textMetrics.ts

export function countLines(content: string): number {
  if (content.length === 0) {
    return 0;
  }

  let lines = 1;
  for (let index = 0; index < content.length; index++) {
    if (content.charCodeAt(index) === 10) {
      lines++;
    }
  }

  return lines;
}

export function buildLineStartIndices(content: string): number[] {
  if (content.length === 0) {
    return [];
  }

  const lineStarts = [0];
  for (let index = 0; index < content.length; index++) {
    if (content.charCodeAt(index) === 10 && index + 1 < content.length) {
      lineStarts.push(index + 1);
    }
  }

  return lineStarts;
}

export function findLineNumber(lineStarts: number[], index: number): number {
  if (lineStarts.length === 0) {
    return 0;
  }

  let low = 0;
  let high = lineStarts.length - 1;

  while (low <= high) {
    const mid = Math.floor((low + high) / 2);
    const current = lineStarts[mid];
    const next = lineStarts[mid + 1] ?? Number.POSITIVE_INFINITY;

    if (index >= current && index < next) {
      return mid + 1;
    }

    if (index < current) {
      high = mid - 1;
    } else {
      low = mid + 1;
    }
  }

  return lineStarts.length;
}