The precision should be 17 (16 bits need after dot) after dot for double type.

The precision is lost when converting double to string.
E.g:
double PI = 3.1415926535897931;
string value = format("%.16g", PI);

The value will be '3.141592653589793' and last 1 is lost after format operation.
But expected value should be '3.1415926535897931'.

Solution:
string value = format("%.17g", PI);