Apocopate "uno"/"veintiuno" before mil and millones in Spanish

Spanish shortens "uno" to "un" and "veintiuno" to "veintiún" immediately
before "mil" and the "-illón/-illones" scale words. So 21000 is
"veintiún mil" and 21000000 is "veintiún millones", not "veintiuno mil"
or "veintiuno millones". The apocope was already applied to currency
output but never to plain cardinals.

Apply it in Num2Word_ES.to_cardinal and add regression tests.

Fixes #515

Author: Sreekant13

num2words/lang_ES.py

@@ -18,6 +18,7 @@
 from __future__ import division, print_function, unicode_literals
 
 import math
+import re
 
 from .lang_EU import Num2Word_EU
 
@@ -302,6 +303,18 @@ def merge(self, curr, next):
 
         return (ctext + ntext, cnum * nnum)
 
+    # "uno" apocopates to "un", and "veintiuno" to "veintiún", right before
+    # "mil" and the "-illón/-illones" scale words: "veintiún mil", "treinta y
+    # un millones", "veintiún mil millones". Source: https://www.rae.es/dpd/uno
+    _APOCOPE_SCALE = r'(?=\s+(?:mil|\w*ll(?:ón|ones))\b)'
+
+    def to_cardinal(self, value):
+        result = super(Num2Word_ES, self).to_cardinal(value)
+        result = re.sub(r'\bveintiuno' + self._APOCOPE_SCALE, 'veintiún',
+                        result)
+        result = re.sub(r'\buno' + self._APOCOPE_SCALE, 'un', result)
+        return result
+
     def to_ordinal(self, value, gender='m'):
         gender_stem = 'a' if gender == 'f' else 'o'
 

tests/test_es.py

@@ -84,7 +84,14 @@
     (100000000000000, 'cien billones'),
     (1000000000000000000, 'un trillón'),
     (1000000000000000000000, 'mil trillones'),
-    (10000000000000000000000000, 'diez cuatrillones')
+    (10000000000000000000000000, 'diez cuatrillones'),
+    # apocope of "uno"/"veintiuno" before mil/millones (issue #515)
+    (21000, 'veintiún mil'),
+    (31000, 'treinta y un mil'),
+    (101000, 'ciento un mil'),
+    (121000, 'ciento veintiún mil'),
+    (21000000, 'veintiún millones'),
+    (21000000000, 'veintiún mil millones'),
 )
 
 TEST_CASES_ORDINAL = (