6 files changed, 237 insertions, 0 deletions
diff --git a/utils/assert.ts b/utils/assert.ts
new file mode 100644
index 0000000..05de651
--- /dev/null
+++ b/utils/assert.ts
@@ -0,0 +1,36 @@
+/**
+ * assert — throw if a condition is false, narrow the type otherwise.
+ * For programmer errors (invariants that should never fail); use
+ * Result<T, E> for recoverable failures.
+ */
+export function assert(condition: unknown, message?: string): asserts condition {
+  if (!condition) {
+    throw new Error(message ?? 'Assertion failed')
+  }
+}
+
+/**
+ * Mark a branch as unreachable in an exhaustive switch. If the compiler
+ * ever allows this to be reached, `value` stops being `never` and the
+ * call site fails to type-check.
+ *
+ * Usage:
+ *   switch (kind) {
+ *     case 'a': ...
+ *     case 'b': ...
+ *     default: assertNever(kind)
+ *   }
+ */
+export function assertNever(value: never, message?: string): never {
+  throw new Error(message ?? `Unhandled case: ${String(value)}`)
+}
+
+/**
+ * Throw if a value is null or undefined; return the narrowed value.
+ */
+export function assertDefined<T>(value: T, message?: string): NonNullable<T> {
+  if (value === null || value === undefined) {
+    throw new Error(message ?? 'Expected value to be defined')
+  }
+  return value as NonNullable<T>
+}
diff --git a/utils/equal.ts b/utils/equal.ts
new file mode 100644
index 0000000..8e12216
--- /dev/null
+++ b/utils/equal.ts
@@ -0,0 +1,55 @@
+/**
+ * Structural deep equality, used by determinism tests to compare
+ * simulation snapshots. Handles plain objects, arrays, Maps, Sets, Dates,
+ * and primitives. Does not handle class instances with custom equality
+ * or cyclic object graphs — simulation state is a tree, not a graph.
+ */
+export function deepEqual(a: unknown, b: unknown): boolean {
+  if (Object.is(a, b)) return true
+
+  if (typeof a !== typeof b) return false
+  if (a === null || b === null) return false
+  if (typeof a !== 'object') return false
+
+  if (a instanceof Date || b instanceof Date) {
+    return a instanceof Date && b instanceof Date && a.getTime() === b.getTime()
+  }
+
+  if (Array.isArray(a) || Array.isArray(b)) {
+    if (!Array.isArray(a) || !Array.isArray(b)) return false
+    if (a.length !== b.length) return false
+    for (let i = 0; i < a.length; i++) {
+      if (!deepEqual(a[i], b[i])) return false
+    }
+    return true
+  }
+
+  if (a instanceof Map || b instanceof Map) {
+    if (!(a instanceof Map) || !(b instanceof Map)) return false
+    if (a.size !== b.size) return false
+    for (const [key, value] of a) {
+      if (!b.has(key) || !deepEqual(value, b.get(key))) return false
+    }
+    return true
+  }
+
+  if (a instanceof Set || b instanceof Set) {
+    if (!(a instanceof Set) || !(b instanceof Set)) return false
+    if (a.size !== b.size) return false
+    for (const value of a) {
+      if (!b.has(value)) return false
+    }
+    return true
+  }
+
+  const aObj = a as Record<string, unknown>
+  const bObj = b as Record<string, unknown>
+  const aKeys = Object.keys(aObj)
+  const bKeys = Object.keys(bObj)
+  if (aKeys.length !== bKeys.length) return false
+  for (const key of aKeys) {
+    if (!Object.prototype.hasOwnProperty.call(bObj, key)) return false
+    if (!deepEqual(aObj[key], bObj[key])) return false
+  }
+  return true
+}
diff --git a/utils/index.ts b/utils/index.ts
new file mode 100644
index 0000000..4449e3e
--- /dev/null
+++ b/utils/index.ts
@@ -0,0 +1,21 @@
+export {
+  err,
+  isErr,
+  isOk,
+  mapErr,
+  mapResult,
+  ok,
+  unwrap,
+  unwrapOr,
+  type Err,
+  type Ok,
+  type Result
+} from 'utils/result'
+
+export { assert, assertDefined, assertNever } from 'utils/assert'
+
+export { deepEqual } from 'utils/equal'
+
+export type { Brand, DeepReadonly, ElementOf, JsonValue, NonEmptyArray } from 'utils/types'
+
+export { log } from 'utils/log'
diff --git a/utils/log.ts b/utils/log.ts
new file mode 100644
index 0000000..01e23f3
--- /dev/null
+++ b/utils/log.ts
@@ -0,0 +1,35 @@
+/**
+ * Structured logging. Uses console internally but carries a consistent
+ * message-plus-context shape so downstream sinks can filter and format.
+ * The game ships with no runtime telemetry — logs are developer-facing
+ * only, visible in the browser devtools.
+ */
+
+type LogLevel = 'debug' | 'info' | 'warn' | 'error'
+
+function emit(level: LogLevel, message: string, context?: Record<string, unknown>): void {
+  const payload = context ? `${message} ${JSON.stringify(context)}` : message
+  switch (level) {
+    case 'debug':
+        console.log(payload)
+      return
+    case 'info':
+        console.info(payload)
+      return
+    case 'warn':
+        console.warn(payload)
+      return
+    case 'error':
+        console.error(payload)
+      return
+  }
+}
+
+export const log = {
+  debug: (message: string, context?: Record<string, unknown>): void =>
+    emit('debug', message, context),
+  info: (message: string, context?: Record<string, unknown>): void => emit('info', message, context),
+  warn: (message: string, context?: Record<string, unknown>): void => emit('warn', message, context),
+  error: (message: string, context?: Record<string, unknown>): void =>
+    emit('error', message, context)
+} as const
diff --git a/utils/result.ts b/utils/result.ts
new file mode 100644
index 0000000..cfe487f
--- /dev/null
+++ b/utils/result.ts
@@ -0,0 +1,53 @@
+/**
+ * Result<T, E> — a recoverable-error return type for functions that can
+ * fail without throwing. Used at system boundaries (save/load, content
+ * validation, manifest fetch) where the caller needs to decide whether a
+ * failure is fatal. Internal pure logic uses plain returns and throws
+ * Error for programmer errors (rules/01-code-style.md).
+ */
+
+export type Ok<T> = { readonly ok: true; readonly value: T }
+export type Err<E> = { readonly ok: false; readonly error: E }
+export type Result<T, E = Error> = Ok<T> | Err<E>
+
+export function ok<T>(value: T): Ok<T> {
+  return { ok: true, value }
+}
+
+export function err<E>(error: E): Err<E> {
+  return { ok: false, error }
+}
+
+export function isOk<T, E>(r: Result<T, E>): r is Ok<T> {
+  return r.ok
+}
+
+export function isErr<T, E>(r: Result<T, E>): r is Err<E> {
+  return !r.ok
+}
+
+/** Map the value of an ok result; pass errors through unchanged. */
+export function mapResult<T, U, E>(r: Result<T, E>, f: (value: T) => U): Result<U, E> {
+  return r.ok ? ok(f(r.value)) : r
+}
+
+/** Map the error of a failed result; pass values through unchanged. */
+export function mapErr<T, E, F>(r: Result<T, E>, f: (error: E) => F): Result<T, F> {
+  return r.ok ? r : err(f(r.error))
+}
+
+/**
+ * Extract the value from an ok result; throw on error. Reserve for tests
+ * and assertion contexts — in production code prefer branching on isOk.
+ */
+export function unwrap<T, E>(r: Result<T, E>): T {
+  if (!r.ok) {
+    throw new Error(`unwrap() called on error result: ${String(r.error)}`)
+  }
+  return r.value
+}
+
+/** Extract the value or return a default. */
+export function unwrapOr<T, E>(r: Result<T, E>, fallback: T): T {
+  return r.ok ? r.value : fallback
+}
diff --git a/utils/types.ts b/utils/types.ts
new file mode 100644
index 0000000..a9629c8
--- /dev/null
+++ b/utils/types.ts
@@ -0,0 +1,37 @@
+/** A non-empty readonly array; the type system guarantees index 0 exists. */
+export type NonEmptyArray<T> = readonly [T, ...T[]]
+
+/** Values that round-trip through JSON.stringify / JSON.parse. */
+export type JsonValue =
+  | string
+  | number
+  | boolean
+  | null
+  | readonly JsonValue[]
+  | { readonly [key: string]: JsonValue | undefined }
+
+/** Element type of a readonly array (e.g., `ElementOf<typeof MONTH_NAMES>`). */
+export type ElementOf<T extends readonly unknown[]> = T extends readonly (infer E)[] ? E : never
+
+/** Deep-readonly version of a structural type. */
+export type DeepReadonly<T> = T extends (infer U)[]
+  ? ReadonlyArray<DeepReadonly<U>>
+  : T extends Map<infer K, infer V>
+    ? ReadonlyMap<DeepReadonly<K>, DeepReadonly<V>>
+    : T extends Set<infer U>
+      ? ReadonlySet<DeepReadonly<U>>
+      : T extends object
+        ? { readonly [K in keyof T]: DeepReadonly<T[K]> }
+        : T
+
+/**
+ * Nominal / branded type: compile-time-only tag to distinguish otherwise
+ * structurally-equal types.
+ *
+ *   type PersonId = Brand<string, 'PersonId'>
+ *   type RelId = Brand<string, 'RelationshipId'>
+ *
+ * PersonId and RelId are still strings at runtime but aren't assignable
+ * to each other without an explicit cast.
+ */
+export type Brand<T, B extends string> = T & { readonly __brand: B }