diff --git a/src/gid.js b/src/gid.js
new file mode 100644
index 0000000..2fc57b2
--- /dev/null
+++ b/src/gid.js
@@ -0,0 +1,30 @@
+'use strict';
+
+const MAX_ATLAS = 255;
+const MAX_TILE = 1048575;
+const MAX_ROT = 3;
+const MAX_U32 = 0xFFFFFFFF;
+
+function checkInt(name, value, min, max) {
+  if (!Number.isInteger(value) || value < min || value > max) {
+    throw new RangeError(`${name} ${value} out of range [${min},${max}]`);
+  }
+}
+
+function encodeGid(atlas, tile, rotation = 0) {
+  checkInt('atlas', atlas, 0, MAX_ATLAS);
+  checkInt('tile', tile, 0, MAX_TILE);
+  checkInt('rotation', rotation, 0, MAX_ROT);
+  return ((atlas << 24) | (tile << 4) | (rotation << 2)) >>> 0;
+}
+
+function decodeGid(gid) {
+  checkInt('gid', gid, 0, MAX_U32);
+  return {
+    atlas: (gid >>> 24) & 0xFF,
+    tile: (gid >>> 4) & 0xFFFFF,
+    rotation: (gid >>> 2) & 0x3,
+  };
+}
+
+module.exports = { encodeGid, decodeGid, MAX_ATLAS, MAX_TILE, MAX_ROT };
diff --git a/tests/gid.test.js b/tests/gid.test.js
new file mode 100644
index 0000000..4e0a4f8
--- /dev/null
+++ b/tests/gid.test.js
@@ -0,0 +1,83 @@
+'use strict';
+
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const { encodeGid, decodeGid } = require('../src/gid');
+
+test('encodeGid: known reference value (0,12,1) = 196', () => {
+  assert.equal(encodeGid(0, 12, 1), 196);
+});
+
+test('encodeGid: default rotation is 0', () => {
+  assert.equal(encodeGid(0, 12), 192);
+});
+
+test('encodeGid: zero tuple yields 0', () => {
+  assert.equal(encodeGid(0, 0, 0), 0);
+});
+
+test('encodeGid: high atlas (255) produces unsigned u32', () => {
+  // 255 << 24 = 0xFF000000 = 4278190080 (must NOT be negative)
+  assert.equal(encodeGid(255, 0, 0), 4278190080);
+});
+
+test('encodeGid: max tile_id (1048575) fits 20 bits', () => {
+  // tile << 4 = 0xFFFFF0 = 16777200
+  assert.equal(encodeGid(0, 1048575, 0), 16777200);
+});
+
+test('encodeGid: throws on out-of-range atlas', () => {
+  assert.throws(() => encodeGid(256, 0, 0), /atlas/);
+  assert.throws(() => encodeGid(-1, 0, 0), /atlas/);
+});
+
+test('encodeGid: throws on out-of-range tile', () => {
+  assert.throws(() => encodeGid(0, 1048576, 0), /tile/);
+  assert.throws(() => encodeGid(0, -1, 0), /tile/);
+});
+
+test('encodeGid: throws on out-of-range rotation', () => {
+  assert.throws(() => encodeGid(0, 0, 4), /rotation/);
+  assert.throws(() => encodeGid(0, 0, -1), /rotation/);
+});
+
+test('encodeGid: throws on non-integer inputs', () => {
+  assert.throws(() => encodeGid(1.5, 0, 0), /atlas/);
+  assert.throws(() => encodeGid(0, '12', 0), /tile/);
+});
+
+test('decodeGid: empty (0) returns zeros', () => {
+  assert.deepEqual(decodeGid(0), { atlas: 0, tile: 0, rotation: 0 });
+});
+
+test('decodeGid: 196 unpacks to (0,12,1)', () => {
+  assert.deepEqual(decodeGid(196), { atlas: 0, tile: 12, rotation: 1 });
+});
+
+test('decodeGid: 16777232 from real demo map unpacks to (1,1,0)', () => {
+  // demo_v2.map.json wall layer uses 16777232 = (1<<24)|(1<<4)
+  assert.deepEqual(decodeGid(16777232), { atlas: 1, tile: 1, rotation: 0 });
+});
+
+test('decodeGid: high atlas (255,0,0) round-trip', () => {
+  assert.deepEqual(decodeGid(4278190080), { atlas: 255, tile: 0, rotation: 0 });
+});
+
+test('decodeGid: throws on non-integer or negative', () => {
+  assert.throws(() => decodeGid(-1), /gid/);
+  assert.throws(() => decodeGid(1.5), /gid/);
+  assert.throws(() => decodeGid(0x100000000), /gid/); // > u32 max
+});
+
+test('round-trip: 100 random tuples encode→decode losslessly', () => {
+  for (let i = 0; i < 100; i++) {
+    const atlas = Math.floor(Math.random() * 256);
+    const tile = Math.floor(Math.random() * 1048576);
+    const rotation = Math.floor(Math.random() * 4);
+    const gid = encodeGid(atlas, tile, rotation);
+    const decoded = decodeGid(gid);
+    assert.equal(decoded.atlas, atlas, `atlas mismatch for ${gid}`);
+    assert.equal(decoded.tile, tile, `tile mismatch for ${gid}`);
+    assert.equal(decoded.rotation, rotation, `rotation mismatch for ${gid}`);
+  }
+});