/** A logical prompt position expressed as a UTF-16 offset within one line. */
export type Position = {
  line: number;
  column: number;
};

/**
 * The pure editing engine's prompt representation.
 *
 * Lines are always non-empty as a collection (an empty prompt is `[""]`),
 * while individual logical lines may be empty. Grapheme-boundary validation is
 * added by the segmentation phase; this phase only establishes line bounds.
 */
export type PromptBuffer = {
  readonly lines: readonly string[];
  readonly cursor: Position;
  /** Pi large-paste marker ids that must remain atomic during edits. */
  readonly pasteMarkerIds?: ReadonlySet<number>;
};

export type PromptBufferOptions = {
  pasteMarkerIds?: ReadonlySet<number>;
};

/** Normalize the line endings Pi accepts before constructing a logical buffer. */
export function normalizePromptText(text: string): string {
  return text.replaceAll("\r\n", "\n").replaceAll("\r", "\n");
}

/** Create a valid immutable logical prompt buffer with the cursor at its end. */
export function createPromptBuffer(
  text: string,
  options: PromptBufferOptions = {},
): PromptBuffer {
  const lines = normalizePromptText(text).split("\n");
  const line = lines.length - 1;
  const lastLine = lines[line];

  if (lastLine === undefined) {
    throw new Error("Prompt buffer must contain at least one line");
  }

  return {
    lines,
    cursor: { line, column: lastLine.length },
    ...(options.pasteMarkerIds === undefined
      ? {}
      : { pasteMarkerIds: new Set(options.pasteMarkerIds) }),
  };
}

/** Return whether a position is within the inclusive cursor bounds of a buffer. */
export function isValidPosition(
  buffer: PromptBuffer,
  position: Position,
): boolean {
  if (!Number.isInteger(position.line) || !Number.isInteger(position.column))
    return false;
  if (
    position.line < 0 ||
    position.line >= buffer.lines.length ||
    position.column < 0
  )
    return false;

  const line = buffer.lines[position.line];
  return line !== undefined && position.column <= line.length;
}