//! Parties: everyone the company deals with, in one directory.
//!
//! A party is a vendor, a contractor, a customer, an employee, a member, a
//! payout platform, a tax agency, a bank or card issuer, a landlord, or
//! several of these. Vendor and customer *records* (bills, invoices,
//! payments, aging) stay where they are and link to their party; a party
//! adds who they are (names, facts) and links entries to them.
//!
//! ```text
//! GOAL: answer "who is this, and what has the company done with them?"
//!       for anyone the books touch, without a CRM's sprawl.
//! SOURCE: docs/parties-and-attachments.md; DECISION (owner,
//!         2026-10-15); IRS truncated TINs: Treas. Reg. §301.6109-4.
//! INVARIANTS:
//!   1. No party field holds a full taxpayer identification number: any
//!      run of nine digits (single spaces or hyphens between them allowed)
//!      is refused. A W-9's TIN is kept as its last four digits only.
//!   2. W-9 facts need a 1099 role (contractor or landlord); W-4 facts need
//!      the employee role. Taxpayer-ID facts (`TinFacts`: the TIN's type,
//!      last four, and vault record) are allowed for any role: members
//!      need theirs for K-1s.
//!   2a. A party's TIN facts and W-9 facts never disagree: both on file
//!      means the same last four (DECISION, owner, 2026-10-15).
//!   3. Names are unique, ignoring case and surrounding space, among parties
//!      not archived.
//!   4. Every vendor and customer record links to exactly one party.
//! ```

use chrono::NaiveDate;
use serde::{Deserialize, Serialize};
use std::collections::BTreeSet;

/// Opaque party handle.
pub type PartyId = u64;

/// What a party is to the company.
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Serialize, Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum Role {
    /// Sells to the company (has a vendor record: bills, payments).
    Vendor,
    /// A vendor paid for services who gets a 1099-NEC.
    Contractor,
    /// Buys from the company (has a customer record: invoices).
    Customer,
    /// Works for the company on payroll.
    Employee,
    /// A member (owner) of the company, as in wanderbooks.toml.
    Member,
    /// Pays the company out of sales (an app store, a payment processor).
    PayoutPlatform,
    /// Taxes the company (the IRS, a state).
    TaxAgency,
    /// Regulates or registers the company without taxing it (a secretary
    /// of state, a division of corporations, a licensing board).
    GovernmentAgency,
    /// Holds the company's money or credit (a bank, a card issuer).
    FinancialInstitution,
    /// Rents property to the company (gets a 1099-MISC for rent).
    Landlord,
}

impl Role {
    /// Every role, in display order.
    pub const ALL: [Role; 10] = [
        Role::Vendor,
        Role::Contractor,
        Role::Customer,
        Role::Employee,
        Role::Member,
        Role::PayoutPlatform,
        Role::TaxAgency,
        Role::GovernmentAgency,
        Role::FinancialInstitution,
        Role::Landlord,
    ];

    /// Its wire name, e.g. `payout_platform`.
    pub fn key(self) -> &'static str {
        match self {
            Role::Vendor => "vendor",
            Role::Contractor => "contractor",
            Role::Customer => "customer",
            Role::Employee => "employee",
            Role::Member => "member",
            Role::PayoutPlatform => "payout_platform",
            Role::TaxAgency => "tax_agency",
            Role::GovernmentAgency => "government_agency",
            Role::FinancialInstitution => "financial_institution",
            Role::Landlord => "landlord",
        }
    }

    /// The role named `key`.
    pub fn parse(key: &str) -> Option<Role> {
        Role::ALL.into_iter().find(|r| r.key() == key)
    }

    /// Whether this role files a 1099 (and so may carry W-9 facts).
    pub fn gets_1099(self) -> bool {
        matches!(self, Role::Contractor | Role::Landlord)
    }
}

/// What a W-9 said, without the number itself.
#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct W9Facts {
    /// When it was received.
    pub received: NaiveDate,
    /// Line 3 federal tax classification, e.g. "individual/sole proprietor".
    pub entity_type: String,
    /// Line 4 exempt payee code, if any.
    #[serde(default)]
    pub exempt_payee_code: Option<String>,
    /// Line 4 FATCA exemption code, if any.
    #[serde(default)]
    pub fatca_code: Option<String>,
    /// The TIN's last four digits (exactly four digits).
    pub tin_last4: String,
    /// Where the form itself is kept, e.g. "wanderbooks vault" or "CPA portal".
    pub kept_at: String,
    /// SHA-256 of the vault record holding the full TIN, when one exists:
    /// proves the vault held it without the books holding it.
    #[serde(default)]
    pub vault_record: Option<String>,
}

/// A taxpayer identification number's type.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "lowercase")]
pub enum TinType {
    /// Social Security number (an individual).
    Ssn,
    /// Employer identification number (a business).
    Ein,
    /// IRS individual taxpayer identification number.
    Itin,
}

impl TinType {
    /// Its wire name: ssn, ein, or itin.
    pub fn key(self) -> &'static str {
        match self {
            TinType::Ssn => "ssn",
            TinType::Ein => "ein",
            TinType::Itin => "itin",
        }
    }

    /// How a form names it: SSN, EIN, or ITIN.
    pub fn label(self) -> &'static str {
        match self {
            TinType::Ssn => "SSN",
            TinType::Ein => "EIN",
            TinType::Itin => "ITIN",
        }
    }
}

/// What the books know of a party's taxpayer identification number,
/// without the number: its type, its last four digits, and the hash of
/// the TIN vault record that holds it in full. Any role may have one
/// (a member's SSN for their K-1, a contractor's from their W-9).
#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct TinFacts {
    /// SSN, EIN, or ITIN.
    pub tin_type: TinType,
    /// The last four digits (exactly four digits).
    pub last4: String,
    /// SHA-256 of the vault record holding the full TIN; `None` once the
    /// record is purged (or when the vault never held it).
    #[serde(default)]
    pub vault_record: Option<String>,
    /// When and why the vault's copy was purged, if it was.
    #[serde(default)]
    pub purged: Option<String>,
}

impl TinFacts {
    /// Check the facts' form. Returns why not.
    pub fn check(&self) -> Result<(), String> {
        if self.last4.len() != 4 || !self.last4.chars().all(|c| c.is_ascii_digit()) {
            return Err("a TIN's last four is exactly four digits".into());
        }
        if let Some(h) = &self.vault_record
            && (h.len() != 64 || !h.chars().all(|c| c.is_ascii_hexdigit()))
        {
            return Err("vault_record is a SHA-256 in hex".into());
        }
        if let Some(t) = &self.purged {
            check_text("purged", t)?;
        }
        Ok(())
    }
}

/// What a W-4 said, without the SSN.
#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct W4Facts {
    /// When it was received.
    pub received: NaiveDate,
    /// The work state, two letters.
    pub state: String,
    /// Where the form itself is kept.
    pub kept_at: String,
}

/// One party.
#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct Party {
    /// Opaque handle.
    pub id: PartyId,
    /// Legal or trade name, as on documents.
    pub name: String,
    /// A shorter name to show, if any.
    #[serde(default)]
    pub display_name: Option<String>,
    /// Roles set by hand (vendor and customer also follow their records).
    pub roles: BTreeSet<Role>,
    /// Website.
    #[serde(default)]
    pub website: Option<String>,
    /// Business address.
    #[serde(default)]
    pub address: Option<String>,
    /// Notes.
    #[serde(default)]
    pub notes: Option<String>,
    /// No longer dealt with; history stays.
    #[serde(default)]
    pub archived: bool,
    /// W-9 facts (contractor, landlord).
    #[serde(default)]
    pub w9: Option<W9Facts>,
    /// W-4 facts (employee).
    #[serde(default)]
    pub w4: Option<W4Facts>,
    /// Taxpayer-ID facts (any role): the TIN's type and last four, and
    /// its vault record. The full number is only ever in the vault.
    #[serde(default)]
    pub tin: Option<TinFacts>,
    /// Its vendor record, if it bills the company.
    #[serde(default)]
    pub vendor: Option<crate::VendorId>,
    /// Its customer record, if the company invoices it.
    #[serde(default)]
    pub customer: Option<crate::CustomerId>,
}

impl Party {
    /// The name to show: the display name, else the name.
    pub fn label(&self) -> &str {
        self.display_name.as_deref().unwrap_or(&self.name)
    }

    /// Its roles: the ones set by hand, plus vendor and customer when it
    /// has those records.
    pub fn all_roles(&self) -> BTreeSet<Role> {
        let mut r = self.roles.clone();
        if self.vendor.is_some() {
            r.insert(Role::Vendor);
        }
        if self.customer.is_some() {
            r.insert(Role::Customer);
        }
        r
    }
}

/// The key names are compared by: case and surrounding space ignored.
pub fn name_key(name: &str) -> String {
    name.trim().to_lowercase()
}

/// The characters that may sit between a TIN's digits: space, hyphen,
/// dot, slash, underscore, and the Unicode dashes and no-break space
/// (`291-12-2846`, `291.12.2846`, `291/12/2846`, `291 - 12 - 2846`).
fn tin_separator(c: char) -> bool {
    matches!(
        c,
        ' ' | '-'
            | '.'
            | '/'
            | '_'
            | '\u{a0}'
            | '\u{2010}'
            | '\u{2011}'
            | '\u{2012}'
            | '\u{2013}'
            | '\u{2014}'
            | '\u{2212}'
    )
}

/// The most separators that may sit between two of a TIN's digits
/// (`" - "` is three).
const MAX_TIN_GAP: usize = 3;

/// Each run in `chars` that holds nine digits or more, as (first, last)
/// char indices of its digits: digits with at most [`MAX_TIN_GAP`]
/// separators ([`tin_separator`]) between neighbors.
fn full_tin_runs(chars: &[char]) -> Vec<(usize, usize)> {
    let mut runs = Vec::new();
    let mut i = 0;
    while i < chars.len() {
        if !chars[i].is_numeric() {
            i += 1;
            continue;
        }
        let (mut digits, mut last, mut j) = (1, i, i + 1);
        while j < chars.len() {
            if chars[j].is_numeric() {
                digits += 1;
                last = j;
                j += 1;
                continue;
            }
            let gap = chars[j..].iter().take_while(|c| tin_separator(**c)).count();
            if (1..=MAX_TIN_GAP).contains(&gap)
                && chars.get(j + gap).is_some_and(|c| c.is_numeric())
            {
                j += gap;
            } else {
                break;
            }
        }
        if digits >= 9 {
            runs.push((i, last));
        }
        i = last + 1;
    }
    runs
}

/// Whether `text` holds a run of nine digits, with at most three
/// separators (space, hyphen, dot, slash, underscore, a Unicode dash or
/// no-break space) between neighbors: an SSN, EIN, or ITIN however it's
/// written (`291-12-2846`, `291.12.2846`, `291 - 12 - 2846`). Any Unicode
/// numeral counts as a digit (full-width, Arabic-Indic, ...), so a number
/// can't slip through in another script.
pub fn has_full_tin(text: &str) -> bool {
    let chars: Vec<char> = text.chars().collect();
    !full_tin_runs(&chars).is_empty()
}

/// `text` with every run [`has_full_tin`] finds masked: each of its
/// digits becomes `•`, separators kept. For refusals that quote what they
/// were given, so no error echoes a full TIN (docs/spec/rules.md B-26).
pub fn redact_full_tins(text: &str) -> std::borrow::Cow<'_, str> {
    let mut chars: Vec<char> = text.chars().collect();
    let runs = full_tin_runs(&chars);
    if runs.is_empty() {
        return std::borrow::Cow::Borrowed(text);
    }
    for (first, last) in runs {
        for c in &mut chars[first..=last] {
            if c.is_numeric() {
                *c = '•';
            }
        }
    }
    std::borrow::Cow::Owned(chars.into_iter().collect())
}

/// Check one party text field: not a full TIN, at most 500 characters, no
/// control characters except newline (addresses and notes run several
/// lines). Returns why not.
pub fn check_text(field: &str, text: &str) -> Result<(), String> {
    if has_full_tin(text) {
        return Err(format!(
            "{field} looks like it holds a full taxpayer identification number; \
             the books keep only its last four digits (the full number belongs in the vault)"
        ));
    }
    if text.chars().count() > 500 {
        return Err(format!("{field} is at most 500 characters"));
    }
    if text.chars().any(|c| c.is_control() && c != '\n') {
        return Err(format!("{field} can't contain control characters"));
    }
    Ok(())
}

impl W9Facts {
    /// Check the facts' form. Returns why not.
    pub fn check(&self) -> Result<(), String> {
        if self.tin_last4.len() != 4 || !self.tin_last4.chars().all(|c| c.is_ascii_digit()) {
            return Err("tin_last4 is exactly four digits".into());
        }
        if self.entity_type.trim().is_empty() {
            return Err("a W-9's entity type (line 3) is required".into());
        }
        if self.kept_at.trim().is_empty() {
            return Err("say where the W-9 itself is kept".into());
        }
        for (f, t) in [
            ("entity_type", Some(self.entity_type.as_str())),
            ("exempt_payee_code", self.exempt_payee_code.as_deref()),
            ("fatca_code", self.fatca_code.as_deref()),
            ("kept_at", Some(self.kept_at.as_str())),
        ] {
            if let Some(t) = t {
                check_text(f, t)?;
            }
        }
        if let Some(h) = &self.vault_record
            && (h.len() != 64 || !h.chars().all(|c| c.is_ascii_hexdigit()))
        {
            return Err("vault_record is a SHA-256 in hex".into());
        }
        Ok(())
    }
}

impl W4Facts {
    /// Check the facts' form. Returns why not.
    pub fn check(&self) -> Result<(), String> {
        if self.state.len() != 2 || !self.state.chars().all(|c| c.is_ascii_uppercase()) {
            return Err("state is two capital letters, e.g. UT".into());
        }
        if self.kept_at.trim().is_empty() {
            return Err("say where the W-4 itself is kept".into());
        }
        check_text("kept_at", &self.kept_at)
    }
}

/// One fact of a party to change ([`crate::Ledger::update_party`]).
#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "snake_case", tag = "field", content = "value")]
pub enum PartyField {
    /// The legal or trade name.
    Name(String),
    /// The display name (`None` clears it).
    DisplayName(Option<String>),
    /// The website.
    Website(Option<String>),
    /// The business address.
    Address(Option<String>),
    /// Notes.
    Notes(Option<String>),
    /// Archived (no longer dealt with) or not.
    Archived(bool),
}

/// What the books did with one party over a span of dates
/// ([`activity`]).
#[derive(Clone, Debug, Default, PartialEq, Eq)]
pub struct Activity {
    /// The party's entries, drafts included, by id.
    pub entries: BTreeSet<crate::EntryId>,
    /// Paid to it in the span (net of refunds and reversals of payments).
    pub paid: rust_decimal::Decimal,
    /// Received from it in the span (net of reversals of receipts).
    pub received: rust_decimal::Decimal,
    /// Posted entries in the span naming it among several parties whose
    /// shares don't resolve against the entry and the members given
    /// ([`crate::shares::resolve`]): counted in neither total.
    pub unresolved: BTreeSet<crate::EntryId>,
}

/// The accounts [`entry_flow`] reads money from: the cash accounts (every
/// asset account but receivables when `cash` is empty), and the
/// liabilities other than payables and the payroll withholdings (cards,
/// what members paid for the company and are owed back, loans).
#[derive(Clone, Debug, Default, PartialEq, Eq)]
pub struct MoneyAccounts {
    /// Cash: bank, savings, cash on hand.
    pub cash: std::collections::HashSet<crate::AccountId>,
    /// Liabilities that pay for the company or are paid down.
    pub liabilities: std::collections::HashSet<crate::AccountId>,
}

/// The [`MoneyAccounts`] of `ledger`, with `cash` the books' cash accounts.
pub fn money_accounts(ledger: &crate::Ledger, cash: &[crate::AccountId]) -> MoneyAccounts {
    use crate::AccountType;
    let not_money: std::collections::HashSet<crate::AccountId> = crate::PAYROLL[2..]
        .iter()
        .chain([&crate::PAYABLES, &crate::RECEIVABLES])
        .filter_map(|c| ledger.role_account(c))
        .collect();
    let mut out = MoneyAccounts::default();
    for a in ledger.chart().all().filter(|a| !not_money.contains(&a.id)) {
        match a.account_type {
            AccountType::Asset if cash.is_empty() || cash.contains(&a.id) => {
                out.cash.insert(a.id);
            }
            AccountType::Liability => {
                out.liabilities.insert(a.id);
            }
            _ => {}
        }
    }
    out
}

/// What one entry moved with its counterparty ([`entry_flow`]).
#[derive(Clone, Debug, Default, PartialEq, Eq)]
pub struct EntryFlow {
    /// Paid to the counterparty (positive) or received from it (negative).
    pub net: rust_decimal::Decimal,
    /// What for, by account (debits − credits): positive is what the money
    /// paid for, negative where money received came from. Sums to `net`.
    pub what_for: Vec<(crate::AccountId, rust_decimal::Decimal)>,
}

/// The money one entry moved with its counterparty, and what for.
///
/// ```text
/// GOAL: "how much did we pay them" right however it was paid: from the
///       bank, on a card, by a member for the company, or by paying down
///       what the company owes them (a reimbursement, a card payment).
/// SOURCE: DECISION (owner, 2026-10-15), after a reimbursement to the
///         members netted to zero under the earlier rule (cash and
///         liabilities alike as money).
/// DEFINITIONS (over the entry's lines, in `money`):
///   cash_cr, cash_dr: credits and debits on cash accounts;
///   liab_cr, liab_dr: credits and debits on money liabilities;
///   down = min(liab_dr, cash_cr): cash paying down a liability (its
///     creditor is paid; the liability is what for);
///   up = min(liab_cr, cash_dr): a liability raising cash (borrowing: the
///     lender pays the company);
///   left(a), for each money liability a: its debits less what of them
///     `down` takes, less its credits less what of them `up` takes (`down`
///     and `up` taken from its lines in line order), so one account's
///     debits and credits offset first;
///   across = min(Σ left(a) > 0, Σ −left(a) < 0): one liability paying
///     down another (a member paying the card from their own funds: Dr
///     2050 / Cr 2100; a loan paying off a card): the debited liability's
///     creditor is paid, by the credited one's.
/// INVARIANTS:
///   1. net = (cash_cr − cash_dr) + (liab_cr − liab_dr) + down − up
///      + across.
///   2. what_for: every line not on a money account (debits − credits),
///      plus `down` on the debited liabilities and −`up` on the credited
///      ones (in line order), plus `across` on the liabilities with
///      left(a) > 0 (in order of first appearance), zero sums left out,
///      accounts merged; its sum is net exactly, because the entry
///      balances.
/// SCENARIOS:
///   1. Expense from the bank, on a card, or paid by a member for the
///      company (Dr expense / Cr 1000, 2050, or 2100): net is the amount.
///   2. Reimbursing members (Dr 2100 / Cr 1000) or paying a card (Dr 2050 /
///      Cr 1000): net is the amount, what for is the liability.
///   3. A loan (Dr 1000 / Cr loan): net is minus the amount (received).
///   4. A refund (Dr 1000 or Dr card / Cr expense): negative net. A charge
///      and a refund on the same card in one entry net on that card: no
///      card paid down.
///   5. A transfer between the company's own cash accounts: zero.
///   6. A member contributing property (Dr equipment / Cr capital): zero.
///   7. A member paying the card bill from their own funds (Dr 2050 / Cr
///      2100), or a loan paying it off (Dr 2050 / Cr loan): net is the
///      amount, what for is the card, as if paid from the bank.
/// ```
pub fn entry_flow(
    entry: &crate::JournalEntry,
    money: &MoneyAccounts,
) -> Result<EntryFlow, crate::LedgerError> {
    use crate::Side;
    use rust_decimal::Decimal;
    // A draft's lines are as journaled, of any size (rules.md B-28): every
    // sum is checked, and one past exact arithmetic is refused.
    let too_large =
        || crate::LedgerError::TooLargeToCompute(format!("entry {}'s money flow", entry.id()));
    let plus = |a: Decimal, b: Decimal| a.checked_add(b).ok_or_else(too_large);
    let minus = |a: Decimal, b: Decimal| a.checked_sub(b).ok_or_else(too_large);
    let (mut cash_cr, mut cash_dr, mut liab_cr, mut liab_dr) =
        (Decimal::ZERO, Decimal::ZERO, Decimal::ZERO, Decimal::ZERO);
    for l in entry.lines() {
        let a = l.amount.amount;
        match (
            money.cash.contains(&l.account),
            money.liabilities.contains(&l.account),
            l.side,
        ) {
            (true, _, Side::Credit) => cash_cr = plus(cash_cr, a)?,
            (true, _, Side::Debit) => cash_dr = plus(cash_dr, a)?,
            (false, true, Side::Credit) => liab_cr = plus(liab_cr, a)?,
            (false, true, Side::Debit) => liab_dr = plus(liab_dr, a)?,
            _ => {}
        }
    }
    let down = liab_dr.min(cash_cr);
    let up = liab_cr.min(cash_dr);
    let mut by: std::collections::BTreeMap<crate::AccountId, Decimal> = Default::default();
    let (mut down_left, mut up_left) = (down, up);
    let mut order: Vec<crate::AccountId> = Vec::new();
    // left(a) per money liability, in order of first appearance.
    let mut left: Vec<(crate::AccountId, Decimal)> = Vec::new();
    let add = |by: &mut std::collections::BTreeMap<crate::AccountId, Decimal>,
               order: &mut Vec<crate::AccountId>,
               account: crate::AccountId,
               part: Decimal|
     -> Result<(), crate::LedgerError> {
        if !part.is_zero() {
            if !by.contains_key(&account) {
                order.push(account);
            }
            let sum = by.entry(account).or_default();
            *sum = plus(*sum, part)?;
        }
        Ok(())
    };
    for l in entry.lines() {
        let a = l.amount.amount;
        let part = if money.cash.contains(&l.account) {
            Decimal::ZERO
        } else if money.liabilities.contains(&l.account) {
            let (part, rest) = match l.side {
                Side::Debit => {
                    let take = a.min(down_left);
                    down_left -= take;
                    (take, a - take)
                }
                Side::Credit => {
                    let take = a.min(up_left);
                    up_left -= take;
                    (-take, -(a - take))
                }
            };
            match left.iter_mut().find(|(x, _)| *x == l.account) {
                Some((_, v)) => *v = plus(*v, rest)?,
                None => left.push((l.account, rest)),
            }
            part
        } else {
            match l.side {
                Side::Debit => a,
                Side::Credit => -a,
            }
        };
        add(&mut by, &mut order, l.account, part)?;
    }
    let owed_down = left
        .iter()
        .map(|(_, v)| *v)
        .filter(|v| *v > Decimal::ZERO)
        .try_fold(Decimal::ZERO, &plus)?;
    let owed_up = -left
        .iter()
        .map(|(_, v)| *v)
        .filter(|v| *v < Decimal::ZERO)
        .try_fold(Decimal::ZERO, &plus)?;
    let across = owed_down.min(owed_up);
    let mut across_left = across;
    for (account, v) in left {
        if v > Decimal::ZERO && !across_left.is_zero() {
            let take = v.min(across_left);
            across_left -= take;
            add(&mut by, &mut order, account, take)?;
        }
    }
    let net = plus(
        plus(minus(cash_cr, cash_dr)?, minus(liab_cr, liab_dr)?)?,
        plus(minus(down, up)?, across)?,
    )?;
    Ok(EntryFlow {
        net,
        what_for: order
            .into_iter()
            .map(|a| (a, by[&a]))
            .filter(|(_, v)| !v.is_zero())
            .collect(),
    })
}

/// One party's entries, and the money paid to and received from it in
/// `start..=end`, with no members ([`activity_with`] with none: an entry
/// naming several parties then needs explicit shares to count).
pub fn activity(
    ledger: &crate::Ledger,
    party: PartyId,
    span: (NaiveDate, NaiveDate),
    cash: &[crate::AccountId],
) -> Activity {
    activity_with(ledger, party, span, cash, &[])
}

/// One party's entries, and its share of the money paid to and received
/// from the entries' parties in `start..=end`.
///
/// ```text
/// GOAL: one answer to "what did we pay them this year?" for the MCP
///       server and the viewer alike, right when an entry was with
///       several parties.
/// SOURCE: docs/parties-and-attachments.md (Parties, Shares); DECISION
///         (owner, 2026-10-15).
/// INVARIANTS:
///   1. entries: every entry Ledger::entry_parties names the party in
///      (hand tags, vendor tags, bills and their payments, invoices and
///      their payments, and reversals of any of these carry it).
///   2. Each posted entry dated in the span whose entry_flow net is not
///      zero counts the party's resolved share of it
///      (crate::shares::resolve with `members`; the whole net for a sole
///      party): positive is a payment, negative a receipt. A reversal
///      counts on its root original's side (the first entry of
///      Ledger::reversal_chain), so a reversal subtracts and a reversal
///      of that reversal adds back, on the same side.
///   3. An entry moving no money (net zero, like property contributed)
///      counts nothing. An entry whose shares don't resolve counts
///      nothing and is listed in `unresolved`.
///   4. Over all the parties an entry names, the shares counted sum to its
///      net exactly (crate::shares INVARIANT 6).
///   5. A card charge counts as paid (the company paid them, by card); a
///      bill doesn't until it is paid. Not the 1099 amount: that is
///      form1099::vendor_1099_report_with.
/// ```
pub fn activity_with(
    ledger: &crate::Ledger,
    party: PartyId,
    (start, end): (NaiveDate, NaiveDate),
    cash: &[crate::AccountId],
    members: &[crate::PartyMember],
) -> Activity {
    use rust_decimal::Decimal;
    let entries: BTreeSet<crate::EntryId> = (0..ledger.next_entry_id())
        .filter(|&id| ledger.entry_names_party(id, party))
        .collect();
    let money = money_accounts(ledger, cash);
    // A posted entry's lines are in a line's range, so its flow computes;
    // one that doesn't counts nothing and is listed as unresolved.
    let net_out = |e: &crate::JournalEntry| entry_flow(e, &money).map(|f| f.net);
    let (mut paid, mut received) = (Decimal::ZERO, Decimal::ZERO);
    let mut unresolved = BTreeSet::new();
    for &id in &entries {
        let Some(e) = ledger.get_entry(id) else {
            continue;
        };
        if e.status() != crate::EntryStatus::Posted || e.date() < start || e.date() > end {
            continue;
        }
        let Ok(net) = net_out(e) else {
            unresolved.insert(id);
            continue;
        };
        if net.is_zero() {
            continue;
        }
        let share = match crate::shares::resolve(e, ledger.entry_parties(id), &money, members) {
            Ok(parts) => parts
                .into_iter()
                .find(|(p, _)| *p == party)
                .map(|(_, v)| v)
                .unwrap_or_default(),
            Err(_) => {
                unresolved.insert(id);
                continue;
            }
        };
        let side = match e.reversal_of() {
            Some(_) => ledger
                .reversal_chain(id)
                .first()
                .and_then(|&root| ledger.get_entry(root))
                .map_or(Ok(net), net_out)
                .unwrap_or(net),
            None => net,
        };
        if side > Decimal::ZERO {
            paid += share;
        } else {
            received -= share;
        }
    }
    Activity {
        entries,
        paid,
        received,
        unresolved,
    }
}