read_opus_univ <- function(file_path){
   
   `%do%` <- foreach::`%do%`
   extract <- 'spc'
 
  # Avoid `R CMD check` NOTE: no visible binding for global variable ...
   x <- y <- i <- npt <- NULL
   if (!file.exists(file_path)){
     stop(paste0("File does not exist"))
   }
   
   try({
     
     # file_path <- "data/soilspec_background/yamsys_bg_gold/BF_lo_15_soil_cal.0"
     # Read entire content of file as bytes
     pa <- hexView::readRaw(file_path, offset = 0,
                            nbytes = file.info(file_path)$size, human = "char",
                            size = 1, endian = "little")
     # Get raw vector
     pr <- pa$fileRaw
     
     # Read byte positions for selected 3 letter strings that flag important
     # spectral information -----------------------------------------------------
     
     # Get positions of "END" strings
     end <- grepRaw("END", pr, all = TRUE) + 11
     # Get all positions of "NPT" (number of points) string
     npt_all <- grepRaw("NPT", pr, all = TRUE) + 3
     # Get frequency of first (FXV) and last point (LXV) positions
     fxv_all <- grepRaw("FXV", pr, all = TRUE) + 7
     lxv_all <- grepRaw("LXV", pr, all = TRUE) + 7
     
     # For some files, the number of positions where "FXV" and "LXV" occur
     # are not equal, e.g. for the file in
     # data/soilspec_esal_bin/BF_mo_01_soil_cal.0 ; As a consequence, the
     # fist and last point numbers (e.g. wavenumber or points for interferograms)
     # are not correctly read. This results in an error when trying to calculate
     # the wavenumbers; The below code is a quick and dirty fix to remove
     # FXV values that don't have LXV values and vice versa
     # (difference between "LXV" and "FXV" for a spectral data block
     # should be 16) ------------------------------------------------------------
     if (length(fxv_all) > length(lxv_all)) {
       diff_lxv_fxv <- lapply(lxv_all, function(x) x - fxv_all)
       # Return list of logical vectors indicating whether difference of fxv
       # and lxv is 16 (distance of 16 bytes)
       lxv_fxv_min <- lapply(diff_lxv_fxv, function(x) x == 16)
       fxv_list <- rep(list(fxv_all), length(fxv_all))
       fxv_all <- foreach::foreach(
         x = 1:length(fxv_list), y = 1:length(lxv_fxv_min),
         .combine = 'c') %do% {
           fxv_list[[x]][lxv_fxv_min[[y]]]
         }
     }
     
     if (length(lxv_all) > length(fxv_all)) {
       diff_fxv_lxv <- lapply(fxv_all, function(x) x - lxv_all)
       # Return list of logical vectors indicating whether difference of fxv
       # and lxv is 16 (distance of 16 bytes)
       fxv_lxv_min <- lapply(diff_fxv_lxv, function(x) x == -16)
       lxv_list <- rep(list(lxv_all), length(lxv_all))
       lxv_all <- foreach::foreach(
         x = 1:length(lxv_list), y = 1:length(fxv_lxv_min),
         .combine = 'c') %do% {
           lxv_list[[x]][fxv_lxv_min[[y]]]
         }
     }
     
     # Reduce size of npt_all -----------------------------------------------------
     # Some files have an extra "NPT" string without FXV, LXV, and spectral block
     if (length(npt_all) != length(fxv_all)) {
       diff_npt_fxv <- lapply(npt_all, function(x) fxv_all - x)
       min_bigger0_smallerequal40 <- lapply(diff_npt_fxv, function(x) {
         which_min_bigger0 <- x == min(x[x > 0])
         which_smallerequal40 <- x <= 40
         which_min_bigger0 & which_smallerequal40
       }
       )
       which_npt_valid <- sapply(min_bigger0_smallerequal40,
                                 function(x) any(x == TRUE))
       npt_all <- npt_all[which_npt_valid]
     }
     
     # --------------------------------------------------------------------------
     
     ## Read basic spectral information =========================================
     
     # Read all number of points (NPT) at once
     NPT <- foreach::foreach(npt = npt_all, .combine = 'c') %do% {
       hexView::readRaw(
         file_path, offset = npt, nbytes = 12, human = "int", size = 4)[[5]][2]
     }
     
     # Specific error for file: <"data/soilspec_eth_bin/CI_tb_05_soil_cal.2">
     # "Invalid number of bytes" when trying to read spectra
     # -> Reason: NPT at position 1 is 995236000 !!!
     # Omit this entry in NPT and corresponding byte position in npt_all
     # Quick fix ----------------------------------------------------------------
     npt_all <- npt_all[NPT < 40000]
     NPT <- NPT[NPT < 40000]
     # --------------------------------------------------------------------------
     
     # Figure out how many spectral blocks exist and select final spectra
     # positions; end_spc is vector of offsets where spectra start
     if (length(end) == 1) {
       end_spc <- end
     } else {
       end_spc <- end[diff(end) > 4 * min(NPT)]
     }
     
     ## Find final spectra information block positions
     ## that belong to spectra data =============================================
     
     # Save positions that contain possible spectra data block
     # standard parameters
     spc_param_list <- list(
       'npt' = npt_all,
       'fxv' = fxv_all,
       'lxv' = lxv_all
     )
     
     ## Return list of final parameters corresponding to data blocks that contain
     ## spectra, elements are npt (number of points),
     ## fxv (frequency of first point) and lxv (frequency of last point);
     ## returned values represent byte positions in the file where spectra
     ## parameters are stored. --------------------------------------------------
     return_spc_param <- function(end_spc, spc_param_list) {
       
       # Difference between any NPT position vector elements end_spc element
       # (end_spc[i] is a scalar, constant value at iteration i)
       diff_l <- lapply(end_spc, function(x) npt_all - x)
       # Test of any vector in list contains -164 (returns list of vectors
       # TRUE or FALSE)
       isminus164 <- lapply(diff_l, function(x) x == -164)
       
       # Find minimum positive difference within each list
       if (length(diff_l) == 1) {sel_min <- list(TRUE)} else {
         sel_min <- lapply(diff_l,
                           function(x) {if (any(x > 0)) {x == min(x[x > 0])} else {x == -164}})
       }
       # Set FALSE repeated vector in sel_min element where TRUE positions are
       # duplicated
       which_elem_dupl <- which(duplicated(sapply(sel_min, which)))
       if (length(which_elem_dupl) > 1) {
         sel_min[which_elem_dupl] <- NULL
         # Reduce end_spc with duplicated elements
         end_spc <- end_spc[- which_elem_dupl]
       }
       
       # Select minimum difference NPT position for each END position
       npt_min <- Map(function(x, y) x[y],
                      rep(list(npt_all), length(end_spc)), sel_min)
       npt_min <- Filter(length, npt_min)
       
       # Select spectra parameters that immediately follow END positions before
       # corresponding spectra
       param_min <- foreach::foreach(i = 1:length(spc_param_list),
                                     .final = function(i) setNames(i, names(spc_param_list))) %do% {
                                       Map(function(x, y) x[y],
                                           rep(list(spc_param_list[[i]]), length(end_spc)), sel_min)
                                     }
       
       # Test if any difference in list is -164
       if (any(unlist(isminus164) == TRUE)) {
         # Find all list element that contain TRUE in logical vector
         minus164 <- lapply(isminus164, function(x) Find(isTRUE, x))
         # Return element position of last TRUE in list
         where <- function(f, x) {
           vapply(x, f, logical(1))
         }
         last_minus164 <- Position(isTRUE, where(isTRUE, minus164),
                                   right = TRUE)
         # Replace positions in parameter list are at positions of last
         # -164 difference between end_spc element and NPT position
         param_min <- foreach::foreach(i = 1:length(spc_param_list),
                                       .final = function(i) setNames(i, names(spc_param_list))) %do% {
                                         param_min[[i]][[last_minus164]] <-
                                           spc_param_list[[i]][isminus164[[last_minus164]]]
                                         param_min[[i]]
                                       }
       }
       # Return list of final parameters corresponding to data blocks that
       # contain spectra
       param_spc <- lapply(param_min, unlist)
       param_spc$end_spc <- end_spc
       param_spc
     }
     # Save spectra parameter list
     param_spc <- return_spc_param(end_spc, spc_param_list)
     
     # Create individual vectors containing spectra parameters
     npt_spc <- param_spc[["npt"]]
     fxv_spc <- param_spc[["fxv"]]
     lxv_spc <- param_spc[["lxv"]]
     end_spc <- param_spc[["end_spc"]]
     
     # Read number of points corresponding to spectra in file -------------------
     
     NPT_spc <- foreach::foreach(i = 1:length(npt_spc), .combine = 'c') %do% {
       hexView::readRaw(
         file_path, offset = npt_spc[i],
         nbytes = 12, human = "int", size = 4)[[5]][2]
     }
     
     # Delete NPT with negative signs
     NPT_spc <- NPT_spc[NPT_spc > 0]
     
     ## Read all spectra ========================================================
     
     spc <- Map(function(end, NPT) hexView::readRaw(file_path, width = NULL,
                                                    offset = end - 4, nbytes = NPT * 4,
                                                    human = "real", size = 4, endian = "little")[[5]], end_spc, NPT_spc)
     
     # Read FXV and LXV and calculate wavenumbers  ------------------------------
     
     FXV_spc <- foreach::foreach(i = 1:length(fxv_spc), .combine = 'c') %do% {
       hexView::readRaw(file_path,
                        offset = fxv_spc[i], nbytes = 16, human = "real", size = 8)[[5]][1]
     }
     LXV_spc <- foreach::foreach(i = 1:length(lxv_spc), .combine = 'c') %do% {
       hexView::readRaw(file_path,
                        offset = lxv_spc[i], nbytes = 16, human = "real", size = 8)[[5]][1]
     }
     # Calculate wavenumbers
     wavenumbers <- foreach::foreach(i = 1:length(FXV_spc)) %do% {
       rev(seq(LXV_spc[i], FXV_spc[i],
               (FXV_spc[i] - LXV_spc[i]) / (NPT_spc[i] - 1)))
     }
     
     ## Assigning list of intially read spectra depending on block type =========
     
     # Assign an index name to the spectra and parameters for reading
     names(end_spc) <- paste0("idx", 1:length(end_spc))
     names(spc) <- paste0("idx", 1:length(spc))
     names(NPT_spc) <- paste0("idx", 1:length(NPT_spc))
     names(FXV_spc) <- paste0("idx", 1:length(FXV_spc))
     names(wavenumbers) <- paste0("idx", 1:length(wavenumbers))
     
     # Check if elements in FXV_spc (frequency of first point) are equal to 0;
     # these are interferogram spectra ------------------------------------------
     which_Ig <- FXV_spc[which(FXV_spc == 0)]
     Ig_assigned <- if (length(which_Ig) == 0) {
       NULL
     } else if (length(which_Ig) == 1) {
       list(
         spc_idx = names(which_Ig),
         spc_code = "IgSm"
       )
     } else if (length(which_Ig) == 3) {
       list(
         spc_idx = names(which_Ig)[c(1, 3)],
         spc_code = c("IgSm", "IgRf")
       )
     } else {
       list(
         spc_idx = names(which_Ig),
         spc_code = c("IgSm", "IgRf")
       )
     }
     
     na_assigned <- list(
       spc_idx = NULL,
       spc_code = NULL
     )
     if (length(which_Ig) == 3) {
       # Assign duplicated interferogram spectrum to 'not available' assigned
       na_assigned <- list(
         spc_idx = names(which_Ig)[2],
         spc_code = NA
       )
     }
     
     # Remove NA assigned spectra in spc list -------------------------------------
     if (!is.null(na_assigned$spc_idx)) {
       spc[na_assigned$spc_idx] <- NULL
       # Remove wavenumbers with NA assigned spectra in spc list
       wavenumbers[na_assigned$spc_idx] <- NULL
     }
     
     # Assign single channel spectra if present in file -------------------------
     # Return idx (index names) of all remaining spectra that are not
     # interferograms
     notIg <- names(spc)[!names(spc) %in%
                           c(Ig_assigned$spc_idx, na_assigned$spc_idx)]
     # Check if the MIR range was measured
     wavenumbers_mir <- lapply(names(wavenumbers[notIg]),
                               function(i) spc[[i]][wavenumbers[notIg][[i]] < 2392 &
                                                      wavenumbers[notIg][[i]] > 2358])
     is_mir <- any(sapply(wavenumbers_mir, function(x) length(x) != 0))
     if (isTRUE(is_mir)) {
       # Calculate peak ratio for absorbance at around 2392 cm^(-1)
       # and 2358 cm^(-1)
       peak_ratio <- lapply(
         lapply(names(wavenumbers[notIg]),
                function(i) spc[[i]][wavenumbers[notIg][[i]] < 2392 &
                                       wavenumbers[notIg][[i]] > 2358]),
         function(j) j[[1]] / j[[length(j)]]
       )
       names(peak_ratio) <- names(spc[notIg])
       # Single channel (Sc) assignment list
       which_Sc <- names(which(peak_ratio > 2))
     } else {
       peak_ratio <- lapply(
         lapply(names(wavenumbers[notIg]),
                function(i) spc[[i]][wavenumbers[notIg][[i]] < 5340 &
                                       wavenumbers[notIg][[i]] > 5318]),
         function(j) j[[1]] / j[[length(j)]]
       )
       names(peak_ratio) <- names(spc[notIg])
       # Single channel (Sc) assignment list
       which_Sc <- names(which(peak_ratio < 0.9))
     }
     
     # Check for single channel, exclude spectral blocks already assigned to
     # interferograms
     Sc_assigned <- if (length(which_Sc) == 0) {
       NULL
     } else if (length(which_Sc) == 1) {
       list(
         spc_idx = which_Sc,
         spc_code = "ScSm"
       )
     } else {
       list(
         spc_idx = which_Sc,
         spc_code = c("ScSm", "ScRf")
       )
     }
     # Assign corrected and uncorrected (if present) ----------------------------
     # AB spectra list
     which_AB <- names(spc)[!names(spc) %in%
                              c(Ig_assigned[["spc_idx"]], na_assigned[["spc_idx"]],
                                Sc_assigned[["spc_idx"]])]
     AB_assigned <- if (length(which_AB) == 1) {
       list(
         spc_idx = which_AB,
         spc_code = "spc"
       )
     } else {
       list(
         spc_idx = which_AB,
         spc_code = c("spc_nocomp", "spc")
       )
     }
     
     # Read result spectrum with new offset (no `-4`) when atmospheric
     # compensation was done by the OPUS software; replace the spectrum position
     # with index name idx that corresponds to final spectrum after atmospheric
     # compensation; OPUS files from particular spectrometers/OPUS software
     # versions do still need the same offset end_spc[[spc_idx]] - 4 as the other
     # spectra types; new argument atm_comp_minus4offset (default FALSE) is a
     # quick fix to read files with different offsets after atmospheric
     # compensation -------------------------------------------------------------
     if (length(which_AB) == 2 && !atm_comp_minus4offset) {
       spc[[which_AB[length(which_AB)]]] <-
         hexView::readRaw(file_path, width = NULL,
                          offset = end_spc[which_AB[length(which_AB)]],
                          nbytes = NPT_spc[which_AB[length(which_AB)]] * 4,
                          human = "real", size = 4, endian = "little")[[5]]
     }
     
     # Assign spectra type for final spectra in element names of spc list -------
     # Combine spectral assignments lists
     list_assigned <- list(
       'Ig' = Ig_assigned,
       'Sc' = Sc_assigned,
       'AB' = AB_assigned
     )
     # Transpose spectra assignment list, first remove NULL elements in list
     list_assigned_t <- purrr::transpose(
       Filter(Negate(function(x) is.null(unlist(x))), list_assigned)
     )
     # Save spectra index (spc_idx) and spectra code (spc_code)
     # in character vector
     spc_idx <- unlist(list_assigned_t[["spc_idx"]])
     spc_code <- unlist(list_assigned_t[["spc_code"]])
     # Order spc_idx from 1 to n spectra (n = length of end_spc)
     order_spc <- as.numeric(
       sub(".*idx", "", unlist(list_assigned_t[["spc_idx"]])))
     spc_type <- spc_code[order(order_spc)]
     # Set spectrum type as element names of spectra list (spc)
     names(spc) <- spc_type
     # Set spectrum type in wavenumbers list
     names(wavenumbers) <- spc_type
     
     # Read with new offset when first value of
     # ScSm  single channel sample spectrumspectrum is 0 and replace previous ---
     if (any(names(spc) %in% "ScSm" & spc[["ScSm"]][1] == 0)) {
       spc[["ScSm"]] <-
         hexView::readRaw(file_path, width = NULL,
                          offset = end_spc[Sc_assigned$spc_idx[Sc_assigned$spc_code == "ScSm"]],
                          nbytes = NPT_spc[Sc_assigned$spc_idx[Sc_assigned$spc_code == "ScSm"]]
                          * 4,
                          human = "real", size = 4, endian = "little")[[5]]
     }
     
     ## Get additional parameters from OPUS binary file =========================
     
     # Instrument parameters ----------------------------------------------------
     ins <- grepRaw("INS", pr, all = TRUE) # Instrument type
     INS <- hexView::blockString(
       hexView::readRaw(
         file_path, offset = ins[length(ins)] + 7,
         nbytes = 10, human = "char", size = 1, endian = "little"))
     lwn <- grepRaw("LWN", pr, all = TRUE)[1] + 7 # Laser wavenumber
     LWN <- hexView::readRaw(file_path, offset = lwn,
                             nbytes = 8, human = "real", size=8)[[5]][1]
     tsc <- grepRaw("TSC", pr, all = TRUE) + 7 # Scanner temperature
     TSC_all <- lapply(tsc, function(tsc)
       hexView::readRaw(file_path, offset = tsc,
                        nbytes = 16, human = "real", size = 8)[[5]][[1]] # can include sample
       # and background temperature
     )
     # Read relative humidity of the interferometer during measurement
     hum_rel <- grepRaw("HUM", pr, all = TRUE) + 7
     HUM_rel <- lapply(hum_rel, function(hum_rel)
       hexView::readRaw(
         file_path, offset = hum_rel, nbytes = 16,
         human = "int", size = 8)[[5]][[1]] # can include sample and background
       # humidity
     )
     # Read absolute humidity of the interferometer during measurement
     hum_abs <- grepRaw("HUA", pr, all = TRUE) + 7
     HUM_abs <- lapply(hum_abs, function(hum_abs)
       hexView::readRaw(
         file_path, offset = hum_abs, nbytes = 16,
         human = "real", size = 8)[[5]][[1]] # can include sample and background
       # humidity
     )
     
     # Optics parameters --------------------------------------------------------
     src <- grepRaw("SRC", pr, all = TRUE) # Source: MIR or NIR
     SRC <- hexView::blockString(
       hexView::readRaw(
         file_path, offset = src[length(src)] + 4,
         nbytes = 3, human = "char", size = 1, endian = "little"))
     instr_range <- tolower(paste(INS, SRC, sep = "-")) # instrument range
     bms <- grepRaw("BMS", pr, all = TRUE) # Beamsplitter
     BMS <- hexView::blockString(
       hexView::readRaw(file_path, offset = bms[length(bms)] + 4,
                        nbytes = 3, human = "char", size = 1, endian = "little"))
     
     # Fourier transform parameters ---------------------------------------------
     zff <- grepRaw("ZFF", pr, all = TRUE)[1] + 5 # Zero filling factor (numeric)
     ZFF <- hexView::readRaw(file_path, offset = zff,
                             nbytes = 4, human = "int", size=2)[[5]][1]
     
     # (Additional) Standard parameters -----------------------------------------
     csf_all <- grepRaw("CSF", pr, all = TRUE) + 7 # y-scaling factor
     # Read only CSF byte positions that correspond to final spectra
     CSF <- lapply(csf_all[npt_all %in% npt_spc],
                   function(csf) hexView::readRaw(
                     file_path, offset = csf, nbytes = 8, human = "real", size = 8)[[5]][1])
     mxy_all <- grepRaw("MXY", pr, all = TRUE) + 7 # Y-maximum
     MXY <- unlist(lapply(mxy_all[npt_all %in% npt_spc],
                          function(mxy) hexView::readRaw(
                            file_path, offset = mxy, nbytes = 8, human = "real", size = 8)[[5]][1]))
     mny <- grepRaw("MNY", pr, all = TRUE) + 7 # Y-minimum
     dxu_all <- grepRaw("DXU", pr, all = TRUE) + 7 # X units
     DXU <- lapply(dxu_all, function(dxu)
       hexView::blockString(
         hexView::readRaw(file_path, offset = dxu,
                          nbytes = 3, human = "char", size = 1, endian = "little")
       )
     )
     # Y units -> there is no DYU present in file
     dyu_all <- grepRaw("DYU", pr, all = TRUE) + 7
     dat <- grepRaw("DAT", pr, all = TRUE) + 7 # Date
     tim <- grepRaw("TIM", pr, all = TRUE) + 7 # Time
     time <- unlist(lapply(tim, function(tim)
       hexView::blockString(
         hexView::readRaw(file_path, offset = tim,
                          nbytes = 22, human = "char",
                          size = 1, endian = "little")))
     )
     # Only select "DAT" string positions that are immediately before time
     dat_sel <- foreach::foreach(i = 1:length(tim), .combine = 'c') %do% {
       diff_sel <- dat - tim[i]
       dat[which(diff_sel <= 32 & diff_sel >= -20)]
     }
     date <- lapply(dat_sel, function(dat) hexView::blockString(
       hexView::readRaw(file_path, offset = dat,
                        nbytes = 10, human = "char", size = 1,
                        endian = "little"))
     )
     
     date_time <- unique(paste(date, time))
     # Convert date_time from character to class POSIXct (calendar date and time)
     date_time <- as.POSIXct(date_time, format = "%d/%m/%Y %H:%M:%S")
     # , tz = "GMT+1") # tz is argument for time zone
     
     # Scale all spectra with y-scaling factor if any of spectra types present
     # in file are not 1 --------------------------------------------------------
     # Set names of CSF elements equal to spectra list element names
     names(CSF) <- names(spc)
     if (any(unlist(CSF) != 1)) {
       # Return all elements in CSF that have scaling value not equal to 1
       CSF_toscale <- Filter(function(x) x != 1, CSF)
       # Apply scaling for spectra with CSF value not equal to 1;
       # Map() returns list
       spc_scaled <- Map(function(CSF, spc) CSF * spc,
                         unlist(CSF_toscale), spc[names(CSF_toscale)])
       # Replace all spc list elements that have CSF not equal 1 with
       # scaled values
       spc <- replace(x = spc, list = names(CSF_toscale), values = spc_scaled)
     }
     
     # Data aquisition parameters -----------------------------------------------
     
     plf <- grepRaw("PLF", pr, all = TRUE) + 4 # Result spectrum
     PLF_all <- lapply(plf, function(plf) hexView::blockString(
       hexView::readRaw(file_path, offset = plf,
                        nbytes = 2, human = "char", size = 1,
                        endian = "little"))
     )
     # Select only result spectra abbreviations that are more than 0 characters
     # long
     PLF <- unlist(PLF_all[lapply(PLF_all, nchar) > 0])
     res <- grepRaw("RES", pr, all = TRUE)[1] + 5 # Resolution (wavenumber)
     RES <- hexView::readRaw(
       file_path, offset = res, nbytes = 4, human = "int", size = 2)[[5]][1]
     
     ## Create sample metadata objects ==========================================
     # File name
     file_name_nopath <- sub(".+/(.+)", "\\1", file_path)
     # Create sample id from file name;
     # remove extension .0, .1 etc. from OPUS files
     sample_id <- sub("(.+)\\.[[:digit:]]+$", "\\1", file_name_nopath)
     # Extract sample repetition number (rep_no) from file name
     rep_no <- sub(".+\\.([[:digit:]])+$", "\\1", file_path)
     snm <- grepRaw("SNM", pr, all = TRUE)[1] + 7
     SNM <- hexView::blockString(
       hexView::readRaw(file_path, offset = snm,
                        nbytes = 30, human = "char", size = 1, endian = "little")
     )
     # Create unique_id using file_name and time
     # ymd_id <- format(max(date_time), "%Y%m%d")
     ymdhms_id <- max(date_time)
     unique_id <- paste0(file_name_nopath, "_", ymdhms_id)
     
     ## Convert all spectra in list spc into a matrix of 1 row ==================
     spc_m <- lapply(spc,
                     function(x) matrix(x, ncol = length(x), byrow = FALSE))
     # Add dimnames (wavenumbers for columns and unique_id for rows
     spc_m <- foreach::foreach(i = 1:length(spc_m),
                               .final = function(i) setNames(i, names(spc_m))) %do% {
                                 colnames(spc_m[[i]]) <- round(wavenumbers[[i]], 1)
                                 rownames(spc_m[[i]]) <- unique_id
                                 data.table::as.data.table(spc_m[[i]])
                               }
     
     # Save all relevant data parameters (metadata)
     # in tibble data frame (class "data.frame" and "tbl_diff" ==================
     metadata <- tibble::tibble(
       unique_id = unique_id,
       file_id = file_name_nopath, # pb (20170514): changed `scan_id` to `file_id`
       sample_id = sample_id,
       rep_no = as.numeric(rep_no),
       date_time_sm = max(date_time),
       date_time_rf = min(date_time),
       sample_name = SNM,
       instr_name_range = instr_range,
       resolution_wn = RES,
       # Result spectrum; e.g. "AB" = Absorbance
       # result_spc = ifelse(length(unique(PLF)) == 1, unique(PLF), unique(PLF)[2]),
       # // pb: 2019-11-19: allow NULL value for PLF
       result_spc <- if (length(unique(PLF)) == 1) {
         unique(PLF)} else if (length(unique(PLF)) > 1) {
           unique(PLF)[2]} else { NA },
       beamspl = BMS,
       laser_wn = LWN,
       # `spc_in_file`: character vector of spectra found in OPUS file
       spc_in_file = paste(unlist(list_assigned_t[["spc_code"]]),
                           collapse = ";", sep = ";"),
       zero_filling = ZFF, # Zero filling factor for fourier transformation
       # Temperature of scanner during sample measurement
       temp_scanner_sm = TSC_all[[length(TSC_all)]], # select last element
       # Temperature of scanner during reference measurement;
       # if there is only one element in TSC_all, temperature during reference
       # mesurement is not saved
       temp_scanner_rf = ifelse(length(TSC_all) == 1, NA, TSC_all[[1]]),
       # Relative humidity
       hum_rel_sm = HUM_rel[[length(HUM_rel)]], # sample measurement
       hum_rel_rf = ifelse(length(HUM_rel) == 1, NA, HUM_rel[[1]]), # reference
       # measurement
       # Absolute humidity; sample measurement (sm); reference measurment (rf);
       # note: for Vertex 70 instrument HUA is not present, in this case,
       # HUM_abs is a list without elements
       hum_abs_sm = ifelse(length(HUM_abs) != 0, HUM_abs[[length(HUM_abs)]], NA),
       hum_abs_rf = ifelse(length(HUM_abs) == 1 | length(HUM_abs) == 0, NA,
                           HUM_abs[[1]]) # reference measurement
     )
     
     ## Allocate and return data from spectra in output list (out) ==============
     out <- list(
       'metadata' = metadata,
       'spc' = spc_m[["spc"]],
       'spc_nocomp' = if ("spc_nocomp" %in% extract &&
                          "spc_nocomp" %in% names(spc_m)) {
         spc_m[["spc_nocomp"]]} else {NULL},
       'sc_sm' = if ("ScSm" %in% extract && "ScSm" %in% names(spc_m)) {
         spc_m[["ScSm"]]} else {NULL},
       'sc_rf' = if ("ScRf" %in% extract && "ScRf" %in% names(spc_m)) {
         spc_m[["ScRf"]]} else {NULL},
       'ig_sm' = if ("IgSm" %in% extract && "IgSm" %in% names(spc_m)) {
         spc_m[["IgSm"]]} else {NULL},
       'ig_rf' = if ("IgRf" %in% extract && "IgRf" %in% names(spc_m)) {
         spc_m[["IgRf"]]} else {NULL},
       # Wavenumbers of final AB spectra
       wavenumbers = wavenumbers[["spc"]],
       wavenumbers_sc_sm = if ("ScSm" %in% extract) {
         wavenumbers[["ScSm"]]} else {NULL},
       wavenumbers_sc_rf = if ("ScRf" %in% extract) {
         wavenumbers[["ScRf"]]} else {NULL}
     )
     # Return spectra data and metadata contained as elements in list out
     out
   }) # closes try() function
}