#!/bin/bash

# Tom Hale, 2016. MIT Licence.
# Print out 256 colours, with each number printed in its corresponding colour
# See http://askubuntu.com/questions/821157/print-a-256-color-test-pattern-in-the-terminal/821163#821163

set -eu # Fail on errors or undeclared variables

printable_colours=256

# Return a colour that contrasts with the given colour
# Bash only does integer division, so keep it integral
function contrast_colour {
  local r g b luminance
  colour="$1"

  if ((colour < 16)); then # Initial 16 ANSI colours
    ((colour == 0)) && printf "15" || printf "0"
    return
  fi

  # Greyscale # rgb_R = rgb_G = rgb_B = (number - 232) * 10 + 8
  if ((colour > 231)); then # Greyscale ramp
    ((colour < 244)) && printf "15" || printf "0"
    return
  fi

  # All other colours:
  # 6x6x6 colour cube = 16 + 36*R + 6*G + B  # Where RGB are [0..5]
  # See http://stackoverflow.com/a/27165165/5353461

  # r=$(( (colour-16) / 36 ))
  g=$((((colour - 16) % 36) / 6))
  # b=$(( (colour-16) % 6 ))

  # If luminance is bright, print number in black, white otherwise.
  # Green contributes 587/1000 to human perceived luminance - ITU R-REC-BT.601
  ((g > 2)) && printf "0" || printf "15"
  return

  # Uncomment the below for more precise luminance calculations

  # # Calculate percieved brightness
  # # See https://www.w3.org/TR/AERT#color-contrast
  # # and http://www.itu.int/rec/R-REC-BT.601
  # # Luminance is in range 0..5000 as each value is 0..5
  # luminance=$(( (r * 299) + (g * 587) + (b * 114) ))
  # (( $luminance > 2500 )) && printf "0" || printf "15"
}

# Print a coloured block with the number of that colour
function print_colour {
  local colour="$1" contrast
  contrast=$(contrast_colour "$1")
  printf "\e[48;5;%sm" "$colour"                # Start block of colour
  printf "\e[38;5;%sm%3d" "$contrast" "$colour" # In contrast, print number
  printf "\e[0m "                               # Reset colour
}

# Starting at $1, print a run of $2 colours
function print_run {
  local i
  for ((i = "$1"; i < "$1" + "$2" && i < printable_colours; i++)); do
    print_colour "$i"
  done
  printf "  "
}

# Print blocks of colours
function print_blocks {
  local start="$1" i
  local end="$2" # inclusive
  local block_cols="$3"
  local block_rows="$4"
  local blocks_per_line="$5"
  local block_length=$((block_cols * block_rows))

  # Print sets of blocks
  for ((i = start; i <= end; i += (blocks_per_line - 1) * block_length)); do
    printf "\n" # Space before each set of blocks
    # For each block row
    for ((row = 0; row < block_rows; row++)); do
      # Print block columns for all blocks on the line
      for ((block = 0; block < blocks_per_line; block++)); do
        print_run $((i + (block * block_length))) "$block_cols"
      done
      ((i += block_cols)) # Prepare to print the next row
      printf "\n"
    done
  done
}

print_run 0 16 # The first 16 colours are spread over the whole spectrum
printf "\n"
print_blocks 16 231 6 6 3   # 6x6x6 colour cube between 16 and 231 inclusive
print_blocks 232 255 12 2 1 # Not 50, but 24 Shades of Grey
