// Copyright 2018 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

use crate::Hash;
use crate::BLOCK_SIZE;
use std::io;

/// A `MerkleTree` contains levels of hashes that can be used to verify the integrity of data.
///
/// While a single hash could be used to integrity check some data, if the data (or hash) is
/// corrupt, a single hash can not determine what part of the data is corrupt. A `MerkleTree`,
/// however, contains a hash for every 8K block of data, allowing it to identify which 8K blocks of
/// data are corrupt. A `MerkleTree` also allows individual 8K blocks of data to be verified
/// without having to verify the rest of the data.
///
/// Furthermore, a `MerkleTree` contains multiple levels of hashes, where each level
/// contains hashes of 8K blocks of hashes of the lower level. The top level always contains a
/// single hash, the merkle root. This tree of hashes allows a `MerkleTree` to determine which of
/// its own hashes are corrupt, if any.
///
/// # Structure Details
///
/// A merkle tree contains levels. A level is a row of the tree, starting at 0 and counting upward.
/// Level 0 represents the leaves of the tree which contain hashes of chunks of the input stream.
///
/// Each level consists of a hash for each 8K block of hashes from the previous level (or, for
/// level 0, each 8K block of data). When computing a hash, the 8K block is prepended with a block
/// identity.
///
/// A block identity is the binary OR of the starting byte index of the block within
/// the current level and the current level index, followed by the length of the block. For level
/// 0, the length of the block is 8K, except for the last block, which may be less than 8K. All
/// other levels use a block length of 8K.
#[derive(Clone, Eq, PartialEq, Ord, PartialOrd, Hash, Debug)]
pub struct MerkleTree {
    levels: Vec<Vec<Hash>>,
}

impl MerkleTree {
    /// Creates a `MerkleTree` from a well-formed tree of hashes.
    ///
    /// A tree of hashes is well-formed iff:
    /// - The length of the last level is 1.
    /// - The length of every hash level is the length of the prior hash level divided by
    ///   `HASHES_PER_BLOCK`, rounded up to the nearest integer.
    pub fn from_levels(levels: Vec<Vec<Hash>>) -> MerkleTree {
        MerkleTree { levels }
    }

    /// The root hash of the merkle tree.
    pub fn root(&self) -> Hash {
        self.levels[self.levels.len() - 1][0]
    }

    /// Creates a `MerkleTree` from all of the bytes of a `Read`er.
    ///
    /// # Examples
    /// ```
    /// # use fuchsia_merkle::MerkleTree;
    /// let data_to_hash = [0xffu8; 8192];
    /// let tree = MerkleTree::from_reader(&data_to_hash[..]).unwrap();
    /// assert_eq!(
    ///     tree.root(),
    ///     "68d131bc271f9c192d4f6dcd8fe61bef90004856da19d0f2f514a7f4098b0737".parse().unwrap()
    /// );
    /// ```
    pub fn from_reader(mut reader: impl std::io::Read) -> Result<MerkleTree, io::Error> {
        let mut builder = crate::builder::MerkleTreeBuilder::new();
        let mut buf = [0u8; BLOCK_SIZE];
        loop {
            let size = reader.read(&mut buf)?;
            if size == 0 {
                break;
            }
            builder.write(&buf[0..size]);
        }
        Ok(builder.finish())
    }
}

impl AsRef<[Vec<Hash>]> for MerkleTree {
    fn as_ref(&self) -> &[Vec<Hash>] {
        &self.levels[..]
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::util::HASHES_PER_BLOCK;
    use crate::util::{hash_block, hash_hashes};

    impl MerkleTree {
        /// Given the index of a block of data, lookup its hash.
        fn leaf_hash(&self, block: usize) -> Hash {
            self.levels[0][block]
        }
    }

    #[test]
    fn test_single_full_hash_block() {
        let mut leafs = Vec::new();
        {
            let block = vec![0xFF; BLOCK_SIZE];
            for i in 0..HASHES_PER_BLOCK {
                leafs.push(hash_block(&block, i * BLOCK_SIZE));
            }
        }
        let root = hash_hashes(&leafs, 1, 0);

        let tree = MerkleTree::from_levels(vec![leafs.clone(), vec![root]]);

        assert_eq!(tree.root(), root);
        for (i, leaf) in leafs.iter().enumerate().take(HASHES_PER_BLOCK) {
            assert_eq!(&tree.leaf_hash(i), leaf);
        }
    }

    #[test]
    fn test_from_reader_empty() {
        let data_to_hash = [0x00u8; 0];
        let tree = MerkleTree::from_reader(&data_to_hash[..]).unwrap();
        let expected: Hash =
            "15ec7bf0b50732b49f8228e07d24365338f9e3ab994b00af08e5a3bffe55fd8b".parse().unwrap();
        assert_eq!(tree.root(), expected);
    }

    #[test]
    fn test_from_reader_oneblock() {
        let data_to_hash = [0xffu8; 8192];
        let tree = MerkleTree::from_reader(&data_to_hash[..]).unwrap();
        let expected: Hash =
            "68d131bc271f9c192d4f6dcd8fe61bef90004856da19d0f2f514a7f4098b0737".parse().unwrap();
        assert_eq!(tree.root(), expected);
    }

    #[test]
    fn test_from_reader_unaligned() {
        let size = 2_109_440usize;
        let mut the_bytes = Vec::with_capacity(size);
        the_bytes.extend(std::iter::repeat(0xff).take(size));
        let tree = MerkleTree::from_reader(&the_bytes[..]).unwrap();
        let expected: Hash =
            "7577266aa98ce587922fdc668c186e27f3c742fb1b732737153b70ae46973e43".parse().unwrap();
        assert_eq!(tree.root(), expected);
    }

    #[test]
    fn test_from_reader_error_propagation() {
        const CUSTOM_ERROR_MESSAGE: &str = "merkle tree custom error message";
        struct ReaderSuccessThenError {
            been_called: bool,
        }

        impl std::io::Read for ReaderSuccessThenError {
            fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
                if !self.been_called {
                    self.been_called = true;
                    buf[0] = 0;
                    Ok(1)
                } else {
                    Err(io::Error::new(io::ErrorKind::Other, CUSTOM_ERROR_MESSAGE))
                }
            }
        }

        let reader = ReaderSuccessThenError { been_called: false };
        let result = MerkleTree::from_reader(reader);
        assert_eq!(result.unwrap_err().to_string(), CUSTOM_ERROR_MESSAGE);
    }
}