local _M = {}

_M.source, _M.resolution, _M.gettime = (function()
    local has_syscall, syscall = pcall(require, "syscall")
    if has_syscall and syscall.clock_gettime and syscall.c.CLOCK then
        local clock_id = syscall.c.CLOCK.REALTIME
        local function timespec_to_number(timespec)
            return tonumber(timespec.tv_sec) + tonumber(timespec.tv_nsec) * 1e-9
        end
        return "syscall.clock_gettime(CLOCK_REALTIME)",
            syscall.clock_getres and timespec_to_number(syscall.clock_getres(clock_id)) or 1e-9, function()
                return timespec_to_number(syscall.clock_gettime(clock_id))
            end
    end

    local has_unix, unix = pcall(require, "unix")
    -- On Apple devices lunix only uses gettimeofday()
    if has_unix and unix.clock_gettime and unix.uname and unix.uname().sysname ~= "Darwin" then
        return "unix.clock_gettime(CLOCK_REALTIME)", 1e-9, function()
            return unix.clock_gettime()
        end
    end

    if has_syscall and syscall.gettimeofday then
        local function timeval_to_number(timeval)
            return tonumber(timeval.tv_sec) + tonumber(timeval.tv_nsec) * 1e-6
        end
        return "syscall.gettimeofday()", 1e-6, function()
            return timeval_to_number(syscall.gettimeofday())
        end
    end

    if has_unix and unix.gettimeofday then
        return "unix.gettimeofday()", 1e-6, unix.gettimeofday
    end

    local has_socket, socket = pcall(require, "socket")
    if has_socket and socket.gettime then
        -- on windows, this uses GetSystemTimeAsFileTime, which has resolution of 1e-7
        -- on linux, this uses gettimeofday, which has resolution of 1e-6
        return "socket.gettime()", 1e-6, socket.gettime
    end

    if ngx and ngx.now then -- luacheck: ignore 113
        return "ngx.now()", 1e-3, ngx.now -- luacheck: ignore 113
    end

    return "os.time()", 1, os.time
end)()

return _M
