FlowBudget MVP: Chrome extension with SKU-level parsing, sinking funds, seasonal baselines, lifestyle creep heatmap, net worth dashboard

src/lib/budget.ts

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+import type { Category, Transaction, MonthlySnapshot } from '../types'
+
+export function getTotalBudget(categories: Category[]): number {
+  return categories.reduce((sum, c) => sum + getEffectiveBudget(c), 0)
+}
+
+export function getTotalSpent(categories: Category[]): number {
+  return categories.reduce((sum, c) => sum + c.spent, 0)
+}
+
+export function getEffectiveBudget(category: Category, month?: number): number {
+  const base = month !== undefined && category.seasonalBaseline
+    ? getSeasonalValue(category.seasonalBaseline, month)
+    : category.budget
+  return base + (category.rolloverEnabled ? category.rolloverAmount : 0)
+}
+
+function getSeasonalValue(baseline: { jan: number; feb: number; mar: number; apr: number; may: number; jun: number; jul: number; aug: number; sep: number; oct: number; nov: number; dec: number }, month: number): number {
+  const map = [baseline.jan, baseline.feb, baseline.mar, baseline.apr, baseline.may, baseline.jun, baseline.jul, baseline.aug, baseline.sep, baseline.oct, baseline.nov, baseline.dec]
+  return map[month] ?? baseline.jan
+}
+
+export function getRemaining(category: Category): number {
+  return getEffectiveBudget(category) - category.spent
+}
+
+export function getProgressPercent(category: Category): number {
+  const budget = getEffectiveBudget(category)
+  if (budget <= 0) return 0
+  return Math.min(100, Math.round((category.spent / budget) * 100))
+}
+
+export function addTransaction(
+  categories: Category[],
+  transactions: Transaction[],
+  tx: Transaction
+): { categories: Category[]; transactions: Transaction[] } {
+  const cats = categories.map(c => ({ ...c }))
+  const txs = [...transactions, tx]
+
+  if (tx.isSplit && tx.items) {
+    for (const item of tx.items) {
+      const cat = cats.find(c => c.id === item.categoryId)
+      if (cat) {
+        cat.spent += item.amount
+      }
+    }
+  } else {
+    const cat = cats.find(c => c.id === tx.categoryId)
+    if (cat) {
+      cat.spent += tx.amount
+    }
+  }
+
+  return { categories: cats, transactions: txs }
+}
+
+export function removeTransaction(
+  categories: Category[],
+  transactions: Transaction[],
+  txId: string
+): { categories: Category[]; transactions: Transaction[] } {
+  const tx = transactions.find(t => t.id === txId)
+  if (!tx) return { categories, transactions }
+
+  const cats = categories.map(c => ({ ...c }))
+  const txs = transactions.filter(t => t.id !== txId)
+
+  if (tx.isSplit && tx.items) {
+    for (const item of tx.items) {
+      const cat = cats.find(c => c.id === item.categoryId)
+      if (cat) {
+        cat.spent = Math.max(0, cat.spent - item.amount)
+      }
+    }
+  } else {
+    const cat = cats.find(c => c.id === tx.categoryId)
+    if (cat) {
+      cat.spent = Math.max(0, cat.spent - tx.amount)
+    }
+  }
+
+  return { categories: cats, transactions: txs }
+}
+
+export function rollOverMonth(categories: Category[]): Category[] {
+  return categories.map(c => {
+    const remaining = getEffectiveBudget(c) - c.spent
+    return {
+      ...c,
+      rolloverAmount: c.rolloverEnabled ? c.rolloverAmount + remaining : c.rolloverAmount,
+      spent: 0,
+    }
+  })
+}
+
+export function computeLifestyleCreep(
+  snapshots: MonthlySnapshot[],
+  currentMonth: string
+): { categoryId: string; changePercent: number; isInflation: boolean }[] {
+  const current = snapshots.find(s => s.month === currentMonth)
+  const lastYear = currentMonth.slice(0, 4)
+  const prevYear = String(Number(lastYear) - 1)
+  const prevMonthKey = prevYear + currentMonth.slice(4)
+  const previous = snapshots.find(s => s.month === prevMonthKey)
+
+  if (!current || !previous) return []
+
+  return Object.entries(current.categories).map(([catId, curr]) => {
+    const prev = previous.categories[catId]
+    if (!prev || prev.budget <= 0) return { categoryId: catId, changePercent: 0, isInflation: false }
+    const change = ((curr.budget - prev.budget) / prev.budget) * 100
+    return { categoryId: catId, changePercent: Math.round(change * 10) / 10, isInflation: change > 5 }
+  })
+}
+
+export function getNetWorth(accounts: { balance: number }[]): number {
+  return accounts.reduce((sum, a) => sum + a.balance, 0)
+}