buffer feature to a fixed percentage area increase

abuff.R

abuff <- function(f, percent, precision, maxsteps = 20){
    require(rgeos)
    A0 = gArea(f)

    targetA = A0 + A0 * (percent/100)
    e = bbox(f)
    w0 = 0
    w2 = diff(e[1,])/10 # initial w

    ## initial expansion until we pass the solution:

    repeat{
        b1 = gBuffer(f, width=w2)
        if(gArea(b1) < targetA){
            w2 = w2 * 2
        }else{
            break
        }
    }
    w1 = (w0 + w2)/2
       
    ## solution is between w0 and w2
   
    steps = 1
   
    repeat{
        b = gBuffer(f, width=w1)
        A = gArea(b)
        if(abs(100*(A-targetA)/targetA) < precision){
            return(list(steps = steps, poly = b, target=targetA))
        }
        if(A < targetA){
            w0 = w1
            w1 = (w1+w2)/2
        }
        if(A > targetA){
            w2 = w1
            w1 = (w2+w0)/2
        }
        steps = steps + 1
        if(steps > maxsteps){
            return(NA)
        }
    }
}