# Zhang-Suen thinning algorithm

``````class ZhangSuen(str, black="1") {
const NEIGHBOURS = [[-1,0],[-1,1],[0,1],[1,1],[1,0],[1,-1],[0,-1],[-1,-1]]  # 8 neighbors
const CIRCULARS = (NEIGHBOURS + [NEIGHBOURS.first])                         # P2, ... P9, P2

has r = 0
has image = [[]]

method init {
var s1 = str.lines.map{|line| line.chars.map{|c| c==black ? 1 : 0 }}
var s2 = s1.len.of { s1[0].len.of(0) }
var xr = range(1, s1.end-1)
var yr = range(1, s1[0].end-1)
do {
r = 0
xr.each{|x| yr.each{|y| s2[x][y] = (s1[x][y] - self.zs(s1,x,y,1)) }}  # Step 1
xr.each{|x| yr.each{|y| s1[x][y] = (s2[x][y] - self.zs(s2,x,y,0)) }}  # Step 2
} while !r.is_zero
image = s1
}

method zs(ng,x,y,g) {
(ng[x][y] == 0)                                   ->
|| (ng[x-1][y] + ng[x][y+1] + ng[x+g][y+g - 1] == 3) ->
|| (ng[x+g - 1][y+g] + ng[x+1][y] + ng[x][y-1] == 3) ->
&& return 0

var bp1 = NEIGHBOURS.map {|p| ng[x+p[0]][y+p[1]] }.sum  # B(P1)
return 0 if ((bp1 < 2) || (6 < bp1))

var ap1 = 0
CIRCULARS.map {|p| ng[x+p[0]][y+p[1]] }.each_cons(2, {|a,b|
++ap1 if (a < b)                                    # A(P1)
})

return 0 if (ap1 != 1)
r = 1
}

method display {
image.each{|row| say row.map{|col| col ? '#' : ' ' }.join }
}
}

var text = <<EOS
00000000000000000000000000000000
01111111110000000111111110000000
01110001111000001111001111000000
01110000111000001110000111000000
01110001111000001110000000000000
01111111110000001110000000000000
01110111100000001110000111000000
01110011110011101111001111011100
01110001111011100111111110011100
00000000000000000000000000000000
EOS

ZhangSuen.new(text, black: "1").display
``````

#### Output:

``````
#######         ######
#     #        ##
#      #       #
#     #        #
##### #        #
##        #
#    #   ##    ##   #
#         ####
``````