float rounding mistake when optimizing Rails

[toaster]

Oj version: 3.3.9
Ruby version: 2.3.3
Rails version: 5.0.1

[1] pry(main)> f = -1.7976931348623157e+308
=> -1.7976931348623157e+308
[2] pry(main)> require 'rails'
=> true
[3] pry(main)> f.to_json
=> "-1.7976931348623157e+308"
[4] pry(main)> require 'oj'
=> true
[5] pry(main)> Oj.dump(f)
=> "-1.79769313486232e+308"
[6] pry(main)> Oj.default_options = {float_precision: 0}
=> {:float_precision=>0}
[7] pry(main)> Oj.dump(f)
=> "-1.7976931348623157e+308"
[8] pry(main)> Oj.optimize_rails
=> nil
[9] pry(main)> f.to_json
=> "-1.797693134862316e+308"

As you can see, the default Rails to_json does not round the number.
Oj rounds because of its float_precision but this can be turned of using float_precision: 0.
But when Oj optimizes the Rails implementation, it uses a precision independent from its default_options and not fully compatible to the standard Rails behaviour.

[ohler55]

I can bump up the precision for the optimized version to 17 significant digits but you will then find that the 17th digit is sometimes different. Optimization uses a different conversion from a float (C double) to a string. Any more than 16 digits will vary since the binary encoding for a float does not support more than 16 digits across all numbers. If exact behavior with rails out to 17 significant digits is more important than performance then the extra step of optimizing should not be taken.

[toaster]

Okay, I understand that.
We will skip the optimization part, then.

[ohler55]

Thanks for understanding. Float precision has been trouble for quite a while.