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New FP Parsing: Use relevant part of a substring #86018

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16 changes: 11 additions & 5 deletions stdlib/public/core/FloatingPointParsing.swift.gyb
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Original file line number Diff line number Diff line change
Expand Up @@ -256,15 +256,21 @@ extension ${Self}: LosslessStringConvertible {
// Use the all-Swift `parse_float${bits}()` implementation for Float16/32/64
@available(SwiftStdlib 5.3, *)
public init?(_ text: Substring) {
// TODO: Someday, this whole function should simplify down to just:
// ${Self}(text.utf8.span)
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@glessard glessard Dec 17, 2025 •
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Can we spell it that way for the platforms that don't have availability issues?

#if _pointerBitWidth(_16)
// Always fail on 16-bit targets
return nil
#else
// Work around span availability limits
let parsed = unsafe text.base._guts.withFastUTF8 { chars -> ${Self}? in
unsafe parse_float${bits}(chars.span)
let parsed: ${Self}?
if text.base._guts.isFastUTF8 {
parsed = unsafe text.base._guts.withFastUTF8 { chars -> ${Self}? in
unsafe parse_float${bits}(chars.span.extracting(text._offsetRange))
}
} else {
// Otherwise, make a copy so we have contiguous UTF8...
let string = String(text)
parsed = unsafe string._guts.withFastUTF8 { chars -> ${Self}? in
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@tbkka tbkka Dec 12, 2025

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In particular, can I reliably assume that a copied string will always support withFastUTF8?

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@Catfish-Man Catfish-Man Dec 15, 2025

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Why not just use https://developer.apple.com/documentation/swift/string/makecontiguousutf8() ?

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@glessard glessard Dec 16, 2025

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The current implementation does always produce a string that supports withFastUTF8, but we might as well be sure and call makeContiguousUTF8(). I would suggest withUTF8, since it is a wrapper for makeContiguousUTF8() followed by _guts.withFastUTF8…

unsafe parse_float${bits}(chars.span)
}
}

if let parsed {
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8 changes: 8 additions & 0 deletions test/stdlib/ParseFloat64.swift
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Expand Up @@ -328,6 +328,14 @@ tests.test("Decimal Floats") {
expectParse("999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999.999999999999999999999999999", Float64.infinity)
}

tests.test("Substring") {
let s1 = "1.02.03.0"
let s1sub = s1[s1.firstIndex(of: "2")!..<s1.firstIndex(of: "3")!]
let parsed = Float64(s1sub)
expectNotNil(parsed)
expectEqual(parsed!.bitPattern, (2.0).bitPattern)
}

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@tbkka tbkka Dec 12, 2025

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Do I need to test bridged strings specially here?

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@Catfish-Man Catfish-Man Dec 15, 2025

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I'm not going to turn down free test coverage of my code if it's offered, but I don't think it's at all necessary.

@_extern(c, "_swift_stdlib_strtod_clocale")
func _swift_stdlib_strtod_clocale(
_: Optional<UnsafePointer<CChar>>,
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