feat: synchronize recipe completion controls

Fixes #5\nRefs #1, #2

lib/recipe-progress.js

@@ -0,0 +1,35 @@
+function flattenInstructions(instructions) {
+  return instructions.flatMap((instruction) => {
+    if (typeof instruction === "string") return [instruction];
+    if (Array.isArray(instruction?.itemListElement)) return flattenInstructions(instruction.itemListElement);
+    return instruction?.text ? [instruction.text] : [];
+  });
+}
+
+function normalizeProgress(progress = {}) {
+  return {
+    completedIngredients: [...new Set((progress.completedIngredients || []).filter(Number.isInteger))],
+    completedSteps: [...new Set((progress.completedSteps || []).filter(Number.isInteger))]
+  };
+}
+
+function updateProgress(recipe, progress, { kind, index, completed }) {
+  if (!recipe || !["ingredient", "step"].includes(kind)
+    || !Number.isInteger(index) || typeof completed !== "boolean") {
+    throw new Error("Invalid progress update");
+  }
+  const limit = kind === "ingredient"
+    ? (recipe.recipeIngredient || []).length
+    : flattenInstructions(recipe.recipeInstructions || []).length;
+  if (index < 0 || index >= limit) throw new Error("Progress item does not exist");
+
+  const key = kind === "ingredient" ? "completedIngredients" : "completedSteps";
+  const next = normalizeProgress(progress);
+  const items = new Set(next[key]);
+  if (completed) items.add(index);
+  else items.delete(index);
+  next[key] = [...items].sort((left, right) => left - right);
+  return next;
+}
+
+module.exports = { flattenInstructions, normalizeProgress, updateProgress };