diff --git a/src/gameState.js b/src/gameState.js
new file mode 100644
index 0000000..b9687f3
--- /dev/null
+++ b/src/gameState.js
@@ -0,0 +1,101 @@
+import {
+  ROUTES,
+  JOURNAL,
+  INITIAL_STATE,
+  OTIUM_BASE_AUT,
+} from './constants.js'
+
+// Returns a fresh game state with a generated token
+export function createState(token) {
+  return {
+    ...INITIAL_STATE,
+    token,
+    created_at: new Date().toISOString(),
+  }
+}
+
+// Returns current chapter based on den and aut thresholds
+export function getChapter(den, aut) {
+  if (aut >= 30 && den >= 800) return 5
+  if (aut >= 15 && den >= 350) return 4
+  if (aut >= 5 && den >= 120) return 3
+  if (den >= 40) return 2
+  return 1
+}
+
+// Returns routes available to this state (unlocked, not locked)
+export function getUnlockedRoutes(state) {
+  return ROUTES.filter(
+    (r) => state.den >= r.unlock_den && state.aut >= r.unlock_aut
+  )
+}
+
+// Returns true if a route is unlocked
+export function isRouteUnlocked(state, routeId) {
+  const r = ROUTES.find((x) => x.id === routeId)
+  if (!r) return false
+  return state.den >= r.unlock_den && state.aut >= r.unlock_aut
+}
+
+// Apply a completed dispatch — returns new state
+export function applyDispatch(state, routeId) {
+  const r = ROUTES.find((x) => x.id === routeId)
+  if (!r) return state
+
+  const route_dispatches = {
+    ...state.route_dispatches,
+    [routeId]: (state.route_dispatches[routeId] || 0) + 1,
+  }
+
+  const den = state.den + r.profit
+  const dispatches = state.dispatches + 1
+  const chapter = getChapter(den, state.aut)
+
+  // Check for new journal entry
+  const dispatchCount = route_dispatches[routeId]
+  const entries = JOURNAL[routeId] || []
+  const entry = entries.find((e) => e.dispatch === dispatchCount)
+  const journal_seen = entry
+    ? [...state.journal_seen, { routeId, dispatch: dispatchCount }]
+    : state.journal_seen
+
+  return {
+    ...state,
+    den,
+    dispatches,
+    chapter,
+    route_dispatches,
+    journal_seen,
+  }
+}
+
+// Apply cost of launching a dispatch — returns new state or null if insufficient funds
+export function applyDispatchCost(state, routeId) {
+  const r = ROUTES.find((x) => x.id === routeId)
+  if (!r) return null
+  if (state.den < r.cost) return null
+  return { ...state, den: state.den - r.cost }
+}
+
+// Apply completed otium — returns new state
+export function applyOtium(state) {
+  const gain = OTIUM_BASE_AUT + Math.floor(state.dispatches / 3)
+  const aut = state.aut + gain
+  const chapter = getChapter(state.den, aut)
+  return { ...state, aut, chapter }
+}
+
+// Returns the journal entry to show for a completed dispatch, or null
+export function getNewJournalEntry(state, routeId) {
+  const dispatchCount = (state.route_dispatches[routeId] || 0)
+  const entries = JOURNAL[routeId] || []
+  return entries.find((e) => e.dispatch === dispatchCount) || null
+}
+
+// Returns all journal entries seen so far, in order seen
+export function getSeenJournalEntries(state) {
+  return state.journal_seen.map(({ routeId, dispatch }) => {
+    const entries = JOURNAL[routeId] || []
+    return entries.find((e) => e.dispatch === dispatch)
+  }).filter(Boolean)
+}