package txscript

import (
	"sync"
	
	"github.com/p9c/p9/pkg/chainhash"
	"github.com/p9c/p9/pkg/ecc"
)

// sigCacheEntry represents an entry in the SigCache. Entries within the SigCache are keyed according to the sigHash of
// the signature. In the scenario of a cache-hit (according to the sigHash), an additional comparison of the signature,
// and public key will be executed in order to ensure a complete match. In the occasion that two sigHashes collide, the
// newer sigHash will simply overwrite the existing entry.
type sigCacheEntry struct {
	sig    *ecc.Signature
	pubKey *ecc.PublicKey
}

// SigCache implements an ECDSA signature verification cache with a randomized entry eviction policy. Only valid
// signatures will be added to the cache. The benefits of SigCache are two fold. Firstly, usage of SigCache mitigates a
// DoS attack wherein an attack causes a victim's client to hang due to worst-case behavior triggered while processing
// attacker crafted invalid transactions. A detailed description of the mitigated DoS attack can be found here:
// https://bitslog.wordpress.com/2013/01/23/fixed-bitcoin-vulnerability-explanation-why-the-signature-cache-is-a-dos-protection/.
// Secondly, usage of the SigCache introduces a signature verification optimization which speeds up the validation of
// transactions within a block, if they've already been seen and verified within the mempool.
type SigCache struct {
	sync.RWMutex
	validSigs  map[chainhash.Hash]sigCacheEntry
	maxEntries uint
}

// NewSigCache creates and initializes a new instance of SigCache. Its sole parameter 'maxEntries' represents the
// maximum number of entries allowed to exist in the SigCache at any particular moment. Random entries are evicted make
// room for new entries that would cause the number of entries in the cache to exceed the max.
func NewSigCache(maxEntries uint) *SigCache {
	return &SigCache{
		validSigs:  make(map[chainhash.Hash]sigCacheEntry, maxEntries),
		maxEntries: maxEntries,
	}
}

// Exists returns true if an existing entry of 'sig' over 'sigHash' for public key 'pubKey' is found within the
// SigCache. Otherwise, false is returned. NOTE: This function is safe for concurrent access. Readers won't be blocked
// unless there exists a writer, adding an entry to the SigCache.
func (s *SigCache) Exists(sigHash chainhash.Hash, sig *ecc.Signature, pubKey *ecc.PublicKey) bool {
	s.RLock()
	entry, ok := s.validSigs[sigHash]
	s.RUnlock()
	return ok && entry.pubKey.IsEqual(pubKey) && entry.sig.IsEqual(sig)
}

// Add adds an entry for a signature over 'sigHash' under public key 'pubKey' to the signature cache. In the event that
// the SigCache is 'full', an existing entry is randomly chosen to be evicted in order to make space for the new entry.
// NOTE: This function is safe for concurrent access. Writers will block simultaneous readers until function execution
// has concluded.
func (s *SigCache) Add(sigHash chainhash.Hash, sig *ecc.Signature, pubKey *ecc.PublicKey) {
	s.Lock()
	defer s.Unlock()
	if s.maxEntries <= 0 {
		return
	}
	// If adding this new entry will put us over the max number of allowed entries, then evict an entry.
	if uint(len(s.validSigs)+1) > s.maxEntries {
		// Remove a random entry from the map. Relying on the random starting point of Go's map iteration. It's worth
		// noting that the random iteration starting point is not 100% guaranteed by the spec, however most Go compilers
		// support it. Ultimately, the iteration order isn't important here because in order to manipulate which items
		// are evicted, an adversary would need to be able to execute preimage attacks on the hashing function in order
		// to start eviction at a specific entry.
		for sigEntry := range s.validSigs {
			delete(s.validSigs, sigEntry)
			break
		}
	}
	s.validSigs[sigHash] = sigCacheEntry{sig, pubKey}
}