# This file is a part of Julia. License is MIT: https://julialang.org/license

if Sys.iswindows()
    const ERROR_ENVVAR_NOT_FOUND = UInt32(203)

    _getenvlen(var::Vector{UInt16}) = ccall(:GetEnvironmentVariableW,stdcall,UInt32,(Ptr{UInt16},Ptr{UInt16},UInt32),var,C_NULL,0)
    _hasenv(s::Vector{UInt16}) = _getenvlen(s) != 0 || Libc.GetLastError() != ERROR_ENVVAR_NOT_FOUND
    _hasenv(s::AbstractString) = _hasenv(cwstring(s))

    function access_env(onError::Function, str::AbstractString)
        var = cwstring(str)
        len = _getenvlen(var)
        if len == 0
            return Libc.GetLastError() != ERROR_ENVVAR_NOT_FOUND ? "" : onError(str)
        end
        val = zeros(UInt16,len)
        ret = ccall(:GetEnvironmentVariableW,stdcall,UInt32,(Ptr{UInt16},Ptr{UInt16},UInt32),var,val,len)
        if (ret == 0 && len != 1) || ret != len-1 || val[end] != 0
            error(string("getenv: ", str, ' ', len, "-1 != ", ret, ": ", Libc.FormatMessage()))
        end
        pop!(val) # NUL
        return transcode(String, val)
    end

    function _setenv(svar::AbstractString, sval::AbstractString, overwrite::Bool=true)
        var = cwstring(svar)
        val = cwstring(sval)
        if overwrite || !_hasenv(var)
            ret = ccall(:SetEnvironmentVariableW,stdcall,Int32,(Ptr{UInt16},Ptr{UInt16}),var,val)
            systemerror(:setenv, ret == 0)
        end
    end

    function _unsetenv(svar::AbstractString)
        var = cwstring(svar)
        ret = ccall(:SetEnvironmentVariableW,stdcall,Int32,(Ptr{UInt16},Ptr{UInt16}),var,C_NULL)
        systemerror(:setenv, ret == 0)
    end
else # !windows
    _getenv(var::AbstractString) = ccall(:getenv, Cstring, (Cstring,), var)
    _hasenv(s::AbstractString) = _getenv(s) != C_NULL

    function access_env(onError::Function, var::AbstractString)
        val = _getenv(var)
        val == C_NULL ? onError(var) : unsafe_string(val)
    end

    function _setenv(var::AbstractString, val::AbstractString, overwrite::Bool=true)
        ret = ccall(:setenv, Int32, (Cstring,Cstring,Int32), var, val, overwrite)
        systemerror(:setenv, ret != 0)
    end

    function _unsetenv(var::AbstractString)
        ret = ccall(:unsetenv, Int32, (Cstring,), var)
        systemerror(:unsetenv, ret != 0)
    end
end # os test

## ENV: hash interface ##

"""
    EnvDict() -> EnvDict

A singleton of this type provides a hash table interface to environment variables.
"""
struct EnvDict <: AbstractDict{String,String}; end

"""
    ENV

Reference to the singleton `EnvDict`, providing a dictionary interface to system environment
variables.
"""
const ENV = EnvDict()

getindex(::EnvDict, k::AbstractString) = access_env(k->throw(KeyError(k)), k)
get(::EnvDict, k::AbstractString, def) = access_env(k->def, k)
get(f::Callable, ::EnvDict, k::AbstractString) = access_env(k->f(), k)
in(k::AbstractString, ::KeySet{String, EnvDict}) = _hasenv(k)
pop!(::EnvDict, k::AbstractString) = (v = ENV[k]; _unsetenv(k); v)
pop!(::EnvDict, k::AbstractString, def) = haskey(ENV,k) ? pop!(ENV,k) : def
delete!(::EnvDict, k::AbstractString) = (_unsetenv(k); ENV)
setindex!(::EnvDict, v, k::AbstractString) = _setenv(k,string(v))
push!(::EnvDict, kv::Pair{<:AbstractString}) = setindex!(ENV, kv.second, kv.first)

if Sys.iswindows()
    GESW() = (pos = ccall(:GetEnvironmentStringsW,stdcall,Ptr{UInt16},()); (pos,pos))
    function iterate(hash::EnvDict, block::Tuple{Ptr{UInt16},Ptr{UInt16}} = GESW())
        if unsafe_load(block[1]) == 0
            ccall(:FreeEnvironmentStringsW, stdcall, Int32, (Ptr{UInt16},), block[2])
            return nothing
        end
        pos = block[1]
        blk = block[2]
        len = ccall(:wcslen, UInt, (Ptr{UInt16},), pos)
        buf = Vector{UInt16}(undef, len)
        GC.@preserve buf unsafe_copyto!(pointer(buf), pos, len)
        env = transcode(String, buf)
        m = match(r"^(=?[^=]+)=(.*)$"s, env)
        if m === nothing
            error("malformed environment entry: $env")
        end
        return (Pair{String,String}(m.captures[1], m.captures[2]), (pos+(len+1)*2, blk))
    end
else # !windows
    function iterate(::EnvDict, i=0)
        env = ccall(:jl_environ, Any, (Int32,), i)
        env === nothing && return nothing
        env = env::String
        m = match(r"^(.*?)=(.*)$"s, env)
        if m === nothing
            error("malformed environment entry: $env")
        end
        return (Pair{String,String}(m.captures[1], m.captures[2]), i+1)
    end
end # os-test

#TODO: Make these more efficient
function length(::EnvDict)
    i = 0
    for (k,v) in ENV
        i += 1
    end
    return i
end

function show(io::IO, ::EnvDict)
    for (k,v) = ENV
        println(io, "$k=$v")
    end
end

"""
    withenv(f::Function, kv::Pair...)

Execute `f` in an environment that is temporarily modified (not replaced as in `setenv`)
by zero or more `"var"=>val` arguments `kv`. `withenv` is generally used via the
`withenv(kv...) do ... end` syntax. A value of `nothing` can be used to temporarily unset an
environment variable (if it is set). When `withenv` returns, the original environment has
been restored.
"""
function withenv(f::Function, keyvals::Pair{T}...) where T<:AbstractString
    old = Dict{T,Any}()
    for (key,val) in keyvals
        old[key] = get(ENV,key,nothing)
        val !== nothing ? (ENV[key]=val) : delete!(ENV, key)
    end
    try f()
    finally
        for (key,val) in old
            val !== nothing ? (ENV[key]=val) : delete!(ENV, key)
        end
    end
end
withenv(f::Function) = f() # handle empty keyvals case; see #10853