export function readInteger(value: unknown, fallback: number, maximum = Number.MAX_SAFE_INTEGER): number {
  const result = value ?? fallback;
  if (typeof result !== 'number' || !Number.isSafeInteger(result) || result < 1 || result > maximum)
    throw new Error('Invalid read range');
  return result;
}

export function textChunks(text: string): string[] {
  return [text.replaceAll('\r\n', '\n')];
}

// JSON contains contiguous original text; columns count Unicode code points, starting at 1.
export async function readPage(
  source: string,
  chunks: AsyncIterable<string> | Iterable<string>,
  args: Record<string, unknown>,
  maxBytes: number,
): Promise<{ content: string; truncated: boolean }> {
  const start = readInteger(args.start_line, 1);
  const column = readInteger(args.start_column, 1);
  const count = readInteger(args.line_count, 120, 400);
  const chars: string[] = [];
  let lineNumber = 1, position = 1, more = false;
  outer: for await (const chunk of chunks) {
    for (const char of chunk) {
      if (lineNumber === start && char === '\n' && position < column)
        throw new Error('Column is past the line');
      if (lineNumber > start || (lineNumber === start && position >= column)) {
        if (lineNumber >= start + count || chars.length >= maxBytes) { more = true; break outer; }
        chars.push(char);
      }
      if (char === '\n') { lineNumber++; position = 1; } else position++;
    }
  }
  if (lineNumber < start || (lineNumber === start && position < column))
    throw new Error('Read position is past the source');
  const render = (length: number) => {
    let nextLine = start, nextColumn = column;
    for (let i = 0; i < length; i++) {
      if (chars[i] === '\n') { nextLine++; nextColumn = 1; } else nextColumn++;
    }
    const next = more || length < chars.length ? { start_line: nextLine, start_column: nextColumn } : null;
    return { content: JSON.stringify({ source, start_line: start, start_column: column,
      text: chars.slice(0, length).join(''), next }), truncated: next !== null };
  };
  const complete = render(chars.length);
  if (Buffer.byteLength(complete.content) <= maxBytes) return complete;
  let low = 0, high = chars.length;
  while (low < high) {
    const middle = Math.ceil((low + high) / 2);
    if (Buffer.byteLength(render(middle).content) <= maxBytes) low = middle;
    else high = middle - 1;
  }
  const page = render(low);
  if ((chars.length && !low) || Buffer.byteLength(page.content) > maxBytes)
    throw new Error('Read budget too small for a page');
  return page;
}