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fix: correct carry for large negative 64-bit values #215

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fix: correct carry for large negative 64-bit values #215

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7 changes: 7 additions & 0 deletions packages/protons-runtime/package.json
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Original file line number Diff line number Diff line change
Expand Up @@ -128,6 +128,13 @@
"dep-check": "aegir dep-check",
"doc-check": "aegir doc-check",
"build": "aegir build",
"test": "aegir test",
"test:chrome": "aegir test -t browser --cov",
"test:chrome-webworker": "aegir test -t webworker",
"test:firefox": "aegir test -t browser -- --browser firefox",
"test:firefox-webworker": "aegir test -t webworker -- --browser firefox",
"test:node": "aegir test -t node --cov",
"test:electron-main": "aegir test -t electron-main",
"release": "aegir release"
},
"dependencies": {
Expand Down
24 changes: 3 additions & 21 deletions packages/protons-runtime/src/utils/longbits.ts
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Expand Up @@ -122,26 +122,10 @@ export class LongBits {
return this.fromNumber(Number(value))
}

const negative = value < 0n
// two's complement in one step, so a zero low word carries into the high word
const bits = BigInt.asUintN(64, value)

if (negative) {
value = -value
}

let hi = value >> 32n
let lo = value - (hi << 32n)

if (negative) {
hi = ~hi | 0n
lo = ~lo | 0n

if (++lo > TWO_32) {
lo = 0n
if (++hi > TWO_32) { hi = 0n }
}
}

return new LongBits(Number(lo), Number(hi))
return new LongBits(Number(bits & 0xffffffffn), Number(bits >> 32n))
}

/**
Expand Down Expand Up @@ -185,5 +169,3 @@ const zero = new LongBits(0, 0)
zero.toBigInt = function () { return 0n }
zero.zzEncode = zero.zzDecode = function () { return this }
zero.length = function () { return 1 }

const TWO_32 = 4294967296n
51 changes: 51 additions & 0 deletions packages/protons-runtime/test/longbits.spec.ts
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@@ -0,0 +1,51 @@
import { expect } from 'aegir/chai'
import { reader, writer } from '../src/index.ts'

// negative values below -(2^53 - 1) whose low 32 bits are zero used to lose
// the two's-complement carry into the high word and encode as value - 2^32
describe('negative 64-bit values', () => {
const cases: Record<'int64' | 'sint64' | 'sfixed64', Array<[bigint, number[]]>> = {
int64: [
[-(2n ** 53n), [0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0xf0, 0xff, 0x01]],
[-(2n ** 53n + 2n ** 32n), [0x80, 0x80, 0x80, 0x80, 0xf0, 0xff, 0xff, 0xef, 0xff, 0x01]],
[-(2n ** 60n), [0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0xf0, 0x01]],
[-(2n ** 63n - 2n ** 32n), [0x80, 0x80, 0x80, 0x80, 0x90, 0x80, 0x80, 0x80, 0x80, 0x01]],
[-(2n ** 63n), [0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x01]],
[-(2n ** 53n + 1n), [0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xef, 0xff, 0x01]]
],
sint64: [
[-(2n ** 53n), [0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x1f]],
[-(2n ** 53n + 2n ** 32n), [0xff, 0xff, 0xff, 0xff, 0x9f, 0x80, 0x80, 0x20]],
[-(2n ** 60n), [0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x1f]],
[-(2n ** 63n - 2n ** 32n), [0xff, 0xff, 0xff, 0xff, 0xdf, 0xff, 0xff, 0xff, 0xff, 0x01]],
[-(2n ** 63n), [0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x01]],
[-(2n ** 53n + 1n), [0x81, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x20]]
],
sfixed64: [
[-(2n ** 53n), [0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xe0, 0xff]],
[-(2n ** 53n + 2n ** 32n), [0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xdf, 0xff]],
[-(2n ** 60n), [0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xf0]],
[-(2n ** 63n - 2n ** 32n), [0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x80]],
[-(2n ** 63n), [0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80]],
[-(2n ** 53n + 1n), [0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xdf, 0xff]]
]
}

for (const method of ['int64', 'sint64', 'sfixed64'] as const) {
for (const [value, bytes] of cases[method]) {
it(`should encode ${method} ${value}`, () => {
const w = writer()
w[method](value)

expect(w.finish()).to.equalBytes(bytes)
})

it(`should round trip ${method} ${value}`, () => {
const w = writer()
w[method](value)

expect(reader(w.finish())[method]()).to.equal(value)
})
}
}
})
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