Function ansi256FromRgb

src/native-ts/color-diff/index.ts:105–127

(r: number, g: number, b: number)

// comparing cube vs grey-ramp candidates, like the Rust crate.
const CUBE_LEVELS = [0, 95, 135, 175, 215, 255]
function ansi256FromRgb(r: number, g: number, b: number): number {
  const q = (c: number) =>
    c < 48 ? 0 : c < 115 ? 1 : c < 155 ? 2 : c < 195 ? 3 : c < 235 ? 4 : 5
  const qr = q(r)
  const qg = q(g)
  const qb = q(b)
  const cubeIdx = 16 + 36 * qr + 6 * qg + qb
  // Grey ramp candidate (232-255, levels 8..238 step 10). Beyond the ramp's
  // range the cube corner is the only option — ansi_colours snaps 248,248,242
  // to 231 (cube white), not 255 (ramp top).
  const grey = Math.round((r + g + b) / 3)
  if (grey < 5) return 16
  if (grey > 244 && qr === qg && qg === qb) return cubeIdx
  const greyLevel = Math.max(0, Math.min(23, Math.round((grey - 8) / 10)))
  const greyIdx = 232 + greyLevel
  const greyRgb = 8 + greyLevel * 10
  const cr = CUBE_LEVELS[qr]!
  const cg = CUBE_LEVELS[qg]!
  const cb = CUBE_LEVELS[qb]!
  const dCube = (r - cr) ** 2 + (g - cg) ** 2 + (b - cb) ** 2
  const dGrey = (r - greyRgb) ** 2 + (g - greyRgb) ** 2 + (b - greyRgb) ** 2
  return dGrey < dCube ? greyIdx : cubeIdx
}

function colorToEscape(c: Color, fg: boolean, mode: ColorMode): string {
  // alpha=0: palette index encoded in .r (bat's ansi-theme convention)