package vaultik

import (
	"errors"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"sync"
)

// Sentinel errors for blob cache lookups.
var (
	errCacheKeyMissing     = errors.New("key not in cache")
	errCacheReadBeyondBlob = errors.New("read beyond blob size")
)

// blobCacheFileMode is the permission mode for cached blob files.
const blobCacheFileMode = 0o600

// blobDiskCacheEntry tracks a cached blob on disk.
type blobDiskCacheEntry struct {
	key  string
	size int64
	prev *blobDiskCacheEntry
	next *blobDiskCacheEntry
}

// blobDiskCache stores blobs on disk keyed by hash. It exposes ReadAt
// for slice reads (the restore path uses this so chunk extraction
// never reads a whole blob into memory) plus Get/Put for whole-blob
// access.
//
// Eviction policy is caller-controlled. The cache keeps an LRU list
// internally and will fall back to LRU eviction if curBytes exceeds
// maxBytes. Restore passes math.MaxInt64 as maxBytes and drives
// eviction itself via Delete() through restoreSweeper, which deletes
// each blob the moment every file that references its chunks has been
// written. LRU never fires under that configuration; it is kept as a
// safety net for callers that don't manage eviction themselves.
//
// Get/ReadAt/peak-Len counters are debugging instrumentation used by
// tests to assert that the restore code path uses ReadAt rather than
// Get and to bound peak disk-cache occupancy.
type blobDiskCache struct {
	mu       sync.Mutex
	dir      string
	maxBytes int64
	curBytes int64
	items    map[string]*blobDiskCacheEntry
	head     *blobDiskCacheEntry // most recent
	tail     *blobDiskCacheEntry // least recent

	// Instrumentation. Mutated under mu; readable via the methods below.
	getCalls    int
	readAtCalls int
	peakLen     int
}

// newBlobDiskCache creates a new disk-based blob cache with the given max size.
func newBlobDiskCache(maxBytes int64) (*blobDiskCache, error) {
	dir, err := os.MkdirTemp("", "vaultik-blobcache-*")
	if err != nil {
		return nil, fmt.Errorf("creating blob cache dir: %w", err)
	}

	return &blobDiskCache{
		dir:      dir,
		maxBytes: maxBytes,
		items:    make(map[string]*blobDiskCacheEntry),
	}, nil
}

// Put writes blob data to disk cache. Entries larger than maxBytes are
// silently skipped.
func (c *blobDiskCache) Put(key string, data []byte) error {
	entrySize := int64(len(data))

	c.mu.Lock()
	defer c.mu.Unlock()

	if entrySize > c.maxBytes {
		return nil
	}

	// Remove old entry if updating
	if e, ok := c.items[key]; ok {
		c.unlink(e)
		c.curBytes -= e.size
		_ = os.Remove(c.path(key))
		delete(c.items, key)
	}

	err := os.WriteFile(c.path(key), data, blobCacheFileMode)
	if err != nil {
		return fmt.Errorf("writing blob to cache: %w", err)
	}

	e := &blobDiskCacheEntry{key: key, size: entrySize}
	c.pushFront(e)
	c.items[key] = e
	c.curBytes += entrySize

	for c.curBytes > c.maxBytes && c.tail != nil {
		c.evictLRU()
	}

	if n := len(c.items); n > c.peakLen {
		c.peakLen = n
	}

	return nil
}

// PutFromReader streams r into the cache file for key, returning the
// total number of bytes written. Unlike Put, the data never has to
// reside fully in memory at any point — io.Copy uses an internal
// 32 KiB buffer. Used by restore to land a freshly decrypted blob on
// disk without buffering its entire plaintext (which may be tens of GB)
// in RAM.
func (c *blobDiskCache) PutFromReader(key string, r io.Reader) (int64, error) {
	c.mu.Lock()
	// Remove any prior entry first; we'll re-link after the file is
	// written successfully.
	if e, ok := c.items[key]; ok {
		c.unlink(e)
		c.curBytes -= e.size
		_ = os.Remove(c.path(key))
		delete(c.items, key)
	}
	c.mu.Unlock()

	f, err := os.OpenFile(
		c.path(key), os.O_CREATE|os.O_TRUNC|os.O_WRONLY, blobCacheFileMode)
	if err != nil {
		return 0, fmt.Errorf("creating cache file: %w", err)
	}

	written, copyErr := io.Copy(f, r)
	closeErr := f.Close()

	if copyErr != nil {
		_ = os.Remove(c.path(key))

		return written, fmt.Errorf("streaming to cache file: %w", copyErr)
	}

	if closeErr != nil {
		_ = os.Remove(c.path(key))

		return written, fmt.Errorf("closing cache file: %w", closeErr)
	}

	c.mu.Lock()
	defer c.mu.Unlock()

	// If the entry would exceed maxBytes outright, drop it on the
	// floor — but the restore path passes math.MaxInt64 as maxBytes
	// so this branch is effectively unreachable there.
	if written > c.maxBytes {
		_ = os.Remove(c.path(key))

		return written, nil
	}

	e := &blobDiskCacheEntry{key: key, size: written}
	c.pushFront(e)
	c.items[key] = e
	c.curBytes += written

	for c.curBytes > c.maxBytes && c.tail != nil {
		c.evictLRU()
	}

	if n := len(c.items); n > c.peakLen {
		c.peakLen = n
	}

	return written, nil
}

// Get reads a cached blob from disk. Returns data and true on hit.
func (c *blobDiskCache) Get(key string) ([]byte, bool) {
	c.mu.Lock()
	c.getCalls++

	e, ok := c.items[key]
	if !ok {
		c.mu.Unlock()

		return nil, false
	}

	c.unlink(e)
	c.pushFront(e)
	c.mu.Unlock()

	data, err := os.ReadFile(c.path(key))
	if err != nil {
		c.mu.Lock()
		if e2, ok2 := c.items[key]; ok2 && e2 == e {
			c.unlink(e)
			delete(c.items, key)
			c.curBytes -= e.size
		}
		c.mu.Unlock()

		return nil, false
	}

	return data, true
}

// ReadAt reads a slice of a cached blob without loading the entire blob into memory.
func (c *blobDiskCache) ReadAt(key string, offset, length int64) ([]byte, error) {
	c.mu.Lock()
	c.readAtCalls++

	e, ok := c.items[key]
	if !ok {
		c.mu.Unlock()

		return nil, fmt.Errorf("%w: %q", errCacheKeyMissing, key)
	}

	if offset+length > e.size {
		c.mu.Unlock()

		return nil, fmt.Errorf("%w: offset=%d length=%d size=%d",
			errCacheReadBeyondBlob, offset, length, e.size)
	}

	c.unlink(e)
	c.pushFront(e)
	c.mu.Unlock()

	f, err := os.Open(c.path(key))
	if err != nil {
		return nil, err
	}

	defer func() { _ = f.Close() }()

	buf := make([]byte, length)

	_, err = f.ReadAt(buf, offset)
	if err != nil {
		return nil, err
	}

	return buf, nil
}

// Has returns whether a key exists in the cache.
func (c *blobDiskCache) Has(key string) bool {
	c.mu.Lock()
	defer c.mu.Unlock()

	_, ok := c.items[key]

	return ok
}

// Delete removes a blob from the cache and its disk file. No-op if absent.
// Used by restore's sweep logic to free blobs whose chunks have all been
// restored (so they will never be needed again during this restore).
func (c *blobDiskCache) Delete(key string) {
	c.mu.Lock()
	defer c.mu.Unlock()

	e, ok := c.items[key]
	if !ok {
		return
	}

	c.unlink(e)
	delete(c.items, key)
	c.curBytes -= e.size
	_ = os.Remove(c.path(key))
}

// Keys returns a snapshot of all cached keys. Safe for iteration without
// holding the cache lock; the cache may change concurrently.
func (c *blobDiskCache) Keys() []string {
	c.mu.Lock()
	defer c.mu.Unlock()

	keys := make([]string, 0, len(c.items))
	for k := range c.items {
		keys = append(keys, k)
	}

	return keys
}

// Size returns current total cached bytes.
func (c *blobDiskCache) Size() int64 {
	c.mu.Lock()
	defer c.mu.Unlock()

	return c.curBytes
}

// Len returns number of cached entries.
func (c *blobDiskCache) Len() int {
	c.mu.Lock()
	defer c.mu.Unlock()

	return len(c.items)
}

// GetCalls returns the number of times Get has been called.
func (c *blobDiskCache) GetCalls() int {
	c.mu.Lock()
	defer c.mu.Unlock()

	return c.getCalls
}

// ReadAtCalls returns the number of times ReadAt has been called.
func (c *blobDiskCache) ReadAtCalls() int {
	c.mu.Lock()
	defer c.mu.Unlock()

	return c.readAtCalls
}

// PeakLen returns the maximum number of cached entries ever held at
// once during this cache's lifetime.
func (c *blobDiskCache) PeakLen() int {
	c.mu.Lock()
	defer c.mu.Unlock()

	return c.peakLen
}

// Close removes the cache directory and all cached blobs.
func (c *blobDiskCache) Close() error {
	c.mu.Lock()
	defer c.mu.Unlock()

	c.items = nil
	c.head = nil
	c.tail = nil
	c.curBytes = 0

	return os.RemoveAll(c.dir)
}

func (c *blobDiskCache) path(key string) string {
	return filepath.Join(c.dir, key)
}

func (c *blobDiskCache) unlink(e *blobDiskCacheEntry) {
	if e.prev != nil {
		e.prev.next = e.next
	} else {
		c.head = e.next
	}

	if e.next != nil {
		e.next.prev = e.prev
	} else {
		c.tail = e.prev
	}

	e.prev = nil
	e.next = nil
}

func (c *blobDiskCache) pushFront(e *blobDiskCacheEntry) {
	e.prev = nil

	e.next = c.head
	if c.head != nil {
		c.head.prev = e
	}

	c.head = e
	if c.tail == nil {
		c.tail = e
	}
}

func (c *blobDiskCache) evictLRU() {
	if c.tail == nil {
		return
	}

	victim := c.tail
	c.unlink(victim)
	delete(c.items, victim.key)
	c.curBytes -= victim.size
	_ = os.Remove(c.path(victim.key))
}