Improve char to int parsing, and add tryparse method (#59603)

This PR does two things:

* Add a new method `tryparse(::Type{T}, c::AbstractChar;
base::Integer=10) where {T <: Integer}` which was previously missing
* Speed up parsing of chars to integer by ~4x

Author: jakobnissen

base/parse.jl

@@ -38,16 +38,38 @@ julia> parse(Complex{Float64}, "3.2e-1 + 4.5im")
 parse(T::Type, str; base = Int)
 parse(::Type{Union{}}, slurp...; kwargs...) = error("cannot parse a value as Union{}")
 
-function parse(::Type{T}, c::AbstractChar; base::Integer = 10) where T<:Integer
-    a::Int = (base <= 36 ? 10 : 36)
-    2 <= base <= 62 || throw(ArgumentError("invalid base: base must be 2 ≤ base ≤ 62, got $base"))
-    d = '0' <= c <= '9' ? c-'0'    :
-        'A' <= c <= 'Z' ? c-'A'+10 :
-        'a' <= c <= 'z' ? c-'a'+a  : throw(ArgumentError("invalid digit: $(repr(c))"))
-    d < base || throw(ArgumentError("invalid base $base digit $(repr(c))"))
-    convert(T, d)
+@noinline function _invalid_base(base)
+    throw(ArgumentError("invalid base: base must be 2 ≤ base ≤ 62, got $base"))
+end
+
+@noinline _invalid_digit(base, char) = throw(ArgumentError("invalid base $base digit $(repr(char))"))
+
+function parse_char(::Type{T}, c::AbstractChar, base::Integer, throw::Bool) where T
+    a::UInt8 = (base <= 36 ? 10 : 36)
+    (2 <= base <= 62) || _invalid_base(base)
+    base = base % UInt8
+    cp = codepoint(c)
+    cp = cp > 0x7a ? 0xff : cp % UInt8
+    d = UInt8('0') ≤ cp ≤ UInt8('9') ? cp - UInt8('0') :
+        UInt8('A') ≤ cp ≤ UInt8('Z') ? cp - UInt8('A') + UInt8(10) :
+        UInt8('a') ≤ cp ≤ UInt8('z') ? cp - UInt8('a') + a :
+        0xff
+    d < base || (throw ? _invalid_digit(base, c) : return nothing)
+    convert(T, d)::T
+end
+
+function parse(::Type{T}, c::AbstractChar; base::Integer=10) where {T <: Integer}
+    @inline parse_char(T, c, base, true)
 end
 
+function tryparse(::Type{T}, c::AbstractChar; base::Integer=10) where {T <: Integer}
+    @inline parse_char(T, c, base, false)
+end
+
+# For consistency with parse(t, AbstractString), support a `base` argument only when T<:Integer
+parse(::Type{T}, c::AbstractChar) where T = @inline parse_char(T, c, 10, true)
+tryparse(::Type{T}, c::AbstractChar) where T = @inline parse_char(T, c, 10, false)
+
 function parseint_iterate(s::AbstractString, startpos::Int, endpos::Int)
     (0 < startpos <= endpos) || (return Char(0), 0, 0)
     j = startpos
@@ -116,8 +138,7 @@ function tryparse_internal(::Type{T}, s::AbstractString, startpos::Int, endpos::
         return nothing
     end
     if !(2 <= base <= 62)
-        raise && throw(ArgumentError(LazyString("invalid base: base must be 2 ≤ base ≤ 62, got ", base)))
-        return nothing
+        raise ? _invalid_base(base) : return nothing
     end
     if i == 0
         raise && throw(ArgumentError("premature end of integer: $(repr(SubString(s,startpos,endpos)))"))
@@ -236,7 +257,7 @@ end
     if 2 <= base <= 62
         return base
     end
-    throw(ArgumentError("invalid base: base must be 2 ≤ base ≤ 62, got $base"))
+    _invalid_base(base)
 end
 
 """

test/parse.jl

@@ -29,6 +29,13 @@
     @test parse(Int, 'a', base=16) == 10
     @test_throws ArgumentError parse(Int, 'a')
     @test_throws ArgumentError parse(Int,typemax(Char))
+
+    @test tryparse(Int, '8') === 8
+    @test tryparse(Int, 'a') === nothing
+    @test tryparse(Int, 'a'; base=11) === 10
+    @test tryparse(Int32, 'a'; base=11) === Int32(10)
+    @test tryparse(UInt8, 'f'; base=16) === 0x0f
+    @test tryparse(UInt8, 'f'; base=15) === nothing
 end
 
 # Issue 29451