planmymeal/app/lib/grocery.ts

import type { GroceryLine, GrocerySummary, ItemId, StoreItem } from "@/app/types";
import { averageGroceryProvider, type PriceProvider } from "./pricing";
import { RECIPES_BY_ID } from "./recipes";
import { toBaseUnit } from "./units";

// ---------------------------------------------------------------------------
// Grocery aggregation + whole-package costing.
//
//   1. For every planned meal, scale its ingredients to the household size
//      and convert to the store item's base unit.
//   2. Sum per store item across all meals.
//   3. Round UP to whole packages. That's the checkout cost.
//   4. Also compute the value actually eaten, so the gap (carryover) is visible.
//
// Why whole packages? You can't buy 0.3 lb of rice. Rounding up is what makes
// ingredient overlap matter: a second rice dish is nearly free once the bag is
// already in the cart.
//
// Two entry points:
//   buildGroceryList  the full, sorted list for the drawer and budget meter
//   Cart              "what would adding this recipe cost?", asked hundreds of
//                     times by auto-fill and the meal picker. It only looks at
//                     the recipe's own ingredients instead of rebuilding the list.
// ---------------------------------------------------------------------------

export interface GroceryOptions {
  householdSize: number;
  /** Items already in the kitchen: listed, but cost $0. */
  onHandItems: ItemId[];
  provider?: PriceProvider;
}

/** Float-safety: 2.0000000001 packages should still be 2. */
const EPSILON = 1e-6;

const packagesFor = (amount: number, item: StoreItem) => Math.max(1, Math.ceil(amount / item.packageSize - EPSILON));

/** What a line costs at checkout and what's actually eaten. On-hand items cost nothing. */
function lineCost(amount: number, item: StoreItem, onHand: boolean) {
  if (onHand || amount <= 0) return { checkout: 0, eaten: 0 };
  return {
    checkout: packagesFor(amount, item) * item.price,
    eaten: (amount / item.packageSize) * item.price,
  };
}

/** One recipe's ingredients for this household, in each item's base unit. */
interface RecipeNeeds {
  title: string;
  amounts: Map<ItemId, number>;
}

function recipeNeeds(recipeId: string, householdSize: number, provider: PriceProvider): RecipeNeeds | null {
  const recipe = RECIPES_BY_ID.get(recipeId);
  if (!recipe) return null;

  const scale = householdSize / recipe.servings;
  const amounts = new Map<ItemId, number>();
  for (const ingredient of recipe.ingredients) {
    const item = provider.getItem(ingredient.itemId);
    if (!item) continue; // unknown to this price source; skip rather than crash the UI
    const amount = toBaseUnit(ingredient.amount, ingredient.unit, item) * scale;
    amounts.set(item.id, (amounts.get(item.id) ?? 0) + amount);
  }
  return { title: recipe.title, amounts };
}

// --- Full list ----------------------------------------------------------------

export function buildGroceryList(recipeIds: string[], options: GroceryOptions): GrocerySummary {
  const provider = options.provider ?? averageGroceryProvider;
  const onHandItems = new Set(options.onHandItems);

  // itemId → running total + which recipes use it
  const needs = new Map<ItemId, { amount: number; usedBy: Set<string> }>();
  for (const recipeId of recipeIds) {
    const recipe = recipeNeeds(recipeId, options.householdSize, provider);
    if (!recipe) continue;
    for (const [itemId, amount] of recipe.amounts) {
      const entry = needs.get(itemId) ?? { amount: 0, usedBy: new Set<string>() };
      entry.amount += amount;
      entry.usedBy.add(recipe.title);
      needs.set(itemId, entry);
    }
  }

  const lines: GroceryLine[] = [];
  for (const [itemId, { amount, usedBy }] of needs) {
    const item = provider.getItem(itemId)!;
    const onHand = onHandItems.has(itemId);
    const packages = packagesFor(amount, item);
    // Still compute packages for on-hand items: the list shows them so you can check your shelf.
    const cost = lineCost(amount, item, onHand);

    lines.push({
      item,
      neededAmount: amount,
      packages,
      checkoutCost: cost.checkout,
      usedCost: cost.eaten,
      leftoverAmount: packages * item.packageSize - amount,
      onHand,
      usedBy: [...usedBy],
    });
  }

  // Group by aisle, on-hand items last, then most expensive first: easy to scan in-store.
  lines.sort(
    (a, b) => a.item.aisle.localeCompare(b.item.aisle) || Number(a.onHand) - Number(b.onHand) || b.checkoutCost - a.checkoutCost,
  );

  const checkoutTotal = sum(lines.map((l) => l.checkoutCost));
  const usedTotal = sum(lines.map((l) => l.usedCost));

  return {
    lines,
    checkoutTotal: round2(checkoutTotal),
    usedTotal: round2(usedTotal),
    carryoverValue: round2(checkoutTotal - usedTotal),
    sharedItemCount: lines.filter((l) => l.usedBy.length > 1).length,
  };
}

// --- Incremental cart ---------------------------------------------------------

/**
 * A running cart for "what does this recipe add?" questions.
 *
 *   const cart = new Cart(options, plannedIds);
 *   cart.marginal("beef-stroganoff")  // { checkout: 2.48, eaten: 3.10 }
 *   cart.add("beef-stroganoff")
 *
 * Gives the same answer as building the list with and without the recipe and
 * subtracting, but only touches the recipe's own ingredients: every other line
 * in the cart cancels out.
 */
export class Cart {
  private readonly totals = new Map<ItemId, number>();
  private readonly needsCache = new Map<string, RecipeNeeds | null>();
  private readonly provider: PriceProvider;
  private readonly onHandItems: Set<ItemId>;

  constructor(
    private readonly options: GroceryOptions,
    recipeIds: string[] = [],
  ) {
    this.provider = options.provider ?? averageGroceryProvider;
    this.onHandItems = new Set(options.onHandItems);
    for (const id of recipeIds) this.add(id);
  }

  add(recipeId: string): void {
    for (const [itemId, amount] of this.needsOf(recipeId)) {
      this.totals.set(itemId, (this.totals.get(itemId) ?? 0) + amount);
    }
  }

  /** Extra checkout cost and extra eaten value if this recipe were added. */
  marginal(recipeId: string): { checkout: number; eaten: number } {
    let checkout = 0;
    let eaten = 0;
    for (const [itemId, amount] of this.needsOf(recipeId)) {
      const item = this.provider.getItem(itemId)!;
      const onHand = this.onHandItems.has(itemId);
      const before = this.totals.get(itemId) ?? 0;
      const withIt = lineCost(before + amount, item, onHand);
      const without = lineCost(before, item, onHand);
      checkout += withIt.checkout - without.checkout;
      eaten += withIt.eaten - without.eaten;
    }
    return { checkout, eaten };
  }

  private needsOf(recipeId: string): Map<ItemId, number> {
    if (!this.needsCache.has(recipeId)) {
      this.needsCache.set(recipeId, recipeNeeds(recipeId, this.options.householdSize, this.provider));
    }
    return this.needsCache.get(recipeId)?.amounts ?? new Map();
  }
}

const sum = (values: number[]) => values.reduce((total, v) => total + v, 0);
const round2 = (value: number) => Math.round(value * 100) / 100;