string to character code hex string

[Ali Çehreli via Digitalmars-d-learn]

On 09/02/2017 11:02 AM, lithium iodate wrote:
 > On Saturday, 2 September 2017 at 17:41:34 UTC, Ali Çehreli wrote:
 >> You're right but I think there is no intention of interpreting the
 >> result as UTF-8. "f62026" is just to be used as "f62026", which can be
 >> converted byte-by-byte back to "ö…". That's how understand the
 >> requirement anyway.
 >>
 >> Ali
 >
 > That is not possible, because you cannot know whether "f620" and "26" or
 > "f6" and "2026" (or any other combination) should form a code point
 > each. Additional padding to constant width (8 hex chars) is needed.

Ok, I see that I made a mistake but I still don't think the conversion 
is one way. If we can convert byte-by-byte, we should be able to convert 
back byte-by-byte, right? What I failed to ensure was to iterate by code 
units. The following is able to get the same string back:

import std.stdio;
import std.string;
import std.algorithm;
import std.range;
import std.utf;
import std.conv;

auto toHex(R)(R input) {
     // As Moritz Maxeiner says, this format is expensive
     return input.byCodeUnit.map!(c => format!"%02x"(c)).joiner;
}

int hexValue(C)(C c) {
     switch (c) {
     case '0': .. case '9':
         return c - '0';
     case 'a': .. case 'f':
         return c - 'a' + 10;
     default:
         assert(false);
     }
}

auto fromHex(R, Dst = char)(R input) {
     return input.chunks(2).map!((ch) {
             auto high = ch.front.hexValue * 16;
             ch.popFront();
             return high + ch.front.hexValue;
         }).map!(value => cast(Dst)value);
}

void main() {
     assert("AAA".toHex.fromHex.equal("AAA"));

     assert("ö…".toHex.fromHex.equal("ö…".byCodeUnit));
     // Alternative check:
     assert("ö…".toHex.fromHex.text.equal("ö…"));
}

Ali