updated 2019 q1 to more concise solution

2019/q1.py

@@ -1,97 +1,11 @@
-"""
-BIO 2019 Q1: Palindromes
+def next_palindrome(n: str) -> int:
+    l, k = len(n), int(n)
+    if not l - 1: return k + 1 if k < 9 else 11
+    o, m = l % 2, l // 2
+    b = m + 1 if o else m
+    s1, s2 = n[:b], n[b:]
 
-This ended up being surprisingly difficult, for whatever reason I found it surprisingly difficult
-to reason about.
+    if int(s1[m - 1::-1]) <= int(s2): s1 = str(int(s1) + 1)
+    return s1 + s1[m - 1::-1]
 
-I found it easier to think about how, given a palindrome, I would calculate the following
-palindrome. There are ~2 cases:
-
-Odd number of digits: [left][middle][right = reversed(right)]
-Even number of digits: [left][right = reversed(right)]
-
-In the first case, we can (hopefully) obviously generate the next palindrome by adding one to the
-middle digit, and carrying the one into the left hand side as if you were doing regular addition,
-and then reflecting the new value to produce a new palindrome.
-
-In the second case, we can basically do the same thing, but without the middle digit.
-
-And then if we are still 'carrying' anything by the time we get to the end, this becomes a new
-left-most digit, and the right most digit becomes the new middle digit.
-"""
-
-
-class Palindrome:
-    def __init__(self, left, middle):
-        assert middle is None or middle < 10 and middle >= 0
-
-        self.left = list(int(x) for x in str(left))
-        self.middle = middle
-
-    def add_one_left(self, carry):
-        for i in range(len(self.left)):
-            ix = -(i + 1)
-
-            if self.left[ix] == 9:
-                self.left[ix] = 0
-                carry = True
-            else:
-                self.left[ix] += 1
-                carry = False
-                break
-
-        if carry and self.middle is None:
-            self.middle = self.left[-1]
-            self.left = [1] + self.left[:-1]
-
-        elif carry and self.middle is not None:
-            self.left = [1] + self.left
-            self.middle = None
-
-    def next_palindrome(self):
-        if self.middle is not None:
-            if self.middle == 9:
-                self.middle = 0
-                self.add_one_left(carry=True)
-            else:
-                self.middle += 1
-        else:
-            self.add_one_left(carry=False)
-
-    def as_int(self):
-        if self.middle is None:
-            l = self.left + list(reversed(self.left))
-        else:
-            l = self.left + [self.middle] + list(reversed(self.left))
-
-        return int("".join(str(x) for x in l))
-
-    @staticmethod
-    def of_int(i):
-        s = str(i)
-
-        if len(s) % 2 == 0:
-            left = [int(x) for x in s[: len(s) // 2]]
-            middle = None
-        else:
-            left = [int(x) for x in s[: len(s) // 2]]
-            middle = int(s[len(left)])
-
-        return Palindrome("".join(str(x) for x in left), middle)
-
-    def __str__(self):
-        return str(self.as_int())
-
-
-i = input()
-in_int = int(i)
-
-p = Palindrome.of_int(i)
-
-p_int = p.as_int()
-
-if p_int > in_int:
-    print(p_int)
-else:
-    p.next_palindrome()
-    print(p)
+print(next_palindrome(input()))