package main

import (
	"bufio"
	"encoding/binary"
	"fmt"
	"io"
	"os"
	"path/filepath"
)

const jobStateMagic = "CTJ1"

// JobState is the manager-local trust anchor for one named job: the
// per-block hashes of the destination as of the last confirmed write. It is
// never derived by reading the destination — only by recording what this
// tool itself wrote there (or confirmed already matched).
type JobState struct {
	BlockSize int64
	SrcSpec   string
	DstSpec   string
	Size      int64
	Hashes    [][32]byte // zero hash = never synced
}

func statePathFor(stateDir, job string) string {
	return filepath.Join(stateDir, job+".state")
}

func defaultStateDir() string {
	home, err := os.UserHomeDir()
	if err != nil {
		return ".clonetool/jobs"
	}
	return filepath.Join(home, ".clonetool", "jobs")
}

// LoadJobState reads the job's state file. If it doesn't exist yet, an
// empty state is returned (first run). If it exists but was recorded
// against a different source/dest spec, an error is returned unless force
// is set, in which case the old hash history is discarded.
func LoadJobState(path, srcSpec, dstSpec string, blockSize int64, force bool) (*JobState, error) {
	f, err := os.Open(path)
	if err != nil {
		if os.IsNotExist(err) {
			return &JobState{BlockSize: blockSize, SrcSpec: srcSpec, DstSpec: dstSpec}, nil
		}
		return nil, err
	}
	defer f.Close()

	r := bufio.NewReader(f)
	magic := make([]byte, 4)
	if _, err := io.ReadFull(r, magic); err != nil {
		return nil, fmt.Errorf("read job state %s: %w", path, err)
	}
	if string(magic) != jobStateMagic {
		return nil, fmt.Errorf("job state %s: bad magic", path)
	}
	var version uint8
	if err := binary.Read(r, binary.BigEndian, &version); err != nil {
		return nil, err
	}
	if version != 1 {
		return nil, fmt.Errorf("job state %s: unsupported version %d", path, version)
	}
	var storedBlockSize uint32
	if err := binary.Read(r, binary.BigEndian, &storedBlockSize); err != nil {
		return nil, err
	}
	var blockCount uint64
	if err := binary.Read(r, binary.BigEndian, &blockCount); err != nil {
		return nil, err
	}
	storedSrc, err := readLPString(r)
	if err != nil {
		return nil, err
	}
	storedDst, err := readLPString(r)
	if err != nil {
		return nil, err
	}
	var size int64
	if err := binary.Read(r, binary.BigEndian, &size); err != nil {
		return nil, err
	}

	if !force && (storedSrc != srcSpec || storedDst != dstSpec) {
		return nil, fmt.Errorf(
			"job state %s was recorded for source=%q dest=%q, but this run uses source=%q dest=%q; "+
				"reusing its hash history against different endpoints could skip blocks never written there. "+
				"Pass --force to rebind this job (discards hash history)",
			path, storedSrc, storedDst, srcSpec, dstSpec)
	}

	hashes := make([][32]byte, blockCount)
	for i := range hashes {
		if _, err := io.ReadFull(r, hashes[i][:]); err != nil {
			return nil, fmt.Errorf("read job state %s: %w", path, err)
		}
	}

	js := &JobState{BlockSize: int64(storedBlockSize), SrcSpec: srcSpec, DstSpec: dstSpec, Size: size, Hashes: hashes}
	if force {
		js.SrcSpec, js.DstSpec = srcSpec, dstSpec
	}
	if js.BlockSize != blockSize {
		// Block size changed: the stored per-index hashes no longer line up
		// with block boundaries, so start over rather than misinterpret them.
		js.BlockSize = blockSize
		js.Hashes = nil
	}
	return js, nil
}

// Resize grows (with zero/"unknown" hashes) or shrinks the hash table to
// match a new block count.
func (js *JobState) Resize(blockCount uint64) {
	if uint64(len(js.Hashes)) == blockCount {
		return
	}
	grown := make([][32]byte, blockCount)
	copy(grown, js.Hashes)
	js.Hashes = grown
}

func (js *JobState) Save(path string) error {
	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
		return err
	}
	tmp := path + ".tmp"
	f, err := os.OpenFile(tmp, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0644)
	if err != nil {
		return err
	}
	w := bufio.NewWriter(f)
	writeErr := func() error {
		if _, err := w.WriteString(jobStateMagic); err != nil {
			return err
		}
		if err := binary.Write(w, binary.BigEndian, uint8(1)); err != nil {
			return err
		}
		if err := binary.Write(w, binary.BigEndian, uint32(js.BlockSize)); err != nil {
			return err
		}
		if err := binary.Write(w, binary.BigEndian, uint64(len(js.Hashes))); err != nil {
			return err
		}
		if err := writeLPString(w, js.SrcSpec); err != nil {
			return err
		}
		if err := writeLPString(w, js.DstSpec); err != nil {
			return err
		}
		if err := binary.Write(w, binary.BigEndian, js.Size); err != nil {
			return err
		}
		for _, h := range js.Hashes {
			if _, err := w.Write(h[:]); err != nil {
				return err
			}
		}
		return w.Flush()
	}()
	if writeErr != nil {
		f.Close()
		os.Remove(tmp)
		return writeErr
	}
	if err := f.Sync(); err != nil {
		f.Close()
		os.Remove(tmp)
		return err
	}
	if err := f.Close(); err != nil {
		os.Remove(tmp)
		return err
	}
	return os.Rename(tmp, path)
}

func readLPString(r io.Reader) (string, error) {
	var n uint16
	if err := binary.Read(r, binary.BigEndian, &n); err != nil {
		return "", err
	}
	buf := make([]byte, n)
	if _, err := io.ReadFull(r, buf); err != nil {
		return "", err
	}
	return string(buf), nil
}

func writeLPString(w io.Writer, s string) error {
	if len(s) > 65535 {
		return fmt.Errorf("string too long to store (%d bytes)", len(s))
	}
	if err := binary.Write(w, binary.BigEndian, uint16(len(s))); err != nil {
		return err
	}
	_, err := io.WriteString(w, s)
	return err
}