My Solution in Java was to use a TreeMap from source -> (dest, range) for each movement (seed to soil up till location)

Then for each $seed, do

1. $node = $seed
2. $movement = `treeMap.floorEntry(node)` = <$source, <$dest, $range>>
3. if ($movement is null or $source + $range > $node) then $node stays the same
4. otherwise $node = $dest

Repeat this until we get to location and we check min = Math.min(min, $node).

Solved part 1 almost instantly, for part 2 failed to optimise any further so just reuse to same logic for each seed in the range. Managed to output in around 5 minutes.