json-validator/include/jvalidate/detail/relative_pointer.h

#pragma once

#include <ostream>
#include <string>
#include <string_view>

#include <jvalidate/compat/expected.h>
#include <jvalidate/detail/expect.h>
#include <jvalidate/detail/number.h>
#include <jvalidate/detail/pointer.h>
#include <jvalidate/forward.h>

namespace jvalidate::detail {
/**
 * @brief A special form of json pointer that is allowed to specify moving up
 * into the parent scope of the current location.
 * A relative pointer has two forms:
 * - Nth parent-key, which takes the ABNF form:
 *   non-negative-integer "#"
 * - Nth parent followed by a JSON-Pointer as specified by RFC6901
 *
 * This does not comply with array neighbor offsets as described in
 * Section 4 Paragraph 3 (the "index-manipulation" ABNF).
 *
 * https://json-schema.org/draft/2020-12/relative-json-pointer
 * https://datatracker.ietf.org/doc/html/rfc6901
 */
class RelativePointer {
public:
  static expected<RelativePointer, std::string> parse(std::string_view path) {
    if (path == "0") { // A literal RelativePointer of "0" simply means "here"
      return RelativePointer();
    }

    RelativePointer rval;
    if (size_t pos = path.find('/'); pos != path.npos) {
      // Handle the JSON-Pointer version
      expected ptr = Pointer::parse(path.substr(pos));
      JVALIDATE_PROPIGATE_UNEXPECTED(ptr);
      rval.pointer_ = *std::move(ptr);
      path.remove_suffix(path.size() - pos);
    } else if (path.ends_with('#')) {
      rval.requests_key_ = true;
      path.remove_suffix(1);
    }

    if (path.starts_with('0') && path != "0") {
      return unexpected("Cannot zero-prefix a relative pointer");
    }

    size_t read = 0;
    expected parent_steps = parse_integer<size_t>(path, {.read = read});
    // Will return unexpected if path contains any non-numeric characters, or
    // if path represents a number that is out of the bounds of a size_t.
    JVALIDATE_PROPIGATE_UNEXPECTED(parent_steps.transform_error(to_message));
    rval.parent_steps_ = *parent_steps;
    return rval;
  }

  /**
   * @brief Acquire either the key of the nth parent or a JSON value at some "cousin"
   * location in the overall document.
   *
   * @tparam A JSON Adapter type
   *
   * @param where The pointer representing the current location.
   * @param root The root document location (accessed by where == Pointer())
   *
   * @return The evaluation result as described by
   * https://json-schema.org/draft/2020-12/relative-json-pointer#rfc.section.4
   */
  template <Adapter A>
  std::variant<std::string, A> inspect(Pointer const & where, A const & root) const {
    if (requests_key_) {
      return where.parent(parent_steps_).back();
    }
    auto rval = where.parent(parent_steps_).walk(root);
    return pointer_.walk(rval);
  }

  friend std::ostream & operator<<(std::ostream & os, RelativePointer const & rel) {
    os << rel.parent_steps_;
    if (rel.requests_key_) {
      return os << '#';
    }
    os << rel.pointer_;
    return os;
  }

private:
  size_t parent_steps_ = 0;
  bool requests_key_ = false;
  Pointer pointer_ = {};
};
}