fix: seq_hi shift for byte 6 (#775)

Author: doomgirl

src/v7.js

@@ -125,7 +125,7 @@ function v7(options, buf, offset) {
   b[i++] = _msecs & 0xff;
 
   // [byte 6] - set 4 bits of version (7) with first 4 bits seq_hi
-  b[i++] = ((seqHigh >>> 4) & 0x0f) | 0x70;
+  b[i++] = ((seqHigh >>> 8) & 0x0f) | 0x70;
 
   // [byte 7] remaining 8 bits of seq_hi
   b[i++] = seqHigh & 0xff;

test/unit/v7.test.js

@@ -1,5 +1,6 @@
 import assert from 'assert';
 import v7 from '../../src/v7.js';
+import stringify from '../../src/stringify.js';
 
 /**
  * fixture bit layout:
@@ -154,7 +155,7 @@ describe('v7', () => {
       seq,
     });
 
-    assert.strictEqual(uuid.substr(0, 25), '017f22e2-79b0-7fff-bfff-f');
+    assert.strictEqual(uuid.substr(0, 25), '017f22e2-79b0-7dff-bfff-f');
   });
 
   test('internal seq is reset upon timestamp change', () => {
@@ -169,4 +170,46 @@ describe('v7', () => {
 
     assert(uuid.indexOf('fff') !== 15);
   });
+
+  test('flipping bits changes the result', () => {
+    // convert uint8array to BigInt (BE)
+    const asBigInt = (buf) => buf.reduce((acc, v) => (acc << 8n) | BigInt(v), 0n);
+
+    // convert the given number of bits (LE) to number
+    const asNumber = (bits, data) => Number(BigInt.asUintN(bits, data));
+
+    // flip the nth bit  (BE) in a BigInt
+    const flip = (data, n) => data ^ (1n << BigInt(127 - n));
+
+    // Extract v7 `options` from a (BigInt) UUID
+    const optionsFrom = (data) => {
+      const ms = asNumber(48, data >> (128n - 48n));
+      const hi = asNumber(12, data >> (43n + 19n + 2n));
+      const lo = asNumber(19, data >> 43n);
+      const r = BigInt.asUintN(43, data);
+      return {
+        msecs: ms,
+        seq: (hi << 19) | lo,
+        random: [
+          ...Array(10).fill(0),
+          ...Array(6)
+            .fill(0)
+            .map((_, i) => asNumber(8, r >> (BigInt(i) * 8n)))
+            .reverse(),
+        ],
+      };
+    };
+    const buf = new Uint8Array(16);
+    const data = asBigInt(v7({}, buf));
+    const id = stringify(buf);
+    const reserved = [48, 49, 50, 51, 64, 65];
+    for (let i = 0; i < 128; ++i) {
+      if (reserved.includes(i)) {
+        continue; // skip bits used for version and variant
+      }
+      const flipped = flip(data, i);
+      assert.strictEqual(asBigInt(v7(optionsFrom(flipped), buf)), flipped, i);
+      assert.notStrictEqual(stringify(buf), id);
+    }
+  });
 });