code_01.R

library(sf)

## Linking to GEOS 3.8.0, GDAL 3.0.4, PROJ 7.0.0

# Sample data
url = "https://geobgu.xyz/data/examples2/nafot.gpkg"
pol = st_read(url)

## Reading layer `nafot' from data source `https://geobgu.xyz/data/examples2/nafot.gpkg' using driver `GPKG'
## Simple feature collection with 15 features and 4 fields
## geometry type:  POLYGON
## dimension:      XY
## bbox:           xmin: 620662.1 ymin: 3263494 xmax: 770624.4 ymax: 3691834
## projected CRS:  WGS 84 / UTM zone 36N

pol = pol[pol$name_eng == "Jerusalem", ]
pol = st_geometry(pol)

# Optimization function
f = function(d, prop) {

    # Target area
    area0 = st_area(pol) * prop
    area0 = as.numeric(area0)

    # Current area
    pol1 = st_buffer(pol, d)
    area1 = st_area(pol1) 
    area1 = as.numeric(area1)

    # Difference
    return(abs(area0 - area1))

}

# Example
d = optimize(f, prop = 0.5, interval = c(-100000, 0))
d

## $minimum
## [1] -2625.665
## 
## $objective
## [1] 3.099185

st_area(pol) * 0.5

## 326873415 [m^2]

st_area(st_buffer(pol, d$minimum))

## 326873412 [m^2]

# Plot
plot(pol)
plot(st_buffer(pol, d$minimum), border = "red", add = TRUE)