LibPosition.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.15;

import "../../libraries/DisputeTypes.sol";

/// @title LibPosition
/// @notice This library contains helper functions for working with the `Position` type.
library LibPosition {

    /// @notice Computes a generalized index (2^{depth} + indexAtDepth).
    /// @param _depth The depth of the position.
    /// @param _indexAtDepth The index at the depth of the position.
    /// @return position_ The computed generalized index.
    function wrap(uint64 _depth, uint64 _indexAtDepth) internal pure returns (Position position_) {
        assembly {
            // gindex = 2^{_depth} + _indexAtDepth
            position_ := add(shl(_depth, 1), _indexAtDepth)
        }
    }

    /// @notice Pulls the `depth` out of a `Position` type.
    /// @param _position The generalized index to get the `depth` of.
    /// @return depth_ The `depth` of the `position` gindex.
    /// @custom:attribution Solady <https://github.com/Vectorized/Solady>
    function depth(Position _position) internal pure returns (uint64 depth_) {
        // Return the most significant bit offset, which signifies the depth of the gindex.
        assembly {
            depth_ := or(depth_, shl(6, lt(0xffffffffffffffff, shr(depth_, _position))))
            depth_ := or(depth_, shl(5, lt(0xffffffff, shr(depth_, _position))))

            // For the remaining 32 bits, use a De Bruijn lookup.
            _position := shr(depth_, _position)
            _position := or(_position, shr(1, _position))
            _position := or(_position, shr(2, _position))
            _position := or(_position, shr(4, _position))
            _position := or(_position, shr(8, _position))
            _position := or(_position, shr(16, _position))

            depth_ := or(
                depth_,
                byte(
                    shr(251, mul(_position, shl(224, 0x07c4acdd))),
                    0x0009010a0d15021d0b0e10121619031e080c141c0f111807131b17061a05041f
                )
            )
        }
    }

    /// @notice Pulls the `indexAtDepth` out of a `Position` type.
    ///         The `indexAtDepth` is the left/right index of a position at a specific depth within
    ///         the binary tree, starting from index 0. For example, at gindex 2, the `depth` = 1
    ///         and the `indexAtDepth` = 0.
    /// @param _position The generalized index to get the `indexAtDepth` of.
    /// @return indexAtDepth_ The `indexAtDepth` of the `position` gindex.
    function indexAtDepth(Position _position) internal pure returns (uint64 indexAtDepth_) {
        // Return bits p_{msb-1}...p_{0}. This effectively pulls the 2^{depth} out of the gindex,
        // leaving only the `indexAtDepth`.
        uint256 msb = depth(_position);
        assembly {
            indexAtDepth_ := sub(_position, shl(msb, 1))
        }
    }

    /// @notice Get the left child of `_position`.
    /// @param _position The position to get the left position of.
    /// @return left_ The position to the left of `position`.
    function left(Position _position) internal pure returns (Position left_) {
        assembly {
            left_ := shl(1, _position)
        }
    }

    /// @notice Get the right child of `_position`
    /// @param _position The position to get the right position of.
    /// @return right_ The position to the right of `position`.
    function right(Position _position) internal pure returns (Position right_) {
        assembly {
            right_ := or(1, shl(1, _position))
        }
    }

    /// @notice Get the parent position of `_position`.
    /// @param _position The position to get the parent position of.
    /// @return parent_ The parent position of `position`.
    function parent(Position _position) internal pure returns (Position parent_) {
        assembly {
            parent_ := shr(1, _position)
        }
    }

    /// @notice Get the deepest, right most gindex relative to the `position`. This is equivalent to
    ///         calling `right` on a position until the maximum depth is reached.
    /// @param _position The position to get the relative deepest, right most gindex of.
    /// @param _maxDepth The maximum depth of the game.
    /// @return rightIndex_ The deepest, right most gindex relative to the `position`.
    function rightIndex(
        Position _position,
        uint256 _maxDepth
    ) internal pure returns (Position rightIndex_) {
        uint256 msb = depth(_position);
        assembly {
            let remaining := sub(_maxDepth, msb)
            rightIndex_ := or(shl(remaining, _position), sub(shl(remaining, 1), 1))
        }
    }

    /// @notice Get the deepest, right most trace index relative to the `position`. This is
    ///         equivalent to calling `right` on a position until the maximum depth is reached and
    ///         then finding its index at depth.
    /// @param _position The position to get the relative trace index of.
    /// @param _maxDepth The maximum depth of the game.
    /// @return traceIndex_ The trace index relative to the `position`.
    function traceIndex(
        Position _position,
        uint256 _maxDepth
    ) internal pure returns (uint256 traceIndex_) {
        uint256 msb = depth(_position);
        assembly {
            let remaining := sub(_maxDepth, msb)
            traceIndex_ := sub(
                or(shl(remaining, _position), sub(shl(remaining, 1), 1)),
                shl(_maxDepth, 1)
            )
        }
    }

    /// @notice Gets the position of the highest ancestor of `_position` that commits to the same
    ///         trace index.
    /// @param _position The position to get the highest ancestor of.
    /// @return ancestor_ The highest ancestor of `position` that commits to the same trace index.
    function traceAncestor(Position _position) internal pure returns (Position ancestor_) {
        // Create a field with only the lowest unset bit of `_position` set.
        Position lsb;
        assembly {
            lsb := and(not(_position), add(_position, 1))
        }
        // Find the index of the lowest unset bit within the field.
        uint256 msb = depth(lsb);
        // The highest ancestor that commits to the same trace index is the original position
        // shifted right by the index of the lowest unset bit.
        assembly {
            let a := shr(msb, _position)
            // Bound the ancestor to the minimum gindex, 1.
            ancestor_ := or(a, iszero(a))
        }
    }

    /// @notice Get the move position of `_position`, which is the left child of:
    ///         1. `_position + 1` if `_isAttack` is true.
    ///         1. `_position` if `_isAttack` is false.
    /// @param _position The position to get the relative attack/defense position of.
    /// @param _isAttack Whether or not the move is an attack move.
    /// @return move_ The move position relative to `position`.
    function move(Position _position, bool _isAttack) internal pure returns (Position move_) {
        assembly {
            move_ := shl(1, or(iszero(_isAttack), _position))
        }
    }
}