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

package mgl32

import (
	"sync"
)

var (
	slicePools []*sync.Pool
	listLock   sync.RWMutex
)

var shouldPool = true

func DisableMemoryPooling() {
	shouldPool = false
}

// Returns the given memory pool. If the pool doesn't exist, it will
// create all pools up to element i. The number "i" corresponds to "p"
// in most other comments. That is, it's Ceil(log_2(whatever)). So i=0
// means you'll get the pool for slices of size 1, i=1 for size 2, i=2 for size 4,
// and so on.
//
// This is concurrency safe and uses an RWMutex to protect the list expansion.
func getPool(i int) *sync.Pool {
	listLock.RLock()
	if i >= len(slicePools) {

		// Promote to a write lock because we now
		// need to mutate the pool
		listLock.RUnlock()
		listLock.Lock()
		defer listLock.Unlock()

		for n := i - len(slicePools); n >= 0; n-- {
			newFunc := genPoolNew(1 << uint(len(slicePools)))
			slicePools = append(slicePools, &sync.Pool{New: newFunc})
		}
	} else {
		defer listLock.RUnlock()
	}

	return slicePools[i]
}

func genPoolNew(i int) func() interface{} {
	return func() interface{} {
		return make([]float32, 0, i)
	}
}

// Grabs a slice from the memory pool, such that its cap
// is 2^p where p is Ceil(log_2(size)). It will be downsliced
// such that the len is size.
func grabFromPool(size int) []float32 {
	pool, exact := binLog(size)

	// Tried to grab something of size
	// zero or less
	if pool == -1 {
		return nil
	}

	// If the log is not exact, we
	// need to "overallocate" so we have
	// log+1
	if !exact {
		pool++
	}

	slice := getPool(pool).Get().([]float32)
	slice = slice[:size]
	return slice
}

// Returns a slice to the appropriate pool. If the slice does not have a cap that's precisely
// a power of 2, this will panic.
func returnToPool(slice []float32) {
	if cap(slice) == 0 {
		return
	}

	pool, exact := binLog(cap(slice))

	if !exact {
		panic("attempt to pool slice with non-exact cap. If you're a user, please file an issue with github.com/go-gl/mathgl about this bug. This should never happen.")
	}

	getPool(pool).Put(slice)
}

// This returns the integer base 2 log of the value
// and whether the log is exact or rounded down.
//
// This is only for positive integers.
//
// There are faster ways to do this, I'm open to suggestions. Most rely on knowing system endianness
// which Go makes hard to do. I'm hesistant to use float conversions and the math package because of off-by-one errors.
func binLog(val int) (int, bool) {
	if val <= 0 {
		return -1, false
	}

	exact := true
	l := 0
	for ; val > 1; val = val >> 1 {
		// If the current lsb is 1 and the number
		// is not equal to 1, this is not an exact
		// log, but rather a rounding of it
		if val&1 != 0 {
			exact = false
		}
		l++
	}

	return l, exact
}