more OO way to do hex string to bytes conversion

[Steven Schveighoffer]

On 2/6/18 1:46 PM, H. S. Teoh wrote:
> Of course, this eagerly constructs an array to store the result, which
> allocates, and also requires the hex string to be fully constructed
> first.  You can make this code lazy by turning it into a range
> algorithm, then you can actually generate the hex digits lazily from
> somewhere else, and process the output bytes as they are generated, no
> allocation necessary:
> 
> 	/* Run this example by putting this in a file called 'test.d'
> 	 * and invoking `dmd -unittest -main -run test.d`
> 	 */
> 	import std.array;
> 	import std.algorithm;
> 	import std.conv;
> 	import std.format;
> 	import std.range;
> 	import std.stdio;
> 
> 	auto hexToBytes(R)(R hex)
> 		if (isInputRange!R && is(ElementType!R : dchar))
> 	{
> 		return hex.chunks(2)
> 		          .map!(s => s.to!ubyte(16));
> 	}
> 
> 	unittest
> 	{
> 		// Infinite stream of hex digits
> 		auto digits = "0123456789abcdef".cycle;
> 
> 		digits.take(100)	// take the first 100 digits
> 		      .hexToBytes	// turn them into bytes
> 		      .map!(b => format("%02X", b)) // print in uppercase
> 		      .joiner(" ")	// nicely delimit bytes with spaces
> 		      .chain("\n")	// end with a nice newline
> 		      .copy(stdout.lockingTextWriter);
> 		      			// write output directly to stdout

Hm... format in a loop? That returns strings, and allocates. Yuck! ;)

writefln("%(%02X %)", digits.take(100).hexToBytes);

-Steve