fix(utils): make mToNsBigint tolerate fractional milliseconds

`mToNsBigint` did `BigInt(ms) * 1_000_000n`, which throws "RangeError: ...
not an integer" for a fractional `ms`. This is reachable via
`new Bench({ concurrency: 'task', time })` with a non-integer `time` /
`warmupTime` on the hrtime provider (Node's default): `withConcurrency`
calls `fromMs(time)` and the task silently ends up in `state: 'errored'`.

Round to the nearest nanosecond with `BigInt(Math.round(ms * 1e6))` —
identical for integer ms, no longer throwing on fractional ms. Add a
regression test.

Also document that the exported `hrtimeNow` narrows the absolute timestamp
to a number (lossy past ~104 days of uptime); prefer
`hrtimeNowTimestampProvider`.

Author: jerome-benoit

src/utils.ts

@@ -151,7 +151,7 @@ export const nBigintToMs = (ns: bigint) => Number(ns) / 1e6
  * @param ms - milliseconds
  * @returns nanoseconds as bigint
  */
-export const mToNsBigint = (ms: number) => BigInt(ms) * 1_000_000n
+export const mToNsBigint = (ms: number) => BigInt(Math.round(ms * 1e6))
 
 /**
  * Formats a number with the specified significant digits and maximum fraction digits.
@@ -906,6 +906,11 @@ const hrtimeBigint =
 
 /**
  * Returns the current timestamp in milliseconds using `process.hrtime.bigint()`.
+ *
+ * Narrows the absolute nanosecond value to a `number`, which loses precision
+ * once it exceeds `Number.MAX_SAFE_INTEGER` (~104 days of uptime). For
+ * benchmarking prefer `hrtimeNowTimestampProvider`, which keeps the bigint
+ * until after the delta is taken.
  * @returns the current timestamp in milliseconds
  */
 export const hrtimeNow = () => nToMs(Number(hrtimeBigint()))

test/utils-ms-to-n.test.ts

@@ -1,10 +1,17 @@
 import { expect, test } from 'vitest'
 
-import { mToNs } from '../src/utils'
+import { mToNs, mToNsBigint } from '../src/utils'
 
 test('mToNs', () => {
   expect(mToNs(1)).toBe(1000000)
   expect(mToNs(0.000001)).toBe(1)
   expect(mToNs(1234.56789)).toBe(1234567890)
   expect(mToNs(-1)).toBe(-1000000)
 })
+
+test('mToNsBigint', () => {
+  expect(mToNsBigint(1)).toBe(1000000n)
+  expect(mToNsBigint(1000)).toBe(1000000000n)
+  expect(mToNsBigint(100.5)).toBe(100500000n)
+  expect(mToNsBigint(1234.56789)).toBe(1234567890n)
+})