1 files changed, 215 insertions, 0 deletions
diff --git a/day9/main.go b/day9/main.go
new file mode 100644
index 0000000..1ecb78a
--- /dev/null
+++ b/day9/main.go
@@ -0,0 +1,215 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+)
+
+const SENTINEL int = -2
+const FREE_BLOCK int = -1
+
+type Block struct {
+	Prev, Next *Block
+	Size       int
+	FileId     int
+}
+
+func (before *Block) InsertAfter(after *Block) {
+	before.Next.Prev = after
+	after.Next, before.Next = before.Next, after
+	after.Prev = before
+}
+
+func (block *Block) Detach() {
+	block.Prev.Next, block.Next.Prev = block.Next, block.Prev
+	block.Prev, block.Next = nil, nil
+}
+
+func (block *Block) TryMergeFreeWithNext() {
+	if block.FileId == FREE_BLOCK && block.Next.FileId == FREE_BLOCK {
+		block.Size += block.Next.Size
+		block.Next.Detach()
+	}
+}
+
+func (block *Block) TryMergeFree() *Block {
+	block.TryMergeFreeWithNext()
+	if block.Prev.FileId == FREE_BLOCK {
+		block = block.Prev
+		block.TryMergeFreeWithNext()
+	}
+	return block
+}
+
+func (block *Block) Split(Size int) {
+	if Size >= block.Size {
+		panic("please")
+	}
+
+	if Size == block.Size {
+		return
+	}
+
+	newBlock := &Block{nil, nil, block.Size - Size, block.FileId}
+	block.Size = Size
+	block.InsertAfter(newBlock)
+}
+
+type Disk struct {
+	Sentinel *Block
+}
+
+func readInt(b byte) int {
+	return int(b - '0')
+}
+
+func MakeDisk(description string) (disk Disk) {
+	disk.Sentinel = new(Block)
+	disk.Sentinel.Size = 0
+	disk.Sentinel.FileId = SENTINEL
+	disk.Sentinel.Next = disk.Sentinel
+	disk.Sentinel.Prev = disk.Sentinel
+
+	nextFileId := 0
+	nextIsFree := false
+
+	head := disk.Sentinel
+
+	for _, b := range []byte(description) {
+		block := &Block{
+			Prev: nil, Next: nil, Size: readInt(b),
+		}
+
+		if block.Size == 0 {
+			nextIsFree = !nextIsFree
+			continue
+		}
+
+		if nextIsFree {
+			block.FileId = FREE_BLOCK
+			nextIsFree = false
+		} else {
+			block.FileId = nextFileId
+			nextFileId++
+			nextIsFree = true
+		}
+
+		head.InsertAfter(block)
+		head = block
+	}
+
+	return
+}
+
+func (disk *Disk) Print() {
+	fmt.Println("===============")
+	for b := disk.Sentinel.Next; b != disk.Sentinel; b = b.Next {
+		fmt.Println(b)
+	}
+	fmt.Println("===============")
+	bufio.NewReader(os.Stdin).ReadBytes('\n')
+}
+
+func (disk *Disk) Compactify() {
+	head := disk.Sentinel.Next
+	tail := disk.Sentinel.Prev
+
+	for head != tail {
+		if head.FileId >= 0 {
+			head = head.Next
+			continue
+		}
+
+		if tail.FileId == FREE_BLOCK {
+			tail = tail.Prev
+			continue
+		}
+
+		if tail.Size > head.Size {
+			tail.Split(tail.Size - head.Size)
+			tail = tail.Next
+		}
+
+		if head.Size > tail.Size {
+			head.Split(tail.Size)
+		}
+
+		head.FileId, tail.FileId = tail.FileId, head.FileId
+		tail = tail.TryMergeFree()
+	}
+}
+
+func (disk *Disk) Defragment() {
+	tail := disk.Sentinel.Prev
+
+	for tail != disk.Sentinel {
+		if tail.FileId == FREE_BLOCK {
+			tail = tail.Prev
+			continue
+		}
+
+		var freeBlock *Block = nil
+		for head := disk.Sentinel.Next; head != tail; head = head.Next {
+			if head.FileId == FREE_BLOCK && head.Size >= tail.Size {
+				freeBlock = head
+				break
+			}
+		}
+
+		if freeBlock == nil {
+			tail = tail.Prev
+			continue
+		}
+
+		if freeBlock.Size > tail.Size {
+			freeBlock.Split(tail.Size)
+		}
+
+		freeBlock.FileId, tail.FileId = tail.FileId, freeBlock.FileId
+		tail = tail.TryMergeFree()
+	}
+}
+
+func (disk *Disk) ComputeChecksum() (sum int64) {
+	pos := 0
+	for b := disk.Sentinel.Next; b != disk.Sentinel; b = b.Next {
+		if b.FileId >= 0 {
+			for i := 0; i < b.Size; i++ {
+				sum += int64(pos * b.FileId)
+				pos++
+			}
+		} else {
+			pos += b.Size
+		}
+	}
+	return
+}
+
+func main() {
+	disk := MakeDisk(readData("example.txt"))
+	disk.Compactify()
+	fmt.Println(disk.ComputeChecksum(), 1928)
+
+	disk = MakeDisk(readData("data.txt"))
+	disk.Compactify()
+	fmt.Println(disk.ComputeChecksum(), 6415184586041)
+
+	disk = MakeDisk(readData("example.txt"))
+	disk.Defragment()
+	fmt.Println(disk.ComputeChecksum(), 2858)
+
+	disk = MakeDisk(readData("data.txt"))
+	disk.Defragment()
+	fmt.Println(disk.ComputeChecksum())
+}
+
+func readData(fileName string) string {
+	resultBytes, err := os.ReadFile(fileName)
+	if err != nil {
+		panic(err)
+	}
+
+	return strings.TrimSpace(string(resultBytes))
+}