# Copyright (c) 2018, Neil Booth
#
# All rights reserved.
#
# The MIT License (MIT)
#
# Permission is hereby granted, free of charge, to any person obtaining
# a copy of this software and associated documentation files (the
# "Software"), to deal in the Software without restriction, including
# without limitation the rights to use, copy, modify, merge, publish,
# distribute, sublicense, and/or sell copies of the Software, and to
# permit persons to whom the Software is furnished to do so, subject to
# the following conditions:
#
# The above copyright notice and this permission notice shall be
# included in all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
# LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
# OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
# WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
# and warranty status of this software.

"""Merkle trees, branches, proofs and roots."""

from asyncio import Event
from math import ceil, log

from scribe.common import double_sha256


class Merkle:
    """Perform merkle tree calculations on binary hashes using a given hash
    function.

    If the hash count is not even, the final hash is repeated when
    calculating the next merkle layer up the tree.
    """

    def __init__(self, hash_func=double_sha256):
        self.hash_func = hash_func

    @staticmethod
    def tree_depth(hash_count):
        return Merkle.branch_length(hash_count) + 1

    @staticmethod
    def branch_length(hash_count):
        """Return the length of a merkle branch given the number of hashes."""
        if not isinstance(hash_count, int):
            raise TypeError('hash_count must be an integer')
        if hash_count < 1:
            raise ValueError('hash_count must be at least 1')
        return ceil(log(hash_count, 2))

    @staticmethod
    def branch_and_root(hashes, index, length=None, hash_func=double_sha256):
        """Return a (merkle branch, merkle_root) pair given hashes, and the
        index of one of those hashes.
        """
        hashes = list(hashes)
        if not isinstance(index, int):
            raise TypeError('index must be an integer')
        # This also asserts hashes is not empty
        if not 0 <= index < len(hashes):
            raise ValueError(f"index '{index}/{len(hashes)}' out of range")
        natural_length = Merkle.branch_length(len(hashes))
        if length is None:
            length = natural_length
        else:
            if not isinstance(length, int):
                raise TypeError('length must be an integer')
            if length < natural_length:
                raise ValueError('length out of range')

        branch = []
        for _ in range(length):
            if len(hashes) & 1:
                hashes.append(hashes[-1])
            branch.append(hashes[index ^ 1])
            index >>= 1
            hashes = [hash_func(hashes[n] + hashes[n + 1])
                      for n in range(0, len(hashes), 2)]

        return branch, hashes[0]

    @staticmethod
    def root(hashes, length=None):
        """Return the merkle root of a non-empty iterable of binary hashes."""
        branch, root = Merkle.branch_and_root(hashes, 0, length)
        return root

    # @staticmethod
    # def root_from_proof(hash, branch, index, hash_func=double_sha256):
    #     """Return the merkle root given a hash, a merkle branch to it, and
    #     its index in the hashes array.
    #
    #     branch is an iterable sorted deepest to shallowest.  If the
    #     returned root is the expected value then the merkle proof is
    #     verified.
    #
    #     The caller should have confirmed the length of the branch with
    #     branch_length().  Unfortunately this is not easily done for
    #     bitcoin transactions as the number of transactions in a block
    #     is unknown to an SPV client.
    #     """
    #     for elt in branch:
    #         if index & 1:
    #             hash = hash_func(elt + hash)
    #         else:
    #             hash = hash_func(hash + elt)
    #         index >>= 1
    #     if index:
    #         raise ValueError('index out of range for branch')
    #     return hash

    @staticmethod
    def level(hashes, depth_higher):
        """Return a level of the merkle tree of hashes the given depth
        higher than the bottom row of the original tree."""
        size = 1 << depth_higher
        root = Merkle.root
        return [root(hashes[n: n + size], depth_higher)
                for n in range(0, len(hashes), size)]

    @staticmethod
    def branch_and_root_from_level(level, leaf_hashes, index,
                                   depth_higher):
        """Return a (merkle branch, merkle_root) pair when a merkle-tree has a
        level cached.

        To maximally reduce the amount of data hashed in computing a
        markle branch, cache a tree of depth N at level N // 2.

        level is a list of hashes in the middle of the tree (returned
        by level())

        leaf_hashes are the leaves needed to calculate a partial branch
        up to level.

        depth_higher is how much higher level is than the leaves of the tree

        index is the index in the full list of hashes of the hash whose
        merkle branch we want.
        """
        if not isinstance(level, list):
            raise TypeError("level must be a list")
        if not isinstance(leaf_hashes, list):
            raise TypeError("leaf_hashes must be a list")
        leaf_index = (index >> depth_higher) << depth_higher
        leaf_branch, leaf_root = Merkle.branch_and_root(
            leaf_hashes, index - leaf_index, depth_higher)
        index >>= depth_higher
        level_branch, root = Merkle.branch_and_root(level, index)
        # Check last so that we know index is in-range
        if leaf_root != level[index]:
            raise ValueError('leaf hashes inconsistent with level')
        return leaf_branch + level_branch, root


class MerkleCache:
    """A cache to calculate merkle branches efficiently."""

    def __init__(self, merkle, source_func):
        """Initialise a cache hashes taken from source_func:

           async def source_func(index, count):
              ...
        """
        self.merkle = merkle
        self.source_func = source_func
        self.length = 0
        self.depth_higher = 0
        self.initialized = Event()

    def _segment_length(self):
        return 1 << self.depth_higher

    def _leaf_start(self, index):
        """Given a level's depth higher and a hash index, return the leaf
        index and leaf hash count needed to calculate a merkle branch.
        """
        depth_higher = self.depth_higher
        return (index >> depth_higher) << depth_higher

    def _level(self, hashes):
        return self.merkle.level(hashes, self.depth_higher)

    async def _extend_to(self, length):
        """Extend the length of the cache if necessary."""
        if length <= self.length:
            return
        # Start from the beginning of any final partial segment.
        # Retain the value of depth_higher; in practice this is fine
        start = self._leaf_start(self.length)
        hashes = await self.source_func(start, length - start)
        self.level[start >> self.depth_higher:] = self._level(hashes)
        self.length = length

    async def _level_for(self, length):
        """Return a (level_length, final_hash) pair for a truncation
        of the hashes to the given length."""
        if length == self.length:
            return self.level
        level = self.level[:length >> self.depth_higher]
        leaf_start = self._leaf_start(length)
        count = min(self._segment_length(), length - leaf_start)
        hashes = await self.source_func(leaf_start, count)
        level += self._level(hashes)
        return level

    async def initialize(self, length):
        """Call to initialize the cache to a source of given length."""
        self.length = length
        self.depth_higher = self.merkle.tree_depth(length) // 2
        self.level = self._level(await self.source_func(0, length))
        self.initialized.set()

    def truncate(self, length):
        """Truncate the cache so it covers no more than length underlying
        hashes."""
        if not isinstance(length, int):
            raise TypeError('length must be an integer')
        if length <= 0:
            raise ValueError('length must be positive')
        if length >= self.length:
            return
        length = self._leaf_start(length)
        self.length = length
        self.level[length >> self.depth_higher:] = []

    async def branch_and_root(self, length, index):
        """Return a merkle branch and root.  Length is the number of
        hashes used to calculate the merkle root, index is the position
        of the hash to calculate the branch of.

        index must be less than length, which must be at least 1."""
        if not isinstance(length, int):
            raise TypeError('length must be an integer')
        if not isinstance(index, int):
            raise TypeError('index must be an integer')
        if length <= 0:
            raise ValueError('length must be positive')
        if index >= length:
            raise ValueError('index must be less than length')
        await self.initialized.wait()
        await self._extend_to(length)
        leaf_start = self._leaf_start(index)
        count = min(self._segment_length(), length - leaf_start)
        leaf_hashes = await self.source_func(leaf_start, count)
        if length < self._segment_length():
            return self.merkle.branch_and_root(leaf_hashes, index)
        level = await self._level_for(length)
        return self.merkle.branch_and_root_from_level(
            level, leaf_hashes, index, self.depth_higher)