package notify

import (
	"context"
	"sync"
)

type streamFlowController struct {
	mu             sync.Mutex
	queue          []*streamFlowRequest
	inFlightBytes  int
	inFlightChunks int
	windowBytes    int
	maxChunks      int
}

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

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

func (c *streamFlowController) applyConfig(cfg streamConfig) {
	if c == nil {
		return
	}
	cfg = normalizeStreamConfig(cfg)
	c.mu.Lock()
	c.windowBytes = cfg.OutboundWindowBytes
	c.maxChunks = cfg.OutboundMaxInFlightChunks
	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()
	}
	req := &streamFlowRequest{
		size:  size,
		ready: make(chan struct{}),
	}

	c.mu.Lock()
	c.queue = append(c.queue, req)
	c.drainLocked()
	c.mu.Unlock()

	select {
	case <-req.ready:
		released := false
		return func() {
			c.mu.Lock()
			if released {
				c.mu.Unlock()
				return
			}
			released = true
			c.inFlightBytes -= size
			if c.inFlightBytes < 0 {
				c.inFlightBytes = 0
			}
			if c.inFlightChunks > 0 {
				c.inFlightChunks--
			}
			c.drainLocked()
			c.mu.Unlock()
		}, nil
	case <-ctx.Done():
		c.mu.Lock()
		if req.admitted {
			c.mu.Unlock()
			released := false
			return func() {
				c.mu.Lock()
				if released {
					c.mu.Unlock()
					return
				}
				released = true
				c.inFlightBytes -= size
				if c.inFlightBytes < 0 {
					c.inFlightBytes = 0
				}
				if c.inFlightChunks > 0 {
					c.inFlightChunks--
				}
				c.drainLocked()
				c.mu.Unlock()
			}, nil
		}
		c.removeLocked(req)
		c.drainLocked()
		c.mu.Unlock()
		return nil, ctx.Err()
	}
}

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]
		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.maxChunks > 0 && c.inFlightChunks >= c.maxChunks {
			return
		}
		if !c.canAdmitLocked(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]
		req.admitted = true
		c.inFlightBytes += req.size
		c.inFlightChunks++
		close(req.ready)
	}
}

func (c *streamFlowController) canAdmitLocked(size int) bool {
	if c == nil {
		return true
	}
	if size <= 0 {
		return true
	}
	if c.windowBytes <= 0 {
		return true
	}
	if c.inFlightBytes+size <= c.windowBytes {
		return true
	}
	return c.inFlightBytes == 0 && c.inFlightChunks == 0
}