Apply a stamp (text string) to each page of a PDF file

my.pdf.stamp <- function(stamp.text="Stamp", pos=c(0.4, 0.9), pdf.in.file,  pdf.out.file)
{ 
    ## Purpose: Apply a stamp (text string) to each page of a PDF file.
    ##          Requires Linux program "pdftk". 
    ## Arguments:
    ##   pdf.in.file: Input PDF file. 
    ##   stamp.text: a vector of strings (stamps). Each value will be stamped on the corresponding page of the PDF file.
    ##               Its values will be recycled to match the total number of pages in the input PDF file.
    ##   pos: position of the stamp text (located between [0,1] X [0,1])
    ##   pdf.out.file: Output PDF file with the stamps. 
    ## Return: PDF file with the stamps. 
    ## Author: Feiming Chen, Date: 25 Apr 2017, 10:42
    ## ________________________________________________
    
    p1 <- pdf.file.page.number(pdf.in.file)
    p2 <- length(stamp.text)
    if (p1 != p2) stamp.text <- rep(stamp.text, length.out = p1)

    ## burst the PDF file into single page PDF's
    f <- "page_%05d.pdf"
    fs <- "stamp_%05d"
    fs2 <- "stamp_%05d-Plot.pdf"
    fo <- "out_%05d.pdf"
    system(paste("pdftk", pdf.in.file, "burst output", f))
    
    for (i in 1:p1) {
        X(text.in.plot(stamp.text[i], cex = 2, pos = pos), file =  sprintf(fs, i), open.pdf=F)
        system(paste("pdftk", sprintf(f, i), "stamp", sprintf(fs2, i), "output", sprintf(fo, i)))
    }

    merge.file.list <- paste(sapply(1:p1, function(i) sprintf(fo, i)), collapse = " ")

    system(paste("pdftk", merge.file.list, "cat output", pdf.out.file))
}
if (F) {                                # Unit Test
    dir.create("tmp"); setwd("tmp")
    pdf("test.pdf"); replicate(3, plot(rnorm(100))); dev.off()

    pdf.in.file <- "test.pdf"
    pdf.out.file <- "result.pdf"
    stamp.text <- LETTERS[1:3]
    pos <- c(0.4, 0.9)

    my.pdf.stamp(stamp.text, pos=pos, pdf.in.file,  pdf.out.file)
}

pdf.file.page.number <- function(fname) {
    ## Return the total number of pages in a PDF file.
    ## Require Linux program "pdfinfo"
    a <- pipe(paste0("pdfinfo '", fname, "' | grep Pages | cut -d: -f2"))
    page.number <- as.numeric(readLines(a))
    close(a)
    page.number
}
if (F) {
    ## pdf.file.page.number("ALL-Plots.pdf")
}

text.in.plot <- function(x, cex=1, pos=c(0.5, 0.5)) {
    ## "x" is a string.  Put "x" in the center of a plot.  For putting
    ## text summary in a PDF file that is mostly plots.
    ## "pos" is the position of the text string
    ## "cex" is the expansion ratio of the text string
  plot(c(0, 1), c(0, 1), type = "n", main = "", xlab = "", ylab = "", xaxt="n", yaxt="n", axes = F, mar=c(0,0,0,0), oma=c(0,0,0,0))
  text(pos[1], pos[2], x, cex=cex)
}
if (F) {
    text.in.plot("good", cex=5)
    text.in.plot("bad", pos=c(0.1, 0.1))
}

X <- function(expr, file="test", open.pdf = TRUE) {
    ## USED IN UNIX!
    ## Generate a PDF file based on evaluation of expr and open it
    ## Put multiple plot commands like "{ cmd1; cmd2; cmd3; cmd4 }".  
    ## USAGE: xpdf(plot(sig1))
    graphics.off()
    ## png("~/tmp/plot.png", width=600, height=600)
    r <- my.pdf(file)
    ans <- eval(expr)
    dev.off()
    ## system("convert ~/tmp/plot.pdf ~/tmp/plot.png")
    if (open.pdf) open.pdf(r$plot)
    invisible(ans)
}
if (F) {
    X(plot(rnorm(100)))
    a <- X({plot(a <- rnorm(100)); a})
    X({plot(a <- rnorm(100)); hist(a); boxplot(a); dotchart(a); qqnorm(a); plot(lm(a~seq(a)))})
}

open.pdf <- function(file) {
    ## Open a PDF file.
    system(paste("evince ", file, " &", sep=""))
    cat("See Plot In: ", file, "\n")
}