// Copyright (C) 2018 Marius Schellenberger

package filter

import (
	"math/rand"
	"sync"
)

// DefaultCacheSize is the default cache size
const DefaultCacheSize = 100

// QueryCache can be used to cache queries
type QueryCache struct {
	// protects m
	sync.RWMutex

	m    map[string]*Query
	size int
}

// NewQueryCache returns a new QueryCache
func NewQueryCache(size int) *QueryCache {
	if size <= 0 {
		size = DefaultCacheSize
	}
	return &QueryCache{
		m:    make(map[string]*Query),
		size: size,
	}
}

// NewQuery looks for the given query in the cache and returns it.
// If the query is not found it will be generated and inserted in the cache..
func (qc *QueryCache) NewQuery(s string) (q *Query, err error) {
	q = qc.Lookup(s)
	if q != nil {
		return
	}
	q, err = NewQuery(s)
	if err != nil {
		return
	}
	qc.Lock()
	defer qc.Unlock()
	if len(qc.m) == qc.size {
		qc.clean()
	}
	qc.m[s] = q
	return
}

// Lookup looks for a given query string and returns the query object or nil.
func (qc *QueryCache) Lookup(s string) *Query {
	qc.RLock()
	defer qc.RUnlock()
	if q, ok := qc.m[s]; ok {
		return q
	}
	return nil
}

func (qc *QueryCache) clean() {
	i := rand.Intn(len(qc.m))
	var k string
	for k = range qc.m {
		if i == 0 {
			break
		}
		i--
	}
	delete(qc.m, k)
}