feat: add impact delta computation with TDD tests

Implements computeImpactDeltas pure function with 8 TDD tests covering replace/add/none modes and null weight/price handling. Adds useImpactDeltas hook, categoryId to ThreadWithCandidates, and selectedSetupId state to uiStore.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/client/hooks/useImpactDeltas.ts

@@ -0,0 +1,22 @@
+import { useMemo } from "react";
+import {
+	type CandidateDelta,
+	type CandidateInput,
+	computeImpactDeltas,
+	type DeltaMode,
+	type ImpactDeltas,
+	type SetupItemInput,
+} from "../lib/impactDeltas";
+
+export type { CandidateDelta, DeltaMode, ImpactDeltas };
+
+export function useImpactDeltas(
+	candidates: CandidateInput[],
+	setupItems: SetupItemInput[] | undefined,
+	threadCategoryId: number,
+): ImpactDeltas {
+	return useMemo(
+		() => computeImpactDeltas(candidates, setupItems, threadCategoryId),
+		[candidates, setupItems, threadCategoryId],
+	);
+}

src/client/hooks/useItems.ts

@@ -2,6 +2,21 @@ import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import type { CreateItem } from "../../shared/types";
 import { apiDelete, apiGet, apiPost, apiPut } from "../lib/api";
 
+interface Item {
+	id: number;
+	name: string;
+	weightGrams: number | null;
+	priceCents: number | null;
+	quantity: number;
+	categoryId: number;
+	notes: string | null;
+	productUrl: string | null;
+	imageFilename: string | null;
+	imageSourceUrl: string | null;
+	createdAt: string;
+	updatedAt: string;
+}
+
 interface ItemWithCategory {
 	id: number;
 	name: string;
@@ -70,3 +85,14 @@ export function useDeleteItem() {
 		},
 	});
 }
+
+export function useDuplicateItem() {
+	const queryClient = useQueryClient();
+	return useMutation({
+		mutationFn: (id: number) => apiPost<Item>(`/api/items/${id}/duplicate`, {}),
+		onSuccess: () => {
+			queryClient.invalidateQueries({ queryKey: ["items"] });
+			queryClient.invalidateQueries({ queryKey: ["totals"] });
+		},
+	});
+}

src/client/hooks/useThreads.ts

@@ -40,6 +40,7 @@ interface ThreadWithCandidates {
 	name: string;
 	status: "active" | "resolved";
 	resolvedCandidateId: number | null;
+	categoryId: number;
 	createdAt: string;
 	updatedAt: string;
 	candidates: CandidateWithCategory[];

src/client/lib/impactDeltas.ts

@@ -0,0 +1,69 @@
+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 };
+}

src/client/stores/uiStore.ts

@@ -52,6 +52,10 @@ interface UIState {
 	// Candidate view mode
 	candidateViewMode: "list" | "grid" | "compare";
 	setCandidateViewMode: (mode: "list" | "grid" | "compare") => void;
+
+	// Setup impact preview
+	selectedSetupId: number | null;
+	setSelectedSetupId: (id: number | null) => void;
 }
 
 export const useUIStore = create<UIState>((set) => ({
@@ -111,4 +115,8 @@ export const useUIStore = create<UIState>((set) => ({
 	// Candidate view mode
 	candidateViewMode: "list",
 	setCandidateViewMode: (mode) => set({ candidateViewMode: mode }),
+
+	// Setup impact preview
+	selectedSetupId: null,
+	setSelectedSetupId: (id) => set({ selectedSetupId: id }),
 }));