use crate::rpc::RpcError;

/// Decides whether the cursor may advance to `to_block` after a batch response.
///
/// A non-empty batch implies the provider has indexed the queried range; the
/// cursor advances unconditionally. An empty batch is ambiguous — some providers
/// (notably dev nodes) return `Ok([])` for ranges past their actual tip instead
/// of erroring — so advancement requires explicit confirmation from the
/// provider's reported `eth_blockNumber`.
pub(crate) fn next_cursor_after_batch(
    to_block: u64,
    logs_empty: bool,
    reported_tip: u64,
) -> Result<u64, RpcError> {
    if logs_empty && to_block > reported_tip {
        return Err(RpcError::CursorPastTip {
            to_block,
            reported_tip,
        });
    }
    Ok(to_block)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn next_cursor_advances_on_non_empty_batch_regardless_of_tip() {
        // Non-empty batches imply the provider indexed the range; the reported
        // tip is not consulted.
        assert_eq!(next_cursor_after_batch(100, false, 0).unwrap(), 100);
        assert_eq!(next_cursor_after_batch(100, false, 50).unwrap(), 100);
        assert_eq!(next_cursor_after_batch(100, false, u64::MAX).unwrap(), 100);
    }

    #[test]
    fn next_cursor_advances_on_empty_batch_within_tip() {
        assert_eq!(next_cursor_after_batch(100, true, 100).unwrap(), 100);
        assert_eq!(next_cursor_after_batch(100, true, 101).unwrap(), 100);
        assert_eq!(next_cursor_after_batch(100, true, u64::MAX).unwrap(), 100);
    }

    #[test]
    fn next_cursor_rejects_empty_batch_past_tip() {
        match next_cursor_after_batch(100, true, 99) {
            Err(RpcError::CursorPastTip {
                to_block,
                reported_tip,
            }) => {
                assert_eq!(to_block, 100);
                assert_eq!(reported_tip, 99);
            }
            other => panic!("expected CursorPastTip, got {:?}", other),
        }
        assert!(next_cursor_after_batch(100, true, 0).is_err());
        assert!(next_cursor_after_batch(u64::MAX, true, u64::MAX - 1).is_err());
    }
}
