2023 - Day 5

2023/05/PART1.md

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+--- Day 5: If You Give A Seed A Fertilizer ---
+
+You take the boat and find the gardener right where you were told he would be:
+managing a giant "garden" that looks more to you like a farm.
+
+"A water source? Island Island is the water source!" You point out that Snow
+Island isn't receiving any water.
+
+"Oh, we had to stop the water because we ran out of sand to filter it with!
+Can't make snow with dirty water. Don't worry, I'm sure we'll get more sand
+soon; we only turned off the water a few days... weeks... oh no." His face sinks
+into a look of horrified realization.
+
+"I've been so busy making sure everyone here has food that I completely forgot
+to check why we stopped getting more sand! There's a ferry leaving soon that is
+headed over in that direction - it's much faster than your boat. Could you
+please go check it out?"
+
+You barely have time to agree to this request when he brings up another. "While
+you wait for the ferry, maybe you can help us with our food production problem.
+The latest Island Island Almanac just arrived and we're having trouble making
+sense of it."
+
+The almanac (your puzzle input) lists all of the seeds that need to be planted.
+It also lists what type of soil to use with each kind of seed, what type of
+fertilizer to use with each kind of soil, what type of water to use with each
+kind of fertilizer, and so on. Every type of seed, soil, fertilizer and so on is
+identified with a number, but numbers are reused by each category - that is,
+soil 123 and fertilizer 123 aren't necessarily related to each other.
+
+For example:
+
+```
+seeds: 79 14 55 13
+
+seed-to-soil map:
+50 98 2
+52 50 48
+
+soil-to-fertilizer map:
+0 15 37
+37 52 2
+39 0 15
+
+fertilizer-to-water map:
+49 53 8
+0 11 42
+42 0 7
+57 7 4
+
+water-to-light map:
+88 18 7
+18 25 70
+
+light-to-temperature map:
+45 77 23
+81 45 19
+68 64 13
+
+temperature-to-humidity map:
+0 69 1
+1 0 69
+
+humidity-to-location map:
+60 56 37
+56 93 4
+```
+
+The almanac starts by listing which seeds need to be planted: seeds 79, 14, 55,
+and 13.
+
+The rest of the almanac contains a list of maps which describe how to convert
+numbers from a source category into numbers in a destination category. That is,
+the section that starts with seed-to-soil map: describes how to convert a seed
+number (the source) to a soil number (the destination). This lets the gardener
+and his team know which soil to use with which seeds, which water to use with
+which fertilizer, and so on.
+
+Rather than list every source number and its corresponding destination number
+one by one, the maps describe entire ranges of numbers that can be converted.
+Each line within a map contains three numbers: the destination range start, the
+source range start, and the range length.
+
+Consider again the example seed-to-soil map:
+
+```
+50 98 2
+52 50 48
+```
+
+The first line has a destination range start of 50, a source range start of 98,
+and a range length of 2. This line means that the source range starts at 98 and
+contains two values: 98 and 99. The destination range is the same length, but it
+starts at 50, so its two values are 50 and 51. With this information, you know
+that seed number 98 corresponds to soil number 50 and that seed number 99
+corresponds to soil number 51.
+
+The second line means that the source range starts at 50 and contains 48 values:
+50, 51, ..., 96, 97. This corresponds to a destination range starting at 52 and
+also containing 48 values: 52, 53, ..., 98, 99. So, seed number 53 corresponds
+to soil number 55.
+
+Any source numbers that aren't mapped correspond to the same destination number.
+So, seed number 10 corresponds to soil number 10.
+
+So, the entire list of seed numbers and their corresponding soil numbers looks
+like this:
+
+```
+seed  soil
+0     0
+1     1
+...   ...
+48    48
+49    49
+50    52
+51    53
+...   ...
+96    98
+97    99
+98    50
+99    51
+```
+
+With this map, you can look up the soil number required for each initial seed
+number:
+
+- Seed number 79 corresponds to soil number 81.
+- Seed number 14 corresponds to soil number 14.
+- Seed number 55 corresponds to soil number 57.
+- Seed number 13 corresponds to soil number 13.
+
+The gardener and his team want to get started as soon as possible, so they'd
+like to know the closest location that needs a seed. Using these maps, find the
+lowest location number that corresponds to any of the initial seeds. To do this,
+you'll need to convert each seed number through other categories until you can
+find its corresponding location number. In this example, the corresponding types
+are:
+
+- Seed 79, soil 81, fertilizer 81, water 81, light 74, temperature 78, humidity
+  78, location 82.
+- Seed 14, soil 14, fertilizer 53, water 49, light 42, temperature 42, humidity
+  43, location 43.
+- Seed 55, soil 57, fertilizer 57, water 53, light 46, temperature 82, humidity
+  82, location 86.
+- Seed 13, soil 13, fertilizer 52, water 41, light 34, temperature 34, humidity
+  35, location 35.
+
+So, the lowest location number in this example is 35.
+
+What is the lowest location number that corresponds to any of the initial seed
+numbers?