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use crate::{
    evm_circuit::{
        execution::ExecutionGadget,
        step::ExecutionState,
        util::{
            common_gadget::SameContextGadget,
            constraint_builder::{
                ConstrainBuilderCommon, EVMConstraintBuilder, StepStateTransition,
                Transition::Delta,
            },
            CachedRegion, U64Cell,
        },
        witness::{Block, Call, Chunk, ExecStep, Transaction},
    },
    util::{word::WordExpr, Expr},
};
use eth_types::{evm_types::OpcodeId, Field};
use halo2_proofs::plonk::Error;

#[derive(Clone, Debug)]
pub(crate) struct GasGadget<F> {
    same_context: SameContextGadget<F>,
    gas_left: U64Cell<F>,
}

impl<F: Field> ExecutionGadget<F> for GasGadget<F> {
    const NAME: &'static str = "GAS";

    const EXECUTION_STATE: ExecutionState = ExecutionState::GAS;

    fn configure(cb: &mut EVMConstraintBuilder<F>) -> Self {
        // The gas passed to a transaction is a 64-bit number.
        let gas_left = cb.query_u64();

        // The `gas_left` in the current state has to be deducted by the gas
        // used by the `GAS` opcode itself.
        cb.require_equal(
            "Constraint: gas left equal to stack value",
            gas_left.expr(),
            cb.curr.state.gas_left.expr() - OpcodeId::GAS.constant_gas_cost().expr(),
        );

        // Construct the value and push it to stack.
        cb.stack_push(gas_left.to_word());

        let step_state_transition = StepStateTransition {
            rw_counter: Delta(1.expr()),
            program_counter: Delta(1.expr()),
            stack_pointer: Delta((-1).expr()),
            gas_left: Delta(-OpcodeId::GAS.constant_gas_cost().expr()),
            ..Default::default()
        };
        let opcode = cb.query_cell();
        let same_context = SameContextGadget::construct(cb, opcode, step_state_transition);

        Self {
            same_context,
            gas_left,
        }
    }

    fn assign_exec_step(
        &self,
        region: &mut CachedRegion<'_, '_, F>,
        offset: usize,
        _: &Block<F>,
        _: &Chunk<F>,
        _: &Transaction,
        _call: &Call,
        step: &ExecStep,
    ) -> Result<(), Error> {
        self.same_context.assign_exec_step(region, offset, step)?;

        // The GAS opcode takes into account the reduction of gas available due
        // to the instruction itself.
        self.gas_left.assign(
            region,
            offset,
            Some(
                step.gas_left
                    .saturating_sub(OpcodeId::GAS.constant_gas_cost())
                    .to_le_bytes(),
            ),
        )?;

        Ok(())
    }
}

#[cfg(test)]
mod test {
    use crate::test_util::CircuitTestBuilder;
    use eth_types::{address, bytecode, Word};
    use mock::TestContext;

    fn test_ok() {
        let bytecode = bytecode! {
            GAS
            STOP
        };

        CircuitTestBuilder::new_from_test_ctx(
            TestContext::<2, 1>::simple_ctx_with_bytecode(bytecode).unwrap(),
        )
        .run();
    }

    #[test]
    fn gas_gadget_simple() {
        test_ok();
    }

    #[test]
    fn gas_gadget_incorrect_deduction() {
        let bytecode = bytecode! {
            GAS
            STOP
        };

        // Create a custom tx setting Gas to
        let ctx = TestContext::<2, 1>::new(
            None,
            |accs| {
                accs[0]
                    .address(address!("0x0000000000000000000000000000000000000010"))
                    .balance(Word::from(1u64 << 20))
                    .code(bytecode);
                accs[1]
                    .address(address!("0x0000000000000000000000000000000000000000"))
                    .balance(Word::from(1u64 << 20));
            },
            |mut txs, accs| {
                txs[0]
                    .to(accs[0].address)
                    .from(accs[1].address)
                    .gas(Word::from(1_000_000u64));
            },
            |block, _tx| block.number(0xcafeu64),
        )
        .unwrap();

        CircuitTestBuilder::<2, 1>::new_from_test_ctx(ctx)
            .block_modifier(Box::new(|block, _chunk| {
                // The above block has 2 steps (GAS and STOP). We forcefully assign a
                // wrong `gas_left` value for the second step, to assert that
                // the circuit verification fails for this scenario.
                assert_eq!(block.txs.len(), 1);
                assert_eq!(block.txs[0].steps().len(), 4);
                block.txs[0].steps_mut()[2].gas_left -= 1;
            }))
            .run_with_result()
            .unwrap_err()
            .assert_evm_failure()
    }
}