aoc2024/day-09/internal/two/disk.go

package two

import (
	"fmt"
	"strings"

	log "github.com/sirupsen/logrus"
)

type disk struct {
	blocks []block
	data   []int
}

func newDisk(blocks []block) disk {
	offset := 0
	data := []int{}
	for id, block := range blocks {
		blocks[id].offset = offset
		for range block.used {
			data = append(data, id)
		}
		for range block.available() {
			data = append(data, empty)
		}
		offset += block.used + block.available()
	}

	return disk{
		blocks: blocks,
		data:   data,
	}
}

func (d *disk) sort() {
	for i := len(d.blocks) - 1; i >= 0; i-- {
		log.Debugf("searching for space for block %d of size %d\n", i, d.blocks[i].used)

		for j := 0; j < len(d.data); j++ {
			if j >= d.blocks[i].offset+d.blocks[i].used {
				break
			}

			if d.data[j] != empty {
				continue
			}

			var sz int
			if d.data[j] == empty {
				sz = d.sizeOfEmptyBlock(j)
			}

			if d.blocks[i].used <= sz {
				d.writeBlockToDisk(j, d.blocks[i])
				// don't attempt to write block again
				break
			}

			// skip to end of empty block
			j += sz
		}
	}
}

func (d *disk) sizeOfEmptyBlock(j int) int {
	var sz int
	for k := j; k < len(d.data) && d.data[k] == empty; k++ {
		sz++
	}
	log.Debugf("found empty space of size %d\n", sz)
	return sz
}

func (d *disk) writeBlockToDisk(start int, b block) {
	for i := b.offset; i < b.offset+b.used; i, start = i+1, start+1 {
		d.data[start] = d.data[i]
		log.Debugf("writing %d to disk at pos %d\n", d.data[i], start)
	}

	for i := b.offset; i < len(d.data) && i < b.offset+b.used; i++ {
		d.data[i] = empty
		log.Debugf("truncating space on disk at pos %d", i)
	}

	log.Debug(d.debug())
}

func (d *disk) debug() string {
	var sb strings.Builder
	for _, n := range d.data {
		if n == empty {
			sb.WriteRune('.')
		} else {
			sb.WriteString(fmt.Sprintf("%d", n))
		}
	}
	return sb.String()
}