/** UTF-16 code-unit offsets into the parsed source. */
export interface Span {
  readonly start: number;
  readonly end: number;
}

/** malformed input | bad Unicode escape | nesting exceeds limit */
export type FjsonErrorKind = "syntax" | "unicode" | "nesting";

function location(name: string, source: string, offset: number): string {
  const prefix = source.slice(0, Math.max(0, Math.min(offset, source.length)));
  let line = 1;
  let column = 1;
  let previousCr = false;
  for (const ch of prefix) {
    if (ch === "\r") {
      line += 1;
      column = 1;
    } else if (ch === "\n") {
      if (!previousCr) line += 1;
      column = 1;
    } else {
      column += 1;
    }
    previousCr = ch === "\r";
  }
  return `${name}:${line}:${column}`;
}

/** Represents a failure to parse an fJSON document */
export class FjsonError extends SyntaxError {
  readonly kind: FjsonErrorKind;
  readonly span: Span;
  readonly previous?: Span;

  constructor(
    kind: FjsonErrorKind,
    message: string,
    span: Span,
    previous?: Span,
  ) {
    super(message);
    this.kind = kind;
    this.span = span;
    if (previous !== undefined) this.previous = previous;
  }

  /** Render against the same source that was parsed. */
  render(name: string, source: string): string {
    let message = `${location(name, source, this.span.start)}: ${this.message}`;
    if (this.previous !== undefined) {
      message += `\n${
        location(name, source, this.previous.start)
      }: First defined here`;
    }
    return message;
  }
}