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day08: replace static foreach with workaround
author Jordi Gutiérrez Hermoso <>
date Tue, 16 Jan 2018 11:28:55 -0500
parents 049fb8e56025
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--- Day 14: Disk Defragmentation ---

Suddenly, a scheduled job activates the system's disk defragmenter.
Were the situation different, you might sit and watch it for a while,
but today, you just don't have that kind of time. It's soaking up
valuable system resources that are needed elsewhere, and so the only
option is to help it finish its task as soon as possible.

The disk in question consists of a 128x128 grid; each square of the
grid is either free or used. On this disk, the state of the grid is
tracked by the bits in a sequence of knot hashes.

A total of 128 knot hashes are calculated, each corresponding to a
single row in the grid; each hash contains 128 bits which correspond
to individual grid squares. Each bit of a hash indicates whether that
square is free (0) or used (1).

The hash inputs are a key string (your puzzle input), a dash, and a
number from 0 to 127 corresponding to the row. For example, if your
key string were flqrgnkx, then the first row would be given by the
bits of the knot hash of flqrgnkx-0, the second row from the bits of
the knot hash of flqrgnkx-1, and so on until the last row,

The output of a knot hash is traditionally represented by 32
hexadecimal digits; each of these digits correspond to 4 bits, for a
total of 4 * 32 = 128 bits. To convert to bits, turn each hexadecimal
digit to its equivalent binary value, high-bit first: 0 becomes 0000,
1 becomes 0001, e becomes 1110, f becomes 1111, and so on; a hash that
begins with a0c2017... in hexadecimal would begin with
10100000110000100000000101110000... in binary.

Continuing this process, the first 8 rows and columns for key flqrgnkx
appear as follows, using # to denote used squares, and . to denote
free ones:

|      |   
V      V   

In this example, 8108 squares are used across the entire 128x128 grid.

Given your actual key string, how many squares are used?

Your puzzle answer was 8292.

--- Part Two ---

Now, all the defragmenter needs to know is the number of regions. A
region is a group of used squares that are all adjacent, not including
diagonals. Every used square is in exactly one region: lone used
squares form their own isolated regions, while several adjacent
squares all count as a single region.

In the example above, the following nine regions are visible, each
marked with a distinct digit:

|      |   
V      V   

Of particular interest is the region marked 8; while it does not
appear contiguous in this small view, all of the squares marked 8 are
connected when considering the whole 128x128 grid. In total, in this
example, 1242 regions are present.

How many regions are present given your key string?

Your puzzle answer was 1069.

Both parts of this puzzle are complete! They provide two gold stars: **