Add basic support for expressing algorithms in positionless form.

.clang-format

@@ -11,3 +11,4 @@ AllowAllParametersOfDeclarationOnNextLine: true
 BinPackArguments: false
 BinPackParameters: false
 AlignAfterOpenBracket: BlockIndent
+AlwaysBreakTemplateDeclarations: Yes

.vscode/settings.json

@@ -7,6 +7,7 @@
         "--header-insertion=iwyu"
     ],
     "files.associations": {
-        "algorithm": "cpp"
+        "algorithm": "cpp",
+        "vector": "cpp"
     }
 }

CMakeLists.txt

@@ -51,6 +51,7 @@ add_executable(unit_tests
     test/tests_main.cpp
     test/partitioning_tests.cpp
     test/algorithms_tests.cpp
+    test/translation_tests.cpp
 )
 target_link_libraries(unit_tests PRIVATE positionless doctest::doctest rapidcheck)
 

include/positionless/detail/algorithm_data.hpp

@@ -0,0 +1,298 @@
+#pragma once
+
+#include "positionless/detail/algorithm_data_fwd.hpp"
+#include "positionless/detail/precondition.hpp"
+#include "positionless/partitioning.hpp"
+
+#include <vector>
+
+#define POSITIONLESS_DEBUG_PRINT false
+
+#if POSITIONLESS_DEBUG_PRINT
+#include <iostream>
+#endif
+
+namespace positionless::detail {
+
+/// The algorithm data shared between the iterators.
+///
+/// Invariants: the number of parts is one less than the number of iterators we have valid.
+template <std::forward_iterator BaseIterator>
+class algorithm_data {
+public:
+  /// The base iterator type.
+  using base_iterator = BaseIterator;
+
+  /// An instance of the algorithm data that covers the range [begin, end).
+  ///
+  /// Creates two iterators corresponding to `begin` and `end`, but never exposes them directly.
+  explicit algorithm_data(base_iterator begin, base_iterator end);
+
+  /// Creates a new iterator pointing to the begin of the range, and returns its index.
+  size_t create_begin_iterator();
+
+  /// Creates a new iterator pointing to the end of the range, and returns its index.
+  size_t create_end_iterator();
+
+  /// Returns the base iterator corresponding to `iterator_index`.
+  ///
+  /// - Precondition: iterator `iterator_index` is valid.
+  base_iterator base(size_t iterator_index) const;
+
+  /// Creates a copy of the base iterator at `iterator_index`, and return the index of the new
+  /// iterator.
+  ///
+  /// - Precondition: iterator `iterator_index` is valid.
+  size_t copy_iterator(size_t iterator_index);
+
+  /// Removes iterator at `iterator_index` from algorithm data.
+  ///
+  /// - Precondition: `iterator_index > 1`.
+  /// - Precondition: iterator `iterator_index` is valid.
+  void destroy_iterator(size_t iterator_index);
+
+  /// Increments the iterator at `iterator_index` to point to the next element.
+  ///
+  /// - Precondition: iterator `iterator_index` is valid.
+  /// - Precondition: `base(iterator_index)` != `base(end_index())`.
+  void increment(size_t iterator_index);
+
+  /// Increments the iterator at `iterator_index` by `n` elements.
+  ///
+  /// - Precondition: iterator `iterator_index` is valid.
+  /// - Precondition: `base(iterator_index) + n` <= `base(end_index())`.
+  void increment_by(size_t iterator_index, size_t n)
+    requires std::random_access_iterator<BaseIterator>;
+
+  /// Decrements the iterator at `iterator_index` to point to the next element.
+  ///
+  /// - Precondition: iterator `iterator_index` is valid.
+  /// - Precondition: `base(iterator_index)` != `base(end_index())`.
+  void decrement(size_t iterator_index);
+
+  /// Decrements the iterator at `iterator_index` by `n` elements.
+  ///
+  /// - Precondition: iterator `iterator_index` is valid.
+  /// - Precondition: `base(iterator_index) + n` <= `base(end_index())`.
+  void decrement_by(size_t iterator_index, size_t n)
+    requires std::random_access_iterator<BaseIterator>;
+
+private:
+  /// The partitioning applied to the input slice.
+  partitioning<BaseIterator> partitioning_;
+
+  /// Mapping from iterator index to part index.
+  /// We reserve here a tombstone value to indicate destroyed iterators.
+  /// This can contain values that are equal to `partitioning_.parts_count()` for iterators that
+  /// point to the element past the end of the slice.
+  std::vector<size_t> parts_mapping_;
+
+#if POSITIONLESS_DEBUG_PRINT
+  /// Prints the current state of the data for debugging purposes.
+  void print_debug() const;
+#endif
+};
+
+/// The value we use to indicate a destroyed iterator.
+static constexpr size_t tombstone_part = static_cast<size_t>(-1);
+
+template <std::forward_iterator BaseIterator>
+inline algorithm_data<BaseIterator>::algorithm_data(BaseIterator begin, BaseIterator end)
+    : partitioning_(begin, end), parts_mapping_{0, 1} {
+#if POSITIONLESS_DEBUG_PRINT
+  std::cout << "Initialized algorithm_data\n";
+  print_debug();
+#endif
+}
+
+template <std::forward_iterator BaseIterator>
+inline size_t algorithm_data<BaseIterator>::create_begin_iterator() {
+  size_t r = copy_iterator(0);
+#if POSITIONLESS_DEBUG_PRINT
+  std::cout << "created begin iterator: " << r << "\n";
+  print_debug();
+#endif
+  return r;
+}
+
+template <std::forward_iterator BaseIterator>
+inline size_t algorithm_data<BaseIterator>::create_end_iterator() {
+  size_t r = copy_iterator(1);
+#if POSITIONLESS_DEBUG_PRINT
+  std::cout << "created end iterator: " << r << "\n";
+  print_debug();
+#endif
+  return r;
+}
+
+template <std::forward_iterator BaseIterator>
+inline typename algorithm_data<BaseIterator>::base_iterator
+algorithm_data<BaseIterator>::base(size_t iterator_index) const {
+  PRECONDITION(iterator_index < parts_mapping_.size());
+  const size_t part = parts_mapping_[iterator_index];
+  PRECONDITION(part != tombstone_part);
+  if (part == partitioning_.parts_count()) {
+    return partitioning_.part(partitioning_.parts_count() - 1).second;
+  } else {
+    return partitioning_.part(part).first;
+  }
+}
+
+template <std::forward_iterator BaseIterator>
+inline size_t algorithm_data<BaseIterator>::copy_iterator(size_t iterator_index) {
+  PRECONDITION(iterator_index < parts_mapping_.size());
+  const size_t part = parts_mapping_[iterator_index];
+  PRECONDITION(part != tombstone_part);
+  // Add an empty part before the part corresponding to `iterator_index`.
+  partitioning_.add_part_begin(part);
+  // Shift all the mappings that corresponds to parts >= `part` by 1.
+  std::transform(
+      parts_mapping_.cbegin(),
+      parts_mapping_.cend(),
+      parts_mapping_.begin(),
+      [part](size_t p) { return p >= part && p != tombstone_part ? p + 1 : p; }
+  );
+
+  // Try to find a tombstone to reuse.
+  size_t r;
+  auto it = std::find(parts_mapping_.begin(), parts_mapping_.end(), tombstone_part);
+  if (it != parts_mapping_.end()) {
+    *it = part;
+    r = static_cast<size_t>(std::distance(parts_mapping_.begin(), it));
+  } else {
+    // Add a new mapping for the new iterator.
+    parts_mapping_.push_back(part);
+    // Return the index of the new iterator.
+    r = parts_mapping_.size() - 1;
+  }
+#if POSITIONLESS_DEBUG_PRINT
+  std::cout << "copied iterator " << iterator_index << " to " << r << "\n";
+  print_debug();
+#endif
+  return r;
+}
+
+template <std::forward_iterator BaseIterator>
+inline void algorithm_data<BaseIterator>::destroy_iterator(size_t iterator_index) {
+  PRECONDITION(iterator_index < parts_mapping_.size());
+  parts_mapping_[iterator_index] = tombstone_part;
+#if POSITIONLESS_DEBUG_PRINT
+  std::cout << "destroyed iterator: " << iterator_index << "\n";
+  print_debug();
+#endif
+}
+
+template <std::forward_iterator BaseIterator>
+inline void algorithm_data<BaseIterator>::increment(size_t iterator_index) {
+  PRECONDITION(iterator_index < parts_mapping_.size());
+  const size_t part = parts_mapping_[iterator_index];
+  PRECONDITION(part != tombstone_part);
+  PRECONDITION(partitioning_.part(part).first != base(1));
+
+  if (!partitioning_.is_part_empty(part)) {
+    // Simple case: we just grow the previous part, which will move the iterator.
+    partitioning_.grow(part - 1);
+  } else {
+    // We need to shift some parts, to make space for growing.
+    size_t next_non_empty_part = part + 1;
+    while (next_non_empty_part < partitioning_.parts_count() &&
+           partitioning_.is_part_empty(next_non_empty_part)) {
+      next_non_empty_part++;
+    }
+    PRECONDITION(next_non_empty_part < partitioning_.parts_count());
+
+    // Swap iterators that map to `part` and `next_non_empty_part-1`.
+    auto it = std::find(parts_mapping_.begin(), parts_mapping_.end(), next_non_empty_part);
+    PRECONDITION(it != parts_mapping_.end());
+    std::swap(*it, parts_mapping_[iterator_index]);
+
+    // Now grow the right part.
+    partitioning_.grow(next_non_empty_part - 1);
+  }
+
+#if POSITIONLESS_DEBUG_PRINT
+  std::cout << "incremented iterator: " << iterator_index << "\n";
+  print_debug();
+#endif
+}
+
+template <std::forward_iterator BaseIterator>
+inline void algorithm_data<BaseIterator>::increment_by(size_t iterator_index, size_t n)
+  requires std::random_access_iterator<BaseIterator>
+{
+  // TODO: improve this
+  for (size_t i = 0; i < n; ++i) {
+    increment(iterator_index);
+  }
+}
+
+template <std::forward_iterator BaseIterator>
+inline void algorithm_data<BaseIterator>::decrement(size_t iterator_index) {
+  PRECONDITION(iterator_index < parts_mapping_.size());
+  const size_t part = parts_mapping_[iterator_index];
+  PRECONDITION(part != tombstone_part);
+  PRECONDITION(part > 0);
+  PRECONDITION(partitioning_.part(part).first != base(0));
+
+  if (part > 0 && !partitioning_.is_part_empty(part - 1)) {
+    // Simple case: we just shrink the previous part, which will move the iterator backward.
+    partitioning_.shrink(part - 1);
+  } else {
+    // We need to find a previous non-empty part to shrink.
+    size_t prev_non_empty_part = part - 1;
+    while (prev_non_empty_part > 0 && partitioning_.is_part_empty(prev_non_empty_part)) {
+      prev_non_empty_part--;
+    }
+    PRECONDITION(!partitioning_.is_part_empty(prev_non_empty_part));
+
+    // Swap iterators that map to `part` and `prev_non_empty_part+1`.
+    auto it = std::find(parts_mapping_.begin(), parts_mapping_.end(), prev_non_empty_part + 1);
+    PRECONDITION(it != parts_mapping_.end());
+    std::swap(*it, parts_mapping_[iterator_index]);
+
+    // Now shrink the non-empty part.
+    partitioning_.shrink(prev_non_empty_part);
+  }
+
+#if POSITIONLESS_DEBUG_PRINT
+  std::cout << "decremented iterator: " << iterator_index << "\n";
+  print_debug();
+#endif
+}
+
+template <std::forward_iterator BaseIterator>
+inline void algorithm_data<BaseIterator>::decrement_by(size_t iterator_index, size_t n)
+  requires std::random_access_iterator<BaseIterator>
+{
+  // TODO: improve this
+  for (size_t i = 0; i < n; ++i) {
+    decrement(iterator_index);
+  }
+}
+
+#if POSITIONLESS_DEBUG_PRINT
+template <std::forward_iterator BaseIterator>
+inline void algorithm_data<BaseIterator>::print_debug() const {
+  std::cout << "  - data: ";
+  for (size_t i = 0; i < partitioning_.parts_count(); ++i) {
+    auto [begin, end] = partitioning_.part(i);
+    std::cout << "[";
+    for (auto it = begin; it != end; ++it) {
+      std::cout << *it << " ";
+    }
+    std::cout << "]";
+  }
+  std::cout << "\n";
+  std::cout << "  - mapping: [";
+  for (size_t i = 0; i < parts_mapping_.size(); ++i) {
+    if (parts_mapping_[i] == tombstone_part) {
+      std::cout << "x ";
+    } else {
+      std::cout << parts_mapping_[i] << " ";
+    }
+  }
+  std::cout << "]\n";
+}
+#endif
+
+} // namespace positionless::detail

include/positionless/detail/algorithm_data_fwd.hpp

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <iterator>
+#include <memory>
+
+namespace positionless::detail {
+
+template <std::forward_iterator BaseIterator>
+class algorithm_data;
+
+template <std::forward_iterator BaseIterator>
+using algorithm_data_ptr = std::shared_ptr<algorithm_data<BaseIterator>>;
+
+} // namespace positionless::detail

include/positionless/partitioning.hpp

@@ -16,7 +16,8 @@ namespace positionless {
 /// constructor most not be invalidated.
 ///
 /// - Invariant: parts_count() >= 1
-template <std::forward_iterator Iterator> class partitioning {
+template <std::forward_iterator Iterator>
+class partitioning {
 public:
   using iterator = Iterator;
   using value_type = std::iter_value_t<Iterator>;
@@ -74,12 +75,20 @@ template <std::forward_iterator Iterator> class partitioning {
   void add_part_end(size_t i);
 
   /// Adds a new empty part at the beginning of the `i`th part.
+  ///
+  /// Equivalent to `add_part_end(parts_count() - 1)` if `i == parts_count()`.
+  ///
+  /// - Precondition: `i <= parts_count()`
   void add_part_begin(size_t i);
 
   /// Adds `count` new empty parts at the end of the `i`th part.
   void add_parts_end(size_t i, size_t count);
 
   /// Adds `count` new empty parts at the beginning of the `i`th part.
+  ///
+  /// Equivalent to `add_parts_end(parts_count() - 1, count)` if `i == parts_count()`.
+  ///
+  /// - Precondition: `i <= parts_count()`
   void add_parts_begin(size_t i, size_t count);
 
   /// Removes the `i`th part, growing the previous part to cover its range.
@@ -145,7 +154,8 @@ inline size_t partitioning<Iterator>::part_size(size_t i) const {
   return std::distance(boundaries_[i], boundaries_[i + 1]);
 }
 
-template <std::forward_iterator Iterator> inline void partitioning<Iterator>::grow(size_t i) {
+template <std::forward_iterator Iterator>
+inline void partitioning<Iterator>::grow(size_t i) {
   PRECONDITION(i + 1 < parts_count());
   PRECONDITION(!is_part_empty(i + 1));
   boundaries_[i + 1]++;
@@ -192,7 +202,7 @@ inline void partitioning<Iterator>::add_part_end(size_t i) {
 
 template <std::forward_iterator Iterator>
 inline void partitioning<Iterator>::add_part_begin(size_t i) {
-  PRECONDITION(i < parts_count());
+  PRECONDITION(i <= parts_count());
   boundaries_.insert(boundaries_.begin() + i, boundaries_[i]);
 }
 
@@ -204,7 +214,7 @@ inline void partitioning<Iterator>::add_parts_end(size_t i, size_t count) {
 
 template <std::forward_iterator Iterator>
 inline void partitioning<Iterator>::add_parts_begin(size_t i, size_t count) {
-  PRECONDITION(i < parts_count());
+  PRECONDITION(i <= parts_count());
   boundaries_.insert(boundaries_.begin() + i, count, boundaries_[i]);
 }