package deque

// minCapacity is the smallest capacity that deque may have.
// Must be power of 2 for bitwise modulus: x % n == x & (n - 1).
const minCapacity = 16

// Deque represents a single instance of the deque data structure.
type Deque struct {
	buf    []int
	head   int
	tail   int
	count  int
	minCap int
}

// New creates a new Deque, optionally setting the current and minimum capacity
// when non-zero values are given for these.
//
// To create a Deque with capacity to store 2048 items without resizing, and
// that will not resize below space for 32 items when removing itmes:
//   d := deque.New(2048, 32)
//
// To create a Deque that has not yet allocated memory, but after it does will
// never resize to have space for less than 64 items:
//   d := deque.New(0, 64)
//
// Note that any values supplied here are rounded up to the nearest power of 2.
func New(size ...int) *Deque {
	var capacity, minimum int
	if len(size) >= 1 {
		capacity = size[0]
		if len(size) >= 2 {
			minimum = size[1]
		}
	}

	minCap := minCapacity
	for minCap < minimum {
		minCap <<= 1
	}

	var buf []int
	if capacity != 0 {
		bufSize := minCap
		for bufSize < capacity {
			bufSize <<= 1
		}
		buf = make([]int, bufSize)
	}

	return &Deque{
		buf:    buf,
		minCap: minCap,
	}
}

// Cap returns the current capacity of the Deque.
func (q *Deque) Cap() int {
	return len(q.buf)
}

// Len returns the number of elements currently stored in the queue.
func (q *Deque) Len() int {
	return q.count
}

// PushBack appends an element to the back of the queue.  Implements FIFO when
// elements are removed with PopFront(), and LIFO when elements are removed
// with PopBack().
func (q *Deque) PushBack(elem int) {
	q.growIfFull()

	q.buf[q.tail] = elem
	// Calculate new tail position.
	q.tail = q.next(q.tail)
	q.count++
}

// PushFront prepends an element to the front of the queue.
func (q *Deque) PushFront(elem int) {
	q.growIfFull()

	// Calculate new head position.
	q.head = q.prev(q.head)
	q.buf[q.head] = elem
	q.count++
}

// PopFront removes and returns the element from the front of the queue.
// Implements FIFO when used with PushBack().  If the queue is empty, the call
// panics.
func (q *Deque) PopFront() int {
	if q.count <= 0 {
		panic("deque: PopFront() called on empty queue")
	}
	ret := q.buf[q.head]
	// Calculate new head position.
	q.head = q.next(q.head)
	q.count--

	q.shrinkIfExcess()
	return ret
}

// PopBack removes and returns the element from the back of the queue.
// Implements LIFO when used with PushBack().  If the queue is empty, the call
// panics.
func (q *Deque) PopBack() int {
	if q.count <= 0 {
		panic("deque: PopBack() called on empty queue")
	}

	// Calculate new tail position
	q.tail = q.prev(q.tail)

	// Remove value at tail.
	ret := q.buf[q.tail]
	q.count--

	q.shrinkIfExcess()
	return ret
}

// Front returns the element at the front of the queue.  This is the element
// that would be returned by PopFront().  This call panics if the queue is
// empty.
func (q *Deque) Front() int {
	if q.count <= 0 {
		panic("deque: Front() called when empty")
	}
	return q.buf[q.head]
}

// Back returns the element at the back of the queue.  This is the element
// that would be returned by PopBack().  This call panics if the queue is
// empty.
func (q *Deque) Back() int {
	if q.count <= 0 {
		panic("deque: Back() called when empty")
	}
	return q.buf[q.prev(q.tail)]
}

// Clear removes all elements from the queue, but retains the current capacity.
// This is useful when repeatedly reusing the queue at high frequency to avoid
// GC during reuse.  The queue will not be resized smaller as long as items are
// only added.  Only when items are removed is the queue subject to getting
// resized smaller.
func (q *Deque) Clear() {
	// bitwise modulus
	modBits := len(q.buf) - 1
	for h := q.head; h != q.tail; h = (h + 1) & modBits {
		q.buf[h] = 0
	}
	q.head = 0
	q.tail = 0
	q.count = 0
}

// Reset sets properties to empty, but does not rewrite the buffer
func (q *Deque) Reset() {
	q.head = 0
	q.tail = 0
	q.count = 0
}

// Rotate rotates the deque n steps front-to-back.  If n is negative, rotates
// back-to-front.  Having Deque provide Rotate() avoids resizing that could
// happen if implementing rotation using only Pop and Push methods.
func (q *Deque) Rotate(n int) {
	if q.count <= 1 {
		return
	}
	// Rotating a multiple of q.count is same as no rotation.
	n %= q.count
	if n == 0 {
		return
	}

	modBits := len(q.buf) - 1
	// If no empty space in buffer, only move head and tail indexes.
	if q.head == q.tail {
		// Calculate new head and tail using bitwise modulus.
		q.head = (q.head + n) & modBits
		q.tail = (q.tail + n) & modBits
		return
	}

	if n < 0 {
		// Rotate back to front.
		for ; n < 0; n++ {
			// Calculate new head and tail using bitwise modulus.
			q.head = (q.head - 1) & modBits
			q.tail = (q.tail - 1) & modBits
			// Put tail value at head and remove value at tail.
			q.buf[q.head] = q.buf[q.tail]
		}
		return
	}

	// Rotate front to back.
	for ; n > 0; n-- {
		// Put head value at tail and remove value at head.
		q.buf[q.tail] = q.buf[q.head]
		// Calculate new head and tail using bitwise modulus.
		q.head = (q.head + 1) & modBits
		q.tail = (q.tail + 1) & modBits
	}
}

// SetMinCapacity sets a minimum capacity of 2^minCapacityExp.  If the value of
// the minimum capacity is less than or equal to the minimum allowed, then
// capacity is set to the minimum allowed.  This may be called at anytime to
// set a new minimum capacity.
//
// Setting a larger minimum capacity may be used to prevent resizing when the
// number of stored items changes frequently across a wide range.
func (q *Deque) SetMinCapacity(minCapacityExp uint) {
	if 1<<minCapacityExp > minCapacity {
		q.minCap = 1 << minCapacityExp
	} else {
		q.minCap = minCapacity
	}
}

// prev returns the previous buffer position wrapping around buffer.
func (q *Deque) prev(i int) int {
	return (i - 1) & (len(q.buf) - 1) // bitwise modulus
}

// next returns the next buffer position wrapping around buffer.
func (q *Deque) next(i int) int {
	return (i + 1) & (len(q.buf) - 1) // bitwise modulus
}

// growIfFull resizes up if the buffer is full.
func (q *Deque) growIfFull() {
	if q.count != len(q.buf) {
		return
	}
	if len(q.buf) == 0 {
		if q.minCap == 0 {
			q.minCap = minCapacity
		}
		q.buf = make([]int, q.minCap)
		return
	}
	q.resize()
}

// shrinkIfExcess resize down if the buffer 1/4 full.
func (q *Deque) shrinkIfExcess() {
	if len(q.buf) > q.minCap && (q.count<<2) == len(q.buf) {
		q.resize()
	}
}

// resize resizes the deque to fit exactly twice its current contents.  This is
// used to grow the queue when it is full, and also to shrink it when it is
// only a quarter full.
func (q *Deque) resize() {
	newBuf := make([]int, q.count<<1)
	if q.tail > q.head {
		copy(newBuf, q.buf[q.head:q.tail])
	} else {
		n := copy(newBuf, q.buf[q.head:])
		copy(newBuf[n:], q.buf[:q.tail])
	}

	q.head = 0
	q.tail = q.count
	q.buf = newBuf
}