updates

Signed-off-by: Jess Frazelle <github@jessfraz.com>

src/day13.rs

@@ -1,5 +1,4 @@
 //!  Day 13: Point of Incidence
-
 use anyhow::Result;
 
 /// A terrain data point.
@@ -222,8 +221,6 @@ fn is_reflection(rows: Vec<TerrainRow>, i: usize) -> bool {
 ///
 /// Find the line of reflection in each of the patterns in your notes. *What
 /// number do you get after summarizing all of your notes?*
-///
-/// To begin, get your puzzle input (<13/input>).
 pub fn solve_part_1(input: &str) -> Result<u32> {
     // Split the input ny two new lines.
     let inputs = input.split("\n\n");
@@ -242,7 +239,79 @@ pub fn solve_part_1(input: &str) -> Result<u32> {
     Ok(total)
 }
 
-/// Not yet unlocked.
+/// You resume walking through the valley of mirrors and - *SMACK!* - run
+/// directly into one. Hopefully nobody was watching, because that must have
+/// been pretty embarrassing.
+///
+/// Upon closer inspection, you discover that every mirror has exactly one
+/// *smudge*: exactly one `.` or `#` should be the opposite type.
+///
+/// In each pattern, you'll need to locate and fix the smudge that causes a
+/// *different reflection line* to be valid. (The old reflection line won't
+/// necessarily continue being valid after the smudge is fixed.)
+///
+/// Here's the above example again:
+///
+/// ```ignore
+/// #.##..##.
+/// ..#.##.#.
+/// ##......#
+/// ##......#
+/// ..#.##.#.
+/// ..##..##.
+/// #.#.##.#.
+///
+/// #...##..#
+/// #....#..#
+/// ..##..###
+/// #####.##.
+/// #####.##.
+/// ..##..###
+/// #....#..#
+/// ```
+///
+/// The first pattern's smudge is in the top-left corner. If the top-left `#`
+/// were instead `.`, it would have a different, horizontal line of reflection:
+///
+/// ```ignore
+/// 1 ..##..##. 1
+/// 2 ..#.##.#. 2
+/// 3v##......#v3
+/// 4^##......#^4
+/// 5 ..#.##.#. 5
+/// 6 ..##..##. 6
+/// 7 #.#.##.#. 7
+/// ```
+///
+/// With the smudge in the top-left corner repaired, a new horizontal line of
+/// reflection between rows 3 and 4 now exists. Row 7 has no corresponding
+/// reflected row and can be ignored, but every other row matches exactly: row 1
+/// matches row 6, row 2 matches row 5, and row 3 matches row 4.
+///
+/// In the second pattern, the smudge can be fixed by changing the fifth symbol
+/// on row 2 from `.` to `#`:
+///
+/// ```ignore
+/// 1v#...##..#v1
+/// 2^#...##..#^2
+/// 3 ..##..### 3
+/// 4 #####.##. 4
+/// 5 #####.##. 5
+/// 6 ..##..### 6
+/// 7 #....#..# 7
+/// ```
+///
+/// Now, the pattern has a different horizontal line of reflection between rows
+/// 1 and 2.
+///
+/// Summarize your notes as before, but instead use the new different reflection
+/// lines. In this example, the first pattern's new horizontal line has 3 rows
+/// above it and the second pattern's new horizontal line has 1 row above it,
+/// summarizing to the value `*400*`.
+///
+/// In each pattern, fix the smudge and find the different line of reflection.
+/// *What number do you get after summarizing the new reflection line in each
+/// pattern in your notes?*
 pub fn solve_part_2(_input: &str) -> Result<u32> {
     todo!()
 }