neo-decompiler 0.8.1

use crate::error::{DisassemblyError, Result};
use crate::instruction::{OpCode, Operand, OperandEncoding};

use super::Disassembler;

mod immediates;

const MAX_OPERAND_LEN: usize = 1_048_576;

fn read_i16_le(bytes: &[u8]) -> i16 {
    let mut array = [0u8; 2];
    array.copy_from_slice(bytes);
    i16::from_le_bytes(array)
}

fn read_i32_le(bytes: &[u8]) -> i32 {
    let mut array = [0u8; 4];
    array.copy_from_slice(bytes);
    i32::from_le_bytes(array)
}

fn read_i64_le(bytes: &[u8]) -> i64 {
    let mut array = [0u8; 8];
    array.copy_from_slice(bytes);
    i64::from_le_bytes(array)
}

fn read_u16_le(bytes: &[u8]) -> u16 {
    let mut array = [0u8; 2];
    array.copy_from_slice(bytes);
    u16::from_le_bytes(array)
}

fn read_u32_le(bytes: &[u8]) -> u32 {
    let mut array = [0u8; 4];
    array.copy_from_slice(bytes);
    u32::from_le_bytes(array)
}

impl Disassembler {
    pub(super) fn read_operand(
        &self,
        opcode: OpCode,
        bytecode: &[u8],
        offset: usize,
    ) -> Result<(Option<Operand>, usize)> {
        if let Some(constant) = immediates::immediate_constant(opcode) {
            return Ok((Some(constant), 0));
        }

        match opcode.operand_encoding() {
            OperandEncoding::None => Ok((None, 0)),
            OperandEncoding::I8 => {
                let bytes = self.read_slice(bytecode, offset + 1, 1, offset)?;
                let value = bytes[0] as i8;
                Ok((Some(Operand::I8(value)), 1))
            }
            OperandEncoding::I16 => {
                let bytes = self.read_slice(bytecode, offset + 1, 2, offset)?;
                let value = read_i16_le(bytes);
                Ok((Some(Operand::I16(value)), 2))
            }
            OperandEncoding::I32 => {
                let bytes = self.read_slice(bytecode, offset + 1, 4, offset)?;
                let value = read_i32_le(bytes);
                Ok((Some(Operand::I32(value)), 4))
            }
            OperandEncoding::I64 => {
                let bytes = self.read_slice(bytecode, offset + 1, 8, offset)?;
                let value = read_i64_le(bytes);
                Ok((Some(Operand::I64(value)), 8))
            }
            OperandEncoding::Bytes(len) => {
                let bytes = self.read_slice(bytecode, offset + 1, len, offset)?.to_vec();
                Ok((Some(Operand::Bytes(bytes)), len))
            }
            OperandEncoding::Data1 => {
                let (bytes, consumed) = self.read_bytes_prefixed(bytecode, offset, 1)?;
                Ok((Some(Operand::Bytes(bytes)), consumed))
            }
            OperandEncoding::Data2 => {
                let (bytes, consumed) = self.read_bytes_prefixed(bytecode, offset, 2)?;
                Ok((Some(Operand::Bytes(bytes)), consumed))
            }
            OperandEncoding::Data4 => {
                let (bytes, consumed) = self.read_bytes_prefixed(bytecode, offset, 4)?;
                Ok((Some(Operand::Bytes(bytes)), consumed))
            }
            OperandEncoding::Jump8 => {
                let bytes = self.read_slice(bytecode, offset + 1, 1, offset)?;
                let value = bytes[0] as i8;
                Ok((Some(Operand::Jump(value)), 1))
            }
            OperandEncoding::Jump32 => {
                let bytes = self.read_slice(bytecode, offset + 1, 4, offset)?;
                let value = read_i32_le(bytes);
                Ok((Some(Operand::Jump32(value)), 4))
            }
            OperandEncoding::U16 => {
                let bytes = self.read_slice(bytecode, offset + 1, 2, offset)?;
                let value = read_u16_le(bytes);
                Ok((Some(Operand::U16(value)), 2))
            }
            OperandEncoding::U8 => {
                let bytes = self.read_slice(bytecode, offset + 1, 1, offset)?;
                Ok((Some(Operand::U8(bytes[0])), 1))
            }
            OperandEncoding::U32 => {
                let bytes = self.read_slice(bytecode, offset + 1, 4, offset)?;
                let value = read_u32_le(bytes);
                Ok((Some(Operand::U32(value)), 4))
            }
            OperandEncoding::Syscall => {
                let bytes = self.read_slice(bytecode, offset + 1, 4, offset)?;
                let value = read_u32_le(bytes);
                Ok((Some(Operand::Syscall(value)), 4))
            }
        }
    }

    fn read_bytes_prefixed(
        &self,
        bytecode: &[u8],
        offset: usize,
        prefix_len: usize,
    ) -> Result<(Vec<u8>, usize)> {
        let len_bytes = self.read_slice(bytecode, offset + 1, prefix_len, offset)?;
        let len = match prefix_len {
            1 => len_bytes[0] as usize,
            2 => read_u16_le(len_bytes) as usize,
            4 => read_u32_le(len_bytes) as usize,
            _ => unreachable!("prefix_len is controlled by OperandEncoding"),
        };

        if len > MAX_OPERAND_LEN {
            return Err(DisassemblyError::OperandTooLarge { offset, len }.into());
        }

        let data_start = offset + 1 + prefix_len;
        let data = self.read_slice(bytecode, data_start, len, offset)?.to_vec();
        Ok((data, prefix_len + len))
    }

    fn read_slice<'a>(
        &self,
        bytecode: &'a [u8],
        start: usize,
        len: usize,
        offset: usize,
    ) -> Result<&'a [u8]> {
        let end = start
            .checked_add(len)
            .ok_or(DisassemblyError::UnexpectedEof { offset })?;
        match bytecode.get(start..end) {
            Some(slice) => Ok(slice),
            None => Err(DisassemblyError::UnexpectedEof { offset }.into()),
        }
    }
}