NewJournalEntry {
+    pub date: DateTime<Utc>,
+    pub postings: Vec<NewPosting>,
+    pub description: String
+}
+
+impl NewJournalEntry {
+    fn new(date: DateTime<Utc>, postings: Vec<NewPosting>, description: impl Into<String>) -> NewJournalEntry {
+        NewJournalEntry {
+            date,
+            postings,
+            description: description.into()
+        }
+    }
+    pub fn validate(&self) -> Result<(), InvalidJournalEntry> {
+        if self.postings.len() < 2 {
+            return Err(InvalidJournalEntry::TooFewPostings(self.postings.len()));
+        }
+        let mut debit: u128 = 0;
+        let mut credit: u128 = 0;
+        for posting in &self.postings {
+            if posting.amount == 0 {
+                return Err(InvalidJournalEntry::ZeroAmount);
+            }
+            if posting.amount > MAX_POSTING_AMOUNT {
+                return Err(InvalidJournalEntry::AmountTooLarge(posting.amount));
+            }
+            match posting.posting_type {
+                PostingType::Debit => debit += posting.amount as u128,
+                PostingType::Credit => credit += posting.amount as u128
+            }
+        }
+        if debit != credit {
+            return Err(InvalidJournalEntry::Unbalanced);
+        }
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use ulid::Ulid;
+
+    // validate() is pure and ignores account identity, so any dummy id works.
+    fn posting(posting_type: PostingType, amount: u64) -> NewPosting {
+        NewPosting {
+            posting_type,
+            account_id: AccountId::from_ulid(Ulid::from(1u128)),
+            amount,
+        }
+    }
+
+    fn entry(postings: Vec<NewPosting>) -> NewJournalEntry {
+        NewJournalEntry {
+            date: DateTime::from_timestamp(0, 0).unwrap(),
+            postings,
+            description: String::from("test"),
+        }
+    }
+
+    #[test]
+    fn balanced_two_postings_is_ok() {
+        let e = entry(vec![
+            posting(PostingType::Debit, 1000),
+            posting(PostingType::Credit, 1000),
+        ]);
+        assert!(e.validate().is_ok());
+    }
+
+    #[test]
+    fn balanced_multiple_postings_is_ok() {
+        // two debits summing to one credit
+        let e = entry(vec![
+            posting(PostingType::Debit, 600),
+            posting(PostingType::Debit, 400),
+            posting(PostingType::Credit, 1000),
+        ]);
+        assert!(e.validate().is_ok());
+    }
+
+    #[test]
+    fn unbalanced_is_err() {
+        let e = entry(vec![
+            posting(PostingType::Debit, 1000),
+            posting(PostingType::Credit, 999),
+        ]);
+        assert!(matches!(e.validate(), Err(InvalidJournalEntry::Unbalanced)));
+    }
+
+    #[test]
+    fn single_posting_is_too_few() {
+        let e = entry(vec![posting(PostingType::Debit, 1000)]);
+        assert!(matches!(
+            e.validate(),
+            Err(InvalidJournalEntry::TooFewPostings(1))
+        ));
+    }
+
+    #[test]
+    fn no_postings_is_too_few() {
+        let e = entry(vec![]);
+        assert!(matches!(
+            e.validate(),
+            Err(InvalidJournalEntry::TooFewPostings(0))
+        ));
+    }
+
+    #[test]
+    fn zero_amount_is_err() {
+        let e = entry(vec![
+            posting(PostingType::Debit, 0),
+            posting(PostingType::Credit, 0),
+        ]);
+        assert!(matches!(e.validate(), Err(InvalidJournalEntry::ZeroAmount)));
+    }
+
+    #[test]
+    fn amount_over_cap_is_err() {
+        let e = entry(vec![
+            posting(PostingType::Debit, MAX_POSTING_AMOUNT + 1),
+            posting(PostingType::Credit, MAX_POSTING_AMOUNT + 1),
+        ]);
+        assert!(matches!(
+            e.validate(),
+            Err(InvalidJournalEntry::AmountTooLarge(_))
+        ));
+    }
+
+    #[test]
+    fn amount_at_cap_is_ok() {
+        let e = entry(vec![
+            posting(PostingType::Debit, MAX_POSTING_AMOUNT),
+            posting(PostingType::Credit, MAX_POSTING_AMOUNT),
+        ]);
+        assert!(e.validate().is_ok());
+    }
+
+    #[test]
+    fn too_few_takes_precedence_over_zero_amount() {
+        // a single zero-amount posting: the count check fires first
+        let e = entry(vec![posting(PostingType::Debit, 0)]);
+        assert!(matches!(
+            e.validate(),
+            Err(InvalidJournalEntry::TooFewPostings(1))
+        ));
+    }
+}
diff --git a/src/types/mod.rs b/src/types/mod.rs
new file mode 100644
index 0000000..cb89841
--- /dev/null
+++ b/src/types/mod.rs
@@ -0,0 +1,11 @@
+mod account;
+mod ids;
+mod journal;
+mod path;
+mod repository;
+
+pub use account::*;
+pub use ids::*;
+pub use journal::*;
+pub use path::*;
+pub use repository::*;
diff --git a/src/types/path.rs b/src/types/path.rs
new file mode 100644
index 0000000..1d095cd
--- /dev/null
+++ b/src/types/path.rs
@@ -0,0 +1,37 @@
+use ulid::Ulid;
+
+use super::ids::AccountId;
+
+#[derive(Debug, thiserror::Error)]
+pub enum PathError {
+    #[error("path is empty")]
+    Empty,
+    #[error("invalid ULID segment: {0}")]
+    BadSegment(String),
+}
+
+#[derive(Clone)]
+pub struct MaterializedPath(String);
+
+impl MaterializedPath {
+    pub fn parse(s: &str) -> Result<Self, PathError> {
+        if s.is_empty() {
+            return Err(PathError::Empty);
+        }
+        for seg in s.split('/') {
+            Ulid::from_string(seg).map_err(|_| PathError::BadSegment(seg.to_string()))?;
+        }
+        Ok(MaterializedPath(s.to_string()))
+    }
+    pub fn as_str(&self) -> &str { &self.0 }
+    pub fn depth(&self) -> usize { self.0.split('/').count() }
+    pub fn is_ancestor_of(&self, other: &MaterializedPath) -> bool {
+        other.0.starts_with(&self.0) && other.0.as_bytes().get(self.0.len()) == Some(&b'/')
+    }
+    pub fn with_child(&self, id: AccountId) -> MaterializedPath {
+        MaterializedPath(format!("{}/{}", self.0, id.as_ulid()))
+    }
+    pub fn root(id: AccountId) -> MaterializedPath {
+        MaterializedPath(id.as_ulid().to_string())
+    }
+}
diff --git a/src/types/repository.rs b/src/types/repository.rs
new file mode 100644
index 0000000..0bef30e
--- /dev/null
+++ b/src/types/repository.rs
@@ -0,0 +1,171 @@
+use async_trait::async_trait;
+use std::collections::HashMap;
+use std::sync::Mutex;
+use ulid::Generator;
+
+use super::*;
+
+#[derive(Debug, thiserror::Error)]
+pub enum RepoError {
+    #[error("parent account {0} not found")]
+    ParentNotFound(AccountId),
+}
+
+#[async_trait]
+pub trait AccountRepository: Send + Sync {
+    async fn create(&self, new: NewAccount) -> Result<Account, RepoError>;
+    async fn get(&self, id: AccountId) -> Result<Option<Account>, RepoError>;
+}
+
+#[derive(Default)]
+pub struct InMemoryAccountRepository {
+    accounts: Mutex<HashMap<AccountId, Account>>,
+    generator: Mutex<Generator>,
+}
+
+#[async_trait]
+impl AccountRepository for InMemoryAccountRepository {
+    async fn create(&self, new: NewAccount) -> Result<Account, RepoError> {
+        let id = {
+            let mut generator = self.generator.lock().unwrap();
+            // generate() is monotonic; on the rare same-tick random overflow,
+            // recover into a valid (incremented) Ulid rather than failing.
+            let ulid = generator
+                .generate()
+                .unwrap_or_else(|overflow| overflow.commit_overflow_increment());
+            AccountId::from_ulid(ulid)
+        };
+
+        let mut accounts = self.accounts.lock().unwrap();
+
+        let path = match new.parent {
+            Some(parent_id) => accounts
+                .get(&parent_id)
+                .ok_or(RepoError::ParentNotFound(parent_id))?
+                .path
+                .with_child(id),
+            None => MaterializedPath::root(id),
+        };
+
+        let account =
+            Account::from_parts(id, new.account_type, new.name, new.description, path);
+        accounts.insert(id, account.clone());
+        Ok(account)
+    }
+
+    async fn get(&self, id: AccountId) -> Result<Option<Account>, RepoError> {
+        Ok(self.accounts.lock().unwrap().get(&id).cloned())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn new_account(
+        account_type: AccountType,
+        name: &str,
+        parent: Option<AccountId>,
+    ) -> NewAccount {
+        NewAccount {
+            account_type,
+            name: name.into(),
+            description: String::new(),
+            parent,
+        }
+    }
+
+    // Seed the five standard chart-of-accounts roots, each with no parent.
+    async fn seed_roots(repo: &InMemoryAccountRepository) -> Vec<Account> {
+        let roots = [
+            (AccountType::Asset, "Assets"),
+            (AccountType::Liability, "Liabilities"),
+            (AccountType::Equity, "Equity"),
+            (AccountType::Revenue, "Revenue"),
+            (AccountType::Expense, "Expenses"),
+        ];
+        let mut created = Vec::new();
+        for (account_type, name) in roots {
+            let account = repo
+                .create(new_account(account_type, name, None))
+                .await
+                .unwrap();
+            created.push(account);
+        }
+        created
+    }
+
+    #[tokio::test]
+    async fn seeds_five_distinct_root_accounts() {
+        let repo = InMemoryAccountRepository::default();
+        let roots = seed_roots(&repo).await;
+
+        assert_eq!(roots.len(), 5);
+
+        // each root's path is just its own id — depth 1, no parent segment
+        for root in &roots {
+            assert_eq!(root.path.depth(), 1);
+        }
+
+        // all five ids are distinct
+        let mut ids: Vec<AccountId> = roots.iter().map(|a| a.id()).collect();
+        ids.sort();
+        ids.dedup();
+        assert_eq!(ids.len(), 5);
+    }
+
+    #[tokio::test]
+    async fn child_nests_under_its_parent() {
+        let repo = InMemoryAccountRepository::default();
+        let roots = seed_roots(&repo).await;
+        let assets = &roots[0];
+
+        let cash = repo
+            .create(new_account(AccountType::Asset, "Cash", Some(assets.id())))
+            .await
+            .unwrap();
+
+        // the child's path was built from the parent's
+        assert!(assets.path.is_ancestor_of(&cash.path));
+        assert_eq!(cash.path.depth(), assets.path.depth() + 1);
+
+        // and a grandchild nests under both
+        let checking = repo
+            .create(new_account(AccountType::Asset, "Checking", Some(cash.id())))
+            .await
+            .unwrap();
+
+        assert!(assets.path.is_ancestor_of(&checking.path));
+        assert!(cash.path.is_ancestor_of(&checking.path));
+        assert_eq!(checking.path.depth(), 3);
+    }
+
+    #[tokio::test]
+    async fn get_returns_a_created_account() {
+        let repo = InMemoryAccountRepository::default();
+        let roots = seed_roots(&repo).await;
+        let assets = &roots[0];
+
+        let fetched = repo.get(assets.id()).await.unwrap();
+        assert_eq!(fetched.map(|a| a.id()), Some(assets.id()));
+    }
+
+    #[tokio::test]
+    async fn create_under_missing_parent_fails() {
+        let repo = InMemoryAccountRepository::default();
+        seed_roots(&repo).await;
+
+        // an id minted by a *different* repo is guaranteed absent from `repo`
+        let other = InMemoryAccountRepository::default();
+        let stranger = other
+            .create(new_account(AccountType::Asset, "Elsewhere", None))
+            .await
+            .unwrap();
+
+        let result = repo
+            .create(new_account(AccountType::Asset, "Orphan", Some(stranger.id())))
+            .await;
+
+        assert!(matches!(result, Err(RepoError::ParentNotFound(_))));
+    }
+}