testcolors

#!/usr/bin/env sh
# vim: ft=sh ts=4 sw=4 sts=4 et :

#   This file echoes a bunch of 24-bit color codes
#   to the terminal to demonstrate its functionality.
#   The foreground escape sequence is ^[38;2;<r>;<g>;<b>m
#   The background escape sequence is ^[48;2;<r>;<g>;<b>m
#   <r> <g> <b> range from 0 to 255 inclusive.
#   The escape sequence ^[0m returns output to default

set_bg() {
    printf '\x1b[48;2;%s;%s;%sm' "$1" "$2" "$3"
}

reset_output() {
    printf '\x1b[0m\n'
}

# Get terminal width, default to 80 if unable to determine
get_width() {
    if command -v tput >/dev/null 2>&1; then
        tput cols
    elif command -v stty >/dev/null 2>&1; then
        stty size | cut -d' ' -f2
    else
        echo 80
    fi
}

# Calculate how many lines needed for the gradient
get_lines() {
    width=$(get_width)
    lines=$((256 / width + (256 % width > 0)))
    [ "$lines" -lt 1 ] && lines=1
    echo "$lines"
}

rainbow() {
    h=$(($1 / 43))
    f=$(($1 - 43 * h))
    t=$((f * 255 / 43))
    q=$((255 - t))

    if [ "$h" -eq 0 ]; then
        echo "255 $t 0"
    elif [ "$h" -eq 1 ]; then
        echo "$q 255 0"
    elif [ "$h" -eq 2 ]; then
        echo "0 255 $t"
    elif [ "$h" -eq 3 ]; then
        echo "0 $q 255"
    elif [ "$h" -eq 4 ]; then
        echo "$t 0 255"
    elif [ "$h" -eq 5 ]; then
        echo "255 0 $q"
    else
        # Execution should never reach here
        echo "0 0 0"
    fi
}

# Draw a gradient strip with given color components
# $1: r component (0 or 1)
# $2: g component (0 or 1)
# $3: b component (0 or 1)
# $4: use rainbow colors if 1
draw_gradient() {
    width=$(get_width)
    lines=$(get_lines)
    colors_per_line=$((256 / lines))
    chars_per_line=$((256 / lines + (256 % lines > 0)))

    line=0
    while [ $line -lt $lines ]; do
        start=$((line * colors_per_line))
        end=$((start + colors_per_line))
        [ $line -eq $((lines - 1)) ] && end=256

        i=$start
        count=0
        while [ $count -lt $chars_per_line ]; do
            if [ $i -lt $end ] && [ $i -lt 256 ]; then
                if [ "$4" = "1" ]; then
                    set_bg $(rainbow $i)
                else
                    set_bg $(($1 * i)) $(($2 * i)) $(($3 * i))
                fi
                i=$((i + 1))
            else
                # Pad with last color if needed
                if [ "$4" = "1" ]; then
                    set_bg $(rainbow $((end - 1)))
                else
                    set_bg $(($1 * (end - 1))) $(($2 * (end - 1))) $(($3 * (end - 1)))
                fi
            fi
            printf " "
            count=$((count + 1))
        done
        reset_output
        line=$((line + 1))
    done
}

# Draw all gradients
draw_gradient 1 0 0 0 # red
draw_gradient 0 1 0 0 # green
draw_gradient 0 0 1 0 # blue
draw_gradient 0 0 0 1 # rainbow