diff --git a/README.md b/README.md
index dd55604e..96878d35 100644
--- a/README.md
+++ b/README.md
@@ -26,6 +26,7 @@ evaluation wherever possible.
[accumulate](#accumulate)
[compress](#compress)
[sorted](#sorted)
+[shuffled](#shuffled)
[chain](#chain)
[chain.from\_iterable](#chainfrom_iterable)
[reversed](#reversed)
@@ -570,6 +571,25 @@ The below outputs `0 1 2 3 4`.
```c++
unordered_set nums{4, 0, 2, 1, 3};
for (auto&& i : sorted(nums)) {
+ cout << i << ' ';
+}
+```
+
+shuffled
+--------
+*Additional Requirements*: Input must have a ForwardIterator.
+
+Allows iteration over a sequence in shuffled order. `shuffled` does
+**not** produce a new sequence, copy elements, or modify the original
+sequence. It only provides a way to iterate over existing elements.
+`shuffled` also takes an optional second argument - randomization seed.
+If not provided, defaults to 1.
+
+The below outputs `0 2 4 3 1`.
+
+```c++
+unordered_set nums{4, 0, 2, 1, 3};
+for (auto&& i : shuffled(nums)) {
cout << i << '\n';
}
```
diff --git a/shuffled.hpp b/shuffled.hpp
new file mode 100644
index 00000000..b643041b
--- /dev/null
+++ b/shuffled.hpp
@@ -0,0 +1,175 @@
+#ifndef ITER_SHUFFLED_HPP_
+#define ITER_SHUFFLED_HPP_
+
+#include "internal/iterbase.hpp"
+#include
+#include
+#include
+
+/* Suppose the size of the container is N. We can find the power of two,
+ * that is greater or equal to N - K. Numbers from 1 to K can be easy
+ * shuffled with help of Linear Feedback Shift Register (LFSR) using
+ * special prime poly. Access to every element of the shuffled is
+ * implemented through advance of the first (begin) iterator of the
+ * container, so random access iterator is desirable. In the constructor
+ * we have to calculate the total size of the container, so
+ * std::distance will be used. User can define more effective
+ * std::distance for the container.*/
+
+namespace iter {
+ namespace impl {
+ template
+ class ShuffledView;
+ // linear feedback shift register
+ namespace lfsr {
+ constexpr std::array PRIME_POLY = {
+ {1, 3, 3, 3, 5, 3, 3, 29, 17, 9, 5, 83, 27, 43, 3, 45, 9, 129, 39, 9,
+ 5, 3, 33, 27, 9, 71, 39, 9, 5, 83, 9, 197, 8193, 231, 5, 119, 83,
+ 99, 17, 57, 9, 153, 89, 101, 27, 449, 33, 183, 113, 29, 75, 9, 71,
+ 125, 71, 149, 129, 99, 123, 3, 39, 105, 3,
+ 27}}; // prime poly for full-cycle shift registers
+ constexpr std::uint16_t get_approx(std::uint64_t val);
+ constexpr std::uint64_t shift(std::uint64_t reg, std::uint8_t reg_size);
+ }
+ }
+
+ template
+ impl::ShuffledView shuffled(Container&&, int seed = 1);
+}
+
+// power of 2 approximation (val < pow(2, get_approx(val)+1))
+constexpr inline std::uint16_t iter::impl::lfsr::get_approx(std::uint64_t val) {
+ if (val == 0) {
+ return 0;
+ }
+ constexpr auto LIMIT = std::uint64_t(1UL) << 63;
+ if (val > LIMIT) {
+ return 63;
+ }
+ std::uint16_t pow2_approx = 0;
+ while (pow2_approx < 64 && (val >= (1UL << (pow2_approx + 1)))) {
+ ++pow2_approx;
+ }
+ return pow2_approx;
+}
+
+constexpr inline std::uint64_t iter::impl::lfsr::shift(
+ std::uint64_t reg, std::uint8_t reg_size) {
+ if (reg & 1) {
+ reg = ((reg ^ PRIME_POLY[reg_size]) >> 1) | (1 << reg_size);
+ } else {
+ reg >>= 1;
+ }
+ return reg;
+}
+
+template
+class iter::impl::ShuffledView {
+ private:
+ Container container;
+ Distance size;
+ std::uint8_t size_approx;
+ std::uint64_t seed;
+
+ friend ShuffledView iter::shuffled(Container&&, int);
+
+ using IterDeref = iterator_traits_deref;
+ ShuffledView(Container&& in_container, int in_seed)
+ : container(std::forward(in_container)),
+ size(std::distance(std::begin(container), std::end(container))),
+ size_approx(lfsr::get_approx(size)),
+ seed(in_seed) {
+ if (size > 0) {
+ std::uint64_t mask = -1;
+ mask = (mask >> (64 - size_approx));
+ this->seed = seed & mask;
+ this->seed = lfsr::shift(this->seed, size_approx);
+ while (this->seed >= size) {
+ this->seed = lfsr::shift(this->seed, size_approx);
+ }
+ }
+ }
+
+ public:
+ ShuffledView(ShuffledView&&) = default;
+
+ class Iterator : public std::iterator {
+ private:
+ friend class ShuffledView;
+ ShuffledView* owner{};
+ std::uint64_t state{};
+
+ public:
+ Iterator() = default;
+ Iterator(ShuffledView* in_owner, std::uint64_t in_state)
+ : owner{in_owner}, state{in_state} {}
+
+ Iterator& operator++() {
+ if (*this == owner->end()) {
+ return *this;
+ }
+ do {
+ state = lfsr::shift(state, owner->size_approx);
+ } while (state > owner->size);
+ if (state == owner->seed) {
+ *this = owner->end();
+ }
+ return *this;
+ }
+
+ Iterator operator++(int) {
+ auto ret = *this;
+ ++*this;
+ return ret;
+ }
+
+ bool operator==(const Iterator& other) const {
+ return owner == other.owner && state == other.state;
+ }
+
+ bool operator!=(const Iterator& other) const {
+ return !(*this == other);
+ }
+
+ iterator_deref operator*() {
+ return *std::next(std::begin(owner->container), state - 1);
+ }
+
+ ArrowProxy operator->() {
+ return {**this};
+ }
+ };
+
+ Iterator begin() {
+ return {this, (size == 0 ? 0 : seed)};
+ }
+
+ Iterator end() {
+ return {this, 0};
+ }
+
+ /* @brief restore iteration through a vector in shuffled order from the
+ * index of the last seen element in non shuffled container
+ *
+ * @example:
+ * v = {"apple", "banana", "orange", "peach"}; // original vector
+ * s = shuffled(v); // {"orange", "peach", "banana", "apple"} - shuffled
+ * vector
+ *
+ * We iterated through s. Last seen element was "banana". In the
+ * original vector "banana" had index 1. So to restore shuffling wee
+ * need to do s.restore(1); Operation is zero cost - no iterators
+ * advance is needed.*/
+ Iterator restore(std::uint64_t state) {
+ return {this, (state >= size ? seed : state + 1)};
+ }
+};
+
+
+template
+iter::impl::ShuffledView iter::shuffled(
+ Container&& container, int seed) {
+ return {std::forward(container), seed};
+}
+
+#endif
diff --git a/test/SConstruct b/test/SConstruct
index cbf3cd3c..9809b263 100644
--- a/test/SConstruct
+++ b/test/SConstruct
@@ -37,6 +37,7 @@ progs = Split(
sliding_window
starmap
sorted
+ shuffled
takewhile
unique_everseen
unique_justseen
diff --git a/test/test_shuffled.cpp b/test/test_shuffled.cpp
new file mode 100644
index 00000000..e07c6a46
--- /dev/null
+++ b/test/test_shuffled.cpp
@@ -0,0 +1,224 @@
+#include
+
+#include
+#include
+#include
+#include
+#include
+#include
+#include
+
+#include "helpers.hpp"
+#include "catch.hpp"
+
+using iter::shuffled;
+
+using Vec = const std::vector;
+
+TEST_CASE("shuffled: iterates through a vector in shuffled order", "[shuffled]") {
+ Vec ns = {4, 0, 5, 1, 6, 7, 9, 3, 2, 8};
+ auto s = shuffled(ns);
+ Vec v(std::begin(s), std::end(s));
+ Vec vc = {2, 9, 8, 6, 7, 5, 3, 1, 0, 4};
+ REQUIRE(v == vc);
+}
+
+TEST_CASE("shuffled: iterates through a list of strings in shuffled order",
+ "[shuffled]") {
+ std::list ns = {"apple", "banana", "orange", "peach"};
+ auto s = shuffled(ns);
+ std::list v(std::begin(s), std::end(s));
+ std::list vc = {"orange", "peach", "banana", "apple"};
+ REQUIRE(v == vc);
+}
+
+TEST_CASE("shuffled: restore iteration through a vector in shuffled order "
+ "from the index of the last seen element in non shuffled container",
+ "[shuffled]") {
+ //index: 0 1 2 3 4 5 6 7 8 9
+ Vec ns = {4, 0, 5, 1, 6, 7, 9, 3, 2, 8};
+ auto s = shuffled(ns);
+
+ // overflow - same as default
+ Vec v10(s.restore(10), std::end(s));
+ Vec vc10 = {2, 9, 8, 6, 7, 5, 3, 1, 0, 4};
+ REQUIRE(v10 == vc10);
+
+ Vec v1(s.restore(6), std::end(s));
+ Vec vc1 = {9, 8, 6, 7, 5, 3, 1, 0, 4};
+ REQUIRE(v1 == vc1);
+
+ Vec v2(s.restore(9), std::end(s));
+ Vec vc2 = {8, 6, 7, 5, 3, 1, 0, 4};
+ REQUIRE(v2 == vc2);
+
+ Vec v3(s.restore(4), std::end(s));
+ Vec vc3 = {6, 7, 5, 3, 1, 0, 4};
+ REQUIRE(v3 == vc3);
+
+ Vec v4(s.restore(5), std::end(s));
+ Vec vc4 = {7, 5, 3, 1, 0, 4};
+ REQUIRE(v4 == vc4);
+
+ Vec v5(s.restore(2), std::end(s));
+ Vec vc5 = {5, 3, 1, 0, 4};
+ REQUIRE(v5 == vc5);
+
+ Vec v6(s.restore(7), std::end(s));
+ Vec vc6 = {3, 1, 0, 4};
+ REQUIRE(v6 == vc6);
+
+ Vec v7(s.restore(3), std::end(s));
+ Vec vc7 = {1, 0, 4};
+ REQUIRE(v7 == vc7);
+
+ Vec v8(s.restore(1), std::end(s));
+ Vec vc8 = {0, 4};
+ REQUIRE(v8 == vc8);
+
+ Vec v9(s.restore(0), std::end(s));
+ Vec vc9 = {4};
+ REQUIRE(v9 == vc9);
+}
+
+TEST_CASE("shuffled: restore iteration through a vector in shuffled order "
+ "from the index of the last seen element in non shuffled "
+ "container of strings", "[shuffled]") {
+ std::list ns = {"apple", "banana", "orange", "peach"};
+ auto s = shuffled(ns);
+ std::list v(s.restore(1), std::end(s));
+ std::list vc = {"banana", "apple"};
+ REQUIRE(v == vc);
+}
+
+TEST_CASE("shuffled: iterates through a vector in shuffled order with non"
+ " standard seed", "[shuffled]") {
+ Vec ns = {4, 0, 5, 1, 6, 7, 9, 3, 2, 8};
+ auto s = shuffled(ns, 1234);
+ Vec v(std::begin(s), std::end(s));
+ Vec vc = {4, 2, 9, 8, 6, 7, 5, 3, 1, 0};
+ REQUIRE(v == vc);
+}
+
+TEST_CASE("shuffled: can modify elements through shuffled", "[shuffled]") {
+ std::vector ns(3, 9);
+ for (auto&& n : shuffled(ns)) {
+ n = -1;
+ }
+ Vec vc(3, -1);
+ REQUIRE(ns == vc);
+}
+
+TEST_CASE("shuffled: can iterate over unordered container", "[shuffled]") {
+ std::unordered_set ns = {1, 3, 2, 0, 4};
+ auto s = shuffled(ns);
+
+ Vec v(std::begin(s), std::end(s));
+ Vec vc = {3, 2, 1, 4, 0};
+ REQUIRE(v == vc);
+}
+
+TEST_CASE("shuffled: empty when iterable is empty", "[shuffled]") {
+ Vec ns{};
+ auto s = shuffled(ns);
+ REQUIRE(std::begin(s) == std::end(s));
+}
+
+namespace {
+ template
+ class BasicIterableWithConstDeref {
+ private:
+ T* data;
+ std::size_t size;
+ bool was_moved_from_ = false;
+
+ public:
+ BasicIterableWithConstDeref(std::initializer_list il)
+ : data{new T[il.size()]}, size{il.size()} {
+ // would like to use enumerate, can't because it's for unit
+ // testing enumerate
+ std::size_t i = 0;
+ for (auto&& e : il) {
+ data[i] = e;
+ ++i;
+ }
+ }
+
+ BasicIterableWithConstDeref& operator=(
+ BasicIterableWithConstDeref&&) = delete;
+ BasicIterableWithConstDeref& operator=(
+ const BasicIterableWithConstDeref&) = delete;
+ BasicIterableWithConstDeref(const BasicIterableWithConstDeref&) = delete;
+
+ BasicIterableWithConstDeref(BasicIterableWithConstDeref&& other)
+ : data{other.data}, size{other.size} {
+ other.data = nullptr;
+ other.was_moved_from_ = true;
+ }
+
+ bool was_moved_from() const {
+ return this->was_moved_from_;
+ }
+
+ ~BasicIterableWithConstDeref() {
+ delete[] this->data;
+ }
+
+ class Iterator
+ : public std::iterator {
+ private:
+ T* p;
+
+ public:
+ Iterator(T* b) : p{b} {}
+ bool operator!=(const Iterator& other) const {
+ return this->p != other.p;
+ }
+
+ Iterator& operator++() {
+ ++this->p;
+ return *this;
+ }
+
+ T& operator*() {
+ return *this->p;
+ }
+
+ const T& operator*() const {
+ return *this->p;
+ }
+ };
+
+ Iterator begin() {
+ return {this->data};
+ }
+
+ Iterator end() {
+ return {this->data + this->size};
+ }
+ };
+}
+
+TEST_CASE("shuffled: moves rvalues and binds to lvalues", "[shuffled]") {
+ BasicIterableWithConstDeref bi{1, 2};
+ shuffled(bi);
+ REQUIRE_FALSE(bi.was_moved_from());
+
+ shuffled(std::move(bi));
+ REQUIRE(bi.was_moved_from());
+}
+
+TEST_CASE("shuffled: doesn't move or copy elements of iterable", "[shuffled]") {
+ using itertest::SolidInt;
+ constexpr SolidInt arr[] = {{6}, {7}, {8}};
+ for (auto &&i : shuffled(arr))
+ (void)i;
+}
+
+template
+using ImpT = decltype(shuffled(std::declval()));
+TEST_CASE("shuffled: has correct ctor and assign ops", "[shuffled]") {
+ REQUIRE(itertest::IsMoveConstructibleOnly>::value);
+ REQUIRE(itertest::IsMoveConstructibleOnly>::value);
+}
+