// Shared preview bound: every core-tool preview is capped at 2000 chars so a
// single tool call row can never flood the cell render. Lines that do not fit
// are dropped head/tail (mirroring the pi-fabric previewEntries shape) with a
// hidden-lines marker in between.

export const PREVIEW_BOUND_CHARS = 2000;

export interface BoundedPreview {
  readonly hidden: number;
  readonly lines: readonly string[];
}

export function boundPreviewLines(
  lines: readonly string[],
  bound: number = PREVIEW_BOUND_CHARS
): BoundedPreview {
  const widths = lines.map(codePointLength);
  const total = widths.reduce((sum, width) => sum + width + 1, 0);
  if (total <= bound) {
    return { lines, hidden: 0 };
  }
  const headBudget = Math.floor(bound * 0.65);
  const tailBudget = bound - headBudget;
  let headEnd = 0;
  let headChars = 0;
  while (
    headEnd < lines.length &&
    headChars + widths[headEnd] + 1 <= headBudget
  ) {
    headChars += widths[headEnd] + 1;
    headEnd += 1;
  }
  let tailStart = lines.length;
  let tailChars = 0;
  while (
    tailStart > headEnd &&
    tailChars + widths[tailStart - 1] + 1 <= tailBudget
  ) {
    tailChars += widths[tailStart - 1] + 1;
    tailStart -= 1;
  }
  const hidden = tailStart - headEnd;
  if (headEnd === 0 && tailStart === lines.length) {
    const first = lines[0] ?? "";
    return {
      lines: [`${truncateCodePoints(first, bound)}…`],
      hidden: lines.length - 1,
    };
  }
  const marker = `--- ${hidden} lines hidden ---`;
  return {
    lines: [...lines.slice(0, headEnd), marker, ...lines.slice(tailStart)],
    hidden,
  };
}

function codePointLength(text: string): number {
  return Array.from(text).length;
}

function truncateCodePoints(text: string, max: number): string {
  return Array.from(text).slice(0, max).join("");
}