/* Copyright 2025 The OpenXLA Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
==============================================================================*/

#ifndef XLA_SERVICE_SOURCE_TARGET_PAIRS_H_
#define XLA_SERVICE_SOURCE_TARGET_PAIRS_H_

#include <cstddef>
#include <cstdint>
#include <string>
#include <utility>
#include <vector>

#include "absl/container/inlined_vector.h"
#include "absl/log/check.h"
#include "absl/strings/str_format.h"
#include "absl/strings/string_view.h"

namespace xla {

// SourceTargetPairs is a class that represents a list of (source, target)
// pairs using in a collective permute instruction,
// e.g. {{0,1},{1,2},{2,3},{3,0}}.
class SourceTargetPairs {
  static constexpr int64_t kInlineFactor = 8;

 public:
  struct SourceTargetPair {
    int64_t source;
    int64_t target;
    SourceTargetPair() = default;
    SourceTargetPair(int64_t s, int64_t t) : source(s), target(t) {}
    bool operator==(const SourceTargetPair& other) const {
      return source == other.source && target == other.target;
    }
  };

  // TODO: b/388623407 - rename kUnknown to kNone
  enum class CycleType { kUnknown, kForward, kBackward };

  using STVector = absl::InlinedVector<SourceTargetPair, kInlineFactor>;

  SourceTargetPairs() = default;
  explicit SourceTargetPairs(size_t size) : pairs_(size) {};

  explicit SourceTargetPairs(
      const std::vector<std::pair<int64_t, int64_t>>& pairs) {
    for (const auto& pair : pairs) {
      emplace_back(pair.first, pair.second);
    }
  }

  static absl::StatusOr<SourceTargetPairs> FromString(absl::string_view str);

  // Returns a cannoical string such as {{0,1},{1,2},{2,3},{3,0}}.
  std::string ToString() const;

  STVector& data() { return pairs_; }

  SourceTargetPair& operator[](int64_t i) {
    CHECK_LT(i, pairs_.size())
        << "Index out of bounds. Size: " << pairs_.size() << " Index: " << i;
    return pairs_[i];
  }
  const SourceTargetPair& operator[](int64_t i) const {
    CHECK_LT(i, pairs_.size())
        << "Index out of bounds. Size: " << pairs_.size() << " Index: " << i;
    return pairs_[i];
  }

  bool operator==(const SourceTargetPairs& other) const {
    return pairs_ == other.pairs_;
  }

  template <typename Sink>
  friend void AbslStringify(Sink& sink, const SourceTargetPairs& pairs) {
    absl::Format(&sink, "%s", pairs.ToString());
  }

  int64_t size() const { return pairs_.size(); }
  void emplace_back(int64_t source, int64_t target) {
    pairs_.emplace_back(source, target);
  }
  void push_back(const SourceTargetPair& pair) { pairs_.push_back(pair); }

  // Converts to a vector of pairs of ints.
  std::vector<std::pair<int64_t, int64_t>> expand() const {
    std::vector<std::pair<int64_t, int64_t>> data;
    for (const auto& pair : pairs_) {
      data.push_back({pair.source, pair.target});
    }
    return data;
  }

  // Splits input into backward (first) and forwards (second) edges.
  // In backward cycle, backwards edge is the first element and in forwards
  // cycle, backward edge is the last element.
  std::pair<SourceTargetPairs, SourceTargetPairs> SplitEdges(
      CycleType cycle_type) const;

  // Returns the cycle type of this source-target pairs.
  // TODO: b/388623407 - remove assumptions that pairs are ordered and 0 based.
  CycleType GetCycleType() const;
  bool IsForwardCycle() const;
  bool IsBackwardCycle() const;
  bool HasCycles() const { return GetCycleType() != CycleType::kUnknown; }

  int64_t GetMaxDeviceNum() const {
    int64_t max_device_num = -1;
    for (auto [source, target] : pairs_) {
      max_device_num = std::max(std::max(source, target), max_device_num);
    }
    return max_device_num;
  }

  // Returns true if the (source, target) pairs form a forward cycle with all
  // participants in the cycle, such as {{0,1},{1,2},{2,3},{3,0}}. We assume
  // that the (source, target) pairs are ordered via increasing source IDs, as
  // they are currently generated by SPMD partitioning.
  static bool IsForwardCycle(const SourceTargetPairs& backedge,
                             const SourceTargetPairs& others);

  // Returns true if the (source, target) pairs form a backward cycle with all
  // participants in the cycle, such as {{0,3},{1,0},{2,1},{3,2}}. We assume
  // that the (source, target) pairs are ordered via increasing source IDs, as
  // they are currently generated by SPMD partitioning.
  static bool IsBackwardCycle(const SourceTargetPairs& backedge,
                              const SourceTargetPairs& others);

 private:
  STVector pairs_;
};

}  // namespace xla
#endif  // XLA_SERVICE_SOURCE_TARGET_PAIRS_H_