day4: add pythong solution

README.md

@@ -8,4 +8,5 @@ https://adventofcode.com/2020/
 |---|--------|---------|------|
 | 1 |  `**`  |  `**`   |      |
 | 2 |  `**`  |         | `**` |
-| 3 |  `**`  |         |      |
+| 3 |  `**`  |  `**`   |      |
+| 4 |  `**`  |         |      |

day4/day4.py

@@ -0,0 +1,55 @@
+#!/usr/bin/env python
+
+def within(a, b, x):
+    return a <= x <= b
+
+PASSPORT_FIELDS = {
+        'byr': lambda s: len(s) == 4 and within(1920, 2002, int(s)),
+        'iyr': lambda s: len(s) == 4 and within(2010, 2020, int(s)),
+        'eyr': lambda s: len(s) == 4 and within(2020, 2030, int(s)),
+        'hgt': lambda s: within(*{'cm': (150, 193), 'in': (59, 76)}[s[-2:]], int(s[:-2])),
+        'hcl': lambda s: len(s) == 7 and s[0] == '#' and all(c in "0123456789abcdef" for c in s[1:]),
+        'ecl': lambda s: s in ('amb', 'blu', 'brn', 'gry', 'grn', 'hzl', 'oth'),
+        'pid': lambda s: len(s) == 9 and s.isdigit(),
+}
+
+def parse_passport(line):
+    d = {}
+    for entry in line.split():
+        k, v = entry.split(':', maxsplit=1)
+        d[k] = v
+    return d
+
+def check_passport_fields_exist(passport):
+    return set(PASSPORT_FIELDS.keys()) <= set(passport.keys())
+
+def check_passport_fields_valid(passport):
+    for name, spec in PASSPORT_FIELDS.items():
+        try:
+            valid = spec(passport[name])
+        except:
+            valid = False
+
+        if not valid:
+            return False
+    return True
+
+def split_on(f, it):
+    current_part = []
+
+    for el in it:
+        if f(el):
+            yield current_part
+            current_part = []
+        else:
+            current_part.append(el)
+
+if __name__ == '__main__':
+    with open('input.txt', 'r') as f:
+        lines = [line.strip() for line in f.readlines()]
+
+    specs = list(' '.join(lines) for lines in split_on(lambda l: l == "", lines))
+
+    passports = [parse_passport(spec) for spec in specs]
+    print(len(list(filter(check_passport_fields_exist, passports))))
+    print(len(list(filter(check_passport_fields_valid, passports))))