define ncodeunits(c::Char) as fast equivalent of ncodeunits(string(c))

There was a non-public `codelen(c::Char)` method which previously did
this. This also replaces internal uses of this with `ncodeunits(c)`.

base/char.jl

@@ -50,6 +50,15 @@ Char
 (::Type{T})(x::AbstractChar) where {T<:Union{Number,AbstractChar}} = T(codepoint(x))
 (::Type{T})(x::T) where {T<:AbstractChar} = x
 
+"""
+    ncodeunits(c::Char) -> Int
+
+Return the number of code units required to encode a character as UTF-8.
+This is the number of bytes which will be printed if the character is written
+to an output stream, or `ncodeunits(string(c))` but computed efficiently.
+"""
+ncodeunits(c::Char) = write(devnull, c) # this is surprisingly efficient
+
 """
     codepoint(c::AbstractChar) -> Integer
 
@@ -197,7 +206,6 @@ hash(x::Char, h::UInt) =
     hash_uint64(((reinterpret(UInt32, x) + UInt64(0xd4d64234)) << 32) ⊻ UInt64(h))
 
 first_utf8_byte(c::Char) = (reinterpret(UInt32, c) >> 24) % UInt8
-codelen(c::Char) = 4 - (trailing_zeros(0xff000000 | reinterpret(UInt32, c)) >> 3)
 
 # fallbacks:
 isless(x::AbstractChar, y::AbstractChar) = isless(Char(x), Char(y))

base/io.jl

@@ -974,7 +974,7 @@ function skipchars(predicate, io::IO; linecomment=nothing)
         if c === linecomment
             readline(io)
         elseif !predicate(c)
-            skip(io, -codelen(c))
+            skip(io, -ncodeunits(c))
             break
         end
     end

base/strings/substring.jl

@@ -149,7 +149,7 @@ function string(a::Union{Char, String, SubString{String}}...)
     n = 0
     for v in a
         if v isa Char
-            n += codelen(v)
+            n += ncodeunits(v)
         else
             n += sizeof(v)
         end
@@ -159,7 +159,7 @@ function string(a::Union{Char, String, SubString{String}}...)
     for v in a
         if v isa Char
            x = bswap(reinterpret(UInt32, v))
-           for j in 1:codelen(v)
+           for j in 1:ncodeunits(v)
                unsafe_store!(pointer(out, offs), x % UInt8)
                offs += 1
                x >>= 8

test/char.jl

@@ -256,3 +256,26 @@ Base.codepoint(c::ASCIIChar) = reinterpret(UInt8, c)
     @test_throws MethodError write(IOBuffer(), ASCIIChar('x'))
     @test_throws MethodError read(IOBuffer('x'), ASCIIChar)
 end
+
+@testset "ncodeunits(::Char)" begin
+    # valid encodings
+    @test ncodeunits('\0')       == 1
+    @test ncodeunits('\x1')      == 1
+    @test ncodeunits('\x7f')     == 1
+    @test ncodeunits('\u80')     == 2
+    @test ncodeunits('\uff')     == 2
+    @test ncodeunits('\u7ff')    == 2
+    @test ncodeunits('\u800')    == 3
+    @test ncodeunits('\uffff')   == 3
+    @test ncodeunits('\U10000')  == 4
+    @test ncodeunits('\U10ffff') == 4
+    # invalid encodings
+    @test ncodeunits(reinterpret(Char, 0x80_00_00_00)) == 1
+    @test ncodeunits(reinterpret(Char, 0x81_00_00_00)) == 1
+    @test ncodeunits(reinterpret(Char, 0x80_80_00_00)) == 2
+    @test ncodeunits(reinterpret(Char, 0x80_01_00_00)) == 2
+    @test ncodeunits(reinterpret(Char, 0x80_00_80_00)) == 3
+    @test ncodeunits(reinterpret(Char, 0x80_00_01_00)) == 3
+    @test ncodeunits(reinterpret(Char, 0x80_00_00_80)) == 4
+    @test ncodeunits(reinterpret(Char, 0x80_00_00_01)) == 4
+end

test/iostream.jl

@@ -30,9 +30,9 @@
         @test read(file, Char) == 'n'
 
         # test it correctly handles unicode
-        for (byte,char) in zip(1:4, ('@','߷','࿊','𐋺'))
+        for (byte, char) in zip(1:4, ('@','߷','࿊','𐋺'))
             append_to_file("abcdef$char")
-            @test Base.codelen(char) == byte
+            @test ncodeunits(char) == byte
             @test !eof(skipchars(isletter, file))
             @test read(file, Char) == char
         end