functional

src/engine.ts

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+import type { Config, EstimationResult, MissingPeriod, Plan, Selections } from "./types";
+import { clampDate, daysBetween, inclusiveDayCount, overlapDays } from "./dates";
+
+/** The active missing periods for a set of ids, in config order. */
+function activePeriods(config: Config, ids: Set<string>): MissingPeriod[] {
+  return config.missingPeriods.filter((p) => ids.has(p.id));
+}
+
+/**
+ * Days a student is expected to be on campus across the full meal window,
+ * after removing days covered by the active missing periods.
+ * Overlapping/adjacent periods are handled by summing per-period overlap with
+ * the window; periods in the config are assumed non-overlapping (typical for
+ * distinct breaks). Result is clamped to >= 0.
+ */
+export function totalActiveDays(config: Config, activeIds: Set<string>): number {
+  const { mealStartDate, mealEndDate } = config;
+  const windowDays = inclusiveDayCount(mealStartDate, mealEndDate);
+
+  let excluded = 0;
+  for (const p of activePeriods(config, activeIds)) {
+    excluded += overlapDays(mealStartDate, mealEndDate, p.startDate, p.endDate);
+  }
+
+  return Math.max(0, windowDays - excluded);
+}
+
+/**
+ * Active days from meal start up to (and including) the target date, minus
+ * active missing periods within that window, minus extra excluded days.
+ * The target date is clamped into the meal window first. Result clamped >= 0.
+ */
+export function activeDaysConsumed(config: Config, selections: Selections): number {
+  const { mealStartDate, mealEndDate } = config;
+
+  // Before the meal period even begins, nothing is consumed yet.
+  if (daysBetween(mealStartDate, selections.targetDate) < 0) {
+    return 0;
+  }
+
+  const target = clampDate(selections.targetDate, mealStartDate, mealEndDate);
+
+  const windowDays = inclusiveDayCount(mealStartDate, target);
+
+  let excluded = 0;
+  for (const p of activePeriods(config, selections.activeMissingPeriodIds)) {
+    excluded += overlapDays(mealStartDate, target, p.startDate, p.endDate);
+  }
+
+  const consumed = windowDays - excluded - Math.max(0, selections.extraExcludedDays);
+  return Math.max(0, consumed);
+}
+
+function findPlan(config: Config, planId: string): Plan | undefined {
+  return config.plans.find((p) => p.id === planId);
+}
+
+/**
+ * Master estimate. Distributes a plan's totals evenly across total active days,
+ * then reports per-week rate, consumed-by-target, and remaining for both meal
+ * blocks and FLEX dollars. All output values are clamped to sane bounds.
+ */
+export function estimate(config: Config, selections: Selections): EstimationResult {
+  const plan = findPlan(config, selections.selectedPlanId) ?? config.plans[0];
+
+  const total = totalActiveDays(config, selections.activeMissingPeriodIds);
+  const consumedDays = Math.min(
+    activeDaysConsumed(config, selections),
+    total
+  );
+
+  // Guard against division by zero when every day is excluded.
+  const blockRate = total > 0 ? plan.totalMealBlocks / total : 0;
+  const flexRate = total > 0 ? plan.totalFlexDollars / total : 0;
+
+  const consumedMealBlocks = clamp(blockRate * consumedDays, 0, plan.totalMealBlocks);
+  const consumedFlexDollars = clamp(flexRate * consumedDays, 0, plan.totalFlexDollars);
+
+  return {
+    perWeekMealBlocks: blockRate * 7,
+    perWeekFlexDollars: flexRate * 7,
+    consumedMealBlocks,
+    consumedFlexDollars,
+    remainingMealBlocks: plan.totalMealBlocks - consumedMealBlocks,
+    remainingFlexDollars: plan.totalFlexDollars - consumedFlexDollars,
+    totalActiveDays: total,
+    activeDaysConsumed: consumedDays,
+  };
+}
+
+function clamp(v: number, lo: number, hi: number): number {
+  return Math.min(hi, Math.max(lo, v));
+}