From cff58795720432d97f9301d7b970d29e145d7afc Mon Sep 17 00:00:00 2001
From: Patrick Wuttke <mewin@mewin.de>
Date: Mon, 21 Aug 2023 01:48:43 +0200
Subject: [PATCH] Added filter() and some more iterator utilities.

---
 source/mijin/util/iterators.hpp | 270 +++++++++++++++++++++++++++++++-
 1 file changed, 263 insertions(+), 7 deletions(-)

diff --git a/source/mijin/util/iterators.hpp b/source/mijin/util/iterators.hpp
index 9525e13..e0be6cd 100644
--- a/source/mijin/util/iterators.hpp
+++ b/source/mijin/util/iterators.hpp
@@ -5,6 +5,7 @@
 
 #include <cstddef>
 #include <functional>
+#include <optional>
 #include <span>
 #include <string_view>
 #include <tuple>
@@ -30,6 +31,29 @@ struct RangeRef
     }
 };
 
+template<typename TIterator>
+struct RangeRef<std::pair<TIterator, TIterator>>
+{
+    std::pair<TIterator, TIterator> iterators;
+
+    TIterator begin() const
+    {
+        return iterators.first;
+    }
+
+    TIterator end() const
+    {
+        return iterators.second;
+    }
+};
+
+// in case of pass-by-reference
+template<typename TIterator>
+struct RangeRef<std::pair<TIterator, TIterator>&> : RangeRef<std::pair<TIterator, TIterator>> {};
+
+static_assert(std::is_same_v<decltype(std::declval<RangeRef<std::pair<int*, int*>>>().iterators.first), int*>, "RangeRef for iterator pair not working.");
+static_assert(std::is_same_v<decltype(std::declval<RangeRef<std::pair<std::string_view::iterator, std::string_view::iterator>&>>().iterators.first), std::string_view::iterator>, "RangeRef for iterator pair not working.");
+
 template<typename T>
 struct is_range_adapter_type : std::false_type {};
 
@@ -199,7 +223,7 @@ struct ReplacingIterator
 
     ReplacingIterator operator--(int) noexcept
     {
-        EnumeratingIterator copy(*this);
+        ReplacingIterator copy(*this);
         --(*this);
         return copy;
     }
@@ -254,10 +278,14 @@ struct MappingIterator
     using orig_value_type = typename std::iterator_traits<TIterator>::value_type;
     using difference_type = std::ptrdiff_t;
     using value_type = std::invoke_result_t<TFunctor, orig_value_type>;
+    using reference = value_type&;
     using iterator_category = std::bidirectional_iterator_tag; // TODO?
 
+    static_assert(!std::is_same_v<value_type, void>, "Invalid mapping result type.");
+
     TIterator base;
     TFunctor functor;
+    mutable std::optional<value_type> result;
 
     MappingIterator(TIterator base_, TFunctor functor_) noexcept : base(base_), functor(std::move(functor_)) {}
     MappingIterator(const MappingIterator&) noexcept = default;
@@ -266,20 +294,24 @@ struct MappingIterator
     MappingIterator& operator=(const MappingIterator&) noexcept = default;
     MappingIterator& operator=(MappingIterator&&) noexcept = default;
 
-    value_type operator*() const noexcept
+    reference operator*() const noexcept
     {
-        return functor(*base);
+        if (!result.has_value()) {
+            result = functor(*base);
+        }
+        return *result;
     }
 
     MappingIterator& operator++() noexcept
     {
         ++base;
+        result = std::nullopt;
         return *this;
     }
 
     MappingIterator operator++(int) noexcept
     {
-        ReplacingIterator copy(*this);
+        MappingIterator copy(*this);
         ++(*this);
         return copy;
     }
@@ -287,19 +319,20 @@ struct MappingIterator
     MappingIterator& operator--() noexcept
     {
         --base;
+        result = std::nullopt;
         return *this;
     }
 
     MappingIterator operator--(int) noexcept
     {
-        EnumeratingIterator copy(*this);
+        MappingIterator copy(*this);
         --(*this);
         return copy;
     }
 
     bool operator==(const MappingIterator& other) const noexcept
     {
-        return functor == other.functor && base == other.base;
+        return base == other.base; // TODO: compare functor? -> doesn't always work and may be useless
     }
 
     bool operator!=(const MappingIterator& other) const noexcept
@@ -311,7 +344,8 @@ struct MappingIterator
 template<typename TIterable, typename TFunctor>
 struct MappingRange : RangeAdapter
 {
-    using value_type = typename std::iterator_traits<decltype(std::begin(std::declval<TIterable>()))>::value_type;
+    // using value_type = typename std::iterator_traits<decltype(std::begin(std::declval<TIterable>()))>::value_type;
+    using value_type = typename std::iterator_traits<decltype(std::declval<RangeRef<TIterable>>().begin())>::value_type;
 
     RangeRef<TIterable> base;
     TFunctor functor;
@@ -336,6 +370,184 @@ auto map(TIterable&& iterable, TFunctor&& functor)
     };
 }
 
+template<typename TIterator, typename TFunctor>
+struct OptionalMappingIterator
+{
+    using orig_value_type = typename std::iterator_traits<TIterator>::value_type;
+    using difference_type = std::ptrdiff_t;
+    using optional_type = std::invoke_result_t<TFunctor, orig_value_type>;
+    using value_type = std::decay_t<decltype(std::declval<optional_type>().value())>;
+    using iterator_category = std::forward_iterator_tag; // TODO?
+    using reference = value_type&;
+
+    static_assert(!std::is_same_v<value_type, void>, "Invalid mapping result type.");
+
+    TIterator base;
+    TIterator end;
+    TFunctor functor;
+    mutable optional_type result; // must be mutable so dereferencing can stay const, not nice :/
+
+    OptionalMappingIterator(TIterator base_, TIterator end_, TFunctor functor_) noexcept : base(base_), end(end_), functor(std::move(functor_)) {
+        if (base != end)
+        {
+            result = functor(*base);
+            if (!result.has_value()) {
+                ++(*this);
+            }
+        }
+    }
+    OptionalMappingIterator(const OptionalMappingIterator&) noexcept = default;
+    OptionalMappingIterator(OptionalMappingIterator&&) noexcept = default;
+
+    OptionalMappingIterator& operator=(const OptionalMappingIterator&) noexcept = default;
+    OptionalMappingIterator& operator=(OptionalMappingIterator&&) noexcept = default;
+
+    reference operator*() const noexcept
+    {
+        return *result;
+    }
+
+    OptionalMappingIterator& operator++() noexcept
+    {
+        do
+        {
+            ++base;
+            result = base != end ? functor(*base) : std::nullopt;
+        } while (base != end && !result.has_value());
+        return *this;
+    }
+
+    OptionalMappingIterator operator++(int) noexcept
+    {
+        OptionalMappingIterator copy(*this);
+        ++(*this);
+        return copy;
+    }
+
+    bool operator==(const OptionalMappingIterator& other) const noexcept
+    {
+        return base == other.base && end == other.end; // TODO: compare functor? -> doesn't always work and may be useless
+    }
+
+    bool operator!=(const OptionalMappingIterator& other) const noexcept
+    {
+        return !(*this == other);
+    }
+};
+
+template<typename TIterable, typename TFunctor>
+struct OptionalMappingRange : RangeAdapter
+{
+    // using value_type = typename std::iterator_traits<decltype(std::begin(std::declval<TIterable>()))>::value_type;
+    using value_type = typename std::iterator_traits<decltype(std::declval<RangeRef<TIterable>>().begin())>::value_type;
+
+    RangeRef<TIterable> base;
+    TFunctor functor;
+
+    auto begin() const noexcept
+    {
+        return OptionalMappingIterator(base.begin(), base.end(), functor);
+    }
+
+    auto end() const noexcept
+    {
+        return OptionalMappingIterator(base.end(), base.end(), functor);
+    }
+};
+
+template<typename TIterable, typename TFunctor>
+auto mapOptional(TIterable&& iterable, TFunctor&& functor)
+{
+    return OptionalMappingRange<TIterable, TFunctor>{
+        .base = {std::forward<TIterable>(iterable)},
+        .functor = std::forward<TFunctor>(functor)
+    };
+}
+
+template<typename TIterator, typename TPredicate>
+struct FilteringIterator
+{
+    using difference_type = typename std::iterator_traits<TIterator>::difference_type;
+    using value_type = typename std::iterator_traits<TIterator>::value_type;
+    using iterator_category = std::forward_iterator_tag; // TODO?
+    using reference = typename std::iterator_traits<TIterator>::reference;
+
+    TIterator base;
+    TIterator end;
+    TPredicate predicate;
+
+    FilteringIterator(TIterator base_, TIterator end_, TPredicate predicate_) noexcept : base(base_), end(end_), predicate(std::move(predicate_)) {
+        if (base != end && !predicate(*base)) {
+            ++(*this);
+        }
+    }
+    FilteringIterator(const FilteringIterator&) noexcept = default;
+    FilteringIterator(FilteringIterator&&) noexcept = default;
+
+    FilteringIterator& operator=(const FilteringIterator&) noexcept = default;
+    FilteringIterator& operator=(FilteringIterator&&) noexcept = default;
+
+    reference operator*() const noexcept
+    {
+        return *base;
+    }
+
+    FilteringIterator& operator++() noexcept
+    {
+        do
+        {
+            ++base;
+        } while (base != end && !predicate(*base));
+        return *this;
+    }
+
+    FilteringIterator operator++(int) noexcept
+    {
+        FilteringIterator copy(*this);
+        ++(*this);
+        return copy;
+    }
+
+    bool operator==(const FilteringIterator& other) const noexcept
+    {
+        return base == other.base && end == other.end; // TODO: compare predicate?
+    }
+
+    bool operator!=(const FilteringIterator& other) const noexcept
+    {
+        return !(*this == other);
+    }
+};
+
+template<typename TIterable, typename TPredicate>
+struct FilteringRange : RangeAdapter
+{
+    // using value_type = typename std::iterator_traits<decltype(std::begin(std::declval<TIterable>()))>::value_type;
+    using value_type = typename std::iterator_traits<decltype(std::declval<RangeRef<TIterable>>().begin())>::value_type;
+
+    RangeRef<TIterable> base;
+    TPredicate predicate;
+
+    auto begin() const noexcept
+    {
+        return FilteringIterator(base.begin(), base.end(), predicate);
+    }
+
+    auto end() const noexcept
+    {
+        return FilteringIterator(base.end(), base.end(), predicate);
+    }
+};
+
+template<typename TIterable, typename TPredicate>
+auto filter(TIterable&& iterable, TPredicate&& predicate)
+{
+    return FilteringRange<TIterable, TPredicate>{
+        .base = {std::forward<TIterable>(iterable)},
+        .predicate = std::forward<TPredicate>(predicate)
+    };
+}
+
 template<typename TFirstIterator, typename TSecondIterator>
 struct ChainingIterator
 {
@@ -524,6 +736,50 @@ auto operator|(TIterable&& iterable, Map<TFunctor> mapper)
 {
     return map(std::forward<TIterable>(iterable), std::move(mapper.functor));
 }
+
+template<typename T>
+struct MapOptional
+{
+    T functor;
+
+    explicit MapOptional(T functor_) : functor(std::move(functor_)) {}
+};
+template<typename T>
+MapOptional(T) -> MapOptional<T>;
+
+template<typename TIterable, typename TFunctor>
+auto operator|(TIterable&& iterable, MapOptional<TFunctor> mapper)
+{
+    return mapOptional(std::forward<TIterable>(iterable), std::move(mapper.functor));
+}
+
+template<typename T>
+struct Filter
+{
+    T predicate;
+
+    explicit Filter(T predicate_) : predicate(std::move(predicate_)) {}
+};
+template<typename T>
+Filter(T) -> Filter<T>;
+
+template<typename TIterable, typename TFilter>
+auto operator|(TIterable&& iterable, Filter<TFilter> filterer)
+{
+    return filter(std::forward<TIterable>(iterable), std::move(filterer.predicate));
+}
+
+template<std::size_t idx>
+struct Xth {};
+
+using First = Xth<0>;
+using Second = Xth<1>;
+
+template<typename TIterable, std::size_t idx>
+auto operator|(TIterable&& iterable, Xth<idx>)
+{
+    return map(std::forward<TIterable>(iterable), [](auto&& element) { return std::get<idx>(element); } );
+}
 }
 } // namespace mijin