util/TransformN: new library

src/util/TransformN.hxx

+/*
+ * Copyright 2019 Max Kellermann <max.kellermann@gmail.com>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the
+ * distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
+ * FOUNDATION OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef TRANSFORM_N_HXX
+#define TRANSFORM_N_HXX
+
+#include <stddef.h>
+
+/**
+ * The std::transform_n() function that is missing in the C++ standard
+ * library.
+ */
+template<class InputIt, class OutputIt, class UnaryOperation>
+OutputIt
+transform_n(InputIt input, size_t n, OutputIt output,
+	    UnaryOperation unary_op)
+{
+	while (n-- > 0)
+		*output++ = unary_op(*input++);
+	return output;
+}
+
+/**
+ * Optimized overload for the above transform_n() implementation for
+ * the special case that both input and output are regular pointers.
+ * Turns out that most compilers generate better code this way.
+ */
+template<class InputType, class OutputType, class UnaryOperation>
+OutputType *
+transform_n(const InputType *input, size_t n, OutputType *output,
+	    UnaryOperation unary_op)
+{
+	for (size_t i = 0; i < n; ++i)
+		output[i] = unary_op(input[i]);
+	return output + n;
+}
+
+#endif