Module day03

Rucksack Reorganization

The core idea of this puzzle is computing set intersection. We could use the built-in HashSet but as the cardinality of the set is so small (52 maximum including both lowercase and upper case letters) we can instead use a much faster approach of storing each set in a single u128 integer and using bit manipulation.

If a letter is present in the set then the corresponding bit will be 1 otherwise 0. For example to add the letter “a”, logical OR the set with 1 shifted left by 97

set | (1 << 97)

Set intersection is the logical AND of two integers which compiles to a single machine instruction.

a & b

To obtain the score we can use the trailing_zeroes method to find the first set bit. On most architectures this also compiles down to a single instruction (LZCNT on x86 or CLZ on ARM) that is blazing fast.

Notes:

• We could get away with a u64 for the set, but by using an u128 we can shift directly by the raw ASCII codes and not bother computing offsets until the very end.

Functions

mask 🔒

Build a set from a slice of ASCII characters, using the fold function to repeatedly OR bit offsets into an accumulator.

parse

Collect each line into a vec of string slices.

part1

Split each line into 2 equal halves, then compute the set intersection.

part2

Group lines into chunks of 3, then compute the mutual set intersection.

priority 🔒

Find the lowest set bit (there should only be one) then convert to priority using the given rules.