2021 day8 part1

4 files changed, 88 insertions, 0 deletions

diff --git a/src/2021/day8/README.md b/src/2021/day8/README.md
index a98cb6e..1830133 100644
--- a/src/2021/day8/README.md
+++ b/src/2021/day8/README.md
@@ -70,4 +70,52 @@ Because the digits 1, 4, 7, and 8 each use a unique number of segments, you shou
 
 In the output values, how many times do digits 1, 4, 7, or 8 appear?
 
+--- Part Two ---
+Through a little deduction, you should now be able to determine the remaining digits. Consider again the first example above:
 
+acedgfb cdfbe gcdfa fbcad dab cefabd cdfgeb eafb cagedb ab |
+cdfeb fcadb cdfeb cdbaf
+After some careful analysis, the mapping between signal wires and segments only make sense in the following configuration:
+
+ dddd
+e    a
+e    a
+ ffff
+g    b
+g    b
+ cccc
+So, the unique signal patterns would correspond to the following digits:
+
+acedgfb: 8
+cdfbe: 5
+gcdfa: 2
+fbcad: 3
+dab: 7
+cefabd: 9
+cdfgeb: 6
+eafb: 4
+cagedb: 0
+ab: 1
+Then, the four digits of the output value can be decoded:
+
+cdfeb: 5
+fcadb: 3
+cdfeb: 5
+cdbaf: 3
+Therefore, the output value for this entry is 5353.
+
+Following this same process for each entry in the second, larger example above, the output value of each entry can be determined:
+
+fdgacbe cefdb cefbgd gcbe: 8394
+fcgedb cgb dgebacf gc: 9781
+cg cg fdcagb cbg: 1197
+efabcd cedba gadfec cb: 9361
+gecf egdcabf bgf bfgea: 4873
+gebdcfa ecba ca fadegcb: 8418
+cefg dcbef fcge gbcadfe: 4548
+ed bcgafe cdgba cbgef: 1625
+gbdfcae bgc cg cgb: 8717
+fgae cfgab fg bagce: 4315
+Adding all of the output values in this larger example produces 61229.
+
+For each entry, determine all of the wire/segment connections and decode the four-digit output values. What do you get if you add up all of the output values?
diff --git a/src/2021/day8/aoc.cpp b/src/2021/day8/aoc.cpp
index 7a3c895..9f95991 100644
--- a/src/2021/day8/aoc.cpp
+++ b/src/2021/day8/aoc.cpp
@@ -1,5 +1,34 @@
 #include "aoc.h"
+#include <algorithm>
 
 namespace aoc2021 {
 
+static void get_length(const char** pp, int* d) {
+  const char* p = *pp;
+  *d = 0;
+  while (*p >= 'a' && *p <= 'g') {
+    *d += 1;
+    p++;
+  }
+  *pp = p;
 }
+
+int day8(line_view file) {
+  // int digits[] = {6, 2, 5, 5, 4, 5, 6, 3, 7, 6};
+  int total{0};
+  int ls[4] = {0};
+  per_line(file, [&ls, &total](line_view lv) {
+    const char* p = lv.contains("|") + 2;
+    int index{0};
+    while (p < lv.line + lv.length) {
+      get_length(&p, &ls[index]);
+      total += int(ls[index] == 2 || ls[index] == 3 || ls[index] == 4 || ls[index] == 7);
+      index++;
+      p++;
+    }
+    return true;
+  });
+  return total;
+}
+
+} // namespace aoc2021
diff --git a/src/2021/day8/aoc.h b/src/2021/day8/aoc.h
index cd205cd..cc4b869 100644
--- a/src/2021/day8/aoc.h
+++ b/src/2021/day8/aoc.h
@@ -3,4 +3,5 @@
 
 namespace aoc2021 {
 
+int day8(line_view);
 }
diff --git a/src/2021/day8/input0 b/src/2021/day8/input0
new file mode 100644
index 0000000..c9f629b
--- /dev/null
+++ b/src/2021/day8/input0
@@ -0,0 +1,10 @@
+be cfbegad cbdgef fgaecd cgeb fdcge agebfd fecdb fabcd edb | fdgacbe cefdb cefbgd gcbe
+edbfga begcd cbg gc gcadebf fbgde acbgfd abcde gfcbed gfec | fcgedb cgb dgebacf gc
+fgaebd cg bdaec gdafb agbcfd gdcbef bgcad gfac gcb cdgabef | cg cg fdcagb cbg
+fbegcd cbd adcefb dageb afcb bc aefdc ecdab fgdeca fcdbega | efabcd cedba gadfec cb
+aecbfdg fbg gf bafeg dbefa fcge gcbea fcaegb dgceab fcbdga | gecf egdcabf bgf bfgea
+fgeab ca afcebg bdacfeg cfaedg gcfdb baec bfadeg bafgc acf | gebdcfa ecba ca fadegcb
+dbcfg fgd bdegcaf fgec aegbdf ecdfab fbedc dacgb gdcebf gf | cefg dcbef fcge gbcadfe
+bdfegc cbegaf gecbf dfcage bdacg ed bedf ced adcbefg gebcd | ed bcgafe cdgba cbgef
+egadfb cdbfeg cegd fecab cgb gbdefca cg fgcdab egfdb bfceg | gbdfcae bgc cg cgb
+gcafb gcf dcaebfg ecagb gf abcdeg gaef cafbge fdbac fegbdc | fgae cfgab fg bagce