diff --git a/src/session.js b/src/session.js
new file mode 100644
index 0000000..8a61604
--- /dev/null
+++ b/src/session.js
@@ -0,0 +1,104 @@
+// Reines Modul: kein window, kein document, kein localStorage.
+
+import { specsCompatible } from './spec.js';
+
+const STACK_LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
+
+/** @returns {{stacks: Array, entries: Array, nextEntryId: number}} */
+export function createSession() {
+  return { stacks: [], entries: [], nextEntryId: 1 };
+}
+
+function nextStackId(session) {
+  const used = new Set(session.stacks.map((stack) => stack.id));
+  for (const letter of STACK_LETTERS) {
+    if (!used.has(letter)) return letter;
+  }
+  // Nach Z weiter mit A2, B2, ... - in der Praxis nie erreicht.
+  return `A${session.stacks.length}`;
+}
+
+/**
+ * Schlaegt vor, wohin ein Modul gehoert.
+ * Genau ein vertraeglicher Stapel -> Zuweisung.
+ * Keiner -> neuer Stapel.
+ * Mehrere -> mehrdeutig, der Nutzer entscheidet.
+ */
+export function proposeAssignment(session, spec) {
+  const candidates = session.stacks
+    .filter((stack) => specsCompatible(stack.spec, spec))
+    .map((stack) => stack.id);
+
+  if (candidates.length === 1) {
+    return { kind: 'match', stackId: candidates[0], candidates };
+  }
+  if (candidates.length === 0) {
+    return { kind: 'new', stackId: nextStackId(session), candidates: [] };
+  }
+  return { kind: 'ambiguous', stackId: null, candidates };
+}
+
+/** Bucht das Modul auf den angegebenen Stapel und legt ihn bei Bedarf an. */
+export function commitAssignment(session, spec, source, stackId) {
+  let stack = session.stacks.find((candidate) => candidate.id === stackId);
+  if (!stack) {
+    stack = { id: stackId, spec, count: 0 };
+    session.stacks.push(stack);
+    session.stacks.sort((a, b) => a.id.localeCompare(b.id));
+  } else {
+    // Ein spaeterer, vollstaendigerer Scan ergaenzt fehlende Felder des Stapels.
+    for (const field of ['capacityGb', 'formFactor', 'rank', 'speed', 'partNumber']) {
+      if (stack.spec[field] === null && spec[field] !== null) {
+        stack.spec[field] = spec[field];
+      }
+    }
+  }
+
+  stack.count += 1;
+  const entry = { entryId: session.nextEntryId++, spec, stackId, source };
+  session.entries.push(entry);
+  return entry;
+}
+
+function dropEmptyStacks(session) {
+  session.stacks = session.stacks.filter((stack) => stack.count > 0);
+}
+
+/** Nimmt den zuletzt erfassten Eintrag zurueck. */
+export function undoLast(session) {
+  const entry = session.entries.pop();
+  if (!entry) return null;
+  const stack = session.stacks.find((candidate) => candidate.id === entry.stackId);
+  if (stack) stack.count -= 1;
+  dropEmptyStacks(session);
+  return entry;
+}
+
+/** Ordnet einen bereits erfassten Eintrag einem anderen Stapel zu. */
+export function moveEntry(session, entryId, stackId) {
+  const entry = session.entries.find((candidate) => candidate.entryId === entryId);
+  if (!entry || entry.stackId === stackId) return;
+
+  const from = session.stacks.find((candidate) => candidate.id === entry.stackId);
+  if (from) from.count -= 1;
+
+  let to = session.stacks.find((candidate) => candidate.id === stackId);
+  if (!to) {
+    to = { id: stackId, spec: entry.spec, count: 0 };
+    session.stacks.push(to);
+    session.stacks.sort((a, b) => a.id.localeCompare(b.id));
+  }
+  to.count += 1;
+  entry.stackId = stackId;
+  dropEmptyStacks(session);
+}
+
+/** Entfernt einen einzelnen Eintrag aus der Sitzung. */
+export function removeEntry(session, entryId) {
+  const index = session.entries.findIndex((candidate) => candidate.entryId === entryId);
+  if (index === -1) return;
+  const [entry] = session.entries.splice(index, 1);
+  const stack = session.stacks.find((candidate) => candidate.id === entry.stackId);
+  if (stack) stack.count -= 1;
+  dropEmptyStacks(session);
+}
diff --git a/test/session.test.js b/test/session.test.js
new file mode 100644
index 0000000..35e2b82
--- /dev/null
+++ b/test/session.test.js
@@ -0,0 +1,104 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { emptySpec } from '../src/spec.js';
+import {
+  createSession, proposeAssignment, commitAssignment,
+  undoLast, moveEntry, removeEntry,
+} from '../src/session.js';
+
+const s64 = () => ({ ...emptySpec(), capacityGb: 64, formFactor: 'LRDIMM', rank: '4DRx4', speed: 'PC4-2400', partNumber: 'M386A8K40BM1-CRC4Y' });
+const s32 = () => ({ ...emptySpec(), capacityGb: 32, formFactor: 'RDIMM', rank: '2Rx4', speed: 'PC4-2666', partNumber: 'M393A4K40BB1-CTD' });
+
+function scan(session, spec, source = 'barcode') {
+  const plan = proposeAssignment(session, spec);
+  return commitAssignment(session, spec, source, plan.stackId);
+}
+
+test('erster Scan legt Stapel A an', () => {
+  const session = createSession();
+  const plan = proposeAssignment(session, s64());
+  assert.equal(plan.kind, 'new');
+  assert.equal(plan.stackId, 'A');
+});
+
+test('gleiches Modul landet auf demselben Stapel', () => {
+  const session = createSession();
+  scan(session, s64());
+  const plan = proposeAssignment(session, s64());
+  assert.equal(plan.kind, 'match');
+  assert.equal(plan.stackId, 'A');
+});
+
+test('anderes Modul legt Stapel B an', () => {
+  const session = createSession();
+  scan(session, s64());
+  const plan = proposeAssignment(session, s32());
+  assert.equal(plan.kind, 'new');
+  assert.equal(plan.stackId, 'B');
+});
+
+test('eine Verwechslung erzeugt keinen Fast-Duplikat-Stapel', () => {
+  const session = createSession();
+  scan(session, s64());
+  const verrauscht = { ...s64(), partNumber: 'M386A8K4OBM1-CRC4Y' };
+  const plan = proposeAssignment(session, verrauscht);
+  assert.equal(plan.kind, 'match');
+  assert.equal(plan.stackId, 'A');
+});
+
+test('Scan ohne Teilenummer bei zwei passenden Stapeln ist mehrdeutig', () => {
+  const session = createSession();
+  scan(session, { ...s64(), partNumber: 'M386A8K40BM1-CRC4Y' });
+  scan(session, { ...s64(), partNumber: 'HMAA8GL7AMR4N-UH' });
+  const ohnePn = { ...s64(), partNumber: null };
+  const plan = proposeAssignment(session, ohnePn);
+  assert.equal(plan.kind, 'ambiguous');
+  assert.deepEqual(plan.candidates, ['A', 'B']);
+});
+
+test('commitAssignment zaehlt den Stapel hoch und fuehrt die Liste', () => {
+  const session = createSession();
+  scan(session, s64());
+  scan(session, s64());
+  assert.equal(session.stacks.length, 1);
+  assert.equal(session.stacks[0].count, 2);
+  assert.equal(session.entries.length, 2);
+});
+
+test('undoLast nimmt den letzten Eintrag zurueck', () => {
+  const session = createSession();
+  scan(session, s64());
+  scan(session, s64());
+  const entfernt = undoLast(session);
+  assert.equal(entfernt.stackId, 'A');
+  assert.equal(session.stacks[0].count, 1);
+  assert.equal(session.entries.length, 1);
+});
+
+test('undoLast entfernt einen leer gewordenen Stapel', () => {
+  const session = createSession();
+  scan(session, s64());
+  undoLast(session);
+  assert.equal(session.stacks.length, 0);
+  assert.equal(undoLast(session), null);
+});
+
+test('moveEntry sortiert einen Eintrag um', () => {
+  const session = createSession();
+  const a = scan(session, s64());
+  scan(session, s32());
+  moveEntry(session, a.entryId, 'B');
+  const stapelA = session.stacks.find((s) => s.id === 'A');
+  const stapelB = session.stacks.find((s) => s.id === 'B');
+  assert.equal(stapelA, undefined, 'leerer Stapel wird entfernt');
+  assert.equal(stapelB.count, 2);
+});
+
+test('removeEntry entfernt einen einzelnen Eintrag', () => {
+  const session = createSession();
+  const a = scan(session, s64());
+  scan(session, s64());
+  removeEntry(session, a.entryId);
+  assert.equal(session.entries.length, 1);
+  assert.equal(session.stacks[0].count, 1);
+});