Refactor rgbTo256Color function to improve grayscale handling and optimize RGB color mapping

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// cSpell: enable

function rgbTo256Color(r: number, g: number, b: number): number {

  // Handle grayscale colors (colors 232-255)

export function rgbTo256Color(r: number, g: number, b: number): number {

  // Check if it's a grayscale color first (when all RGB values are very close)

  const gray = Math.round((r + g + b) / 3);

  if (

    Math.abs(r - gray) < 10 && Math.abs(g - gray) < 10 &&

    Math.abs(b - gray) < 10

  ) {

    if (gray < 8) return 16; // Black

    if (gray > 248) return 231; // White

    return Math.round(((gray - 8) / 240) * 23) + 232;

  const isGrayscale = Math.abs(r - gray) <= 5 && Math.abs(g - gray) <= 5 &&

    Math.abs(b - gray) <= 5;

  // Handle grayscale colors (colors 232-255) - but exclude exact cube values

  if (isGrayscale) {

    const cubeValues = [0, 95, 135, 175, 215, 255];

    const isExactCubeValue = cubeValues.includes(r) && r === g && g === b;

    if (!isExactCubeValue) {

      if (gray < 8) return 232; // Darkest grayscale

      if (gray > 238) return 255; // Brightest grayscale

      // Map to grayscale range 232-255 (24 levels)

      // XTerm grayscale: 8, 18, 28, ..., 238 maps to 232, 233, 234, ..., 255

      const grayIndex = Math.round((gray - 8) / 10);

      return Math.max(232, Math.min(255, 232 + grayIndex));

    }

  }

  // Handle RGB colors (colors 16-231)

  // Convert to 6x6x6 cube

  const r6 = Math.round((r / 255) * 5);

  const g6 = Math.round((g / 255) * 5);

  const b6 = Math.round((b / 255) * 5);

  // XTerm 256 color cube values: [0, 95, 135, 175, 215, 255]

  const cubeValues = [0, 95, 135, 175, 215, 255];

  const findClosestIndex = (value: number): number => {

    let minDiff = Infinity;

    let closestIndex = 0;

    for (let i = 0; i < cubeValues.length; i++) {

      const diff = Math.abs(value - cubeValues[i]);

      if (diff < minDiff) {

        minDiff = diff;

        closestIndex = i;

      }

    }

    return closestIndex;

  };

  const r6 = findClosestIndex(r);

  const g6 = findClosestIndex(g);

  const b6 = findClosestIndex(b);

  return 16 + (36 * r6) + (6 * g6) + b6;

}