diff --git a/src/sim/loyalty.ts b/src/sim/loyalty.ts
new file mode 100644
index 0000000..b00c773
--- /dev/null
+++ b/src/sim/loyalty.ts
@@ -0,0 +1,124 @@
+export type StampType = 'buy_new' | 'buy_used' | 'trade' | 'buy_back';
+
+export type Reward =
+  | { kind: 'free_trade' }
+  | { kind: 'free_used_game'; maxValueCents: number }
+  | { kind: 'discount_percent'; percent: number }
+  | { kind: 'voucher'; valueCents: number };
+
+export interface StampTrack {
+  stampType: StampType;
+  slots: number;
+  reward: Reward;
+}
+
+export interface LoyaltyProgram {
+  version: number;
+  tracks: StampTrack[];
+}
+
+export interface LoyaltyCard {
+  id: string;
+  programVersion: number;
+  stamps: number[]; // one counter per track of its program
+  pendingRewards: Reward[];
+  voucherCents: number;
+}
+
+export interface LoyaltyState {
+  programs: LoyaltyProgram[];
+  cards: Record<string, LoyaltyCard>;
+  nextCardNumber: number;
+}
+
+export const DEFAULT_TRACKS: StampTrack[] = [
+  { stampType: 'buy_new', slots: 10, reward: { kind: 'free_used_game', maxValueCents: 1000 } },
+];
+
+/** Estimated cost of a reward, used to rate how generous a program is. */
+const FREE_TRADE_VALUE_CENTS = 800;
+const AVERAGE_BASKET_CENTS = 3000;
+
+export function createLoyaltyState(): LoyaltyState {
+  return { programs: [{ version: 1, tracks: DEFAULT_TRACKS }], cards: {}, nextCardNumber: 1 };
+}
+
+export function currentProgram(l: LoyaltyState): LoyaltyProgram {
+  return l.programs[l.programs.length - 1];
+}
+
+export function getProgram(l: LoyaltyState, version: number): LoyaltyProgram {
+  const program = l.programs.find((p) => p.version === version);
+  if (!program) throw new Error(`Unknown loyalty program version: ${version}`);
+  return program;
+}
+
+export function validateTracks(tracks: StampTrack[]): string | null {
+  if (tracks.length < 1 || tracks.length > 2) return '1 ou 2 types de tampons';
+  if (new Set(tracks.map((t) => t.stampType)).size !== tracks.length) return 'Types de tampons en double';
+  if (tracks.some((t) => t.slots < 3 || t.slots > 20)) return 'Entre 3 et 20 cases par type';
+  return null;
+}
+
+export function rewardValueCents(r: Reward): number {
+  switch (r.kind) {
+    case 'free_trade': return FREE_TRADE_VALUE_CENTS;
+    case 'free_used_game': return r.maxValueCents;
+    case 'discount_percent': return (AVERAGE_BASKET_CENTS * r.percent) / 100;
+    case 'voucher': return r.valueCents;
+  }
+}
+
+/** Reward value given back per stamp, in cents. */
+export function generosity(program: LoyaltyProgram): number {
+  return program.tracks.reduce((sum, t) => sum + rewardValueCents(t.reward) / t.slots, 0);
+}
+
+export function returnVisitMultiplier(program: LoyaltyProgram): number {
+  return 1 + Math.min(generosity(program) / 100, 1);
+}
+
+export function joinProbability(program: LoyaltyProgram): number {
+  return Math.min(0.15 + generosity(program) / 1000, 0.6);
+}
+
+export function setProgram(l: LoyaltyState, tracks: StampTrack[]): { reputationDelta: number } {
+  const error = validateTracks(tracks);
+  if (error) throw new Error(error);
+  const previous = currentProgram(l);
+  const next: LoyaltyProgram = { version: previous.version + 1, tracks };
+  l.programs.push(next);
+  return { reputationDelta: generosity(next) < generosity(previous) ? -2 : 0 };
+}
+
+export function issueCard(l: LoyaltyState): LoyaltyCard {
+  const program = currentProgram(l);
+  const card: LoyaltyCard = {
+    id: `C${String(l.nextCardNumber).padStart(4, '0')}`,
+    programVersion: program.version,
+    stamps: program.tracks.map(() => 0),
+    pendingRewards: [],
+    voucherCents: 0,
+  };
+  l.nextCardNumber += 1;
+  l.cards[card.id] = card;
+  return card;
+}
+
+/** Stamps every track of the card matching `type`. Returns the rewards earned. */
+export function addStamp(l: LoyaltyState, cardId: string, type: StampType): Reward[] {
+  const card = l.cards[cardId];
+  if (!card) throw new Error(`Unknown card: ${cardId}`);
+  const program = getProgram(l, card.programVersion);
+  const earned: Reward[] = [];
+  program.tracks.forEach((track, i) => {
+    if (track.stampType !== type) return;
+    card.stamps[i] += 1;
+    if (card.stamps[i] < track.slots) return;
+    card.stamps[i] = 0;
+    earned.push(track.reward);
+    if (track.reward.kind === 'voucher') card.voucherCents += track.reward.valueCents;
+    else card.pendingRewards.push(track.reward);
+  });
+  return earned;
+}
diff --git a/tests/sim/loyalty.test.ts b/tests/sim/loyalty.test.ts
new file mode 100644
index 0000000..9c1ece2
--- /dev/null
+++ b/tests/sim/loyalty.test.ts
@@ -0,0 +1,94 @@
+import { describe, it, expect } from 'vitest';
+import {
+  addStamp, createLoyaltyState, currentProgram, generosity, issueCard,
+  joinProbability, returnVisitMultiplier, setProgram, StampTrack, validateTracks,
+} from '../../src/sim/loyalty';
+
+describe('loyalty', () => {
+  it('starts with 10 purchase stamps for a used game up to 10 €', () => {
+    const l = createLoyaltyState();
+    expect(currentProgram(l)).toEqual({
+      version: 1,
+      tracks: [{ stampType: 'buy_new', slots: 10, reward: { kind: 'free_used_game', maxValueCents: 1000 } }],
+    });
+    expect(generosity(currentProgram(l))).toBe(100);
+    expect(returnVisitMultiplier(currentProgram(l))).toBe(2);
+    expect(joinProbability(currentProgram(l))).toBeCloseTo(0.25);
+  });
+
+  it('issues numbered cards on the current program', () => {
+    const l = createLoyaltyState();
+    const card = issueCard(l);
+    expect(card).toEqual({ id: 'C0001', programVersion: 1, stamps: [0], pendingRewards: [], voucherCents: 0 });
+    expect(issueCard(l).id).toBe('C0002');
+  });
+
+  it('earns the reward on the last stamp and resets the track', () => {
+    const l = createLoyaltyState();
+    const card = issueCard(l);
+    for (let i = 0; i < 9; i++) expect(addStamp(l, card.id, 'buy_new')).toEqual([]);
+    expect(addStamp(l, card.id, 'buy_new')).toEqual([{ kind: 'free_used_game', maxValueCents: 1000 }]);
+    expect(card.stamps).toEqual([0]);
+    expect(card.pendingRewards).toHaveLength(1);
+  });
+
+  it('ignores stamp types the card does not track', () => {
+    const l = createLoyaltyState();
+    const card = issueCard(l);
+    expect(addStamp(l, card.id, 'trade')).toEqual([]);
+    expect(card.stamps).toEqual([0]);
+  });
+
+  it('supports two independent stamp tracks', () => {
+    const l = createLoyaltyState();
+    setProgram(l, [
+      { stampType: 'buy_new', slots: 3, reward: { kind: 'discount_percent', percent: 20 } },
+      { stampType: 'trade', slots: 3, reward: { kind: 'free_trade' } },
+    ]);
+    const card = issueCard(l);
+    addStamp(l, card.id, 'buy_new');
+    addStamp(l, card.id, 'trade');
+    addStamp(l, card.id, 'trade');
+    expect(card.stamps).toEqual([1, 2]);
+    expect(addStamp(l, card.id, 'trade')).toEqual([{ kind: 'free_trade' }]);
+  });
+
+  it('turns voucher rewards into card credit', () => {
+    const l = createLoyaltyState();
+    setProgram(l, [{ stampType: 'buy_used', slots: 3, reward: { kind: 'voucher', valueCents: 500 } }]);
+    const card = issueCard(l);
+    for (let i = 0; i < 3; i++) addStamp(l, card.id, 'buy_used');
+    expect(card.voucherCents).toBe(500);
+    expect(card.pendingRewards).toEqual([]);
+  });
+
+  it('validates tracks', () => {
+    const reward = { kind: 'free_trade' } as const;
+    const t = (stampType: StampTrack['stampType'], slots: number): StampTrack => ({ stampType, slots, reward });
+    expect(validateTracks([])).toBe('1 ou 2 types de tampons');
+    expect(validateTracks([t('buy_new', 5), t('trade', 5), t('buy_back', 5)])).toBe('1 ou 2 types de tampons');
+    expect(validateTracks([t('buy_new', 5), t('buy_new', 5)])).toBe('Types de tampons en double');
+    expect(validateTracks([t('buy_new', 2)])).toBe('Entre 3 et 20 cases par type');
+    expect(validateTracks([t('buy_new', 21)])).toBe('Entre 3 et 20 cases par type');
+    expect(validateTracks([t('buy_new', 3)])).toBeNull();
+    expect(() => setProgram(createLoyaltyState(), [])).toThrow('1 ou 2 types de tampons');
+  });
+
+  it('keeps existing cards on the rules they were issued with', () => {
+    const l = createLoyaltyState();
+    const oldCard = issueCard(l);
+    setProgram(l, [{ stampType: 'buy_new', slots: 3, reward: { kind: 'voucher', valueCents: 500 } }]);
+    for (let i = 0; i < 3; i++) addStamp(l, oldCard.id, 'buy_new');
+    expect(oldCard.stamps).toEqual([3]);
+    expect(oldCard.voucherCents).toBe(0);
+    expect(issueCard(l).programVersion).toBe(2);
+  });
+
+  it('penalizes reputation when the program becomes less generous', () => {
+    const l = createLoyaltyState();
+    const lessGenerous = setProgram(l, [{ stampType: 'buy_new', slots: 20, reward: { kind: 'voucher', valueCents: 500 } }]);
+    expect(lessGenerous.reputationDelta).toBe(-2);
+    const moreGenerous = setProgram(l, [{ stampType: 'buy_new', slots: 5, reward: { kind: 'voucher', valueCents: 1000 } }]);
+    expect(moreGenerous.reputationDelta).toBe(0);
+  });
+});