Add solution to part 2 of day 1

2017/day/1/problem.txt

@@ -7,3 +7,17 @@ For example:
     1234 produces 0 because no digit matches the next.
     91212129 produces 9 because the only digit that matches the next one is the last digit, 9.
 
+--- Part Two ---
+
+You notice a progress bar that jumps to 50% completion. Apparently, the door isn't yet satisfied, but it did emit a star as encouragement. The instructions change:
+
+Now, instead of considering the next digit, it wants you to consider the digit halfway around the circular list. That is, if your list contains 10 items, only include a digit in your sum if the digit 10/2 = 5 steps forward matches it. Fortunately, your list has an even number of elements.
+
+For example:
+
+    1212 produces 6: the list contains 4 items, and all four digits match the digit 2 items ahead.
+    1221 produces 0, because every comparison is between a 1 and a 2.
+    123425 produces 4, because both 2s match each other, but no other digit has a match.
+    123123 produces 12.
+    12131415 produces 4.
+

2017/day/1/src/main.rs

@@ -7,6 +7,7 @@ fn main() {
     file.read_to_end(&mut input).expect("error reading input");
 
     println!("{}", captcha(&input));
+    println!("{}", captcha2(&input));
 }
 
 fn captcha(input: &[u8]) -> u64 {
@@ -24,27 +25,73 @@ fn captcha(input: &[u8]) -> u64 {
     sum
 }
 
+fn captcha2(input: &[u8]) -> u64 {
+    let digits = input.iter()
+        .take_while(|&&chr| chr >= 0x30 && chr <= 0x39)
+        .collect::<Vec<_>>();
+
+    let mut sum = 0;
+    for idx in 0..digits.len() {
+        if digits[idx] == digits[(idx + (digits.len() / 2)) % digits.len()] {
+            sum += *digits[idx] as u64 - 0x30;
+        }
+    }
+
+    sum
+}
+
 // 1122 produces a sum of 3 (1 + 2) because the first digit (1) matches the second digit and the
 // third digit (2) matches the fourth digit.
 #[test]
-fn test_example1() {
+fn test_captcha_example1() {
     assert_eq!(captcha(b"1122"), 3);
 }
 
 // 1111 produces 4 because each digit (all 1) matches the next.
 #[test]
-fn test_example2() {
+fn test_captcha_example2() {
     assert_eq!(captcha(b"1111"), 4);
 }
 
 // 1234 produces 0 because no digit matches the next.
 #[test]
-fn test_example3() {
+fn test_captcha_example3() {
     assert_eq!(captcha(b"1234"), 0);
 }
 
 // 91212129 produces 9 because the only digit that matches the next one is the last digit, 9.
 #[test]
-fn test_example4() {
+fn test_captcha_example4() {
     assert_eq!(captcha(b"91212129"), 9);
 }
+
+// 1212 produces 6: the list contains 4 items, and all four digits match the digit 2 items ahead.
+#[test]
+fn test_captcha2_example1() {
+    assert_eq!(captcha2(b"1212"), 6);
+}
+
+// 1221 produces 0, because every comparison is between a 1 and a 2.
+#[test]
+fn test_captcha2_example2() {
+    assert_eq!(captcha2(b"1221"), 0);
+}
+
+// 123425 produces 4, because both 2s match each other, but no other digit has a match.
+#[test]
+fn test_captcha2_example3() {
+    assert_eq!(captcha2(b"123425"), 4);
+}
+
+// 123123 produces 12.
+#[test]
+fn test_captcha2_example4() {
+    assert_eq!(captcha2(b"123123"), 12);
+}
+
+// 12131415 produces 4.
+#[test]
+fn test_captcha2_example5() {
+    assert_eq!(captcha2(b"12131415"), 4);
+}
+