package docbuf

import (
	"encoding/base64"
	"errors"
	"io"
)

const (
	DemoteFromEmptyCacheError = "Can't demote from empty cache"
	PromoteToFullCacheError   = "Can't promote to full cache"
	WriteEmptyDocumentError   = "Not writing empty document to store"
	PartialWriteError         = "Couldnt complete document write"
	BadReadRequestError       = "Can't read less than one documents"
	BadReadLengthError        = "Unexpected read length"
	PopFromEmptyBufferError   = "Can't pop from empty buffer"
	RemoveOverCapacityError   = "Not enough documents in buffer to remove"
)

type DocumentBuffer interface {
	/* Push
	 * Add a document to the buffer.
	 * If cache is full oldest document is written to backing writer.
	 */
	Push(string) error
	/* Pop
	 * removes a document from the cache and pulls a document back
	 * out of the backing ReaderWriter.
	 */
	Pop() (string, error)
	/* Remove
	 * removes N documents according to arg.
	 * backfills cache by pulling documents back out of backing
	 * ReaderWriter.
	 */
	Remove(int) ([]string, error)
	/* Flush
	 * empties cache by placing all cached documnent into backing 
	 * ReaderWriter.
	 */
	Flush() error
	/* Peek
	 * views first document in either cache or backing ReaderWriter
	 */
	Peek() (string, error)
	/* Read
	 * returns N documents from the cache and backing store without
	 * removing them from either data structure.
	 */
	Read(int) ([]string, error)
	/* Apply
	 * Takes a callable/closure/function that accepts a string (document)
	 * and returns a bool. Apply returns an error, or nil.
	 * Apply will begin applying this callable to each doc from most recent
	 * to least recent so long as the callable continues to return true.
	 * If the callable returns false, Apply ceases iteration and returns.
	 */
	Apply(func(string)bool) error
	/* Close
	 * flushes and then prepares backing store for closure
	 */
	Close() (int64, error)
	/* Cached
	 * returns current length of cache
	 */
	Cached() int
}

type DocBuf struct {
	cache         []string
	store         io.ReadWriteSeeker
	cacheSize     int
	endIndex      int64
}

// put an element from cache into store
func (b *DocBuf) demote() error {
	if len(b.cache) < 1 {
		return errors.New(DemoteFromEmptyCacheError)
	}

	demoted := b.cache[len(b.cache)-1]
	b.cache = b.cache[:len(b.cache)-1]
	return b.writeToBackingStore(demoted)
}

/* bounds here refers to checking the capacity of the internal memory cache
 * Remove() includes a use case where a user might want to evict more events
 * than the internal memory cache can possibly contain, in which case we use
 * promote and turn off bounds checking in order to overpopulate memory cache
 */
func (b *DocBuf) promote(bounds bool) error {
	if bounds && len(b.cache) >= b.cacheSize {
		return errors.New(PromoteToFullCacheError)
	}

	doc, err := b.readDocumentsFromDisk(1, true, false)
	if err != nil {
		return err
	}

	if len(doc) > 0 {
		b.cache = append(b.cache, doc[0])
	}
	return nil
}

// encode as base64 so as to avoid needing to escape things
// then documents can be separated by newlines
func (b *DocBuf) writeToBackingStore(doc string) error {
	if len(doc) == 0 {
		return errors.New(WriteEmptyDocumentError)
	}

	str := base64.StdEncoding.EncodeToString([]byte(doc))

	// seek to end index
	c, err := b.store.Seek(int64(b.endIndex), io.SeekStart);
	if err != nil {
		return err
	}

	// separate documents with a newline
	if c > 0 {
		str = "\n" + str
	}

	n, err := b.store.Write([]byte(str))
	if err != nil || n < len(doc) {
		if n < len(doc) {
			return errors.Join(
					errors.New(PartialWriteError),
					err)
		}
		return err
	}

	b.endIndex += int64(n)

	return nil
}

func (b *DocBuf) readDocumentsFromDisk(
	count int,
	truncate bool,
	continues bool,
) ([]string, error) {
	docs   := []string{}
	cursor := int64(0)
	var err error

	if count < 1 {
		return docs, errors.New(BadReadRequestError)
	}

	if continues {
		cursor, err = b.store.Seek(0, io.SeekCurrent)
		if b.endIndex == 0 || err != nil || cursor == 0 {
			return docs, err
		}
	
	} else {
		// catch store is empty
		cursor, err = b.store.Seek(0, io.SeekEnd)
		if b.endIndex == 0 || err != nil || cursor == 0 {
			return docs, err
		}

		// self repair?
		if b.endIndex > cursor {
			b.endIndex = cursor
		}
		cursor = b.endIndex - 1
	}

	for len(docs) < count && cursor >= 0 {
		doc     := ""
		char    := make([]byte, 1)
		char[0] = 0

		// read bytes backwards from file
		for cursor >= 0 {
			// set cursor
			if _, err := b.store.Seek(cursor, io.SeekStart); err != nil {
				return docs, err
			}

			// read one byte
			n, err := b.store.Read(char)
			if err != nil {
				return docs, err
			}
			if n != 1 {
				return docs, errors.New(BadReadLengthError)
			}

			// break on newline
			if char[0] == 10 {
				break
			}

			cursor -= 1
			doc = string(char) + doc
		}

		/* parse document and add to account (docs)
		 * each read will stop on the newline so make sure
		 * we dont try to account for an empty document
		 */
		if len(doc) > 0 && doc != "\n" {
			str, err := base64.StdEncoding.DecodeString(doc)
			if err != nil {
				return docs, err
			}
			doc = string(str)
			docs = append(docs, doc)
		}

		if len(docs) < count {
			cursor -= 1
		}
	}

	if truncate {
		if cursor < 0 {
			cursor = 0
		}
		b.endIndex = cursor
	}
	return docs, nil
}


/* WARNING: this constructor will promote items
 *   from storage, resulting in a modified backing
 *   as well as a prefilled cache.
 */
func NewDocumentBuffer(
	cacheSize int,
	backingStore io.ReadWriteSeeker,
) (DocumentBuffer, error) {
	newBuf := DocBuf {
		cache: make([]string, 0, cacheSize),
		store: backingStore,
		cacheSize: cacheSize,
	}

	c, err := backingStore.Seek(0, io.SeekEnd)
	newBuf.endIndex = c
	if err != nil {
		return &newBuf, err
	}

	// prefill cache
	for range cacheSize {
		newBuf.promote(true)
	}
	return &newBuf, nil
}

/* Push
 * Add a document to the buffer.
 * If cache is full oldest document is written to backing writer.
 */
func (b *DocBuf) Push(doc string) error {
	if b.cacheSize == 0 {
		b.writeToBackingStore(doc)
		return nil
	}

	if len(b.cache) >= b.cacheSize {
		if err := b.demote(); err != nil {
			return err
		}
	}

	tc := b.cache
	b.cache = append(make([]string, 0, b.cacheSize), doc)
	b.cache = append(b.cache, tc...)
	return nil
}

/* Pop
 * removes a document from the cache and pulls a document back
 * out of the backing ReaderWriter.
 */
func (b *DocBuf) Pop() (string, error) {
	if b.cacheSize == 0 {
		d, e := b.readDocumentsFromDisk(1, true, false)
		if len(d) > 0 {
			return d[0], e
		}
		return "", e
	}

	if len(b.cache) < 1 {
		if err := b.promote(true); err != nil {
			return "", err
		}
	}

	if len(b.cache) < 1 {
		return "", errors.New(PopFromEmptyBufferError)
	}

	candidate := b.cache[0]
	b.cache = b.cache[1:]
	e := b.promote(true)

	return candidate, e
}

/* Remove
 * removes N newest documents in order of newest to oldest.
 * backfills cache by pulling documents back out of backing
 * ReaderWriter (again, newest first).
 */
func (b *DocBuf) Remove(count int) ([]string, error) {
	delta := count - b.cacheSize
	if delta > 0 {
		for range delta {
			if err := b.promote(false); err != nil {
				return []string{}, err
			}
		}
	}

	if count > len(b.cache) {
		tmp := b.cache
		b.cache = make([]string, 0, b.cacheSize)
		return tmp, errors.New(RemoveOverCapacityError)
	}

	candidates := b.cache[:count]
	b.cache = b.cache[count:]

	// refill cache
	delta = b.cacheSize - len(b.cache)
	if delta > 0 {
		for range delta {
			if err := b.promote(true); err != nil {
				return []string{}, err
			}
		}
	}

	return candidates, nil
}

/* Flush
 * empties cache by placing all cached documnent into backing 
 * ReaderWriter.
 */
func (b *DocBuf) Flush() error {
	for range len(b.cache) {
		if err := b.demote(); err != nil {
			return err
		}
	}

	return nil
}

/* Peek
 * views first document in either cache or backing ReaderWriter
 */
func (b *DocBuf) Peek() (string, error) {
	if len(b.cache) > 0 {
		return b.cache[0], nil
	} else {
		d, e := b.readDocumentsFromDisk(1, false, false)
		if len(d) < 1 {
			return "", e
		}
		return d[0], e
	}
}

/* Read
 * returns N documents from the cache and backing store without
 * removing them from either data structure.
 */
func (b *DocBuf) Read(count int) ([]string, error) {
	delta := count - len(b.cache)
	candidates := b.cache[:count - (delta)]

	if delta > 0 {
		fromStorage, err := b.readDocumentsFromDisk(delta, false, false)
		if err != nil {
			return candidates, err
		}

		candidates = append(candidates, fromStorage...)
	}

	return candidates, nil
}

/* Apply
 * Takes a callable/closure/function that accepts a string (document)
 * and returns a bool. Apply returns an error, or nil.
 * Apply will begin applying this callable to each doc from most recent
 * to least recent so long as the callable continues to return true.
 * If the callable returns false, Apply ceases iteration and returns.
 */
func (b *DocBuf) Apply(f func(string)bool) error {
	// iterate over internal cache applying function
	for _, i := range b.cache {
		if !f(i) {
			return nil
		}
	}

	// begin iterating with readDocumentsFromDisk
	first := true
	flag := true
	for flag {
		doc, err := b.readDocumentsFromDisk(1, false, !first)
		if err != nil {
			return err
		}
		if len(doc) == 0 {
			return nil
		} else if len(doc) != 1 {
			return errors.New("improper read from buffer")
		}

		first = false
		flag = f(doc[0])

		if flag {
			var c, d int64
			var err error
			// since continue is on we need to bring the cursor back one
			if c, err = b.store.Seek(0, io.SeekCurrent); err != nil {
				return err
			// dont seek earlier than 0
			} else if c < 1 {
				break
			}
			if d, err = b.store.Seek(-1, io.SeekCurrent); err != nil {
				return err
			}

			if c == d {
				return errors.New("Seek failure!")
			}
		}
	}

	return nil
}

/* Close
 * flushes and then prepares backing store for closure.
 * returns the end index into the underlying stream...
 * it is the callers responsibility to truncate.
 */
func (b *DocBuf) Close() (int64, error) {
	if err := b.Flush(); err != nil {
		return b.endIndex, err
	}

	return b.endIndex, nil
}

/* Cached
 * returns current length of cache
 */
 func (b *DocBuf) Cached() int {
	 return len(b.cache)
 }