#!/usr/bin/ruby

#
## https://rosettacode.org/wiki/Draw_a_sphere
#

# Generates a PGM image.

func normalize (vec)  { vec »/» (vec »*« vec -> sum.sqrt) }
func dot       (x, y) { -(x »*« y -> sum) `max` 0 }

var x = var y = 255
x += 1 if x.is_even   # must be odd

var light = normalize([ 3, 2, -5 ])
var depth = 255

func draw_sphere(rad, k, ambient) {
    var r2 = (rad * rad)
    var r = (-rad .. rad)
    gather {
        for x in (r), y in (r) {
            if ((var x2 = x*x) + (var y2 = y*y) < r2) {
                var vector = normalize([x, y, (r2 - x2 - y2).sqrt])
                var intensity = (dot(light, vector)**k + ambient)
                take(0 `max` (intensity*depth -> int) `min` depth)
            }
            else {
                take(0)
            }
        }
    }
}

var outfile = %f'sphere-sidef.pgm'
var out = outfile.open('>:raw')

out.say("P5\n#{x} #{y}\n#{depth}")    # .pgm header
out.write(draw_sphere((x-1)/2, .9, .2).map{.chr}.join)
out.close