[NUMBERS-201] FP64 equals is not consistent with hashCode

• Type: Bug
• Status: Closed
• Priority: Trivial
• Resolution: Fixed
• Affects Version/s: 1.1
• Fix Version/s: 1.2
• Component/s: field
• difficulty-level: Easy
• Assignee: Unassigned
• Reporter: Alex Herbert
• Created: 26/Jul/23 14:21
• Updated: 12/Aug/24 11:11
• Resolved: 04/Aug/23 08:04

Description

The FP64 class wraps a double so that it can be used as a field. The implementation of equals uses Precision with an allowed difference of up to 2 ULP:

public boolean equals(Object other) {
    if (other instanceof FP64) {
        final FP64 o = (FP64) other;
        return Precision.equals(value, o.value, 1);
    }
    return false;
}

The hashCode is generated using Double.hashCode(value).

The result is that two objects that are equal can generate a different hash code. This breaks the contract of Object.equals where two objects that are equal have the same hash code.

Possible fixes:

1. Use the equivalent of ((Double)value).equals(o.value) for a binary equality check between values that is consistent with double.
2. Document the class as having an equals method that is not consistent with hash code. 

I do not understand the requirement for using Precision. If a user has their own tolerance for field equivalence then this should be left to them to set the threshold. So I would recommend option 1.