Add README and solution for day4

day4/README.md

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+# Day4: Secure Container
+
+## Problem
+
+You arrive at the Venus fuel depot only to discover it's protected by a
+password. The Elves had written the password on a sticky note, but someone
+threw it out.
+
+However, they do remember a few key facts about the password:
+
+* It is a six-digit number.
+* The value is within the range given in your puzzle input.
+* Two adjacent digits are the same (like 22 in 122345).
+* Going from left to right, the digits never decrease; they only ever increase
+or stay the same (like 111123 or 135679).
+
+Other than the range rule, the following are true:
+
+* 111111 meets these criteria (double 11, never decreases).
+* 223450 does not meet these criteria (decreasing pair of digits 50).
+* 123789 does not meet these criteria (no double).
+
+How many different passwords within the range given in your puzzle input meet
+these criteria?

day4/python/day4.py

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+from collections import Counter
+
+low = 152085
+high = 670283
+
+print(len(list( \
+            filter( \
+                lambda x: len(set(x)) != len(x) \
+                    and list(x) == sorted(x) \
+                    and 2 in set(Counter(x).values())\
+                , map(str,range(low,high+1)) \
+            ) \
+          ) \
+      ) \
+)

day4/rust/Cargo.toml

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+[package]
+name = "day4"
+version = "0.1.0"
+authors = ["Tobias Eidelpes <tobias@eidelpes.info>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+itertools = "0.8"

day4/rust/src/main.rs

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+use itertools::Itertools;
+
+fn main() {
+    let passwords = (152_086_usize..670_282_usize)
+        .map(|x| x.to_string().chars().collect::<Vec<char>>())
+        .filter(|x| x.iter().zip(x.iter().skip(1)).all(|(x,y)| x <= y))
+        .filter(|x| {
+            for (_,g) in &x.iter().group_by(|&y| y) {
+                if g.count() == 2 {
+                    return true;
+                }
+            }
+            false
+        })
+    .count();
+
+    println!("Solution: {:?}", passwords);
+}