New function to combine dice, and use this function for hit dice.

Fixes: https://github.com/canismarko/dungeon-sheets/issues/84

dungeonsheets/dice.py

@@ -1,10 +1,12 @@
 import random
 import re
 from collections import namedtuple
+from itertools import groupby
 
 from dungeonsheets.exceptions import DiceError
 
 dice_re = re.compile(r"(\d+)d(\d+)", flags=re.I)
+dice_part_re = re.compile(r"[0-9d]+", flags=re.I)
 Dice = namedtuple("Dice", ("num", "faces"))
 
 
@@ -25,6 +27,36 @@ def read_dice_str(dice_str):
     return dice
 
 
+def combine_dice(dice_str):
+    """Condense a dice string into its simplest representation.
+
+    For example: "1d8 + 5 + 2d8 + 2" would be reduced to "3d8 + 7".
+
+    Returns
+    =======
+    new_dice_str
+      A new, condensed string for the given dice.
+    
+    """
+    dice = []
+    bonuses = 0
+    # Sort out each portion of the dice string
+    for part in dice_part_re.findall(dice_str):
+        try:
+            dice.append(read_dice_str(part))
+        except DiceError:
+            bonuses += int(part)
+    # Recombine the dice into a more concise string
+    new_parts = []
+    dice = sorted(dice, key=lambda x: x.faces)
+    for faces, grp in groupby(dice, key=lambda x: x.faces):
+        new_parts.append(f"{sum([d.num for d in grp])}d{faces}")
+    if bonuses > 0:
+        new_parts.append(str(bonuses))
+    new_dice_str = " + ".join(new_parts)
+    return new_dice_str
+
+
 def roll(a, b=None):
     """roll(20) means roll 1d20, roll(2, 6) means roll 2d6"""
     if b is None: