#!/usr/bin/env sh
# 
# Modified version of bash mandelbrot from Mecki  stackoverflow.com/users/15809
# found at https://stackoverflow.com/a/63749612/1765658
# Avoiding forks and running bc as background task, reducing
# execution time, from many hours to less than 10' (on my desktop)
#
# Modified version for POSIX shell by Léa Gris:
#   https://gist.github.com/leagris/59e1b7e72462024b278652696f375e71
# Improved to addressing fifos by FDs.

BAILOUT=16
MAX_ITERATIONS=1000
ALPHWIDTH=$(( $(tput cols) / 2 ))
AWIDE=$(( ALPHWIDTH -1 ))

bcin=$(mktemp -u)
bcout=$(mktemp -u)
mkfifo "$bcin" "$bcout"

exec 8<>"$bcin"
exec 9<>"$bcout"
bc -l <"$bcin" >"$bcout" &
echo "scale=16;bailout=$BAILOUT;alphwidth=$ALPHWIDTH;0" >&8
read -r foo <&9  # Wait for bc before rm fifos
rm "$bcin" "$bcout"

bc() {
    retval=$1
    shift
    printf %s\\n "$@" >&8
    # shellcheck disable=SC2229 # No nameref support on older Bash
    read -r "$retval" <&9
}

iterate () {
    # $1 is x
    # $2 is y
    zi=0
    zr=0
    i=0

    cr=
    bc cr "$2 - 0.5"
    
    while true
    do
        temp=0
        zr2=0
        zi2=0
        i=$((i + 1))
        bc zr2 "($zr * $zr) - ($zi * $zi) + $cr"
        bc zi2 "(($zr * $zi) * 2) + $1"
        bc temp "(($zi * $zi) + ($zr * $zr)) > bailout"

        [ "$temp" -eq 1 ] && return "$i"

        [ "$i" -gt "$MAX_ITERATIONS" ] && return 0

        zr=$zr2
        zi=$zi2
    done
}

mandelbrot () {
    y=$((0 - AWIDE))
    while [ "$y" -lt "$AWIDE" ]
    do
        printf \\n
        x=$((0 - AWIDE))
        while [ "$x" -lt "$AWIDE" ]
        do
            xi=0
            yi=0
            ires=
            bc xi "$x / alphwidth"
            bc yi "$y / alphwidth"
            iterate "$xi" "$yi"
            ires=$?

            if [ "$ires" -eq 0 ]
            then
                printf "*"
            else
                printf " "
            fi
          x=$((x + 1))
        done
      y=$((y + 1))
    done
    printf \\n
}

mandelbrot