Fun with NaN boxing

nanboxing.ts

/// We pack 64-bit integers into the 64 bytes of a double
type PackedInt64 = number;

const doubleView = new Float64Array(1);
const int32View = new Int32Array(doubleView.buffer);

export function pack(lo: number, hi: number): PackedInt64 {
    int32View[0] = lo;
    int32View[1] = hi;
    return doubleView[0];
}

exort function getLo(value: PackedInt64): number {
    doubleView[0] = value;
    return int32View[0];
}

export function getHi(value: PackedInt64): number {
    doubleView[0] = value;
    return int32View[1];
}

export function toBigInt(value: PackedInt64): bigint {
    return BigInt(getLo(value)) + BigInt(getHi(value)) << BigInt(32);
}

/// And this works in Chrome:
/// > let x = pack (12, 0xffffffff)
/// undefined
/// > x
/// NaN
/// > getLo(x)
/// 12
/// > getHi(x)
/// -1

/// But not in Firefox, because it turns all NaNs into a canonical form so it can use other NaN bitpatterns to store pointers
/// https://brionv.com/log/2018/05/17/javascript-engine-internals-nan-boxing/
///
/// > let x = pack(12,0xffffffff)
/// undefined 
/// > x
/// NaN
/// > getLo(x)
/// 0
/// > getHi(x)
/// 2146959360
///
/// > y = pack (1234, 0xffffffff)
// NaN
/// > getLo(y) 
/// 0 
/// > getHi(y) 
/// 2146959360
/// > getLo(x) == getLo(y) && getHi(x) == getHi(y)
/// true