Code Retreat

#!/bin/bash

set -eu

#declare -i start_time="$(date +'%s%N')"
#function debug_trap {
#    if [[ "${FUNCNAME[1]}" != return_trap ]]; then
#    #(set -o posix ; set) 1>&2
#	local -i time="$(date +'%s%N')"
#	time=$(( (${time} - ${start_time}) / 1000000 ))
#	echo "${time}ms ${FUNCNAME[1]} ${BASH_LINENO[0]}" 1>&2
#    fi
#}
#set -T
#shopt -s extdebug
#trap debug_trap DEBUG
#function return_trap {
#    #(set -o posix ; set) 1>&2
#    local -i time="$(date +'%s%N')"
#    time=$(( (${time} - ${start_time}) / 1000000 ))
#    echo "${time}ms ${FUNCNAME[1]} ${BASH_LINENO[0]} RETURN" 1>&2
#}
#trap return_trap RETURN

## Yes, this is an honest to goddess traceback for bash.
## Useful for debugging.
#function traceback {
#    local -i start=1
#    if [[ -n "${1:-}" ]]; then
#        # Always hide our function call...
#        start=$(( $1 + ${start} ))
#    fi
#    local i
#    local j
#    echo "Traceback (last called is first):"
#    for i in $(seq "${start}" $(( ${#BASH_SOURCE[@]} - 1 ))); do
#        j=$(( $i - 1 ))
#        local function="${FUNCNAME[$i]}"
#        local file="${BASH_SOURCE[$i]}"
#        local line="${BASH_LINENO[$j]}"
#        echo "     ${function}() in ${file}:${line}"
#    done
#}

function print_field() {
  local -i y x
  for y in $(seq 0 $((rows - 1))); do
    for x in $(seq 0 $((columns - 1))); do
      field_get "$x" "$y"
      echo -n " "
    done
    echo
  done
}

function coord_to_idx() {
  local -i x="${1}"
  shift
  local -i y="${1}"
  shift
  echo $((y * columns + x))
}

function field_get() {
  echo -n "${field[$(coord_to_idx "$@")]}"
}

function field_set() {
  local -i x="${1}"
  local -i y="${2}"
  local value="${3:-o}"

  # Assertion
  case "${value}" in
  .) ;;
  o) ;;
  *)
    echo "WRONG"
    exit 1
    ;;
  esac

  field[$(coord_to_idx "$x" "$y")]="${value}"
}

function neighbor_count() {
  local -i x="${1}"
  local -i y="${2}"
  local -i count=0

  for i in "-1" 0 "+1"; do
    for j in "-1" 0 "+1"; do
      if [[ $i == 0 && $j == 0 ]]; then
        continue
      fi
      local -i new_x=$(((x + columns + i) % columns))
      local -i new_y=$(((y + rows + j) % rows))
      if [[ $(field_get ${new_x} ${new_y}) == o ]]; then
        count=$((count + 1))
      fi
    done
  done

  echo "${count}"
}

declare -i rows=6
declare -i columns=8
declare -a field=()
declare -a new_field=()
declare design=gliber
declare -i iterations=6

while (($# > 0)); do
  case "${1}" in
  -i | --iterations)
    iterations="${2}"
    shift
    ;;
  -h | --help)
    cat <<EOF
Usage: $0 [OPTIONS] [pattern] [<columns>x<rows>]

Options:
  -h --help             This help.
  -i --interations NUM  Run for NUM iterations at most.

You can specify the size of the playing field by using <columns>x<rows> (e.g. '5x5').

Patterns:
EOF
    grep -E '^\s*\w\w+\)' "$0" | grep -Eo '\w+' | sort
    exit
    ;;
  -*)
    echo "Unknown flag ${1}"
    exit 1
    ;;
  *[0-9]x[0-9]*)
    rows="${1/x*/}"
    columns="${1/*x/}"
    ;;
  *)
    design="${1}"
    ;;
  esac
  shift
done

# shellcheck disable=SC2207
declare -a seq=(0 $(seq $((rows * columns - 1))))
field=("${seq[@]}")
new_field=("${seq[@]}")

for i in "${seq[@]}"; do
  field[$i]=.
done

case "${design}" in
blank) ;;

random)
  for i in "${seq[@]}"; do
    if ((RANDOM % 3 == 0)); then
      field[$i]=o
    else
      field[$i]=.
    fi
  done
  ;;
blinker)
  #   0 1 2
  # 0 . . .
  # 1 o o o
  # 2 . . .
  field_set 0 1
  field_set 1 1
  field_set 2 1
  ;;
block)
  #   0 1 2
  # 0 o o .
  # 1 o o .
  # 2 . . .
  field_set 0 0
  field_set 1 0
  field_set 0 1
  field_set 1 1
  ;;
beacon)
  #   0 1 2 3
  # 0 o o . .
  # 1 o o . .
  # 2 . . o o
  # 3 . . o o
  field_set 0 0
  field_set 1 0
  field_set 0 1
  field_set 1 1

  field_set 2 2
  field_set 3 2
  field_set 2 3
  field_set 3 3
  ;;
glider)
  #   0 1 2
  # 0 . . o
  # 1 o . o
  # 2 . o o
  field_set 2 0
  field_set 0 1
  field_set 2 1
  field_set 1 2
  field_set 2 2
  ;;
spaceship)
  #   0 1 2 3 4
  # 0 o . . o .
  # 1 . . . . o
  # 2 o . . . o
  # 3 . o o o o
  field_set 0 0
  field_set 3 0

  field_set 4 1

  field_set 0 2
  field_set 4 2

  field_set 1 3
  field_set 2 3
  field_set 3 3
  field_set 4 3
  ;;
diehard)
  #   0 1 2 3 4 5 6 7
  # 0 . . . . . . o .
  # 1 o o . . . . . .
  # 2 . o . . . o o o
  field_set 6 0

  field_set 0 1
  field_set 1 1

  field_set 1 2
  field_set 5 2
  field_set 6 2
  field_set 7 2
  ;;
*)
  echo "specify a shape"
  exit 13
  ;;
esac

echo
echo "Initial:"
print_field

while ((iterations > 0)); do
  echo

  declare -i count idx x y

  # Tick
  # Populate new_field with the next iteration.
  for y in $(seq 0 $((rows - 1))); do
    for x in $(seq 0 $((columns - 1))); do
      count=$(neighbor_count "$x" "$y")
      old=$(field_get "$x" "$y")
      idx=$(coord_to_idx "$x" "$y")
      if [[ ${old} == 'o' ]]; then
        if ((count < 2 || count > 3)); then # under-population
          new_field[$idx]=.
        else
          new_field[$idx]=o
        fi
      else
        if ((count == 3)); then
          new_field[$idx]=o
        else
          new_field[$idx]=.
        fi
      fi
    done
  done

  # Tock
  if [[ ${field[*]} == "${new_field[*]}" ]]; then
    echo "Stopped since nothing changed."
    exit 0
  fi
  # Copy new_field to field, and empty new_field.
  field=("${new_field[@]}")
  new_field=("${seq[0]}")

  echo "Iteration $iterations:"
  print_field
  iterations=$((iterations - 1))
done

# EOF