Flatten budget-app contents into repo root

backend/app/calc.py

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+"""Amount expressions and budget arithmetic. All money math uses Decimal."""
+import ast
+import operator
+from dataclasses import dataclass, field
+from decimal import ROUND_HALF_UP, Decimal, InvalidOperation
+
+MONTHS = 12
+ZERO = Decimal("0")
+
+_OPS = {
+    ast.Add: operator.add,
+    ast.Sub: operator.sub,
+    ast.Mult: operator.mul,
+    ast.Div: operator.truediv,
+}
+
+
+class ExpressionError(ValueError):
+    pass
+
+
+def normalize_number_text(text: str) -> str:
+    """Accept '1 234,56', '1.234,56' and '1,234.56' as well as plain numbers."""
+    s = text.replace(" ", "").replace(" ", "").replace("€", "")
+    if "," in s and "." in s:
+        if s.rfind(",") > s.rfind("."):
+            s = s.replace(".", "").replace(",", ".")
+        else:
+            s = s.replace(",", "")
+    elif "," in s:
+        s = s.replace(",", ".")
+    return s
+
+
+def evaluate(expr: str | None) -> Decimal:
+    """Evaluate a typed amount such as '=5512*12' or '3292.56+(12*839.67)'."""
+    if expr is None:
+        return ZERO
+    s = str(expr).strip()
+    if s.startswith("="):
+        s = s[1:]
+    if not s:
+        return ZERO
+    s = normalize_number_text(s)
+    if len(s) > 400:
+        raise ExpressionError("expression too long")
+    try:
+        tree = ast.parse(s, mode="eval")
+    except SyntaxError as exc:
+        raise ExpressionError(f"invalid expression: {expr}") from exc
+
+    def walk(node: ast.AST) -> Decimal:
+        if isinstance(node, ast.Expression):
+            return walk(node.body)
+        if isinstance(node, ast.Constant) and isinstance(node.value, (int, float)) and not isinstance(node.value, bool):
+            return Decimal(str(node.value))
+        if isinstance(node, ast.BinOp) and type(node.op) in _OPS:
+            left, right = walk(node.left), walk(node.right)
+            if isinstance(node.op, ast.Div) and right == 0:
+                raise ExpressionError("division by zero")
+            return _OPS[type(node.op)](left, right)
+        if isinstance(node, ast.UnaryOp) and isinstance(node.op, (ast.USub, ast.UAdd)):
+            value = walk(node.operand)
+            return -value if isinstance(node.op, ast.USub) else value
+        raise ExpressionError(f"invalid expression: {expr}")
+
+    try:
+        return walk(tree)
+    except InvalidOperation as exc:
+        raise ExpressionError(f"invalid expression: {expr}") from exc
+
+
+def round2(value: Decimal) -> Decimal:
+    return Decimal(value).quantize(Decimal("0.01"), rounding=ROUND_HALF_UP)
+
+
+def factor_for(adj_kind: str, adj_value: Decimal, year_rate: Decimal) -> Decimal:
+    adj_value = Decimal(adj_value or 0)
+    if adj_kind == "year_rate":
+        return Decimal(1) + Decimal(year_rate or 0)
+    if adj_kind == "percent":
+        return Decimal(1) + adj_value / Decimal(100)
+    if adj_kind == "factor":
+        return adj_value
+    return Decimal(1)
+
+
+@dataclass
+class LineFigures:
+    prior: Decimal
+    base: Decimal
+    factor: Decimal
+    adjusted: Decimal
+    months: list[Decimal]
+
+    @property
+    def change(self) -> Decimal:
+        return self.adjusted - self.prior
+
+    @property
+    def change_pct(self) -> Decimal | None:
+        return None if self.prior == 0 else self.change / self.prior * 100
+
+
+def line_figures(line, year_rate: Decimal) -> LineFigures:
+    # Each line is rounded to cents before it is summed, as the finance reports do.
+    prior = round2(Decimal(line.prior_actual or 0))
+    if line.monthly_kind == "manual":
+        months = [Decimal(str(v or 0)) for v in (line.monthly_values or [])][:MONTHS]
+        months += [ZERO] * (MONTHS - len(months))
+        total = sum(months, ZERO)
+        return LineFigures(prior, total, Decimal(1), total, months)
+    base = round2(Decimal(line.base_amount or 0))
+    factor = factor_for(line.adj_kind, line.adj_value, year_rate)
+    adjusted = round2(Decimal(line.base_amount or 0) * factor)
+    if line.monthly_kind == "selected":
+        return LineFigures(prior, base, factor, adjusted, selected_spread(adjusted, line.monthly_months))
+    return LineFigures(prior, base, factor, adjusted, even_spread(adjusted))
+
+
+def even_spread(total: Decimal) -> list[Decimal]:
+    """Twelve months in cents; December takes the rounding remainder so the months add up exactly."""
+    month = round2(total / MONTHS)
+    return [month] * (MONTHS - 1) + [total - month * (MONTHS - 1)]
+
+
+def clean_months(months) -> list[int]:
+    """Month numbers 1-12, sorted and without repeats."""
+    return sorted({int(m) for m in (months or []) if 1 <= int(m) <= MONTHS})
+
+
+def selected_spread(total: Decimal, months) -> list[Decimal]:
+    """Equal amounts in cents over the chosen months only (1 = January); the last chosen month takes the
+    rounding remainder. No months chosen means all twelve."""
+    chosen = clean_months(months) or list(range(1, MONTHS + 1))
+    share = round2(total / len(chosen))
+    out = [ZERO] * MONTHS
+    for m in chosen:
+        out[m - 1] = share
+    out[chosen[-1] - 1] = total - share * (len(chosen) - 1)
+    return out
+
+
+@dataclass
+class Totals:
+    prior: Decimal = ZERO
+    base: Decimal = ZERO
+    adjusted: Decimal = ZERO
+    months: list[Decimal] = field(default_factory=lambda: [ZERO] * MONTHS)
+
+    def add(self, f: LineFigures) -> None:
+        self.prior += f.prior
+        self.base += f.base
+        self.adjusted += f.adjusted
+        self.months = [a + b for a, b in zip(self.months, f.months)]
+
+    @property
+    def change(self) -> Decimal:
+        return self.adjusted - self.prior
+
+    @property
+    def change_pct(self) -> Decimal | None:
+        return None if self.prior == 0 else self.change / self.prior * 100