package notify

import (
	"context"
	"sync"
	"sync/atomic"
)

type streamFlowController struct {
	mu sync.Mutex

	queue []*streamFlowRequest

	inFlightBytes  atomic.Int64
	inFlightChunks atomic.Int64
	windowBytes    atomic.Int64
	maxChunks      atomic.Int64
	waiters        atomic.Int32
}

type streamFlowRequest struct {
	size     int
	ready    chan struct{}
	admitted bool
}

func newStreamFlowController(cfg streamConfig) *streamFlowController {
	cfg = normalizeStreamConfig(cfg)
	controller := &streamFlowController{}
	controller.windowBytes.Store(int64(cfg.OutboundWindowBytes))
	controller.maxChunks.Store(int64(cfg.OutboundMaxInFlightChunks))
	return controller
}

func (c *streamFlowController) applyConfig(cfg streamConfig) {
	if c == nil {
		return
	}
	cfg = normalizeStreamConfig(cfg)
	c.windowBytes.Store(int64(cfg.OutboundWindowBytes))
	c.maxChunks.Store(int64(cfg.OutboundMaxInFlightChunks))
	if c.waiters.Load() == 0 {
		return
	}
	c.mu.Lock()
	c.drainLocked()
	c.mu.Unlock()
}

func (c *streamFlowController) acquire(ctx context.Context, size int) (func(), error) {
	if c == nil || size <= 0 {
		return func() {}, nil
	}
	if ctx == nil {
		ctx = context.Background()
	}
	if c.tryAcquire(size) {
		return c.releaseFunc(size), nil
	}
	req := &streamFlowRequest{
		size:  size,
		ready: make(chan struct{}),
	}

	c.mu.Lock()
	if c.tryAcquireLocked(size) {
		c.mu.Unlock()
		return c.releaseFunc(size), nil
	}
	c.queue = append(c.queue, req)
	c.waiters.Add(1)
	c.drainLocked()
	c.mu.Unlock()

	select {
	case <-req.ready:
		return c.releaseFunc(size), nil
	case <-ctx.Done():
		c.mu.Lock()
		if req.admitted {
			c.mu.Unlock()
			return c.releaseFunc(size), nil
		}
		c.removeLocked(req)
		c.drainLocked()
		c.mu.Unlock()
		return nil, ctx.Err()
	}
}

func (c *streamFlowController) tryAcquire(size int) bool {
	if c == nil || size <= 0 {
		return true
	}
	if c.waiters.Load() != 0 {
		return false
	}
	return c.tryAcquireCAS(size)
}

func (c *streamFlowController) tryAcquireLocked(size int) bool {
	if c == nil || size <= 0 {
		return true
	}
	if len(c.queue) != 0 {
		return false
	}
	return c.tryAcquireCAS(size)
}

func (c *streamFlowController) tryAcquireCAS(size int) bool {
	if c == nil || size <= 0 {
		return true
	}
	size64 := int64(size)
	for {
		window := c.windowBytes.Load()
		maxChunks := c.maxChunks.Load()
		inFlightBytes := c.inFlightBytes.Load()
		inFlightChunks := c.inFlightChunks.Load()

		if maxChunks > 0 && inFlightChunks >= maxChunks {
			return false
		}
		if window > 0 && inFlightBytes+size64 > window {
			if !(inFlightBytes == 0 && inFlightChunks == 0) {
				return false
			}
		}
		if !c.inFlightBytes.CompareAndSwap(inFlightBytes, inFlightBytes+size64) {
			continue
		}
		if c.addChunksCAS(1, maxChunks) {
			return true
		}
		c.subBytesCAS(size64)
		return false
	}
}

func (c *streamFlowController) addChunksCAS(delta int64, maxChunks int64) bool {
	if c == nil || delta <= 0 {
		return true
	}
	for {
		current := c.inFlightChunks.Load()
		if maxChunks > 0 && current+delta > maxChunks {
			return false
		}
		if c.inFlightChunks.CompareAndSwap(current, current+delta) {
			return true
		}
	}
}

func (c *streamFlowController) subBytesCAS(delta int64) {
	if c == nil || delta <= 0 {
		return
	}
	for {
		current := c.inFlightBytes.Load()
		next := current - delta
		if next < 0 {
			next = 0
		}
		if c.inFlightBytes.CompareAndSwap(current, next) {
			return
		}
	}
}

func (c *streamFlowController) subChunksCAS(delta int64) {
	if c == nil || delta <= 0 {
		return
	}
	for {
		current := c.inFlightChunks.Load()
		next := current - delta
		if next < 0 {
			next = 0
		}
		if c.inFlightChunks.CompareAndSwap(current, next) {
			return
		}
	}
}

func (c *streamFlowController) releaseFunc(size int) func() {
	released := false
	return func() {
		if released {
			return
		}
		released = true
		c.release(size)
	}
}

func (c *streamFlowController) release(size int) {
	if c == nil || size <= 0 {
		return
	}
	c.subBytesCAS(int64(size))
	c.subChunksCAS(1)
	if c.waiters.Load() == 0 {
		return
	}
	c.mu.Lock()
	c.drainLocked()
	c.mu.Unlock()
}

func (c *streamFlowController) removeLocked(req *streamFlowRequest) {
	if c == nil || req == nil {
		return
	}
	for i, item := range c.queue {
		if item != req {
			continue
		}
		copy(c.queue[i:], c.queue[i+1:])
		c.queue[len(c.queue)-1] = nil
		c.queue = c.queue[:len(c.queue)-1]
		c.waiters.Add(-1)
		return
	}
}

func (c *streamFlowController) drainLocked() {
	if c == nil {
		return
	}
	for len(c.queue) > 0 {
		req := c.queue[0]
		if req == nil {
			c.queue = c.queue[1:]
			continue
		}
		if !c.canAdmitLocked(req.size) {
			return
		}
		if !c.tryAcquireCAS(req.size) {
			return
		}
		copy(c.queue[0:], c.queue[1:])
		c.queue[len(c.queue)-1] = nil
		c.queue = c.queue[:len(c.queue)-1]
		c.waiters.Add(-1)
		req.admitted = true
		close(req.ready)
	}
}

func (c *streamFlowController) canAdmitLocked(size int) bool {
	if c == nil {
		return true
	}
	if size <= 0 {
		return true
	}
	window := c.windowBytes.Load()
	chunks := c.inFlightChunks.Load()
	bytes := c.inFlightBytes.Load()
	maxChunks := c.maxChunks.Load()
	if maxChunks > 0 && chunks >= maxChunks {
		return false
	}
	if window <= 0 {
		return true
	}
	if bytes+int64(size) <= window {
		return true
	}
	return bytes == 0 && chunks == 0
}