// Copyright (c) 2019 The BFE Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Flow control

package bfe_spdy

import (
	"fmt"
)

// flow is the flow control window's size.
type flow struct {
	// n is the number of DATA bytes we're allowed to send.
	// A flow is kept both on a conn and a per-stream.
	n int32

	// conn points to the shared connection-level flow that is
	// shared by all streams on that conn. It is nil for the flow
	// that's on the conn directly.
	conn *flow
}

func (f *flow) setConnFlow(cf *flow) { f.conn = cf }

func (f *flow) available() int32 {
	n := f.n
	if f.conn != nil && f.conn.n < n {
		n = f.conn.n
	}
	return n
}

func (f *flow) take(n int32) {
	if n > f.available() {
		panic("internal error: took too much")
	}
	f.n -= n
	if f.conn != nil {
		f.conn.n -= n
	}
}

// add adds n bytes (positive or negative) to the flow control window.
// It returns false if the sum would exceed 2^31-1.
func (f *flow) add(n int32) bool {
	remain := (1<<31 - 1) - f.n
	if n > remain {
		return false
	}
	f.n += n
	return true
}

func (f *flow) String() string {
	if f.conn != nil {
		return fmt.Sprintf("flow(stream:%d, conn:%d)", f.n, f.conn.n)
	}
	return fmt.Sprintf("flow(conn:%d)", f.n)
}
