[SR-8409] Decimal initialized with Double loses precision

[swift-ci]

Previous ID	SR-8409
Radar	None
Original Reporter	TomasLinhart (JIRA User)
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Foundation
Labels	Bug
Assignee	None
Priority	Medium

md5: 87fa6caf176bfbf5ed96bbc10110c73b

is duplicated by:

• SR-11514 Decimal.init(_ value: Double) sometimes yields incorrect value

Issue Description:

Decimal precision is a bit strange for a number 456.789. When you initialize Double with 456.789 it holds the number so it is representable, but if you try to convert it to a decimal with Decimal(456.789) it results in 456.7889999999998976 but if you use Decimal(string: "456.789")! it results in a correct decimal with a value 456.789.

I tried also Objective-C, and it works correctly there:

NSDecimalNumber *number = [[NSDecimalNumber alloc] initWithDouble:456.789];
NSLog(@"Number: %f", number.doubleValue);

[belkadan]

cc @stephentyrone

[stephentyrone]

This is mostly expected. `456.789 as Double` has the exact value 456.788999999999987267074175179004669189453125, which is not exactly representable as `Decimal`, so it gets rounded to a representable `Decimal` value.

That second rounding appears to be not quite perfect (that's a Foundation bug), but even if Foundation were rounding correctly, the result would not be 456.789. It would be something like 456.78899999999998726707417517900466919 instead.

[spevans]

I wonder if a better solution would be to use the .description property of Double:

public init(_ value: Double) {
    self = Decimal(string: value.description) ?? Decimal.nan
}

and Decimal.description:

internal var doubleValue: Double {
    return Double(self.description) ?? Double.nan
} 

Whilst there will be a conversion cost going via a String, it may provide less surprising values overall.