// Copyright 2020 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

package trie

import (
	"errors"
	"fmt"
	"sync"

	"github.com/ethereum/go-ethereum/common"
	"github.com/ethereum/go-ethereum/ethdb"
	"github.com/ethereum/go-ethereum/log"
	"github.com/ethereum/go-ethereum/rlp"
)

var ErrCommitDisabled = errors.New("no database for committing")

var stPool = sync.Pool{
	New: func() interface{} {
		return NewStackTrie(nil)
	},
}

func stackTrieFromPool(db ethdb.KeyValueStore) *StackTrie {
	st := stPool.Get().(*StackTrie)
	st.db = db
	return st
}

func returnToPool(st *StackTrie) {
	st.Reset()
	stPool.Put(st)
}

// StackTrie is a trie implementation that expects keys to be inserted
// in order. Once it determines that a subtree will no longer be inserted
// into, it will hash it and free up the memory it uses.
type StackTrie struct {
	nodeType  uint8          // node type (as in branch, ext, leaf)
	val       []byte         // value contained by this node if it's a leaf
	key       []byte         // key chunk covered by this (full|ext) node
	keyOffset int            // offset of the key chunk inside a full key
	children  [16]*StackTrie // list of children (for fullnodes and exts)

	db ethdb.KeyValueStore // Pointer to the commit db, can be nil
}

// NewStackTrie allocates and initializes an empty trie.
func NewStackTrie(db ethdb.KeyValueStore) *StackTrie {
	return &StackTrie{
		nodeType: emptyNode,
		db:       db,
	}
}

func newLeaf(ko int, key, val []byte, db ethdb.KeyValueStore) *StackTrie {
	st := stackTrieFromPool(db)
	st.nodeType = leafNode
	st.keyOffset = ko
	st.key = append(st.key, key[ko:]...)
	st.val = val
	return st
}

func newExt(ko int, key []byte, child *StackTrie, db ethdb.KeyValueStore) *StackTrie {
	st := stackTrieFromPool(db)
	st.nodeType = extNode
	st.keyOffset = ko
	st.key = append(st.key, key[ko:]...)
	st.children[0] = child
	return st
}

// List all values that StackTrie#nodeType can hold
const (
	emptyNode = iota
	branchNode
	extNode
	leafNode
	hashedNode
)

// TryUpdate inserts a (key, value) pair into the stack trie
func (st *StackTrie) TryUpdate(key, value []byte) error {
	k := keybytesToHex(key)
	if len(value) == 0 {
		panic("deletion not supported")
	}
	st.insert(k[:len(k)-1], value)
	return nil
}

func (st *StackTrie) Update(key, value []byte) {
	if err := st.TryUpdate(key, value); err != nil {
		log.Error(fmt.Sprintf("Unhandled trie error: %v", err))
	}
}

func (st *StackTrie) Reset() {
	st.db = nil
	st.key = st.key[:0]
	st.val = nil
	for i := range st.children {
		st.children[i] = nil
	}
	st.nodeType = emptyNode
	st.keyOffset = 0
}

// Helper function that, given a full key, determines the index
// at which the chunk pointed by st.keyOffset is different from
// the same chunk in the full key.
func (st *StackTrie) getDiffIndex(key []byte) int {
	diffindex := 0
	for ; diffindex < len(st.key) && st.key[diffindex] == key[st.keyOffset+diffindex]; diffindex++ {
	}
	return diffindex
}

// Helper function to that inserts a (key, value) pair into
// the trie.
func (st *StackTrie) insert(key, value []byte) {
	switch st.nodeType {
	case branchNode: /* Branch */
		idx := int(key[st.keyOffset])
		// Unresolve elder siblings
		for i := idx - 1; i >= 0; i-- {
			if st.children[i] != nil {
				if st.children[i].nodeType != hashedNode {
					st.children[i].hash()
				}
				break
			}
		}
		// Add new child
		if st.children[idx] == nil {
			st.children[idx] = stackTrieFromPool(st.db)
			st.children[idx].keyOffset = st.keyOffset + 1
		}
		st.children[idx].insert(key, value)
	case extNode: /* Ext */
		// Compare both key chunks and see where they differ
		diffidx := st.getDiffIndex(key)

		// Check if chunks are identical. If so, recurse into
		// the child node. Otherwise, the key has to be split
		// into 1) an optional common prefix, 2) the fullnode
		// representing the two differing path, and 3) a leaf
		// for each of the differentiated subtrees.
		if diffidx == len(st.key) {
			// Ext key and key segment are identical, recurse into
			// the child node.
			st.children[0].insert(key, value)
			return
		}
		// Save the original part. Depending if the break is
		// at the extension's last byte or not, create an
		// intermediate extension or use the extension's child
		// node directly.
		var n *StackTrie
		if diffidx < len(st.key)-1 {
			n = newExt(diffidx+1, st.key, st.children[0], st.db)
		} else {
			// Break on the last byte, no need to insert
			// an extension node: reuse the current node
			n = st.children[0]
		}
		// Convert to hash
		n.hash()
		var p *StackTrie
		if diffidx == 0 {
			// the break is on the first byte, so
			// the current node is converted into
			// a branch node.
			st.children[0] = nil
			p = st
			st.nodeType = branchNode
		} else {
			// the common prefix is at least one byte
			// long, insert a new intermediate branch
			// node.
			st.children[0] = stackTrieFromPool(st.db)
			st.children[0].nodeType = branchNode
			st.children[0].keyOffset = st.keyOffset + diffidx
			p = st.children[0]
		}
		// Create a leaf for the inserted part
		o := newLeaf(st.keyOffset+diffidx+1, key, value, st.db)

		// Insert both child leaves where they belong:
		origIdx := st.key[diffidx]
		newIdx := key[diffidx+st.keyOffset]
		p.children[origIdx] = n
		p.children[newIdx] = o
		st.key = st.key[:diffidx]

	case leafNode: /* Leaf */
		// Compare both key chunks and see where they differ
		diffidx := st.getDiffIndex(key)

		// Overwriting a key isn't supported, which means that
		// the current leaf is expected to be split into 1) an
		// optional extension for the common prefix of these 2
		// keys, 2) a fullnode selecting the path on which the
		// keys differ, and 3) one leaf for the differentiated
		// component of each key.
		if diffidx >= len(st.key) {
			panic("Trying to insert into existing key")
		}

		// Check if the split occurs at the first nibble of the
		// chunk. In that case, no prefix extnode is necessary.
		// Otherwise, create that
		var p *StackTrie
		if diffidx == 0 {
			// Convert current leaf into a branch
			st.nodeType = branchNode
			p = st
			st.children[0] = nil
		} else {
			// Convert current node into an ext,
			// and insert a child branch node.
			st.nodeType = extNode
			st.children[0] = NewStackTrie(st.db)
			st.children[0].nodeType = branchNode
			st.children[0].keyOffset = st.keyOffset + diffidx
			p = st.children[0]
		}

		// Create the two child leaves: the one containing the
		// original value and the one containing the new value
		// The child leave will be hashed directly in order to
		// free up some memory.
		origIdx := st.key[diffidx]
		p.children[origIdx] = newLeaf(diffidx+1, st.key, st.val, st.db)
		p.children[origIdx].hash()

		newIdx := key[diffidx+st.keyOffset]
		p.children[newIdx] = newLeaf(p.keyOffset+1, key, value, st.db)

		// Finally, cut off the key part that has been passed
		// over to the children.
		st.key = st.key[:diffidx]
		st.val = nil
	case emptyNode: /* Empty */
		st.nodeType = leafNode
		st.key = key[st.keyOffset:]
		st.val = value
	case hashedNode:
		panic("trying to insert into hash")
	default:
		panic("invalid type")
	}
}

// hash() hashes the node 'st' and converts it into 'hashedNode', if possible.
// Possible outcomes:
// 1. The rlp-encoded value was >= 32 bytes:
//  - Then the 32-byte `hash` will be accessible in `st.val`.
//  - And the 'st.type' will be 'hashedNode'
// 2. The rlp-encoded value was < 32 bytes
//  - Then the <32 byte rlp-encoded value will be accessible in 'st.val'.
//  - And the 'st.type' will be 'hashedNode' AGAIN
//
// This method will also:
// set 'st.type' to hashedNode
// clear 'st.key'
func (st *StackTrie) hash() {
	/* Shortcut if node is already hashed */
	if st.nodeType == hashedNode {
		return
	}
	// The 'hasher' is taken from a pool, but we don't actually
	// claim an instance until all children are done with their hashing,
	// and we actually need one
	var h *hasher

	switch st.nodeType {
	case branchNode:
		var nodes [17]node
		for i, child := range st.children {
			if child == nil {
				nodes[i] = nilValueNode
				continue
			}
			child.hash()
			if len(child.val) < 32 {
				nodes[i] = rawNode(child.val)
			} else {
				nodes[i] = hashNode(child.val)
			}
			st.children[i] = nil // Reclaim mem from subtree
			returnToPool(child)
		}
		nodes[16] = nilValueNode
		h = newHasher(false)
		defer returnHasherToPool(h)
		h.tmp.Reset()
		if err := rlp.Encode(&h.tmp, nodes); err != nil {
			panic(err)
		}
	case extNode:
		h = newHasher(false)
		defer returnHasherToPool(h)
		h.tmp.Reset()
		st.children[0].hash()
		// This is also possible:
		//sz := hexToCompactInPlace(st.key)
		//n := [][]byte{
		//	st.key[:sz],
		//	st.children[0].val,
		//}
		n := [][]byte{
			hexToCompact(st.key),
			st.children[0].val,
		}
		if err := rlp.Encode(&h.tmp, n); err != nil {
			panic(err)
		}
		returnToPool(st.children[0])
		st.children[0] = nil // Reclaim mem from subtree
	case leafNode:
		h = newHasher(false)
		defer returnHasherToPool(h)
		h.tmp.Reset()
		st.key = append(st.key, byte(16))
		sz := hexToCompactInPlace(st.key)
		n := [][]byte{st.key[:sz], st.val}
		if err := rlp.Encode(&h.tmp, n); err != nil {
			panic(err)
		}
	case emptyNode:
		st.val = st.val[:0]
		st.val = append(st.val, emptyRoot[:]...)
		st.key = st.key[:0]
		st.nodeType = hashedNode
		return
	default:
		panic("Invalid node type")
	}
	st.key = st.key[:0]
	st.nodeType = hashedNode
	if len(h.tmp) < 32 {
		st.val = st.val[:0]
		st.val = append(st.val, h.tmp...)
		return
	}
	// Going to write the hash to the 'val'. Need to ensure it's properly sized first
	// Typically, 'branchNode's will have no 'val', and require this allocation
	if required := 32 - len(st.val); required > 0 {
		buf := make([]byte, required)
		st.val = append(st.val, buf...)
	}
	st.val = st.val[:32]
	h.sha.Reset()
	h.sha.Write(h.tmp)
	h.sha.Read(st.val)
	if st.db != nil {
		// TODO! Is it safe to Put the slice here?
		// Do all db implementations copy the value provided?
		st.db.Put(st.val, h.tmp)
	}
}

// Hash returns the hash of the current node
func (st *StackTrie) Hash() (h common.Hash) {
	st.hash()
	if len(st.val) != 32 {
		// If the node's RLP isn't 32 bytes long, the node will not
		// be hashed, and instead contain the  rlp-encoding of the
		// node. For the top level node, we need to force the hashing.
		ret := make([]byte, 32)
		h := newHasher(false)
		defer returnHasherToPool(h)
		h.sha.Reset()
		h.sha.Write(st.val)
		h.sha.Read(ret)
		return common.BytesToHash(ret)
	}
	return common.BytesToHash(st.val)
}

// Commit will firstly hash the entrie trie if it's still not hashed
// and then commit all nodes to the associated database. Actually most
// of the trie nodes MAY have been committed already. The main purpose
// here is to commit the root node.
//
// The associated database is expected, otherwise the whole commit
// functionality should be disabled.
func (st *StackTrie) Commit() (common.Hash, error) {
	if st.db == nil {
		return common.Hash{}, ErrCommitDisabled
	}
	st.hash()
	h := common.BytesToHash(st.val)
	return h, nil
}