From 8a431820b0b360b9a7c9e086cac05935bee8275a Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Wed, 4 Dec 2019 07:59:18 +0100
Subject: [PATCH] Implement day 04.

---
 2019/inputs/04.txt |  1 +
 2019/src/day04.cpp | 88 ++++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 87 insertions(+), 2 deletions(-)
 create mode 100644 2019/inputs/04.txt

diff --git a/2019/inputs/04.txt b/2019/inputs/04.txt
new file mode 100644
index 0000000..9e35a59
--- /dev/null
+++ b/2019/inputs/04.txt
@@ -0,0 +1 @@
+197487-673251
diff --git a/2019/src/day04.cpp b/2019/src/day04.cpp
index c9f113f..a8dd0b2 100644
--- a/2019/src/day04.cpp
+++ b/2019/src/day04.cpp
@@ -1,10 +1,94 @@
 #include <iostream>
+#include <utility>
 #include "days.hpp"
 
+namespace {
+    constexpr bool is_valid_pass(int num) {
+        bool has_double = false;
+        int prev = 11;
+
+        for (; num != 0; num /= 10) {
+            int digit = num % 10;
+
+            if (digit == prev) {
+                has_double = true;
+            }
+
+            if (digit > prev) {
+                return false;
+            }
+
+            prev = digit;
+        }
+
+        return has_double;
+    }
+
+    constexpr bool is_valid_pass2(int num) {
+        int prev = 11;
+        bool has_double = false;
+        int run = 1;
+
+        for (; num != 0; num /= 10) {
+            int digit = num % 10;
+
+            if (digit == prev) {
+                ++run;
+            } else {
+                if (run == 2) {
+                    has_double = true;
+                }
+                run = 1;
+            }
+
+            if (digit > prev) {
+                return false;
+            }
+
+            prev = digit;
+        }
+
+        return has_double || run == 2;
+    }
+
+    std::pair<int, int> read_input(std::istream& input) {
+        int a, b;
+        input >> a;
+        input.ignore();
+        input >> b;
+
+        return {a, b};
+    }
+}
+
 void aoc2019::day04_part1(std::istream &input, std::ostream &output) {
-	output << "Not implemented\n";
+    auto [start_range, end_range] = read_input(input);
+
+    int num_valid = 0;
+    for (; start_range <= end_range; ++start_range) {
+        num_valid += is_valid_pass(start_range);
+    }
+
+    output << num_valid << std::endl;
 }
 
 void aoc2019::day04_part2(std::istream &input, std::ostream &output) {
-	output << "Not implemented\n";
+    auto [start_range, end_range] = read_input(input);
+
+    int num_valid = 0;
+    for (; start_range <= end_range; ++start_range) {
+        num_valid += is_valid_pass2(start_range);
+    }
+
+    output << num_valid << std::endl;
 }
+
+// Poor man's unit tests
+static_assert(is_valid_pass(122345));
+static_assert(is_valid_pass(111111));
+static_assert(!is_valid_pass(223450));
+static_assert(!is_valid_pass(123678));
+
+static_assert(is_valid_pass2(112233));
+static_assert(!is_valid_pass2(123444));
+static_assert(is_valid_pass2(111122));