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[stdlib] Fix bug in Bit with-overflow arithmetic #680

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Merged
gribozavr merged 5 commits into from
Dec 21, 2015
Merged

[stdlib] Fix bug in Bit with-overflow arithmetic #680

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9f16272
[stdlib] Fix bug in Bit with-overflow arithmetic
PatrickPijnappel Dec 20, 2015
56ed11e
[stdlib] Refactor local variables names
PatrickPijnappel Dec 20, 2015
3e80f8d
[stdlib] Add test for Bit with-overflow arithmetic bug
PatrickPijnappel Dec 20, 2015
f8c155a
[stdlib] Refactor Bit._withOverflow()
PatrickPijnappel Dec 20, 2015
f819683
[stdlib] Adjust formatting to fit 80 columns
PatrickPijnappel Dec 21, 2015
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12 changes: 8 additions & 4 deletions stdlib/public/core/Bit.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -94,11 +94,15 @@ public func < (lhs: Bit, rhs: Bit) -> Bool {
}

extension Bit : IntegerArithmeticType {
static func _withOverflow(v: (Int, overflow: Bool)) -> (Bit, overflow: Bool) {
if let b = Bit(rawValue: v.0) {
return (b, v.overflow)
static func _withOverflow(
intResult: Int, overflow: Bool
) -> (Bit, overflow: Bool) {
if let bit = Bit(rawValue: intResult) {
return (bit, overflow: overflow)
} else {
return (Bit(rawValue: v.0 % 2)!, true)
let bitRaw = intResult % 2 + (intResult < 0 ? 2 : 0)
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@gribozavr gribozavr Dec 21, 2015

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I think this might be simpler, and yet produce the same result: let bitRaw = (intResult + 2) % 2.

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@PatrickPijnappel PatrickPijnappel Dec 21, 2015

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Well, that would rely on the precondition that intResult >= -2. This is true for all current calls to the function, but the way it's written now trivially eliminates this precondition.

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@gribozavr gribozavr Dec 21, 2015

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OK.

let bit = Bit(rawValue: bitRaw)!
return (bit, overflow: true)
}
}

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2 changes: 2 additions & 0 deletions test/1_stdlib/Bit.swift
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Expand Up @@ -27,6 +27,8 @@ print(one.predecessor().rawValue)

// CHECK-NEXT: 0
print((one &+ one).rawValue)
// CHECK-NEXT: 1
print((zero &- one).rawValue)

// CHECK: done.
print("done.")
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