// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "./math/Math.sol";
import "./math/SafeCast.sol";

/**
 * @dev This library defines the `History` struct, for checkpointing values as they change at different points in
 * time, and later looking up past values by block number. See {Votes} as an example.
 *
 * To create a history of checkpoints define a variable type `Checkpoints.History` in your contract, and store a new
 * checkpoint for the current transaction block using the {push} function.
 *
 * _Available since v4.5._
 */
library Checkpoints {
    struct Checkpoint {
        uint32 _blockNumber;
        uint224 _value;
    }

    struct History {
        Checkpoint[] _checkpoints;
    }

    /**
     * @dev Returns the value in the latest checkpoint, or zero if there are no checkpoints.
     */
    function latest(History storage self) internal view returns (uint256) {
        uint256 pos = self._checkpoints.length;
        return pos == 0 ? 0 : self._checkpoints[pos - 1]._value;
    }

    /**
     * @dev Returns the value at a given block number. If a checkpoint is not available at that block, the closest one
     * before it is returned, or zero otherwise.
     */
    function getAtBlock(History storage self, uint256 blockNumber) internal view returns (uint256) {
        require(blockNumber < block.number, "Checkpoints: block not yet mined");

        uint256 high = self._checkpoints.length;
        uint256 low = 0;
        while (low < high) {
            uint256 mid = Math.average(low, high);
            if (self._checkpoints[mid]._blockNumber > blockNumber) {
                high = mid;
            } else {
                low = mid + 1;
            }
        }
        return high == 0 ? 0 : self._checkpoints[high - 1]._value;
    }

    /**
     * @dev Pushes a value onto a History so that it is stored as the checkpoint for the current block.
     *
     * Returns previous value and new value.
     */
    function push(History storage self, uint256 value) internal returns (uint256, uint256) {
        uint256 pos = self._checkpoints.length;
        uint256 old = latest(self);
        if (pos > 0 && self._checkpoints[pos - 1]._blockNumber == block.number) {
            self._checkpoints[pos - 1]._value = SafeCast.toUint224(value);
        } else {
            self._checkpoints.push(
                Checkpoint({_blockNumber: SafeCast.toUint32(block.number), _value: SafeCast.toUint224(value)})
            );
        }
        return (old, value);
    }

    /**
     * @dev Pushes a value onto a History, by updating the latest value using binary operation `op`. The new value will
     * be set to `op(latest, delta)`.
     *
     * Returns previous value and new value.
     */
    function push(
        History storage self,
        function(uint256, uint256) view returns (uint256) op,
        uint256 delta
    ) internal returns (uint256, uint256) {
        return push(self, op(latest(self), delta));
    }
}
