From cd9960db2ab3cdf9434fc5caf954d25aea484108 Mon Sep 17 00:00:00 2001
From: Eugen Wissner <belka@caraus.de>
Date: Sun, 25 Mar 2018 08:17:38 +0200
Subject: [PATCH] Add take range adapter

---
 source/tanya/range/package.d | 284 +++++++++++++++++++++++++++++++++++
 1 file changed, 284 insertions(+)

diff --git a/source/tanya/range/package.d b/source/tanya/range/package.d
index 8894c95..1d03e22 100644
--- a/source/tanya/range/package.d
+++ b/source/tanya/range/package.d
@@ -15,5 +15,289 @@
  */
 module tanya.range;
 
+import tanya.algorithm.mutation;
+import tanya.math;
 public import tanya.range.array;
 public import tanya.range.primitive;
+
+/**
+ *
+ */
+struct Take(R)
+if (isInputRange!R)
+{
+    private R source;
+    size_t length_;
+
+    @disable this();
+
+    private this(R source, size_t length)
+    {
+        this.source = source;
+        static if (hasLength!R)
+        {
+            this.length_ = min(source.length, length);
+        }
+        else
+        {
+            this.length_ = length;
+        }
+    }
+
+    @property auto ref front()
+    in
+    {
+        assert(!empty);
+    }
+    do
+    {
+        return this.source.front;
+    }
+
+    void popFront()
+    in
+    {
+        assert(!empty);
+    }
+    do
+    {
+        this.source.popFront();
+        --this.length_;
+    }
+
+    @property bool empty()
+    {
+        static if (isInfinite!R)
+        {
+            return length == 0;
+        }
+        else
+        {
+            return length == 0 || this.source.empty;
+        }
+    }
+
+    @property size_t length()
+    {
+        return this.length_;
+    }
+
+    static if (hasMobileElements!R)
+    {
+        auto moveFront()
+        in
+        {
+            assert(!empty);
+        }
+        do
+        {
+            return this.source.moveFront();
+        }
+    }
+    static if (hasAssignableElements!R)
+    {
+        @property void front(ref ElementType!R value)
+        in
+        {
+            assert(!empty);
+        }
+        do
+        {
+            this.source.front = value;
+        }
+
+        @property void front(ElementType!R value)
+        in
+        {
+            assert(!empty);
+        }
+        do
+        {
+            this.source.front = move(value);
+        }
+    }
+
+    static if (isForwardRange!R)
+    {
+        typeof(this) save()
+        {
+            return typeof(this)(this.source.save(), length);
+        }
+    }
+    static if (isRandomAccessRange!R)
+    {
+        @property auto ref back()
+        in
+        {
+            assert(!empty);
+        }
+        do
+        {
+            return this.source[this.length - 1];
+        }
+
+        void popBack()
+        in
+        {
+            assert(!empty);
+        }
+        do
+        {
+            --this.length_;
+        }
+
+        auto ref opIndex(size_t i)
+        in
+        {
+            assert(i < length);
+        }
+        do
+        {
+            return this.source[i];
+        }
+
+        static if (hasMobileElements!R)
+        {
+            auto moveBack()
+            in
+            {
+                assert(!empty);
+            }
+            do
+            {
+                return this.source.moveAt(length - 1);
+            }
+
+            auto moveAt(size_t i)
+            in
+            {
+                assert(i < length);
+            }
+            do
+            {
+                return this.source.moveAt(i);
+            }
+        }
+        static if (hasAssignableElements!R)
+        {
+            @property void back(ref ElementType!R value)
+            in
+            {
+                assert(!empty);
+            }
+            do
+            {
+                this.source[length - 1] = value;
+            }
+
+            @property void back(ElementType!R value)
+            in
+            {
+                assert(!empty);
+            }
+            do
+            {
+                this.source[length - 1] = move(value);
+            }
+
+            void opIndexAssign(ref ElementType!R value, size_t i)
+            in
+            {
+                assert(i < length);
+            }
+            do
+            {
+                this.source[i] = value;
+            }
+
+            void opIndexAssign(ElementType!R value, size_t i)
+            in
+            {
+                assert(i < length);
+            }
+            do
+            {
+                this.source[i] = move(value);
+            }
+        }
+    }
+    static if (hasSlicing!R)
+    {
+        auto opSlice(size_t i, size_t j)
+        in
+        {
+            assert(i <= j);
+            assert(j <= length);
+        }
+        do
+        {
+            return take(this.source[i .. j], length);
+        }
+    }
+}
+
+/**
+ * ditto
+ */
+Take!R take(R)(R range, size_t n)
+if (isInputRange!R)
+{
+    return Take!R(range, n);
+}
+
+///
+@nogc nothrow pure @safe unittest
+{
+    static struct InfiniteRange
+    {
+        private size_t front_ = 1;
+
+        enum bool empty = false;
+
+        @property size_t front() @nogc nothrow pure @safe
+        {
+            return this.front_;
+        }
+
+        @property void front(size_t i) @nogc nothrow pure @safe
+        {
+            this.front_ = i;
+        }
+
+        void popFront() @nogc nothrow pure @safe
+        {
+            ++this.front_;
+        }
+        
+        size_t opIndex(size_t i) @nogc nothrow pure @safe
+        {
+            return this.front_ + i;
+        }
+
+        void opIndexAssign(size_t value, size_t i) @nogc nothrow pure @safe
+        {
+            this.front = i + value;
+        }
+
+        InfiniteRange save() @nogc nothrow pure @safe
+        {
+            return this;
+        }
+    }
+
+    auto t = InfiniteRange().take(3);
+    assert(t.length == 3);
+    assert(t.front == 1);
+    assert(t.back == 3);
+
+    t.popFront();
+    assert(t.front == 2);
+    assert(t.back == 3);
+
+    t.popBack();
+    assert(t.front == 2);
+    assert(t.back == 2);
+
+    t.popFront();
+    assert(t.empty);
+}