toSigned abstract method

Returns the least significant width bits of this integer, extending the highest retained bit to the sign. This is the same as truncating the value to fit in width bits using an signed 2-s complement representation. The returned value has the same bit value in all positions higher than width.

var big15 = BigInt.from(15);
var big16 = BigInt.from(16);
var big239 = BigInt.from(239);
                               //     V--sign bit-V
big16.toSigned(5) == -big16;   //  00010000 -> 11110000
big239.toSigned(5) == big15;   //  11101111 -> 00001111
                               //     ^           ^

This operation can be used to simulate arithmetic from low level languages. For example, to increment an 8 bit signed quantity:

q = (q + 1).toSigned(8);

q will count from 0 up to 127, wrap to -128 and count back up to 127.

If the input value fits in width bits without truncation, the result is the same as the input. The minimum width needed to avoid truncation of x is x.bitLength + 1, i.e.

x == x.toSigned(x.bitLength + 1);