website/src/lib/money-tracker/charts.ts

export type ChartPoint = { date: string; value: number };
export type ChartSeries = {
	label: string;
	color: string;
	points: ChartPoint[];
	emphasis?: boolean;
	/** Points after this date are drawn dashed (projected). */
	dashedFrom?: string;
};

const palette = ['#0070f3', '#e5484d', '#46a758', '#ad7f00', '#8e4ec6', '#0091ad', '#e56910'];

/** The account's chosen color, or a palette color based on its position in the list. */
export function accountColor(account: { color?: string | null }, index: number): string {
	return account.color || palette[index % palette.length];
}

export function balanceSeries(
	accounts: { id: number; name: string; openingBalanceCents: number; color?: string | null }[],
	transactions: { accountId: number; date: string; amountCents: number }[]
): ChartSeries[] {
	const ordered = [...transactions].sort((a, b) => a.date.localeCompare(b.date));
	const balances = new Map(accounts.map((account) => [account.id, account.openingBalanceCents]));
	const accountPoints = new Map(accounts.map((account) => [account.id, [] as ChartPoint[]]));
	const combinedPoints: ChartPoint[] = [];
	for (let index = 0; index < ordered.length;) {
		const date = ordered[index].date;
		while (index < ordered.length && ordered[index].date === date) {
			const transaction = ordered[index++];
			balances.set(
				transaction.accountId,
				(balances.get(transaction.accountId) ?? 0) + transaction.amountCents
			);
		}
		for (const account of accounts)
			accountPoints.get(account.id)!.push({ date, value: balances.get(account.id)! });
		combinedPoints.push({
			date,
			value: [...balances.values()].reduce((sum, balance) => sum + balance, 0)
		});
	}
	return [
		...accounts.map((account, index) => ({
			label: account.name,
			color: accountColor(account, index),
			points: accountPoints.get(account.id)!
		})),
		{ label: 'All accounts', color: 'var(--site-muted)', points: combinedPoints }
	];
}

export function accountBalances(
	accounts: { id: number; openingBalanceCents: number }[],
	transactions: { accountId: number; amountCents: number }[]
): Map<number, number> {
	const balances = new Map(accounts.map((account) => [account.id, account.openingBalanceCents]));
	for (const transaction of transactions)
		balances.set(
			transaction.accountId,
			(balances.get(transaction.accountId) ?? 0) + transaction.amountCents
		);
	return balances;
}

export function monthlyCents(amountCents: number, cadence: string): number {
	return cadence === 'weekly'
		? (amountCents * 52) / 12
		: cadence === 'yearly'
			? amountCents / 12
			: amountCents;
}

export function forecastSeries(
	currentCents: number,
	forecasts: { date: string; amountCents: number }[],
	today: string
): ChartSeries[] {
	let projected = currentCents;
	const points: ChartPoint[] = [{ date: today, value: currentCents }];
	for (const item of forecasts
		.filter((entry) => entry.date >= today)
		.sort((a, b) => a.date.localeCompare(b.date))) {
		projected += item.amountCents;
		points.push({ date: item.date, value: projected });
	}
	return [{ label: 'Projected total', color: '#0070f3', points }];
}

type ProjectionAccount = { id: number; kind: string; openingBalanceCents: number };
type ProjectionMovement = { date: string; amountCents: number; accountId: number | null };
type ProjectionRecurring = {
	amountCents: number;
	cadence: string;
	accountId: number | null;
	startsOn?: string | null;
	endsOn?: string | null;
};
type ProjectionSettings = {
	investmentAccountId?: number | null;
	reserveCents: number;
	annualReturnPercent: string | number;
	contributionMode: string;
	fixedMonthlyCents: number;
	includeRecurring: boolean;
	endDate?: string | null;
};

export function wealthProjection(
	accounts: ProjectionAccount[],
	balances: Map<number, number>,
	futureTransactions: ProjectionMovement[],
	forecasts: ProjectionMovement[],
	recurring: ProjectionRecurring[],
	settings: ProjectionSettings | null,
	today: string
): ChartSeries[] {
	const accountKind = new Map(accounts.map((account) => [account.id, account.kind]));
	const investmentId = settings?.investmentAccountId ?? null;
	const validInvestmentId =
		investmentId !== null && accountKind.get(investmentId) === 'investment' ? investmentId : null;
	let spendable = 0;
	let invested = 0;
	for (const account of accounts) {
		const value = balances.get(account.id) ?? 0;
		if (account.kind === 'investment') invested += value;
		else spendable += value;
	}
	let indexBalance = validInvestmentId === null ? 0 : (balances.get(validInvestmentId) ?? 0);
	const totalPoints: ChartPoint[] = [{ date: today, value: spendable + invested }];
	const spendablePoints: ChartPoint[] = [{ date: today, value: spendable }];
	const investedPoints: ChartPoint[] = [{ date: today, value: invested }];
	const movements = [...futureTransactions, ...forecasts].filter((entry) => entry.date > today);
	const latestMovement = movements.reduce(
		(latest, entry) => (entry.date > latest ? entry.date : latest),
		''
	);
	const fallback = new Date(`${today}T12:00:00Z`);
	fallback.setUTCFullYear(fallback.getUTCFullYear() + 1);
	const horizon =
		settings?.endDate && settings.endDate > today
			? settings.endDate
			: latestMovement || fallback.toISOString().slice(0, 10);
	const monthEnd = (year: number, month: number) =>
		new Date(Date.UTC(year, month + 1, 0)).toISOString().slice(0, 10);
	const start = new Date(`${today}T12:00:00Z`);
	let year = start.getUTCFullYear();
	let month = start.getUTCMonth();
	const yearly = Number(settings?.annualReturnPercent ?? 5) / 100;
	const monthlyGrowth = Math.pow(1 + yearly, 1 / 12) - 1;
	for (let index = 0; index < 240; index++) {
		const first = `${year}-${String(month + 1).padStart(2, '0')}-01`;
		const end = monthEnd(year, month);
		const date = end < horizon ? end : horizon;
		if (date > today) {
			for (const entry of movements.filter(
				(item) => (index === 0 ? item.date > today : item.date >= first) && item.date <= date
			)) {
				if (entry.accountId !== null && accountKind.get(entry.accountId) === 'investment') {
					invested += entry.amountCents;
					if (entry.accountId === validInvestmentId) indexBalance += entry.amountCents;
				} else spendable += entry.amountCents;
			}
			if (settings?.includeRecurring !== false && index > 0) {
				for (const item of recurring) {
					if ((item.startsOn && item.startsOn > date) || (item.endsOn && item.endsOn < first))
						continue;
					const value = Math.round(monthlyCents(item.amountCents, item.cadence));
					if (item.accountId !== null && accountKind.get(item.accountId) === 'investment') {
						invested += value;
						if (item.accountId === validInvestmentId) indexBalance += value;
					} else spendable += value;
				}
			}
			if (index > 0) {
				const growth = Math.round(indexBalance * monthlyGrowth);
				invested += growth;
				indexBalance += growth;
				const excess = Math.max(0, spendable - (settings?.reserveCents ?? 100000));
				const contribution =
					settings?.contributionMode === 'fixed'
						? Math.min(excess, settings.fixedMonthlyCents)
						: excess;
				spendable -= contribution;
				invested += contribution;
				indexBalance += contribution;
			}
			totalPoints.push({ date, value: spendable + invested });
			spendablePoints.push({ date, value: spendable });
			investedPoints.push({ date, value: invested });
		}
		if (end >= horizon) break;
		month++;
		if (month === 12) {
			month = 0;
			year++;
		}
	}
	return [
		{ label: 'Total wealth', color: '#0070f3', points: totalPoints, emphasis: true },
		{ label: 'Spendable', color: '#46a758', points: spendablePoints },
		{ label: 'Invested', color: '#8e4ec6', points: investedPoints }
	];
}

export type TimelineView = 'total' | 'split' | 'accounts';

/**
 * Joins actual history (solid, through today) with the wealth projection (dashed, after
 * today) so both read as one continuous line per measure.
 */
export function wealthTimeline(
	accounts: {
		id: number;
		name: string;
		kind: string;
		openingBalanceCents: number;
		color?: string | null;
	}[],
	actualTransactions: { accountId: number; date: string; amountCents: number }[],
	projection: ChartSeries[],
	today: string,
	view: TimelineView
): ChartSeries[] {
	const history = balanceSeries(accounts, actualTransactions);
	const accountLines = history.slice(0, accounts.length);
	const dates = history.at(-1)!.points.map((point) => point.date);
	const sumKind = (investment: boolean) =>
		dates.map((date, index) => ({
			date,
			value: accounts.reduce(
				(sum, account, accountIndex) =>
					(account.kind === 'investment') === investment
						? sum + accountLines[accountIndex].points[index].value
						: sum,
				0
			)
		}));
	const untilToday = (points: ChartPoint[]) => {
		const last = points.at(-1);
		return last && last.date < today ? [...points, { date: today, value: last.value }] : points;
	};
	const join = (actual: ChartPoint[], projected: ChartPoint[]) => {
		const solid = untilToday(actual);
		const cutoff = solid.at(-1)?.date ?? '';
		return [...solid, ...projected.filter((point) => point.date > cutoff)];
	};
	const [totalProjection, spendableProjection, investedProjection] = projection;
	const total: ChartSeries = {
		label: 'Total',
		color: 'var(--site-text)',
		emphasis: true,
		dashedFrom: today,
		points: join(history.at(-1)!.points, totalProjection?.points ?? [])
	};
	if (view === 'accounts')
		return [
			...accountLines
				.filter((line) => line.points.some((point) => point.value !== 0))
				.map((line) => ({ ...line, points: untilToday(line.points) })),
			total
		];
	if (view === 'split')
		return [
			total,
			{
				label: 'Spendable',
				color: '#46a758',
				dashedFrom: today,
				points: join(sumKind(false), spendableProjection?.points ?? [])
			},
			{
				label: 'Invested',
				color: '#8e4ec6',
				dashedFrom: today,
				points: join(sumKind(true), investedProjection?.points ?? [])
			}
		];
	return [total];
}

export function utilityEstimate(
	readings: { date: string; reading: string; kind: string }[],
	kind: string,
	ratePerUnit: string,
	monthlyAdvanceCents: number,
	billingStart?: string | null
) {
	const byKind = readings.filter(
		(reading) => reading.kind === kind && (!billingStart || reading.date >= billingStart)
	);
	if (byKind.length < 2) return null;
	const first = byKind[0];
	const last = byKind.at(-1)!;
	const usage = Number(last.reading) - Number(first.reading);
	const months =
		(Date.parse(`${last.date}T12:00:00Z`) - Date.parse(`${first.date}T12:00:00Z`)) /
		((365.25 / 12) * 86_400_000);
	if (months <= 0 || usage < 0) return null;
	const estimatedCents = Math.round(usage * Number(ratePerUnit) * 100);
	const advancesCents = Math.round(months * monthlyAdvanceCents);
	return {
		usage,
		months,
		estimatedCents,
		advancesCents,
		differenceCents: advancesCents - estimatedCents
	};
}