Implement Base.one for StringView (#24)

* Implement Base.one for StringView

I have a lot of `S where S<:AbstractString` code which uses `one(S)` for the empty string to compare against. I didn't realize until I went to add this that `typemin` also works, but hey, that makes the implementation that much easier, and StringViews gives me a zero-copy drop-in replacement for a string buffer with this addition.

* Update runtests.jl

And a test for identity equality

* Union type signature for Base.one

Co-authored-by: Steven G. Johnson <[email protected]>

* Update test/runtests.jl

Co-authored-by: Steven G. Johnson <[email protected]>

* Adds Base.oneunit

Also defined for Strings

* Test for oneunit

* Adds convert for data type of StringView

As well as a custom typemin for StringView(Base.CodeUnits{etc}}.

This makes the Base implementation of typemin function for both
s and su in the test suite, and I would consider returning the
data field for an attempt to convert a StringView to its underlying
data to be legitimate.

* Remove settings.json

* remove settings.json entirely :/

* Removes Manifest.toml

* Remove convert, add StringViews{S} constructor

* Remove Manifest.toml

---------

Co-authored-by: Steven G. Johnson <[email protected]>

Author: mnemnion

src/StringViews.jl

@@ -39,6 +39,7 @@ Base.Vector{UInt8}(s::StringViewAndSub) = Vector{UInt8}(codeunits(s))
 Base.Array{UInt8}(s::StringViewAndSub) = Vector{UInt8}(s)
 Base.String(s::StringViewAndSub) = String(copyto!(Base.StringVector(ncodeunits(s)), codeunits(s)))
 StringView(s::StringView) = s
+StringView{S}(s::StringView{S}) where {S<:AbstractVector{UInt8}} = s
 StringView(s::String) = StringView(codeunits(s))
 
 # iobuffer constructor (note that buf.data is always 1-based)
@@ -85,7 +86,9 @@ end
 Base.:(==)(s1::StringViewAndSub, s2::StringAndSub) = s2 == s1
 
 Base.typemin(::Type{StringView{Vector{UInt8}}}) = StringView(Vector{UInt8}(undef,0))
+Base.typemin(::Type{StringView{Base.CodeUnits{UInt8, String}}}) = StringView("")
 Base.typemin(::T) where {T<:StringView} = typemin(T)
+Base.one(::Union{T,Type{T}}) where {T<:StringView} = typemin(T)
 
 if VERSION < v"1.10.0-DEV.1007" # JuliaLang/julia#47880
     Base.isvalid(s::DenseStringViewAndSub) = ccall(:u8_isvalid, Int32, (Ptr{UInt8}, Int), s, sizeof(s)) ≠ 0

test/runtests.jl

@@ -157,6 +157,10 @@ end
 
     @test Base.typemin(s) isa StringView{Vector{UInt8}}
     @test Base.typemin(s) == ""
+    @test one(s) == one(typeof(s)) == typemin(s) == ""
+    @test oneunit(s) == oneunit(typeof(s)) == one(s) == ""
+    @test one(su) == one(typeof(su)) == typemin(su) == ""
+    @test oneunit(su) == oneunit(typeof(su)) == one(su) == ""
 
     @test isascii(s)
     @test !isascii(StringView("fööbār"))