// Copyright 2019-2020 Georges Racinet <georges.racinet@octobus.net>

//

// This software may be used and distributed according to the terms of the

// GNU General Public License version 2 or any later version.

//! Definitions and utilities for Revision nodes

//!

//! In Mercurial code base, it is customary to call "a node" the binary SHA

//! of a revision.

use hex::{self, FromHex, FromHexError};

/// The length in bytes of a `Node`

///

/// This constant is meant to ease refactors of this module, and

/// are private so that calling code does not expect all nodes have

/// the same size, should we support several formats concurrently in

/// the future.

const NODE_BYTES_LENGTH: usize = 20;

/// The length in bytes of a `Node`

///

/// see also `NODES_BYTES_LENGTH` about it being private.

const NODE_NYBBLES_LENGTH: usize = 2 * NODE_BYTES_LENGTH;

/// Private alias for readability and to ease future change

type NodeData = [u8; NODE_BYTES_LENGTH];

/// Binary revision SHA

///

/// ## Future changes of hash size

///

/// To accomodate future changes of hash size, Rust callers

/// should use the conversion methods at the boundaries (FFI, actual

/// computation of hashes and I/O) only, and only if required.

///

/// All other callers outside of unit tests should just handle `Node` values

/// and never make any assumption on the actual length, using [`nybbles_len`]

/// if they need a loop boundary.

///

/// All methods that create a `Node` either take a type that enforces

/// the size or fail immediately at runtime with [`ExactLengthRequired`].

///

/// [`nybbles_len`]: #method.nybbles_len

/// [`ExactLengthRequired`]: struct.NodeError#variant.ExactLengthRequired

#[derive(Clone, Debug, PartialEq)]

pub struct Node {

    data: NodeData,

}

/// The node value for NULL_REVISION

pub const NULL_NODE: Node = Node {

    data: [0; NODE_BYTES_LENGTH],

};

impl From<NodeData> for Node {

    fn from(data: NodeData) -> Node {

        Node { data }

    }

}

#[derive(Debug, PartialEq)]

pub enum NodeError {

    ExactLengthRequired(usize, String),

    HexError(FromHexError, String),

}

/// Low level utility function, also for prefixes

fn get_nybble(s: &[u8], i: usize) -> u8 {

    if i % 2 == 0 {

        s[i / 2] >> 4

    } else {

        s[i / 2] & 0x0f

    }

}

impl Node {

    /// Retrieve the `i`th half-byte of the binary data.

    ///

    /// This is also the `i`th hexadecimal digit in numeric form,

    /// also called a [nybble](https://en.wikipedia.org/wiki/Nibble).

    pub fn get_nybble(&self, i: usize) -> u8 {

        get_nybble(&self.data, i)

    }

    /// Length of the data, in nybbles

    pub fn nybbles_len(&self) -> usize {

        // public exposure as an instance method only, so that we can

        // easily support several sizes of hashes if needed in the future.

        NODE_NYBBLES_LENGTH

    }

    /// Convert from hexadecimal string representation

    ///

    /// Exact length is required.

    ///

    /// To be used in FFI and I/O only, in order to facilitate future

    /// changes of hash format.

    pub fn from_hex(hex: &str) -> Result<Node, NodeError> {

        Ok(NodeData::from_hex(hex)

            .map_err(|e| NodeError::from((e, hex)))?

            .into())

    }

    /// Convert to hexadecimal string representation

    ///

    /// To be used in FFI and I/O only, in order to facilitate future

    /// changes of hash format.

    pub fn encode_hex(&self) -> String {

        hex::encode(self.data)

    }

    /// Provide access to binary data

    ///

    /// This is needed by FFI layers, for instance to return expected

    /// binary values to Python.

    pub fn as_bytes(&self) -> &[u8] {

        &self.data

    }

}

        let (err, offender) = err_offender;

        match err {

            FromHexError::InvalidStringLength => {

                NodeError::ExactLengthRequired(

                    NODE_NYBBLES_LENGTH,

                )

            }

        }

    }

}

#[cfg(test)]

mod tests {

    use super::*;

    fn sample_node() -> Node {

        let mut data = [0; NODE_BYTES_LENGTH];

        data.copy_from_slice(&[

            0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef, 0xfe, 0xdc, 0xba,

            0x98, 0x76, 0x54, 0x32, 0x10, 0xde, 0xad, 0xbe, 0xef,

        ]);

        data.into()

    }

    /// Pad an hexadecimal string to reach `NODE_NYBBLES_LENGTH`

    ///

    /// The padding is made with zeros

    fn hex_pad_right(hex: &str) -> String {

        let mut res = hex.to_string();

        while res.len() < NODE_NYBBLES_LENGTH {

            res.push('0');

        }

        res

    }

    fn sample_node_hex() -> String {

        hex_pad_right("0123456789abcdeffedcba9876543210deadbeef")

    }

    #[test]

    fn test_node_from_hex() {

        assert_eq!(Node::from_hex(&sample_node_hex()), Ok(sample_node()));

        let mut short = hex_pad_right("0123");

        short.pop();

        short.pop();

        assert_eq!(

            Node::from_hex(&short),

            Err(NodeError::ExactLengthRequired(NODE_NYBBLES_LENGTH, short)),

        );

        let not_hex = hex_pad_right("012... oops");

        assert_eq!(

            Node::from_hex(&not_hex),

            Err(NodeError::HexError(

                FromHexError::InvalidHexCharacter { c: '.', index: 3 },

                not_hex,

            )),

        );

    }

    #[test]

    fn test_node_encode_hex() {

        assert_eq!(sample_node().encode_hex(), sample_node_hex());

    }

}