// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import {ITwapSource} from "./TwapFallback.sol";

/// @notice Minimal UniswapV2-style pair surface (XSwap V2 on XDC).
interface IV2Pair {
    function token0() external view returns (address);
    function token1() external view returns (address);
    function getReserves() external view returns (uint112 reserve0, uint112 reserve1, uint32 blockTimestampLast);
    function price0CumulativeLast() external view returns (uint256);
    function price1CumulativeLast() external view returns (uint256);
}

interface IERC20Decimals {
    function decimals() external view returns (uint8);
}

/// @title XSwapV2TwapSource
/// @notice ITwapSource over an XSwap V2 pair: price of `base` in `quote` token, using the pair's
///         UQ112x112 cumulative price extended to the current block. `quote()` reverts unless the
///         pair holds at least `minQuoteReserve` of the quote token (raw units).
/// @dev Clean-room implementation of the well-known V2 accumulator formula.
contract XSwapV2TwapSource is ITwapSource {
    IV2Pair public immutable pair;
    bool public immutable baseIsToken0;
    uint256 public immutable minQuoteReserve;
    uint256 internal immutable num; // 10**(outDecimals + baseDecimals)
    uint256 internal immutable den; // 10**quoteDecimals

    error NotInPair();
    error BadConfig();
    error InsufficientLiquidity(uint256 quoteReserve, uint256 minQuoteReserve);

    constructor(IV2Pair _pair, address base, uint8 outDecimals, uint256 _minQuoteReserve) {
        address t0 = _pair.token0();
        address t1 = _pair.token1();
        if (base != t0 && base != t1) revert NotInPair();
        if (_minQuoteReserve == 0) revert BadConfig();
        pair = _pair;
        baseIsToken0 = base == t0;
        uint8 bd = IERC20Decimals(base).decimals();
        uint8 qd = IERC20Decimals(base == t0 ? t1 : t0).decimals();
        if (outDecimals > 18 || bd > 18 || qd > 18) revert BadConfig();
        num = 10 ** (uint256(outDecimals) + bd);
        den = 10 ** uint256(qd);
        minQuoteReserve = _minQuoteReserve;
    }

    function priceCumulative() external view returns (uint256 c, uint256 timestamp) {
        (uint112 r0, uint112 r1, uint32 tsLast) = pair.getReserves();
        c = baseIsToken0 ? pair.price0CumulativeLast() : pair.price1CumulativeLast();
        uint32 nowTs = uint32(block.timestamp);
        if (tsLast != nowTs && r0 != 0 && r1 != 0) {
            unchecked {
                uint256 elapsed = uint32(nowTs - tsLast); // wraps like the pair's own clock
                uint256 px = baseIsToken0 ? (uint256(r1) << 112) / r0 : (uint256(r0) << 112) / r1;
                c += px * elapsed;
            }
        }
        timestamp = block.timestamp;
    }

    /// @param avgRaw average UQ112x112 price (quote raw units per base raw unit)
    function quote(uint256 avgRaw) external view returns (uint256) {
        (uint112 r0, uint112 r1,) = pair.getReserves();
        uint256 q = baseIsToken0 ? r1 : r0;
        if (q < minQuoteReserve) revert InsufficientLiquidity(q, minQuoteReserve);
        uint256 hi = avgRaw >> 112;
        uint256 lo = avgRaw & ((uint256(1) << 112) - 1);
        return hi * num / den + (lo * num) / (den << 112);
    }
}