range bits

/// map only on the specified index of each tuple element of the range `r`
auto mapOnPart(size_t i, alias f, R)(R r)
{
	return r.map!(t => tuple(t[0 .. i].expand, f(t[i]), t[i + 1 .. $].expand));
}

/// map the whole tuple element to one part, copying the rest untouched
auto mapAllToPart(size_t i, alias f, R)(R r)
{
	return r.map!(t => tuple(t[0 .. i].expand, f(t), t[i + 1 .. $].expand));
}

auto unzip(R)(R r)
if (isForwardRange!R
	&& __traits(compiles, { enum size_t _ = ElementType!R.length; }))
{
	import std.range : iota;
	import std.algorithm : map;
	import std.range : save;
	import std.typecons : tuple;
	import std.conv : to;
	import std.string : join;
	return mixin(`tuple(` ~
				iota(ElementType!R.length)
				.map!(i => `r.save.map!"a[` ~ i.to!string ~ `]"`)
				.join(", ")
				~ `)`
				);
}

@("unzip")
unittest
{
	import std.range : zip, ElementType;
	import std.algorithm : equal;
	auto a = [1, 2, 3, 4];
	auto b = [5, 6, 7, 8];
	auto p = zip(a, b).unzip();
	assert(a.equal(p[0]));
	assert(b.equal(p[1]));
}

auto unzipDynamic(R)(R r)
{
	return r.front.walkLength.iota.map!(i => transversal(x, i));
}