//! Chart of accounts: the five account types and the account registry.
//!
//! Every account has exactly one of five types. The type determines the
//! account's *normal balance side*: assets and expenses are debit-normal,
//! liabilities, equity, and revenue are credit-normal. Reports are built by
//! grouping on type, so getting this right is the whole game.

use crate::{Currency, LedgerError};
use rust_decimal::Decimal;
use serde::{Deserialize, Serialize};
use std::collections::HashMap;

/// Opaque account handle. Created by [`ChartOfAccounts`]; never forged.
pub type AccountId = u64;

/// The five kinds of accounts. This list is closed: all of double-entry
/// bookkeeping is expressed with these.
#[derive(Clone, Copy, PartialEq, Eq, Debug, Serialize, Deserialize)]
pub enum AccountType {
    /// What the business owns or is owed: cash, receivables, equipment.
    Asset,
    /// What the business owes: payables, loans.
    Liability,
    /// The owners' claim: capital, retained earnings.
    Equity,
    /// Increases to equity from operations: sales, fees.
    Revenue,
    /// Decreases to equity from operations: rent, wages, supplies.
    Expense,
}

/// Debit or credit: the two sides of every journal line.
#[derive(Clone, Copy, PartialEq, Eq, Debug, Serialize, Deserialize)]
pub enum Side {
    /// Left side. Increases assets and expenses.
    Debit,
    /// Right side. Increases liabilities, equity, and revenue.
    Credit,
}

impl AccountType {
    /// The side that increases an account of this type — its normal balance.
    pub fn normal_side(&self) -> Side {
        match self {
            AccountType::Asset | AccountType::Expense => Side::Debit,
            AccountType::Liability | AccountType::Equity | AccountType::Revenue => Side::Credit,
        }
    }

    /// True for the three types that appear on the balance sheet.
    pub fn is_balance_sheet(&self) -> bool {
        matches!(
            self,
            AccountType::Asset | AccountType::Liability | AccountType::Equity
        )
    }
}

/// The longest name a new account (or a rename) may take, in characters:
/// one line that every report prints, as long as the company's own name
/// may be ([`crate::settings::MAX_TEXT`]). The command's bound, never
/// replay's (rules.md B-28): a journaled name of any length replays.
pub const MAX_ACCOUNT_NAME: usize = crate::settings::MAX_TEXT;

/// A single account in the chart of accounts.
#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
pub struct Account {
    /// Opaque handle.
    pub id: AccountId,
    /// Human-meaningful code, e.g. `"1000"`. Unique within the chart.
    pub code: String,
    /// Display name, e.g. `"Cash"`.
    pub name: String,
    /// One of the five types. Determines reporting and normal side.
    pub account_type: AccountType,
    /// Hierarchy link, e.g. `"1010"` under `"1000"`. `None` for top-level.
    pub parent: Option<AccountId>,
    /// Header accounts organize the chart and take no postings.
    pub placeholder: bool,
    /// Day one: always the ledger's base currency (see R-5).
    pub currency: Currency,
    /// Tax category key (see [`crate::tax`]), for revenue and expense
    /// accounts.
    #[serde(default)]
    pub tax_category: Option<String>,
    /// Percent deductible, overriding the category's default (e.g. 50 for
    /// meals, 0 for penalties).
    #[serde(default)]
    pub deductible_percent: Option<Decimal>,
}

/// The registry of accounts. Owns id allocation for accounts it holds.
#[derive(Clone, Debug, Default, PartialEq)]
pub struct ChartOfAccounts {
    accounts: HashMap<AccountId, Account>,
    by_code: HashMap<String, AccountId>,
}

impl ChartOfAccounts {
    /// Register an account. Codes must be unique and non-empty; a parent, if
    /// given, must already exist.
    pub fn add(&mut self, account: Account) -> Result<(), LedgerError> {
        if account.code.is_empty() {
            return Err(LedgerError::EmptyAccountCode);
        }
        if self.by_code.contains_key(&account.code) {
            return Err(LedgerError::DuplicateAccountCode(account.code));
        }
        if let Some(p) = account.parent
            && !self.accounts.contains_key(&p)
        {
            return Err(LedgerError::UnknownParentAccount(p));
        }
        self.by_code.insert(account.code.clone(), account.id);
        self.accounts.insert(account.id, account);
        Ok(())
    }

    /// Look up an account by id.
    pub fn get(&self, id: AccountId) -> Option<&Account> {
        self.accounts.get(&id)
    }

    /// Look up an account by code.
    pub fn find_by_code(&self, code: &str) -> Option<&Account> {
        self.by_code.get(code).and_then(|id| self.accounts.get(id))
    }

    /// Iterate over all accounts (order unspecified; sort by code for display).
    pub fn all(&self) -> impl Iterator<Item = &Account> {
        self.accounts.values()
    }

    /// Give an account a new code. The code must be non-empty and free.
    pub fn set_code(&mut self, id: AccountId, code: &str) -> Result<(), LedgerError> {
        if code.trim().is_empty() {
            return Err(LedgerError::EmptyAccountCode);
        }
        if self.by_code.contains_key(code) {
            return Err(LedgerError::DuplicateAccountCode(code.to_string()));
        }
        let a = self
            .accounts
            .get_mut(&id)
            .ok_or(LedgerError::UnknownAccount(id))?;
        self.by_code.remove(&a.code);
        a.code = code.to_string();
        self.by_code.insert(a.code.clone(), id);
        Ok(())
    }

    /// Give an account a new name. Only the account must exist: what a
    /// new name may be is the command's to check
    /// ([`crate::Ledger::rename_account`]), never replay's (rules.md B-28).
    pub fn set_name(&mut self, id: AccountId, name: &str) -> Result<(), LedgerError> {
        let a = self
            .accounts
            .get_mut(&id)
            .ok_or(LedgerError::UnknownAccount(id))?;
        a.name = name.to_string();
        Ok(())
    }

    /// Set (or clear) an account's tax category and deductible percent.
    pub fn set_tax(
        &mut self,
        id: AccountId,
        category: Option<String>,
        deductible_percent: Option<Decimal>,
    ) -> Result<(), LedgerError> {
        let a = self
            .accounts
            .get_mut(&id)
            .ok_or(LedgerError::UnknownAccount(id))?;
        a.tax_category = category;
        a.deductible_percent = deductible_percent;
        Ok(())
    }

    /// Number of accounts in the chart.
    pub fn len(&self) -> usize {
        self.accounts.len()
    }

    /// True when the chart holds no accounts.
    pub fn is_empty(&self) -> bool {
        self.accounts.is_empty()
    }
}