#include <stdio.h>
#include <stdlib.h>
#include <inttypes.h>
#include <assert.h>

inline uint32_t sign_extend(uint32_t word, uint32_t size)
{
    const uint32_t mask = 1U << (size - 1);
    return (word ^ mask) - mask;
}

struct Instruction
{
    uint8_t  opcode;
    uint8_t  rs1;
    uint8_t  rs2;
    uint8_t  rd;
    uint8_t  funct3;
    uint8_t  funct7;
    uint32_t imm;
};

struct Instruction decode_r_type(uint32_t word)
{
    struct Instruction instruction = {0};
    instruction.opcode =  word        & 0x7F;
    instruction.rd     = (word >> 7)  & 0x1F;
    instruction.funct3 = (word >> 12) & 0x07;
    instruction.rs1    = (word >> 15) & 0x1F;
    instruction.rs2    = (word >> 20) & 0x1F;
    instruction.funct7 =  word >> 25;
    return instruction;
};

struct Instruction decode_i_type(uint32_t word)
{
    struct Instruction instruction = {0};
    instruction.opcode =  word        & 0x7F;
    instruction.rd     = (word >> 7)  & 0x1F;
    instruction.funct3 = (word >> 12) & 0x07;
    instruction.rs1    = (word >> 15) & 0x1F;
    instruction.imm    = sign_extend(word >> 20, 12);
    return instruction;
};

struct Instruction decode_s_type(uint32_t word)
{
    struct Instruction instruction = {0};
    instruction.opcode =  word        & 0x7F;
    instruction.funct3 = (word >> 12) & 0x07;
    instruction.rs1    = (word >> 15) & 0x1F;
    instruction.rs2    = (word >> 20) & 0x1F;
    instruction.imm    = sign_extend(((word >> 7) & 0x1F) | (word >> 25), 12);
    return instruction;
};

struct Instruction decode_b_type(uint32_t word)
{
    struct Instruction instruction = decode_s_type(word);
    instruction.imm = ((instruction.imm << 11) & 0x800) | (instruction.imm & 0xfffff7ff);
    return instruction;
};

struct Instruction decode_u_type(uint32_t word)
{
    struct Instruction instruction = {0};
    instruction.opcode =  word        & 0x7F;
    instruction.rd     = (word >> 7)  & 0x1F;
    instruction.imm    =  word        & 0xFFFFF000;
    return instruction;
};

struct Instruction decode_j_type(uint32_t word)
{
    struct Instruction instruction = {0};
    instruction.opcode =  word        & 0x7F;
    instruction.rd     = (word >> 7)  & 0x1F;
    instruction.imm    = sign_extend(
        ((word & 0x80000000) >> 11) |
        ((word & 0x000FF000) >> 0)  |
        ((word & 0x00100000) >> 9)  |
        ((word & 0x7FE00000) >> 20), 21);
    return instruction;
}

struct Hart
{
    uint32_t pc;
    uint32_t regs[32];
};

int main(int argc, char* argv[])
{
    uint32_t instruction = 0xfa728213;
    struct Instruction instr = decode_i_type(instruction);
    printf("%u %u %u\n", instr.rd, instr.rs1, instr.imm);
    return EXIT_SUCCESS;
}