fast string trim with C++17 usage

dbj_string_trim.cpp

/*
Copyright 2018 by [email protected]

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

http ://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

#include <string>
#include <string_view>
#include <stdlib.h>
#include <assert.h>

#if ! _HAS_CXX17
#error C++17 required
#endif

namespace {

	constexpr int EOS = int('\0');

	// this actually defines the trimming policy
	extern "C" inline bool dbj_isspace(unsigned char c)
	{
		return c == ' '; // just spaces are trimmed !
	}
        // note: we use unsigned char to avoid accidents with negative char values
	extern "C" inline void string_trim_front_back_finder( unsigned char ** begin_, unsigned char ** end_)
	{
		assert(*end_);
		assert(*begin_);

		if (*begin_ == *end_) return;

		while (dbj_isspace(**begin_)) {
			(*begin_)++;
		};

		if (*begin_ == *end_) return;

		// otherwise the algorithm will not work
		assert(**end_ == EOS);
		// we have requested end_ to be EOS (aka 0)
		// it is not a space (aka 32)
		// thus we have to move it "left" 1 first
		// before the first check
		(*end_)--;
		while (dbj_isspace(**end_)) {
			(*end_)--;
		};
		// move it back
		// one beyond the last non space char
		(*end_)++;
	}

	// must be zero limited string
	// that is with EOS ('\0') a the end
	extern "C" inline void string_trim(const char * text, unsigned char ** p1, unsigned char ** p2)
	{
		assert(text);
		size_t text_len = strlen(text);
		*p1 = (unsigned char *)& text[0];
		// point at EOS position
		// otherwise the algorithm will not work
		*p2 = (unsigned char *)& text[text_len];
		string_trim_front_back_finder(p1, p2);
	}
	// must be limited string
	// that is with EOS ('\0') a the end
	inline std::string trimmer(::std::string_view text)
	{
		unsigned char * p1 = 0;
		unsigned char * p2 = 0;
		string_trim(text.data(), &p1, &p2);
		return { p1, p2 };
	}

} // nspace 

int main()
{
	using namespace ::std::string_view_literals;

	auto target = "LINE O FF    TE   XT"sv ;
	std::string_view text[]{
		{ "   LINE O FF    TE   XT    "sv },
		{ "   LINE O FF    TE   XT"sv },
		{ "LINE O FF    TE   XT"sv },
		{ "     "sv }
	};

	_ASSERTE( target == trimmer(text[0]) );
	_ASSERTE( target == trimmer(text[1]) );
	_ASSERTE( target == trimmer(text[2]) );
	// on trim, spaces are collapsing 
	// to empty string
	_ASSERTE( "" == trimmer(text[3]) );
}

NOTE1: This is just space trimming, of zero limited strings.
NOTE2: extern "C" does NOT mean that is a C code ...