Merge: Bytes from hexdigest

As @xymus requested in #1705, a new method is available in Text, `hexdigest_to_bytes` which transforms a regular hexdigest in its Bytes counterpart.

Ping @ppepos this could please your Pythonneous eyes

Pull-Request: #1716
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Jean Privat <jean@pryen.org>

diff --cc lib/core/bytes.nit
index d589646,75d7ccd..356f386
--- 1/lib/core/bytes.nit
--- 2/lib/core/bytes.nit
+++ b/lib/core/bytes.nit
@@@ -20,13 -20,50 +20,58 @@@ import collection::arra
  intrude import text::flat
  
  redef class Byte
 +      # Write self as a string into `ns` at position `pos`
 +      private fun add_digest_at(ns: NativeString, pos: Int) do
 +              var tmp = (0xF0u8 & self) >> 4
 +              ns[pos] = if tmp >= 0x0Au8 then tmp + 0x37u8 else tmp + 0x30u8
 +              tmp = 0x0Fu8 & self
 +              ns[pos + 1] = if tmp >= 0x0Au8 then tmp + 0x37u8 else tmp + 0x30u8
 +      end
++
+       # Is `self` a valid hexadecimal digit (in ASCII)
+       #
+       # ~~~nit
+       # intrude import core::bytes
+       # assert not '/'.ascii.to_b.is_valid_hexdigit
+       # assert '0'.ascii.to_b.is_valid_hexdigit
+       # assert '9'.ascii.to_b.is_valid_hexdigit
+       # assert not ':'.ascii.to_b.is_valid_hexdigit
+       # assert not '@'.ascii.to_b.is_valid_hexdigit
+       # assert 'A'.ascii.to_b.is_valid_hexdigit
+       # assert 'F'.ascii.to_b.is_valid_hexdigit
+       # assert not 'G'.ascii.to_b.is_valid_hexdigit
+       # assert not '`'.ascii.to_b.is_valid_hexdigit
+       # assert 'a'.ascii.to_b.is_valid_hexdigit
+       # assert 'f'.ascii.to_b.is_valid_hexdigit
+       # assert not 'g'.ascii.to_b.is_valid_hexdigit
+       # ~~~
+       private fun is_valid_hexdigit: Bool do
+               return (self >= 0x30u8 and self <= 0x39u8) or
+                      (self >= 0x41u8 and self <= 0x46u8) or
+                      (self >= 0x61u8 and self <= 0x66u8)
+       end
+ 
+       # `self` as a hexdigit to its byte value
+       #
+       # ~~~nit
+       # intrude import core::bytes
+       # assert 0x39u8.hexdigit_to_byteval == 0x09u8
+       # assert 0x43u8.hexdigit_to_byteval == 0x0Cu8
+       # ~~~
+       #
+       # REQUIRE: `self.is_valid_hexdigit`
+       private fun hexdigit_to_byteval: Byte do
+               if self >= 0x30u8 and self <= 0x39u8 then
+                       return self - 0x30u8
+               else if self >= 0x41u8 and self <= 0x46u8 then
+                       return self - 0x37u8
+               else if self >= 0x61u8 and self <= 0x66u8 then
+                       return self - 0x57u8
+               end
+               # Happens only if the requirement is not met.
+               # i.e. this abort is here to please the compiler
+               abort
+       end
  end
  
  # A buffer containing Byte-manipulation facilities