hex color to cterm color, mainly for vim to translate hex color to cterm colors in colorscheme

hex-to-cterm.rb

#!/usr/bin/env ruby

if ARGV.size != 1 || ARGV[0] !~ /#[0-9a-fA-F]{3,6}/
  abort "USAGE: hex-to-cterm HEX"
end

def hex_to_rgb(hex)
  rgb = hex.gsub('#', '').chars

  if rgb.size == 3
    rgb = rgb.map { |c| "#{c}#{c}" }
  else
    rgb = rgb.each_slice(2).to_a.map(&:join)
  end

  rgb.map { |c| c.to_i(16) }
end

def color_distance(rgb1, rgb2)
  Math.sqrt(
    (rgb1[0] - rgb2[0]) ** 2 +
    (rgb1[1] - rgb2[1]) ** 2 +
    (rgb1[2] - rgb2[2]) ** 2
  )
end

INPUT = hex_to_rgb(ARGV.first)

COLORS = DATA.each_line.to_a.map(&:split).map do |c|
  {
    term: c[0],
    hex: c[1],
    rgb: hex_to_rgb(c[1]),
    distance: color_distance(INPUT, hex_to_rgb(c[1]))
  }
end

color = COLORS.min_by { |c| c[:distance] }

puts color[:term]
print "distance #{color[:hex]} (#{color[:rgb].join(', ')}) "
print "#{ARGV.first} (#{INPUT.join(', ')}): "
puts color[:distance]

# from https://vim.fandom.com/wiki/Xterm256_color_names_for_console_Vim
__END__
16 #000000
17 #00005f
18 #000087
19 #0000af
20 #0000d7
21 #0000ff
22 #005f00
23 #005f5f
24 #005f87
25 #005faf
26 #005fd7
27 #005fff
28 #008700
29 #00875f
30 #008787
31 #0087af
32 #0087d7
33 #0087ff
34 #00af00
35 #00af5f
36 #00af87
37 #00afaf
38 #00afd7
39 #00afff
40 #00d700
41 #00d75f
42 #00d787
43 #00d7af
44 #00d7d7
45 #00d7ff
46 #00ff00
47 #00ff5f
48 #00ff87
49 #00ffaf
50 #00ffd7
51 #00ffff
52 #5f0000
53 #5f005f
54 #5f0087
55 #5f00af
56 #5f00d7
57 #5f00ff
58 #5f5f00
59 #5f5f5f
60 #5f5f87
61 #5f5faf
62 #5f5fd7
63 #5f5fff
64 #5f8700
65 #5f875f
66 #5f8787
67 #5f87af
68 #5f87d7
69 #5f87ff
70 #5faf00
71 #5faf5f
72 #5faf87
73 #5fafaf
74 #5fafd7
75 #5fafff
76 #5fd700
77 #5fd75f
78 #5fd787
79 #5fd7af
80 #5fd7d7
81 #5fd7ff
82 #5fff00
83 #5fff5f
84 #5fff87
85 #5fffaf
86 #5fffd7
87 #5fffff
88 #870000
89 #87005f
90 #870087
91 #8700af
92 #8700d7
93 #8700ff
94 #875f00
95 #875f5f
96 #875f87
97 #875faf
98 #875fd7
99 #875fff
100 #878700
101 #87875f
102 #878787
103 #8787af
104 #8787d7
105 #8787ff
106 #87af00
107 #87af5f
108 #87af87
109 #87afaf
110 #87afd7
111 #87afff
112 #87d700
113 #87d75f
114 #87d787
115 #87d7af
116 #87d7d7
117 #87d7ff
118 #87ff00
119 #87ff5f
120 #87ff87
121 #87ffaf
122 #87ffd7
123 #87ffff
124 #af0000
125 #af005f
126 #af0087
127 #af00af
128 #af00d7
129 #af00ff
130 #af5f00
131 #af5f5f
132 #af5f87
133 #af5faf
134 #af5fd7
135 #af5fff
136 #af8700
137 #af875f
138 #af8787
139 #af87af
140 #af87d7
141 #af87ff
142 #afaf00
143 #afaf5f
144 #afaf87
145 #afafaf
146 #afafd7
147 #afafff
148 #afd700
149 #afd75f
150 #afd787
151 #afd7af
152 #afd7d7
153 #afd7ff
154 #afff00
155 #afff5f
156 #afff87
157 #afffaf
158 #afffd7
159 #afffff
160 #d70000
161 #d7005f
162 #d70087
163 #d700af
164 #d700d7
165 #d700ff
166 #d75f00
167 #d75f5f
168 #d75f87
169 #d75faf
170 #d75fd7
171 #d75fff
172 #d78700
173 #d7875f
174 #d78787
175 #d787af
176 #d787d7
177 #d787ff
178 #d7af00
179 #d7af5f
180 #d7af87
181 #d7afaf
182 #d7afd7
183 #d7afff
184 #d7d700
185 #d7d75f
186 #d7d787
187 #d7d7af
188 #d7d7d7
189 #d7d7ff
190 #d7ff00
191 #d7ff5f
192 #d7ff87
193 #d7ffaf
194 #d7ffd7
195 #d7ffff
196 #ff0000
197 #ff005f
198 #ff0087
199 #ff00af
200 #ff00d7
201 #ff00ff
202 #ff5f00
203 #ff5f5f
204 #ff5f87
205 #ff5faf
206 #ff5fd7
207 #ff5fff
208 #ff8700
209 #ff875f
210 #ff8787
211 #ff87af
212 #ff87d7
213 #ff87ff
214 #ffaf00
215 #ffaf5f
216 #ffaf87
217 #ffafaf
218 #ffafd7
219 #ffafff
220 #ffd700
221 #ffd75f
222 #ffd787
223 #ffd7af
224 #ffd7d7
225 #ffd7ff
226 #ffff00
227 #ffff5f
228 #ffff87
229 #ffffaf
230 #ffffd7
231 #ffffff
232 #080808
233 #121212
234 #1c1c1c
235 #262626
236 #303030
237 #3a3a3a
238 #444444
239 #4e4e4e
240 #585858
241 #626262
242 #6c6c6c
243 #767676
244 #808080
245 #8a8a8a
246 #949494
247 #9e9e9e
248 #a8a8a8
249 #b2b2b2
250 #bcbcbc
251 #c6c6c6
252 #d0d0d0
253 #dadada
254 #e4e4e4
255 #eeeeee