GearBox/src/client/lib/impactDeltas.ts

export interface CandidateInput {
	id: number;
	weightGrams: number | null;
	priceCents: number | null;
}

export interface SetupItemInput {
	categoryId: number;
	weightGrams: number | null;
	priceCents: number | null;
	name: string;
}

export type DeltaMode = "replace" | "add" | "none";

export interface CandidateDelta {
	candidateId: number;
	mode: DeltaMode;
	weightDelta: number | null;
	priceDelta: number | null;
	replacedItemName: string | null;
}

export interface ImpactDeltas {
	mode: DeltaMode;
	deltas: Record<number, CandidateDelta>;
}

export function computeImpactDeltas(
	candidates: CandidateInput[],
	setupItems: SetupItemInput[] | undefined,
	threadCategoryId: number,
): ImpactDeltas {
	if (!setupItems) return { mode: "none", deltas: {} };

	const replacedItem =
		setupItems.find((item) => item.categoryId === threadCategoryId) ?? null;
	const mode: DeltaMode = replacedItem ? "replace" : "add";
	const deltas: Record<number, CandidateDelta> = {};

	for (const candidate of candidates) {
		let weightDelta: number | null = null;
		let priceDelta: number | null = null;

		if (candidate.weightGrams != null) {
			weightDelta =
				replacedItem?.weightGrams != null
					? candidate.weightGrams - replacedItem.weightGrams
					: candidate.weightGrams;
		}

		if (candidate.priceCents != null) {
			priceDelta =
				replacedItem?.priceCents != null
					? candidate.priceCents - replacedItem.priceCents
					: candidate.priceCents;
		}

		deltas[candidate.id] = {
			candidateId: candidate.id,
			mode,
			weightDelta,
			priceDelta,
			replacedItemName: replacedItem?.name ?? null,
		};
	}

	return { mode, deltas };
}