beta.parms.from.quantiles <- function(q, p=c(0.025,0.975),
  precision=0.001, derivative.epsilon=1e-3, start.with.normal.approx=T, start=c(1, 1), plot=F)
{
  # Version 1.3 (February 2017)
  #
  # Function developed by 
  # Lawrence Joseph and pbelisle
  # Division of Clinical Epidemiology
  # Montreal General Hospital
  # Montreal, Qc, Can
  #
  # patrick.belisle@rimuhc.ca
  # http://www.medicine.mcgill.ca/epidemiology/Joseph/PBelisle/BetaParmsFromQuantiles.html
  #
  # Please refer to our webpage for details on each argument.
  
  f <- function(x, theta){dbeta(x, shape1=theta[1], shape2=theta[2])}
  F.inv <- function(x, theta){qbeta(x, shape1=theta[1], shape2=theta[2])}
  f.cum <- function(x, theta){pbeta(x, shape1=theta[1], shape2=theta[2])}
  f.mode <- function(theta){a <- theta[1]; b <- theta[2]; mode <- ifelse(a>1, (a-1)/(a+b-2), NA); mode}
  theta.from.moments <- function(m, v){a <- m*m*(1-m)/v-m; b <- a*(1/m-1); c(a, b)}
  plot.xlim <- c(0, 1)
  
  dens.label <- 'dbeta'
  parms.names <- c('a', 'b')
  
  if (length(p) != 2) stop("Vector of probabilities p must be of length 2.")
  if (length(q) != 2) stop("Vector of quantiles q must be of length 2.")
  p <- sort(p); q <- sort(q)
  
  #_____________________________________________________________________________________________________
        
  print.area.text <- function(p, p.check, q, f, f.cum, F.inv, theta, mode, cex, plot.xlim, M=30, M0=50)
  {
    par.usr <- par('usr')
    par.din <- par('din')

    p.string <- as.character(round(c(0,1) + c(1,-1)*p.check, digits=4))
    str.width <- strwidth(p.string, cex=cex)
    str.height <- strheight("0", cex=cex)

    J <- matrix(1, nrow=M0, ncol=1)
    
    x.units.1in <- diff(par.usr[c(1,2)])/par.din[1]
    y.units.1in <- diff(par.usr[c(3,4)])/par.din[2]
    aspect.ratio <- y.units.1in/x.units.1in

    # --- left area  -----------------------------------------------------------

    scatter.xlim <- c(max(plot.xlim[1], par.usr[1]), q[1])
    scatter.ylim <- c(0, par.usr[4])
    x <- seq(from=scatter.xlim[1], to=scatter.xlim[2], length=M)
    y <- seq(from=scatter.ylim[1], to=scatter.ylim[2], length=M)
    x.grid.index <- rep(seq(M), M)
    y.grid.index <- rep(seq(M), rep(M, M))

    grid.df <- f(x, theta)

    # Estimate mass center
    tmp.p <- seq(from=0, to=p[1], length=M0)
    tmp.x <- F.inv(tmp.p, theta)
    h <- f(tmp.x, theta)
    mass.center <- c(mean(tmp.x), sum(h[-1]*diff(tmp.x))/diff(range(tmp.x)))

    # Identify points under the curve
    # (to eliminate them from the list of candidates)
    gridpoint.under.the.curve <- y[y.grid.index] <= grid.df[x.grid.index]
    w <- which(gridpoint.under.the.curve)
    x <- x[x.grid.index]; y <- y[y.grid.index]
    if (length(w)){x <- x[-w]; y <- y[-w]}

    # Eliminate points to the right of the mode, if any
    w <- which(x>mode)
    if (length(w)){x <- x[-w]; y <- y[-w]}

    # Eliminate points for which the text would fall out of the plot area
    w <- which((par.usr[1]+str.width[1]) <= x & (y + str.height) <= par.usr[4])
    x <- x[w]; y <- y[w]

    # For each height, eliminate the closest point to the curve
    # (we want to stay away from the curve to preserve readability)
    w <- which(!duplicated(y, fromLast=T))
    if (length(w)){x <- x[-w]; y <- y[-w]}

    # For each point, compute distance from mass center and pick the closest point
    d <- ((x-mass.center[1])^2) + ((y-mass.center[2])/aspect.ratio)^2
    w <- which.min(d)
    x <- x[w]; y <- y[w]
    
    if (length(x))
    {
      text(x, y, labels=p.string[1], adj=c(1,0), col='gray', cex=cex)
    }
    else
    {
      text(plot.xlim[1], mean(par.usr[c(3,4)]), labels=p.string[1], col='gray', cex=cex, srt=90, adj=c(1,0))
    }

    # --- right area  ----------------------------------------------------------

    scatter.xlim <- c(q[2], plot.xlim[2])
    scatter.ylim <- c(0, par.usr[4])
    x <- seq(from=scatter.xlim[1], to=scatter.xlim[2], length=M)
    y <- seq(from=scatter.ylim[1], to=scatter.ylim[2], length=M)
    x.grid.index <- rep(seq(M), M)
    y.grid.index <- rep(seq(M), rep(M, M))
    grid.df <- f(x, theta)

    # Estimate mass center
    tmp.p <- seq(from=p[2], to=f.cum(plot.xlim[2], theta), length=M0)
    tmp.x <- F.inv(tmp.p, theta)
    h <- f(tmp.x, theta)
    mass.center <- c(mean(tmp.x), sum(h[-length(h)]*diff(tmp.x))/diff(range(tmp.x)))

    # Identify points under the curve
    # (to eliminate them from the list of candidates)
    gridpoint.under.the.curve <- y[y.grid.index] <= grid.df[x.grid.index]
    w <- which(gridpoint.under.the.curve)
    x <- x[x.grid.index]; y <- y[y.grid.index]
    if (length(w)){x <- x[-w]; y <- y[-w]}

    # Eliminate points to the left of the mode, if any
    w <- which(x<mode)
    if (length(w)){x <- x[-w]; y <- y[-w]}

    # Eliminate points for which the text would fall out of the plot area
    w <- which((par.usr[2]-str.width[2]) >= x & (y + str.height) <= par.usr[4])
    x <- x[w]; y <- y[w]

    # For each height, eliminate the closest point to the curve
    # (we want to stay away from the curve to preserve readability)
    w <- which(!duplicated(y))
    if (length(w)){x <- x[-w]; y <- y[-w]}

    # For each point, compute distance from mass center and pick the closest point
    d <- ((x-mass.center[1])^2) + ((y-mass.center[2])/aspect.ratio)^2
    w <- which.min(d)
    x <- x[w]; y <- y[w]
    
    if (length(x))
    {
      text(x, y, labels=p.string[2], adj=c(0,0), col='gray', cex=cex)
    }
    else
    {
      text(plot.xlim[2], mean(par.usr[c(3,4)]), labels=p.string[2], col='gray', cex=cex, srt=-90, adj=c(1,0))
    }
  }
  
  # ......................................................................................................................................
  
  Newton.Raphson <- function(derivative.epsilon, precision, f.cum, p, q, theta.from.moments, start.with.normal.approx, start)
  {
    Hessian <- matrix(NA, 2, 2)

    if (start.with.normal.approx)
    {
      # Probably not a very good universal choice, but proved good in most cases in practice
      m <-  diff(q)/diff(p)*(0.5-p[1]) + q[1]
      v <- (diff(q)/diff(qnorm(p)))^2
      theta <- theta.from.moments(m, v)
    }
    else theta <- start
  
  
    change <- precision + 1
    niter <- 0
    # Newton-Raphson multivariate algorithm
    while (max(abs(change)) > precision)
    {
      Hessian[,1] <- (f.cum(q, theta) - f.cum(q, theta - c(derivative.epsilon, 0))) / derivative.epsilon
      Hessian[,2] <- (f.cum(q, theta) - f.cum(q, theta - c(0, derivative.epsilon))) / derivative.epsilon
  
      f <- f.cum(q, theta) - p
      change <- solve(Hessian) %*% f
      last.theta <- theta
      theta <- last.theta - change
  
      # If we step out of limits, reduce change
  
      if (any(theta<0))
      {
        w <- which(theta<0)
        k <- min(last.theta[w]/change[w])
        theta <- last.theta - k/2*change
      }
      
      niter <- niter + 1
    }
    
    list(theta=as.vector(theta), niter=niter, last.change=as.vector(change))
  }
  
  # ...............................................................................................................
  
  plot.density <- function(p, q, f, f.cum, F.inv, mode, theta, plot.xlim, dens.label, parms.names, cex)
  {
    if (length(plot.xlim) == 0)
    {
      plot.xlim <- F.inv(c(0, 1), theta)
      
      if (is.infinite(plot.xlim[1]))
      {
        tmp <- min(c(0.001, p[1]/10))
        plot.xlim[1] <- F.inv(tmp, theta)
      }  
      
      if (is.infinite(plot.xlim[2]))
      {
        tmp <- max(c(0.999, 1 - (1-p[2])/10))
        plot.xlim[2] <- F.inv(tmp, theta)
      }
    }
    plot.xlim <- sort(plot.xlim)
  
  
    x <- seq(from=min(plot.xlim), to=max(plot.xlim), length=1000)
    h <- f(x, theta)
    x0 <- x; f0 <- h
    ylab <- paste(c(dens.label, '(x, ', parms.names[1], ' = ', round(theta[1], digits=5), ', ', parms.names[2], ' = ', round(theta[2], digits=5), ')'), collapse='')
    plot(x, h, type='l', ylab=ylab)

    # fill in area on the left side of the distribution
    x <- seq(from=plot.xlim[1], to=q[1], length=1000)
    y <- f(x, theta)
    x <- c(x, q[1], plot.xlim[1]); y <- c(y, 0, 0)
    polygon(x, y, col='lightgrey', border='lightgray')
    # fill in area on the right side of the distribution
    x <- seq(from=max(plot.xlim), to=q[2], length=1000)
    y <- f(x, theta)
    x <- c(x, q[2], plot.xlim[2]); y <- c(y, 0, 0)
    polygon(x, y, col='lightgrey', border='lightgray')
    # draw distrn again
    points(x0, f0, type='l')
    h <- f(q, theta)
    points(rep(q[1], 2), c(0, h[1]), type='l', col='orange')
    points(rep(q[2], 2), c(0, h[2]), type='l', col='orange')
    # place text on both ends areas
    print.area.text(p, p.check, q, f, f.cum, F.inv, theta, mode, cex, plot.xlim)  

    xaxp <- par("xaxp")
    x.ticks <- seq(from=xaxp[1], to=xaxp[2], length=xaxp[3]+1)
    q2print <- as.double(setdiff(as.character(q), as.character(x.ticks)))
    
    mtext(q2print, side=1, col='orange', at=q2print, cex=0.6, line=2.1)
    points(q, rep(par('usr')[3]+0.15*par('cxy')[2], 2), pch=17, col='orange')
  }
  
  #________________________________________________________________________________________________________________


  parms <- Newton.Raphson(derivative.epsilon, precision, f.cum, p, q, theta.from.moments, start.with.normal.approx, start=start)
  p.check <- f.cum(q, parms$theta)
  
  if (plot) plot.density(p, q, f, f.cum, F.inv, f.mode(parms$theta), parms$theta, plot.xlim, dens.label, parms.names, 0.8)

  list(a=parms$theta[1], b=parms$theta[2], last.change=parms$last.change, niter=parms$niter, q=q, p=p, p.check=p.check)
}