export and document transcode (#17323)

* export and document transcode from #16974, add transcode(String, x) and transcode(T, ::String) convenience methods

* docs

* support UTF-32 in transcode

* don't use splatting for UTF-32 to String conversion

* typo

* eliminate method ambiguities

* re-run genstdlib

* doc clarification

* typo

Author: stevengj

NEWS.md

@@ -131,6 +131,9 @@ Library improvements
       `String(s)`, `unsafe_string(ptr)` (formerly `bytestring(ptr)`), and
       `unsafe_wrap(String, ptr)` (formerly `pointer_to_string`) ([#16731]).
 
+    * A `transcode(T, src)` function is now exported for converting data
+      between UTF-xx Unicode encodings ([#17323]).
+
   * Most of the combinatorics functions have been moved from `Base`
     to the [Combinatorics.jl package](https://github.com/JuliaLang/Combinatorics.jl) ([#13897]).
 
@@ -321,4 +324,5 @@ Deprecated or removed
 [#17075]: https://github.com/JuliaLang/julia/issues/17075
 [#17266]: https://github.com/JuliaLang/julia/issues/17266
 [#17300]: https://github.com/JuliaLang/julia/issues/17300
+[#17323]: https://github.com/JuliaLang/julia/issues/17323
 [#17374]: https://github.com/JuliaLang/julia/issues/17374

base/c.jl

@@ -128,20 +128,39 @@ function cwstring(s::AbstractString)
 end
 end
 
-# transcoding between data in UTF-8 and UTF-16 for Windows APIs
+# transcoding between data in UTF-8 and UTF-16 for Windows APIs,
+# and also UTF-32 for APIs using Cwchar_t on other platforms.
+
 """
-    Base.transcode(T,src::Vector{U})
+    transcode(T, src)
+
+Convert string data between Unicode encodings. `src` is either a
+`String` or a `Vector{UIntXX}` of UTF-XX code units, where
+`XX` is 8, 16, or 32. `T` indicates the encoding of the return value:
+`String` to return a (UTF-8 encoded) `String` or `UIntXX`
+to return a `Vector{UIntXX}` of UTF-`XX` data.   (The alias `Cwchar_t`
+can also be used as the integer type, for converting `wchar_t*` strings
+used by external C libraries.)
 
-Transcodes unicode data `src` to a different encoding, where `U` and `T` are the integers
-denoting the input and output code units. Currently supported are UTF-8 and UTF-16, which
-are denoted by integers `UInt8` and `UInt16`, respectively.
+The `transcode` function succeeds as long as the input data can be
+reasonably represented in the target encoding; it always succeeds for
+conversions between UTF-XX encodings, even for invalid Unicode data.
 
-NULs are handled like any other character (i.e. the output will be NUL-terminated if and
-only if the `src` is).
+Only conversion to/from UTF-8 is currently supported.
 """
 function transcode end
-transcode{T<:Union{UInt8,UInt16}}(::Type{T}, src::Vector{T}) = src
-transcode(::Type{Int32}, src::Vector{UInt32}) = reinterpret(Int32, src)
+
+transcode{T<:Union{UInt8,UInt16,UInt32,Int32}}(::Type{T}, src::Vector{T}) = src
+transcode{T<:Union{Int32,UInt32}}(::Type{T}, src::String) = T[T(c) for c in src]
+transcode{T<:Union{Int32,UInt32}}(::Type{T}, src::Vector{UInt8}) = transcode(T, String(src))
+function transcode{S<:Union{Int32,UInt32}}(::Type{UInt8}, src::Vector{S})
+    buf = IOBuffer()
+    for c in src; print(buf, Char(c)); end
+    takebuf_array(buf)
+end
+transcode(::Type{String}, src::String) = src
+transcode(T, src::String) = transcode(T, src.data)
+transcode(::Type{String}, src) = String(transcode(UInt8, src))
 
 function transcode(::Type{UInt16}, src::Vector{UInt8})
     dst = UInt16[]

base/env.jl

@@ -19,7 +19,7 @@ function access_env(onError::Function, str::AbstractString)
         error(string("getenv: ", str, ' ', len, "-1 != ", ret, ": ", Libc.FormatMessage()))
     end
     pop!(val) # NUL
-    return String(transcode(UInt8, val))
+    return transcode(String, val)
 end
 
 function _setenv(svar::AbstractString, sval::AbstractString, overwrite::Bool=true)
@@ -97,7 +97,7 @@ function next(hash::EnvHash, block::Tuple{Ptr{UInt16},Ptr{UInt16}})
     len = ccall(:wcslen, UInt, (Ptr{UInt16},), pos)
     buf = Array{UInt16}(len)
     unsafe_copy!(pointer(buf), pos, len)
-    env = String(transcode(UInt8, buf))
+    env = transcode(String, buf)
     m = match(r"^(=?[^=]+)=(.*)$"s, env)
     if m === nothing
         error("malformed environment entry: $env")

base/exports.jl

@@ -874,6 +874,7 @@ export
     strip,
     strwidth,
     summary,
+    transcode,
     ucfirst,
     unescape_string,
     uppercase,

base/file.jl

@@ -203,7 +203,7 @@ function tempdir()
         error("GetTempPath failed: $(Libc.FormatMessage())")
     end
     resize!(temppath,lentemppath)
-    return String(transcode(UInt8, temppath))
+    return transcode(String, temppath)
 end
 tempname(uunique::UInt32=UInt32(0)) = tempname(tempdir(), uunique)
 const temp_prefix = cwstring("jl_")
@@ -216,7 +216,7 @@ function tempname(temppath::AbstractString,uunique::UInt32)
         error("GetTempFileName failed: $(Libc.FormatMessage())")
     end
     resize!(tname,lentname)
-    return String(transcode(UInt8, tname))
+    return transcode(String, tname)
 end
 function mktemp(parent=tempdir())
     filename = tempname(parent, UInt32(0))

base/interactiveutil.jl

@@ -150,7 +150,7 @@ elseif is_windows()
         len = 0
         while unsafe_load(plock, len+1) != 0; len += 1; end
         # get Vector{UInt16}, transcode data to UTF-8, make a String of it
-        s = String(transcode(UInt8, unsafe_wrap(Array, plock, len)))
+        s = transcode(String, unsafe_wrap(Array, plock, len))
         systemerror(:GlobalUnlock, 0==ccall((:GlobalUnlock, "kernel32"), stdcall, Cint, (Ptr{UInt16},), plock))
         return s
     end

base/libc.jl

@@ -277,7 +277,7 @@ if is_windows()
         buf = Array{UInt16}(len)
         unsafe_copy!(pointer(buf), p, len)
         ccall(:LocalFree,stdcall,Ptr{Void},(Ptr{Void},),p)
-        return String(transcode(UInt8, buf))
+        return transcode(String, buf)
     end
 end
 

base/path.jl

@@ -136,7 +136,7 @@ function realpath(path::AbstractString)
         systemerror(:realpath, n == 0)
         x = n < length(buf) # is the buffer big enough?
         resize!(buf, n) # shrink if x, grow if !x
-        x && return String(transcode(UInt8, buf))
+        x && return transcode(String, buf)
     end
 end
 
@@ -150,7 +150,7 @@ function longpath(path::AbstractString)
         systemerror(:longpath, n == 0)
         x = n < length(buf) # is the buffer big enough?
         resize!(buf, n) # shrink if x, grow if !x
-        x && return String(transcode(UInt8, buf))
+        x && return transcode(String, buf)
     end
 end
 

doc/manual/strings.rst

@@ -352,14 +352,16 @@ exception handling required:
     <BLANKLINE>
     y
 
-Julia uses UTF-8 encoding by default, and support for new encodings can
+Julia uses the UTF-8 encoding by default, and support for new encodings can
 be added by packages. For example, the `LegacyStrings.jl
 <https://github.com/JuliaArchive/LegacyStrings.jl>`_ package implements
 ``UTF16String`` and ``UTF32String`` types. Additional discussion of other
 encodings and how to implement support for them is beyond the scope of this
 document for the time being. For further discussion of UTF-8 encoding issues,
-see the section below on `byte array literals <#Byte+Array+Literals>`_,
-which goes into some greater detail.
+see the section below on `byte array literals <#Byte+Array+Literals>`_.
+The :func:`transcode` function is provided to convert data between
+the various UTF-xx encodings, primarily for working with external
+data and libraries.
 
 .. _man-string-interpolation:
 

doc/stdlib/strings.rst

@@ -56,6 +56,16 @@
 
    Convert a string to a contiguous byte array representation encoded as UTF-8 bytes. This representation is often appropriate for passing strings to C.
 
+.. function:: transcode(T, src)
+
+   .. Docstring generated from Julia source
+
+   Convert string data between Unicode encodings. ``src`` is either a ``String`` or a ``Vector{UIntXX}`` of UTF-XX code units, where ``XX`` is 8, 16, or 32. ``T`` indicates the encoding of the return value: ``String`` to return a (UTF-8 encoded) ``String`` or ``UIntXX`` to return a ``Vector{UIntXX}`` of UTF-``XX`` data.   (The alias ``Cwchar_t`` can also be used as the integer type, for converting ``wchar_t*`` strings used by external C libraries.)
+
+   The ``transcode`` function succeeds as long as the input data can be reasonably represented in the target encoding; it always succeeds for conversions between UTF-XX encodings, even for invalid Unicode data.
+
+   Only conversion to/from UTF-8 is currently supported.
+
 .. function:: unsafe_string(p::Ptr{UInt8}, [length::Integer])
 
    .. Docstring generated from Julia source
@@ -472,4 +482,3 @@
    .. Docstring generated from Julia source
 
    General unescaping of traditional C and Unicode escape sequences. Reverse of :func:`escape_string`\ . See also :func:`unescape_string`\ .
-