13 files changed, 985 insertions, 0 deletions
diff --git a/src/bill.rs b/src/bill.rs
new file mode 100644
index 0000000..16923f9
--- /dev/null
+++ b/src/bill.rs
@@ -0,0 +1,167 @@
+use std::collections::BTreeMap;
+
+use uuid::Uuid;
+
+use crate::who::Who;
+
+#[derive(Default)]
+pub struct Bills {
+    pub items: BTreeMap<Uuid, BillItem>,
+}
+
+pub struct BillItem {
+    pub who_paid: Who,
+    pub reason: String,
+    pub paid: f64,
+    pub split: Vec<Who>,
+}
+
+pub struct SplitResult {
+    pub items: BTreeMap<Who, Vec<SplitResultItem>>,
+    pub final_result: BTreeMap<(Who, Who), f64>,
+}
+
+pub struct SplitResultItem {
+    pub payee: Who,
+    pub bill: f64,
+    pub reason: String,
+}
+
+impl Bills {
+    /// Add a new bill item
+    pub fn add_bill(&mut self, who_paid: &str, reason: &str, paid: f64, split: Vec<&str>) -> Uuid {
+        let item = BillItem {
+            who_paid: who_paid.into(),
+            reason: reason.to_string(),
+            paid,
+            split: split.into_iter().map(|s| s.into()).collect(),
+        };
+        self.add_item(item)
+    }
+
+    /// Add a new bill item
+    pub fn add_item(&mut self, item: BillItem) -> Uuid {
+        let id = Uuid::new_v4();
+        self.items.insert(id, item);
+        id
+    }
+
+    /// Get a bill item by ID (immutable reference)
+    pub fn get_item(&self, id: &Uuid) -> Option<&BillItem> {
+        self.items.get(id)
+    }
+
+    /// Get a bill item by ID (mutable reference)
+    pub fn get_item_mut(&mut self, id: &Uuid) -> Option<&mut BillItem> {
+        self.items.get_mut(id)
+    }
+
+    /// Update the bill item with the specified ID
+    pub fn update_item(&mut self, id: &Uuid, item: BillItem) -> bool {
+        if self.items.contains_key(id) {
+            self.items.insert(*id, item);
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Delete the bill item with the specified ID
+    pub fn delete_item(&mut self, id: &Uuid) -> Option<BillItem> {
+        self.items.remove(id)
+    }
+
+    /// Get all bill items
+    pub fn get_all_items(&self) -> &BTreeMap<Uuid, BillItem> {
+        &self.items
+    }
+
+    /// Check if a bill item with the specified ID exists
+    pub fn contains_item(&self, id: &Uuid) -> bool {
+        self.items.contains_key(id)
+    }
+
+    /// Clear all bill items
+    pub fn clear_items(&mut self) {
+        self.items.clear();
+    }
+}
+
+impl SplitResult {
+    /// Add a bill (who pays whom, amount, reason)
+    pub fn add_bill(&mut self, payer: Who, payee: Who, amount: f64, reason: String) {
+        let result_item = SplitResultItem {
+            payee,
+            bill: amount,
+            reason,
+        };
+
+        self.items
+            .entry(payer)
+            .or_insert_with(Vec::new)
+            .push(result_item);
+    }
+
+    /// Get all bill items for a specified payer (immutable reference)
+    pub fn get_bills(&self, payer: &Who) -> Option<&Vec<SplitResultItem>> {
+        self.items.get(payer)
+    }
+
+    /// Get all bill items for a specified payer (mutable reference)
+    pub fn get_bills_mut(&mut self, payer: &Who) -> Option<&mut Vec<SplitResultItem>> {
+        self.items.get_mut(payer)
+    }
+
+    /// Update the bill list for a specified payer
+    pub fn update_bills(
+        &mut self,
+        payer: Who,
+        bills: Vec<SplitResultItem>,
+    ) -> Option<Vec<SplitResultItem>> {
+        self.items.insert(payer, bills)
+    }
+
+    /// Delete all bill items for a specified payer
+    pub fn delete_bills(&mut self, payer: &Who) -> Option<Vec<SplitResultItem>> {
+        self.items.remove(payer)
+    }
+
+    /// Get all bill items for all payers
+    pub fn get_all_bills(&self) -> &BTreeMap<Who, Vec<SplitResultItem>> {
+        &self.items
+    }
+
+    /// Check if bill items exist for a specified payer
+    pub fn contains_payer(&self, payer: &Who) -> bool {
+        self.items.contains_key(payer)
+    }
+
+    /// Clear all bill items
+    pub fn clear_bills(&mut self) {
+        self.items.clear();
+    }
+    /// Set the simplified result
+    pub fn set_final_result(&mut self, result: BTreeMap<(Who, Who), f64>) {
+        self.final_result = result;
+    }
+
+    /// Get the simplified result (immutable reference)
+    pub fn get_final_result(&self) -> &BTreeMap<(Who, Who), f64> {
+        &self.final_result
+    }
+
+    /// Get the simplified result (mutable reference)
+    pub fn get_final_result_mut(&mut self) -> &mut BTreeMap<(Who, Who), f64> {
+        &mut self.final_result
+    }
+
+    /// Clear the simplified result
+    pub fn clear_final_result(&mut self) {
+        self.final_result.clear();
+    }
+
+    /// Get a specific item from the simplified result (who pays whom, returns Option<f64>)
+    pub fn get_final_result_item(&self, payer: Who, payee: Who) -> Option<f64> {
+        self.final_result.get(&(payer, payee)).copied()
+    }
+}
diff --git a/src/calc.rs b/src/calc.rs
new file mode 100644
index 0000000..fcd0f5b
--- /dev/null
+++ b/src/calc.rs
@@ -0,0 +1,163 @@
+use std::collections::BTreeMap;
+
+use crate::{
+    bill::{Bills, SplitResult, SplitResultItem},
+    error::BillSplitError,
+    who::Who,
+};
+
+pub fn calculate_from(item: Bills) -> Result<SplitResult, BillSplitError> {
+    // Validate input data
+    precheck(&item)?;
+
+    // Calculate each person's net balance and original transactions
+    let (direct_transactions, items) = calculate_balances_and_transactions(&item);
+
+    // Generate the simplest result: net settlement between each pair
+    let final_result = calculate_net_settlements(&direct_transactions);
+
+    // Add "Total" reason to final_result
+    let mut items = items;
+    add_total_reason(&mut items);
+
+    Ok(SplitResult {
+        items,
+        final_result,
+    })
+}
+
+fn precheck(item: &Bills) -> Result<(), BillSplitError> {
+    for (_, bill_item) in &item.items {
+        // Check if the paid amount is negative
+        if bill_item.paid < 0.0 {
+            return Err(BillSplitError::NegativePaidAmount);
+        }
+
+        // Check for duplicate members in the split list
+        let mut seen = std::collections::HashSet::new();
+        for person in &bill_item.split {
+            if !seen.insert(person) {
+                return Err(BillSplitError::DuplicateSplitMembers);
+            }
+        }
+    }
+    Ok(())
+}
+
+fn calculate_balances_and_transactions(
+    item: &Bills,
+) -> (
+    BTreeMap<(Who, Who), f64>,
+    BTreeMap<Who, Vec<SplitResultItem>>,
+) {
+    let mut direct_transactions: BTreeMap<(Who, Who), f64> = BTreeMap::new();
+    let mut items: BTreeMap<Who, Vec<SplitResultItem>> = BTreeMap::new();
+
+    for (_, bill_item) in &item.items {
+        let who_paid = &bill_item.who_paid;
+        let paid = bill_item.paid;
+        let split_count = bill_item.split.len() as f64;
+
+        if split_count == 0.0 {
+            continue;
+        }
+
+        // Round
+        let share = (paid / split_count * 100.0).round() / 100.0;
+
+        // Calculate the amount each person should pay
+        for person in &bill_item.split {
+            // If the payer is also in the split list, deduct their own share
+            if person != who_paid {
+                // Record direct transaction
+                let key = (person.clone(), who_paid.clone());
+                *direct_transactions.entry(key).or_insert(0.0) += share;
+
+                // Add to full record
+                let bill_result_item = SplitResultItem {
+                    payee: who_paid.clone(),
+                    bill: share,
+                    reason: bill_item.reason.clone(),
+                };
+
+                items
+                    .entry(person.clone())
+                    .or_insert_with(Vec::new)
+                    .push(bill_result_item);
+            }
+        }
+    }
+
+    (direct_transactions, items)
+}
+
+fn calculate_net_settlements(
+    direct_transactions: &BTreeMap<(Who, Who), f64>,
+) -> BTreeMap<(Who, Who), f64> {
+    let mut final_result: BTreeMap<(Who, Who), f64> = BTreeMap::new();
+
+    // First, calculate net amounts for each transaction pair
+    let mut net_transactions: BTreeMap<(Who, Who), f64> = BTreeMap::new();
+    for ((from, to), amount) in direct_transactions {
+        let key = (from.clone(), to.clone());
+        *net_transactions.entry(key).or_insert(0.0) += amount;
+    }
+
+    // Now process net transactions, ensuring correct direction
+    let mut processed_pairs = std::collections::HashSet::new();
+
+    for ((from, to), amount) in &net_transactions {
+        // Create a normalized transaction pair key (sorted alphabetically)
+        let pair_key = if from < to {
+            (from.clone(), to.clone())
+        } else {
+            (to.clone(), from.clone())
+        };
+
+        // If this pair has already been processed, skip it
+        if processed_pairs.contains(&pair_key) {
+            continue;
+        }
+        processed_pairs.insert(pair_key.clone());
+
+        // Check for reverse transaction
+        let reverse_key = (to.clone(), from.clone());
+        if let Some(reverse_amount) = net_transactions.get(&reverse_key) {
+            // There is a reverse transaction, calculate net amount
+            let net_amount = *amount - *reverse_amount;
+
+            if net_amount > 0.0001 {
+                // from owes to (net)
+                final_result.insert(
+                    (from.clone(), to.clone()),
+                    (net_amount * 100.0).round() / 100.0,
+                );
+            } else if net_amount < -0.0001 {
+                // to owes from (net)
+                final_result.insert(
+                    (to.clone(), from.clone()),
+                    (-net_amount * 100.0).round() / 100.0,
+                );
+            }
+            // If net amount is close to 0, don't add any transaction
+        } else {
+            // No reverse transaction, add directly
+            if *amount > 0.0001 {
+                final_result.insert(
+                    (from.clone(), to.clone()),
+                    (*amount * 100.0).round() / 100.0,
+                );
+            }
+        }
+    }
+
+    final_result
+}
+
+fn add_total_reason(items: &mut BTreeMap<Who, Vec<SplitResultItem>>) {
+    for (_payer, bills_list) in items.iter_mut() {
+        for bill in bills_list {
+            bill.reason = format!("{} (Total)", bill.reason);
+        }
+    }
+}
diff --git a/src/cli.rs b/src/cli.rs
new file mode 100644
index 0000000..c1d1240
--- /dev/null
+++ b/src/cli.rs
@@ -0,0 +1,6 @@
+pub mod calc_cmd;
+pub mod consts;
+pub mod dispatchers;
+pub mod entry;
+pub mod io_error;
+pub mod ops_cmd;
diff --git a/src/cli/calc_cmd.rs b/src/cli/calc_cmd.rs
new file mode 100644
index 0000000..09f4e03
--- /dev/null
+++ b/src/cli/calc_cmd.rs
@@ -0,0 +1,9 @@
+// use mingling::macros::dispatcher;
+// use mingling::{macros::chain, marker::NextProcess};
+
+// use crate::cli::entry::*;
+
+// dispatcher!("calc", CalculateCommand => CalculateEntry);
+
+// #[chain]
+// pub async fn parse_calc_entry(prev: CalculateEntry) -> NextProcess {}
diff --git a/src/cli/consts.rs b/src/cli/consts.rs
new file mode 100644
index 0000000..254b611
--- /dev/null
+++ b/src/cli/consts.rs
@@ -0,0 +1 @@
+pub const BILL_WORKSPACE_CONFIG_FILE: &str = "cobill.yml";
diff --git a/src/cli/dispatchers.rs b/src/cli/dispatchers.rs
new file mode 100644
index 0000000..27b7747
--- /dev/null
+++ b/src/cli/dispatchers.rs
@@ -0,0 +1,12 @@
+use mingling::{Program, macros::program_setup};
+
+use crate::ThisProgram;
+use crate::cli::ops_cmd::{CreateCommand, InitHereCommand};
+
+#[program_setup]
+pub fn chaos_billing_setup(program: &mut Program<ThisProgram, ThisProgram>) {
+    program.with_dispatcher(InitHereCommand);
+    program.with_dispatcher(CreateCommand);
+
+    // program.with_dispatcher(CalculateCommand);
+}
diff --git a/src/cli/entry.rs b/src/cli/entry.rs
new file mode 100644
index 0000000..e68b7b4
--- /dev/null
+++ b/src/cli/entry.rs
@@ -0,0 +1,21 @@
+use mingling::setup::GeneralRendererSetup;
+
+use crate::__completion_gen::CompletionDispatcher;
+use crate::ThisProgram;
+use crate::cli::dispatchers::*;
+
+pub async fn entry() {
+    let mut program = ThisProgram::new();
+
+    // Add Completion
+    program.with_dispatcher(CompletionDispatcher);
+
+    // Add General Renderer
+    program.with_setup(GeneralRendererSetup);
+
+    // Setup `cobill`
+    program.with_setup(ChaosBillingSetup);
+
+    // Execute
+    program.exec().await;
+}
diff --git a/src/cli/io_error.rs b/src/cli/io_error.rs
new file mode 100644
index 0000000..49b9939
--- /dev/null
+++ b/src/cli/io_error.rs
@@ -0,0 +1,42 @@
+use mingling::{
+    Groupped,
+    macros::{r_println, renderer},
+};
+use serde::Serialize;
+
+use crate::ThisProgram;
+
+#[derive(Groupped)]
+pub struct IOError {
+    inner: std::io::Error,
+}
+
+impl IOError {
+    pub fn new(error: std::io::Error) -> Self {
+        Self { inner: error }
+    }
+}
+
+impl From<std::io::Error> for IOError {
+    fn from(error: std::io::Error) -> Self {
+        Self::new(error)
+    }
+}
+
+impl Serialize for IOError {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        use serde::ser::SerializeStruct;
+        let mut state = serializer.serialize_struct("IOError", 2)?;
+        state.serialize_field("kind", &self.inner.kind().to_string())?;
+        state.serialize_field("info", &self.inner.to_string())?;
+        state.end()
+    }
+}
+
+#[renderer]
+pub fn render_io_error(prev: IOError) {
+    r_println!("{}: {}", prev.inner.kind(), prev.inner.to_string())
+}
diff --git a/src/cli/ops_cmd.rs b/src/cli/ops_cmd.rs
new file mode 100644
index 0000000..4b0eea7
--- /dev/null
+++ b/src/cli/ops_cmd.rs
@@ -0,0 +1,80 @@
+use std::{
+    env::current_dir,
+    fs::{self, create_dir_all},
+    path::PathBuf,
+};
+
+use mingling::{
+    AnyOutput,
+    macros::{chain, dispatcher, pack, r_println, renderer},
+    marker::NextProcess,
+    parser::Picker,
+};
+
+use crate::{
+    ThisProgram,
+    cli::{consts::BILL_WORKSPACE_CONFIG_FILE, io_error::IOError},
+};
+
+dispatcher!("init", InitHereCommand => InitEntry);
+dispatcher!("create", CreateCommand => CreateEntry);
+
+pack!(StateCreateWorkspace = PathBuf);
+
+#[chain]
+pub async fn handle_init_command(_prev: InitEntry) -> NextProcess {
+    let current_dir = match current_dir() {
+        Ok(d) => d,
+        Err(e) => return AnyOutput::new(IOError::from(e)).route_renderer(),
+    };
+    StateCreateWorkspace::new(current_dir).to_chain()
+}
+
+#[chain]
+pub async fn handle_create_command(prev: CreateEntry) -> NextProcess {
+    let path = pick_path(prev.inner);
+    StateCreateWorkspace::new(path).to_chain()
+}
+
+#[chain]
+pub async fn handle_state_create_workspace(prev: StateCreateWorkspace) -> NextProcess {
+    let dir = prev.inner;
+    let file = dir.join(BILL_WORKSPACE_CONFIG_FILE);
+
+    match create_dir_all(&dir) {
+        Ok(d) => d,
+        Err(e) => return AnyOutput::new(IOError::from(e)).route_renderer(),
+    };
+
+    if file.exists() {
+        return AnyOutput::new(WorkspaceConfigAlreadyExists::new(dir)).route_renderer();
+    }
+
+    if let Err(e) = fs::write(file, "") {
+        return AnyOutput::new(IOError::from(e)).route_renderer();
+    }
+
+    StateWorkspaceCreated::new(dir).to_render()
+}
+
+pack!(StateWorkspaceCreated = PathBuf);
+
+#[renderer]
+pub fn render_workspace_created(prev: StateWorkspaceCreated) {
+    r_println!("Workspace created at: {:?}", prev.inner);
+}
+
+pack!(WorkspaceConfigAlreadyExists = PathBuf);
+
+#[renderer]
+pub fn render_workspace_config_already_exists(prev: WorkspaceConfigAlreadyExists) {
+    r_println!("Workspace config already exists: {:?}", prev.inner);
+}
+
+fn pick_path(args: Vec<String>) -> PathBuf {
+    let path = Picker::<()>::new(args)
+        .pick::<String>(())
+        .unpack_directly()
+        .0;
+    PathBuf::from(path)
+}
diff --git a/src/error.rs b/src/error.rs
new file mode 100644
index 0000000..a9c23b1
--- /dev/null
+++ b/src/error.rs
@@ -0,0 +1,8 @@
+#[derive(thiserror::Error, Debug)]
+pub enum BillSplitError {
+    #[error("Paid amount cannot be negative")]
+    NegativePaidAmount,
+
+    #[error("Duplicate split members found")]
+    DuplicateSplitMembers,
+}
diff --git a/src/main.rs b/src/main.rs
new file mode 100644
index 0000000..a97d952
--- /dev/null
+++ b/src/main.rs
@@ -0,0 +1,23 @@
+use mingling::macros::gen_program;
+
+mod bill;
+mod calc;
+mod cli;
+mod error;
+mod who;
+
+#[cfg(test)]
+mod test;
+
+#[tokio::main]
+async fn main() {
+    cli::entry::entry().await
+}
+
+use crate::cli::calc_cmd::*;
+use crate::cli::dispatchers::*;
+use crate::cli::entry::*;
+use crate::cli::io_error::*;
+use crate::cli::ops_cmd::*;
+
+gen_program!();
diff --git a/src/test.rs b/src/test.rs
new file mode 100644
index 0000000..15a7002
--- /dev/null
+++ b/src/test.rs
@@ -0,0 +1,409 @@
+use crate::{bill::Bills, calc::calculate_from};
+
+#[test]
+fn test_no_zero_amount_transactions() {
+    let mut bills = Bills::default();
+
+    // Create some bills where some transaction amounts might be 0
+    // A pays 30, split among A, B, C (10 each)
+    bills.add_bill("A", "Lunch", 30., vec!["A", "B", "C"]);
+    // B pays 30, split among A, B (15 each)
+    bills.add_bill("B", "Coffee", 30., vec!["A", "B"]);
+
+    let result = calculate_from(bills);
+    assert!(result.is_ok(), "calculate should be success");
+    let result = result.unwrap();
+
+    // Check if there are any transactions with amount 0 in the complete record (items)
+    let all_items = result.get_all_bills();
+    for (payer, bills_list) in all_items {
+        for bill in bills_list {
+            assert_ne!(
+                bill.bill, 0.0,
+                "There should be no transactions with amount 0 in the complete record: {} to {} amount is 0",
+                payer, bill.payee
+            );
+        }
+    }
+
+    // Check if there are any transactions with amount 0 in the simplified result (final_result)
+    let final_result = result.get_final_result();
+    for ((payer, payee), amount) in final_result {
+        assert_ne!(
+            *amount, 0.0,
+            "There should be no transactions with amount 0 in the simplified result: {} to {} amount is 0",
+            payer, payee
+        );
+    }
+
+    // Verify transaction count
+    // Should have: C->A (10), A->B (5) after netting
+    assert_eq!(
+        final_result.len(),
+        2,
+        "There should be 2 non-zero transactions"
+    );
+    assert!(
+        final_result.contains_key(&("C".into(), "A".into())),
+        "Should contain transaction C->A"
+    );
+    assert!(
+        final_result.contains_key(&("A".into(), "B".into())),
+        "Should contain transaction A->B"
+    );
+
+    // Verify specific amounts
+    let c_to_a = final_result.get(&("C".into(), "A".into())).unwrap();
+    assert_eq!(*c_to_a, 10.0, "C should pay A 10");
+
+    let a_to_b = final_result.get(&("A".into(), "B".into())).unwrap();
+    assert_eq!(*a_to_b, 5.0, "A should pay B 5");
+}
+
+#[test]
+fn test_zero_amount_edge_cases() {
+    let mut bills = Bills::default();
+
+    // Test perfectly balanced case: A and B prepay the same amount for each other
+    // A prepays 20 for A, B (10 each)
+    bills.add_bill("A", "Dinner", 20., vec!["A", "B"]);
+    // B prepays 20 for A, B (10 each)
+    bills.add_bill("B", "Movie", 20., vec!["A", "B"]);
+
+    let result = calculate_from(bills);
+    assert!(result.is_ok(), "calculate should be success");
+    let result = result.unwrap();
+
+    // Check complete record
+    let all_items = result.get_all_bills();
+    for (payer, bills_list) in all_items {
+        for bill in bills_list {
+            assert_ne!(
+                bill.bill, 0.0,
+                "There should be no transactions with amount 0 in the complete record: {} to {} amount is 0",
+                payer, bill.payee
+            );
+        }
+    }
+
+    // Check simplified result - should be empty because all transactions cancel out
+    let final_result = result.get_final_result();
+    assert_eq!(
+        final_result.len(),
+        0,
+        "The simplified result should be empty because all transaction amounts are 0"
+    );
+
+    // Verify no transactions are included
+    assert!(
+        !final_result.contains_key(&("A".into(), "B".into())),
+        "Should not contain A->B zero amount transaction"
+    );
+    assert!(
+        !final_result.contains_key(&("B".into(), "A".into())),
+        "Should not contain B->A zero amount transaction"
+    );
+}
+
+#[test]
+fn test_items_count() {
+    let mut bills = Bills::default();
+
+    // Add 3 bill items
+    let id1 = bills.add_bill("A", "Lunch", 30., vec!["A", "B"]);
+    let id2 = bills.add_bill("B", "Coffee", 20., vec!["B", "C"]);
+    let id3 = bills.add_bill("C", "Snack", 15., vec!["A", "C"]);
+
+    // Verify items count
+    assert_eq!(bills.get_all_items().len(), 3, "Should have 3 items");
+
+    // Verify each ID exists
+    assert!(bills.contains_item(&id1), "Item 1 should exist");
+    assert!(bills.contains_item(&id2), "Item 2 should exist");
+    assert!(bills.contains_item(&id3), "Item 3 should exist");
+
+    // Verify count after deleting one item
+    let removed = bills.delete_item(&id2);
+    assert!(removed.is_some(), "Should remove item 2");
+    assert_eq!(
+        bills.get_all_items().len(),
+        2,
+        "Should have 2 items after removal"
+    );
+
+    // Verify deleted item no longer exists
+    assert!(
+        !bills.contains_item(&id2),
+        "Item 2 should not exist after removal"
+    );
+
+    // Verify count after clearing
+    bills.clear_items();
+    assert_eq!(
+        bills.get_all_items().len(),
+        0,
+        "Should have 0 items after clear"
+    );
+}
+
+#[test]
+fn test_result() {
+    let mut bills = Bills::default();
+
+    // Define data
+    bills.add_bill("A", "BBQ", 90., vec!["A", "B", "C"]);
+    bills.add_bill("B", "Water", 21., vec!["A", "B", "C"]);
+
+    // Calculate
+    let result = calculate_from(bills);
+
+    // Check result
+    assert!(result.is_ok(), "calculate should be success");
+
+    let result = result.unwrap();
+
+    // Verify split results
+    let c_to_a = result
+        .get_final_result_item("C".into(), "A".into())
+        .expect("Item C to A should be exist");
+    assert_eq!(c_to_a, 30.0, "C should pay A 30 for BBQ");
+
+    let c_to_b = result
+        .get_final_result_item("C".into(), "B".into())
+        .expect("Item C to B should be exist");
+    assert_eq!(c_to_b, 7.0, "C should pay B 7 for Water");
+
+    let b_to_a = result
+        .get_final_result_item("B".into(), "A".into())
+        .expect("Item B to A should be exist");
+    assert_eq!(b_to_a, 23.0, "B should pay A 23 (30 - 7)");
+
+    // Verify count
+    let final_result = result.get_final_result();
+    assert_eq!(final_result.len(), 3, "Should have exactly 3 transactions");
+}
+
+#[test]
+fn test_complex_bills() {
+    let mut bills = Bills::default();
+
+    // A prepays 50 for B and C, B and C should each pay A 25
+    bills.add_bill("A", "Dinner", 50., vec!["B", "C"]);
+
+    let result = calculate_from(bills);
+    assert!(result.is_ok(), "calculate should be success");
+    let result = result.unwrap();
+
+    let b_to_a = result
+        .get_final_result_item("B".into(), "A".into())
+        .expect("Item B to A should be exist");
+    assert_eq!(b_to_a, 25.0, "B should pay A 25");
+
+    let c_to_a = result
+        .get_final_result_item("C".into(), "A".into())
+        .expect("Item C to A should be exist");
+    assert_eq!(c_to_a, 25.0, "C should pay A 25");
+
+    let final_result = result.get_final_result();
+    assert_eq!(final_result.len(), 2, "Should have exactly 2 transactions");
+}
+
+#[test]
+fn test_unrelated_bills() {
+    let mut bills = Bills::default();
+
+    // A prepays 30 split among A, B, C, each should pay A 10
+    // B prepays 30 split among A, B, each should pay B 15
+    // Final result: C only has transaction with A, not with B
+    bills.add_bill("A", "Lunch", 30., vec!["A", "B", "C"]);
+    bills.add_bill("B", "Coffee", 30., vec!["A", "B"]);
+
+    let result = calculate_from(bills);
+    assert!(result.is_ok(), "calculate should be success");
+    let result = result.unwrap();
+
+    // Verify C only has transaction with A, not with B
+    let c_to_a = result.get_final_result_item("C".into(), "A".into());
+    assert!(c_to_a.is_some(), "C should pay A");
+    assert_eq!(c_to_a.unwrap(), 10.0, "C should pay A 10");
+
+    let c_to_b = result.get_final_result_item("C".into(), "B".into());
+    assert!(c_to_b.is_none(), "C should not have any transaction with B");
+
+    // Verify transaction between A and B
+    let a_to_b = result.get_final_result_item("A".into(), "B".into());
+    assert!(a_to_b.is_some(), "A should pay B");
+    assert_eq!(a_to_b.unwrap(), 5.0, "A should pay B 5 (15 - 10)");
+
+    // Verify total transaction count
+    let final_result = result.get_final_result();
+    assert_eq!(final_result.len(), 2, "Should have exactly 2 transactions");
+}
+
+#[test]
+fn test_duplicate_split_members() {
+    let mut bills = Bills::default();
+
+    // Duplicate members in split list, should return Error
+    bills.add_bill("Alice", "Lunch", 60., vec!["Bob", "Bob", "Charlie"]);
+
+    let result = calculate_from(bills);
+    assert!(
+        result.is_err(),
+        "Should return error for duplicate split members"
+    );
+}
+
+#[test]
+fn test_negative_paid_amount() {
+    let mut bills = Bills::default();
+
+    // Negative prepaid amount, should return Error
+    bills.add_bill("Alice", "Refund?", -30., vec!["Alice", "Bob"]);
+
+    let result = calculate_from(bills);
+    assert!(
+        result.is_err(),
+        "Should return error for negative paid amount"
+    );
+}
+
+#[test]
+fn test_rounding() {
+    let mut bills = Bills::default();
+
+    // Test rounding: 51.0333333333 => 51.00, 51.599999999999 => 52.00
+    // 100 / 3 = 33.333..., each should pay 33.33, payer gets back 66.67
+    bills.add_bill("Alice", "Concert", 100., vec!["Alice", "Bob", "Charlie"]);
+
+    let result = calculate_from(bills);
+    assert!(result.is_ok(), "calculate should be success");
+    let result = result.unwrap();
+
+    let bob_to_alice = result
+        .get_final_result_item("Bob".into(), "Alice".into())
+        .expect("Item Bob to Alice should be exist");
+    // 33.333... rounded to 2 decimal places => 33.33
+    assert_eq!(bob_to_alice, 33.33, "Bob should pay Alice 33.33");
+
+    let charlie_to_alice = result
+        .get_final_result_item("Charlie".into(), "Alice".into())
+        .expect("Item Charlie to Alice should be exist");
+    assert_eq!(charlie_to_alice, 33.33, "Charlie should pay Alice 33.33");
+
+    // Another test: 51.599999999999 => 52.00
+    let mut bills2 = Bills::default();
+    bills2.add_bill("Bob", "Dinner", 51.6, vec!["Alice", "Bob"]); // 51.6 / 2 = 25.8
+
+    let result2 = calculate_from(bills2);
+    assert!(result2.is_ok(), "calculate should be success");
+    let result2 = result2.unwrap();
+
+    let alice_to_bob = result2
+        .get_final_result_item("Alice".into(), "Bob".into())
+        .expect("Item Alice to Bob should be exist");
+    // 25.8 rounded to 2 decimal places => 25.80
+    assert_eq!(alice_to_bob, 25.8, "Alice should pay Bob 25.8");
+}
+
+#[test]
+fn test_empty_bills() {
+    let bills = Bills::default();
+    let result = calculate_from(bills);
+    assert!(
+        result.is_ok(),
+        "calculate should be success for empty bills"
+    );
+    let result = result.unwrap();
+
+    assert_eq!(
+        result.get_all_bills().len(),
+        0,
+        "Items should be empty for empty bills"
+    );
+    assert_eq!(
+        result.get_final_result().len(),
+        0,
+        "Final result should be empty for empty bills"
+    );
+}
+
+#[test]
+fn test_single_person_bill() {
+    let mut bills = Bills::default();
+
+    // Single person bill: paying for oneself
+    bills.add_bill("Alice", "Personal", 50., vec!["Alice"]);
+
+    let result = calculate_from(bills);
+    assert!(
+        result.is_ok(),
+        "calculate should be success for single person bill"
+    );
+    let result = result.unwrap();
+
+    // Single person bill should not generate any transactions
+    assert_eq!(
+        result.get_final_result().len(),
+        0,
+        "Should have no transactions for single person bill"
+    );
+}
+
+#[test]
+fn test_split_not_include_payer() {
+    let mut bills = Bills::default();
+
+    // Payer not included in split list
+    bills.add_bill("Alice", "Gift", 100., vec!["Bob", "Charlie"]);
+
+    let result = calculate_from(bills);
+    assert!(
+        result.is_ok(),
+        "calculate should be success when payer not in split"
+    );
+    let result = result.unwrap();
+
+    // Bob and Charlie should each pay Alice 50
+    let bob_to_alice = result
+        .get_final_result_item("Bob".into(), "Alice".into())
+        .expect("Bob should pay Alice");
+    assert_eq!(bob_to_alice, 50.0, "Bob should pay Alice 50");
+
+    let charlie_to_alice = result
+        .get_final_result_item("Charlie".into(), "Alice".into())
+        .expect("Charlie should pay Alice");
+    assert_eq!(charlie_to_alice, 50.0, "Charlie should pay Alice 50");
+}
+
+#[test]
+fn test_large_number_of_people() {
+    let mut bills = Bills::default();
+
+    // Test large group scenario
+    let people = vec!["A", "B", "C", "D", "E", "F", "G", "H", "I", "J"];
+    bills.add_bill("A", "Group Dinner", 1000., people.clone());
+
+    let result = calculate_from(bills);
+    assert!(
+        result.is_ok(),
+        "calculate should be success for large group"
+    );
+    let result = result.unwrap();
+
+    // Each person should pay A 100 (1000/10)
+    for person in &people {
+        if *person != "A" {
+            let amount = result
+                .get_final_result_item(person.to_string().into(), "A".into())
+                .expect(&format!("{} should pay A", person));
+            assert_eq!(amount, 100.0, "{} should pay A 100", person);
+        }
+    }
+
+    assert_eq!(
+        result.get_final_result().len(),
+        9,
+        "Should have 9 transactions"
+    );
+}
diff --git a/src/who.rs b/src/who.rs
new file mode 100644
index 0000000..9d6e5b5
--- /dev/null
+++ b/src/who.rs
@@ -0,0 +1,44 @@
+#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct Who {
+    name: String,
+}
+
+impl std::ops::Deref for Who {
+    type Target = String;
+
+    fn deref(&self) -> &Self::Target {
+        &self.name
+    }
+}
+
+impl std::ops::DerefMut for Who {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.name
+    }
+}
+
+impl From<String> for Who {
+    fn from(s: String) -> Self {
+        Who { name: s }
+    }
+}
+
+impl From<&str> for Who {
+    fn from(s: &str) -> Self {
+        Who {
+            name: s.to_string(),
+        }
+    }
+}
+
+impl Into<String> for Who {
+    fn into(self) -> String {
+        self.name
+    }
+}
+
+impl std::fmt::Display for Who {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.name)
+    }
+}