slice.hpp

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// Copyright (c) Sleipnir contributors
 
#pragma once
 
#include <concepts>
#include <limits>
#include <utility>
 
#include "sleipnir/util/assert.hpp"
#include "sleipnir/util/symbol_exports.hpp"
 
namespace slp {
 
namespace slicing {
 
/// Type tag used to designate an omitted argument of the slice.
struct none_t {};
 
/// Designates an omitted argument of the slice.
static inline constexpr none_t _;
 
}  // namespace slicing
 
/// Represents a sequence of elements in an iterable object.
class SLEIPNIR_DLLEXPORT Slice {
 public:
  /// Start index (inclusive).
  int start = 0;
 
  /// Stop index (exclusive).
  int stop = 0;
 
  /// Step.
  int step = 1;
 
  /// Constructs a Slice.
  constexpr Slice() = default;
 
  /// Constructs a slice.
  // NOLINTNEXTLINE (google-explicit-constructor)
  constexpr Slice(slicing::none_t)
      : Slice(0, std::numeric_limits<int>::max(), 1) {}
 
  /// Constructs a slice.
  ///
  /// @param start Slice start index (inclusive).
  // NOLINTNEXTLINE (google-explicit-constructor)
  constexpr Slice(int start) {
    this->start = start;
    this->stop = (start == -1) ? std::numeric_limits<int>::max() : start + 1;
    this->step = 1;
  }
 
  /// Constructs a slice.
  ///
  /// @param start Slice start index (inclusive).
  /// @param stop Slice stop index (exclusive).
  template <typename Start, typename Stop>
    requires(std::same_as<Start, slicing::none_t> ||
             std::convertible_to<Start, int>) &&
            (std::same_as<Stop, slicing::none_t> ||
             std::convertible_to<Stop, int>)
  constexpr Slice(Start start, Stop stop)
      : Slice(std::move(start), std::move(stop), 1) {}
 
  /// Constructs a slice.
  ///
  /// @param start Slice start index (inclusive).
  /// @param stop Slice stop index (exclusive).
  /// @param step Slice step.
  template <typename Start, typename Stop, typename Step>
    requires(std::same_as<Start, slicing::none_t> ||
             std::convertible_to<Start, int>) &&
            (std::same_as<Stop, slicing::none_t> ||
             std::convertible_to<Stop, int>) &&
            (std::same_as<Step, slicing::none_t> ||
             std::convertible_to<Step, int>)
  constexpr Slice(Start start, Stop stop, Step step) {
    if constexpr (std::same_as<Step, slicing::none_t>) {
      this->step = 1;
    } else {
      slp_assert(step != 0);
 
      this->step = step;
    }
 
    // Avoid UB for step = -step if step is INT_MIN
    if (this->step == std::numeric_limits<int>::min()) {
      this->step = -std::numeric_limits<int>::max();
    }
 
    if constexpr (std::same_as<Start, slicing::none_t>) {
      if (this->step < 0) {
        this->start = std::numeric_limits<int>::max();
      } else {
        this->start = 0;
      }
    } else {
      this->start = start;
    }
 
    if constexpr (std::same_as<Stop, slicing::none_t>) {
      if (this->step < 0) {
        this->stop = std::numeric_limits<int>::min();
      } else {
        this->stop = std::numeric_limits<int>::max();
      }
    } else {
      this->stop = stop;
    }
  }
 
  /// Adjusts start and end slice indices assuming a sequence of the specified
  /// length.
  ///
  /// @param length The sequence length.
  /// @return The slice length.
  constexpr int adjust(int length) {
    slp_assert(step != 0);
    slp_assert(step >= -std::numeric_limits<int>::max());
 
    if (start < 0) {
      start += length;
 
      if (start < 0) {
        start = (step < 0) ? -1 : 0;
      }
    } else if (start >= length) {
      start = (step < 0) ? length - 1 : length;
    }
 
    if (stop < 0) {
      stop += length;
 
      if (stop < 0) {
        stop = (step < 0) ? -1 : 0;
      }
    } else if (stop >= length) {
      stop = (step < 0) ? length - 1 : length;
    }
 
    if (step < 0) {
      if (stop < start) {
        return (start - stop - 1) / -step + 1;
      } else {
        return 0;
      }
    } else {
      if (start < stop) {
        return (stop - start - 1) / step + 1;
      } else {
        return 0;
      }
    }
  }
};
 
}  // namespace slp