Module aoc::year2021::day08

Seven Segment Search

Listing each digit and the number of segments that are lit when that digit is displayed:

0	1	2	3	4	5	6	7	8	9
6	2	5	5	4	5	6	3	7	6

shows that 3 digits share 5 segments and another 3 share 6 segments so we don’t have enough information just yet. Listing the total occurences of each segment summing across all 10 digits:

a	b	c	d	e	f	g
8	6	8	7	4	9	7

shows that 2 segments share 7 occurences and 2 share 8 occurences so this is still not quite enough information. However if we combine these 2 tables by summing the segment occurences for each digit, for example 1 has segments c and f for a total of 17, then the table looks like:

0	1	2	3	4	5	6	7	8	9
42	17	34	39	30	37	41	25	49	45

Now each digit can be uniquely identified. Our algorithm is as follows:

• Calculate the occurences of each scrambled segment letter before the | symbol. Since the cardinality of the set is fixed, we can use an array instead of a HashMap for speed.
• Add the occurences of each scrambled segment for each digit after the | symbol, then lookup the total and map directly to the unscrambled digit.

Functions

• descramble 🔒
• parse
• part1
• part2
• to_digit 🔒

Type Aliases

• Input 🔒