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

#define ET_NONE 0x00
#define ET_REL  0x01
#define ET_EXEC 0x02
#define ET_DYN  0x03
#define ET_CORE 0x04

struct Elf32Header
{
    uint8_t  ident[16];
    uint16_t type;
    uint16_t machine;
    uint32_t version;
    uint32_t entry;
    uint32_t phoff;
    uint32_t shoff;
    uint32_t flags;
    uint16_t ehsize;
    uint16_t phentsize;
    uint16_t phnum;
    uint16_t shentsize;
    uint16_t shnum;
    uint16_t shstrndx;
};

#define PT_NULL    0x00000000
#define PT_LOAD    0x00000001
#define PT_DYNAMIC 0x00000002
#define PT_INTERP  0x00000003
#define PT_NOTE    0x00000004
#define PT_SHLIB   0x00000005
#define PT_PHDR    0x00000006
#define PT_TLS     0x00000007

#define PF_X 0x1
#define PF_W 0x2
#define PF_R 0x4

struct Elf32ProgramHeader
{
    uint32_t type;
    uint32_t offset;
    uint32_t vaddr;
    uint32_t paddr;
    uint32_t filesz;
    uint32_t memsz;
    uint32_t flags;
    uint32_t align;
};

bool load_elf(const char* file, char* mem, uint32_t mem_size, uint32_t* start_address)
{
    FILE* fp = fopen(file, "rb");
    if (fp == NULL)
    {
        printf("Couldn't open input file\n");
        return false;
    }

    struct Elf32Header header;
    size_t read = fread(&header, sizeof(struct Elf32Header), 1, fp);
    if (read != 1)
    {
        printf("ELF header too small\n");
        return false;
    }

    static uint8_t kValidIdent[7] = {
        0x7f, 0x45, 0x4c, 0x46,
        1, 1, 1
    };
    if (memcmp(kValidIdent, header.ident, 7) != 0)
    {
        printf("Invalid ELF header\n");
        return false;
    }

    if (header.type != ET_EXEC)
    {
        printf("Can only link EXEC ELF files\n");
        return false;
    }

    memset(mem, 0, mem_size);
    
    assert(header.phnum == 0 || header.phentsize == sizeof(struct Elf32ProgramHeader));
    for (uint32_t i = 0; i < header.phnum; ++i)
    {
        struct Elf32ProgramHeader pheader;

        fseek(fp, header.phoff + i * header.phentsize, SEEK_SET);
        read = fread(&pheader, sizeof(struct Elf32ProgramHeader), 1, fp);
        if (read != 1)
        {
            printf("Error reading program header at index %u\n", i);
            return false;
        }

        if (pheader.type == PT_LOAD)
        {
            if (pheader.paddr + pheader.memsz > mem_size)
            {
                printf("Memory not large enough for ELF file\n");
                return false;
            }

            fseek(fp, pheader.offset, SEEK_SET);
            read = fread(mem + pheader.paddr, pheader.filesz, 1, fp);
            if (read != 1)
            {
                printf("Error when copying ELF segment\n");
                return false;
            }
        }
    }

    if (header.entry > mem_size)
    {
        printf("Entry address out of bounds\n");
        return false;
    }

    *start_address = header.entry;
    fclose(fp);

    return true;
}

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];
};

void execute_op_imm(struct Hart* hart, uint32_t instruction)
{
    const struct Instruction inst = decode_i_type(instruction);
    if (inst.rd == 0) return;
    
    switch (inst.funct3)
    {
        case 0: // ADDI
            hart->regs[inst.rd] = hart->regs[inst.rs1] + inst.imm;
            break;
        case 1: // SLLI
            hart->regs[inst.rd] = hart->regs[inst.rs1] << (inst.imm & 0x1F);
            break;
        case 2: // SLTI
            hart->regs[inst.rd] = (int32_t)hart->regs[inst.rs1] < (int32_t)inst.imm ? 1 : 0;
            break;
        case 3: // SLTIU
            hart->regs[inst.rd] = hart->regs[inst.rs1] < inst.imm ? 1 : 0;
            break;
        case 4: // XORI
            hart->regs[inst.rd] = hart->regs[inst.rs1] ^ inst.imm;
            break;
        case 5: // SRLI, SRAI
        {
            const uint32_t shamt = inst.imm & 0x1F;
            uint32_t res = hart->regs[inst.rs1] >> shamt;
            if ((inst.imm & 0x400) && shamt > 0) { res = sign_extend(res, 32 - shamt); }
            hart->regs[inst.rd] = res;
            break;
        }
        case 6: // ORI
            hart->regs[inst.rd] = hart->regs[inst.rs1] | inst.imm;
            break;
        case 7: // ANDI
            hart->regs[inst.rd] = hart->regs[inst.rs1] & inst.imm;
            break;
        default:
            assert(!"Unhandled OP-IMM");
    }
}

void execute_op(struct Hart* hart, uint32_t instruction)
{
    const struct Instruction inst = decode_r_type(instruction);
    if (inst.rd == 0) return;
    
    switch (inst.funct3)
    {
        case 0: // ADD, SUB
            if (instruction & 0x40000000)
            {
                hart->regs[inst.rd] = hart->regs[inst.rs1] - hart->regs[inst.rs2];
            }
            else
            {
                hart->regs[inst.rd] = hart->regs[inst.rs1] + hart->regs[inst.rs2];
            }
            break;
        case 1: // SLL
            hart->regs[inst.rd] = hart->regs[inst.rs1] << (hart->regs[inst.rs2] & 0x1F);
            break;
        case 2: // SLT
            hart->regs[inst.rd] = (int32_t)hart->regs[inst.rs1] < (int32_t)hart->regs[inst.rs2] ? 1 : 0;
            break;
        case 3: // SLTU
            hart->regs[inst.rd] = hart->regs[inst.rs1] < hart->regs[inst.rs2] ? 1 : 0;
            break;
        case 4: // XOR
            hart->regs[inst.rd] = hart->regs[inst.rs1] ^ hart->regs[inst.rs2];
            break;
        case 5: // SRL, SRA
        {
            const uint32_t shamt = hart->regs[inst.rs2] & 0x1F;
            uint32_t res = hart->regs[inst.rs1] >> shamt;
            if ((instruction & 0x40000000) && shamt > 0) { res = sign_extend(res, 32 - shamt); }
            hart->regs[inst.rd] = res;
            break;
        }
        case 6: // OR
            hart->regs[inst.rd] = hart->regs[inst.rs1] | hart->regs[inst.rs2];
            break;
        case 7: // AND
            hart->regs[inst.rd] = hart->regs[inst.rs1] & hart->regs[inst.rs2];
            break;
        default:
            assert(!"Unhandled OP-IMM");
    }
}

void execute_branch(struct Hart* hart, uint32_t instruction)
{
    const struct Instruction inst = decode_b_type(instruction);
    const uint32_t r1 = hart->regs[inst.rs1];
    const uint32_t r2 = hart->regs[inst.rs2];
    bool take_branch = false;

    switch (inst.funct3)
    {
        case 0: take_branch = (r1 == r2); break;
        case 1: take_branch = (r1 != r2); break;
        case 4: take_branch = (r1 < r2); break;
        case 5: take_branch = (r1 >= r2); break;
        case 6: take_branch = ((int32_t)r1 < (int32_t)r2); break;
        case 7: take_branch = ((int32_t)r1 >= (int32_t)r2); break;
    }

    if (take_branch)
    {
        hart->pc += inst.imm;
    }
    else
    {
        hart->pc += 4;
    }
}

uint32_t load_byte(struct Hart* hart, uint32_t address)
{
    return 0;
}

uint32_t load_half(struct Hart* hart, uint32_t address)
{
    return 0;
}

uint32_t load_word(struct Hart* hart, uint32_t address)
{
    return 0;
}

void store_byte(struct Hart* hart, uint32_t address, uint8_t value) {}
void store_half(struct Hart* hart, uint32_t address, uint16_t value) {}
void store_word(struct Hart* hart, uint32_t address, uint32_t value) {}

void execute_op_load(struct Hart* hart, uint32_t instruction)
{
    const struct Instruction inst = decode_i_type(instruction);
    const uint32_t address = hart->regs[inst.rs1] + inst.imm;
    
    uint32_t value = 0;
    
    switch (inst.funct3 & 0x03)
    {
        case 0:
            value = load_byte(hart, address);
            if ((inst.funct3 & 0x40) == 0)
            {
                value = sign_extend(value, 8);
            }
            break;
        case 1:
            value = load_half(hart, address);
            if ((inst.funct3 & 0x40) == 0)
            {
                value = sign_extend(value, 16);
            }
            break;
        case 2:
            value = load_byte(hart, address);
            break;
        default:
            assert(!"Unhandled load size");
            break;
    }

    if (inst.rd != 0)
    {
        hart->regs[inst.rd] = value;
    }
}

void execute_op_store(struct Hart* hart, uint32_t instruction)
{
    const struct Instruction inst = decode_s_type(instruction);
    const uint32_t address = hart->regs[inst.rs1] + inst.imm;
    const uint32_t value = hart->regs[inst.rs2];

    switch (inst.funct3 & 0x03)
    {
        case 0: store_byte(hart, address, value & 0xFF); break;
        case 1: store_half(hart, address, value & 0xFFFF); break;
        case 2: store_word(hart, address, value); break;
        default:
            assert(!"Unhandled store size");
            break;
    }
}

void execute_misc_mem(struct Hart* hart, uint32_t instruction)
{
    const struct Instruction inst = decode_i_type(instruction);
    
    if (inst.funct3 == 0)
    {
        // FENCE
    }
    else
    {
        assert(!"Unhandled MISC-MEM instruction");
    }
}

void execute_system(struct Hart* hart, uint32_t instruction)
{
    const struct Instruction inst = decode_i_type(instruction);
    
    if (inst.funct3 == 0 && inst.rs1 == 0 && inst.rd == 0)
    {
        if (inst.imm == 0)
        {
            // ECALL
        }
        else if (inst.imm == 1)
        {
            // EBREAK
        }
        else
        {
            assert(!"Unhandled SYSTEM/PRIV instruction");
        }
    }
    else
    {
        assert(!"Unhandled SYSTEM instruction");
    }
}

void execute(struct Hart* hart, uint32_t instruction)
{
    switch (instruction & 0x7f)
    {
        case 0x03:
            execute_op_load(hart, instruction);
            hart->pc += 4;
            break;
        case 0x0F:
            execute_misc_mem(hart, instruction);
            hart->pc += 4;
            break;
        case 0x13:
            execute_op_imm(hart, instruction);
            hart->pc += 4;
            break;
        case 0x17: // AUIPC
        {
            struct Instruction inst = decode_u_type(instruction);
            if (inst.rd != 0)
            {
                hart->regs[inst.rd] = inst.imm + hart->pc;
            }
            hart->pc += 4;
            break;
        }
        case 0x23:
            execute_op_store(hart, instruction);
            hart->pc += 4;
            break;
        case 0x33:
            execute_op(hart, instruction);
            hart->pc += 4;
            break;
        case 0x37: // LUI
        {
            struct Instruction inst = decode_u_type(instruction);
            if (inst.rd != 0)
            {
                hart->regs[inst.rd] = inst.imm;
            }
            hart->pc += 4;
            break;
        }
        case 0x63:
            execute_branch(hart, instruction);
            break;
        case 0x67: // JALR
        {
            struct Instruction inst = decode_i_type(instruction);
            assert(inst.funct3 == 0);
            if (inst.rd != 0)
            {
                hart->regs[inst.rd] = hart->pc + 4;
            }
            hart->pc = (hart->regs[inst.rs1] + inst.imm) & 0xFFFFFFFE;
            break;
        }
        case 0x6F: // JAL
        {
            struct Instruction inst = decode_j_type(instruction);
            if (inst.rd != 0)
            {
                hart->regs[inst.rd] = hart->pc + 4;
            }
            hart->pc += inst.imm;
            break;
        }
        case 0x73:
        {
            execute_system(hart, instruction);
            hart->pc += 4;
            break;
        }
        default:
            assert(!"Unhandled opcode");
    }

    assert(hart->regs[0] == 0);
}

void test_addi()
{
    struct Hart hart = {0};
    
    execute(&hart, 0x00500093); // addi x1, x0, 5
    assert(hart.regs[1] == 5);

    execute(&hart, 0xffe00093); // addi, x1, x0, -2
    assert(hart.regs[1] == 0xfffffffe);
}

void test_slti_sltiu()
{
    struct Hart hart = {0};
    
    hart.regs[1] = 5;

    execute(&hart, 0x00f0b113); // sltiu x2, x1, 15
    assert(hart.regs[2] == 1);

    execute(&hart, 0x0050b113); // sltiu x2, x1, 15
    assert(hart.regs[2] == 0);

    execute(&hart, 0x0010b113); // sltiu x2, x1, 15
    assert(hart.regs[2] == 0);

    execute(&hart, 0x00f0a113); // slti x2, x1, 15
    assert(hart.regs[2] == 1);

    execute(&hart, 0x0050a113); // slti x2, x1, 15
    assert(hart.regs[2] == 0);

    execute(&hart, 0x0010a113); // slti x2, x1, 15
    assert(hart.regs[2] == 0);

    execute(&hart, 0xffb0a113); // slti x2, x1, -5
    assert(hart.regs[2] == 0);

    hart.regs[1] = (uint32_t)-20;

    execute(&hart, 0xffb0a113); // slti x2, x1, -5
    assert(hart.regs[2] == 1);
}

void test_andi_ori_xori()
{
    struct Hart hart = {0};
    
    hart.regs[1] = 6;

    execute(&hart, 0x00c0c113); // xori x2, x1, 12
    assert(hart.regs[2] == 10);
    
    execute(&hart, 0x00c0e113); // ori x2, x1, 12
    assert(hart.regs[2] == 14);
    
    execute(&hart, 0x00c0f113); // andi x2, x1, 12
    assert(hart.regs[2] == 4);
}

void test_slli_srli_srai()
{
    struct Hart hart = {0};
    
    hart.regs[1] = 6;

    execute(&hart, 0x00209113); // slli x2, x1, 2
    assert(hart.regs[2] == 24);

    execute(&hart, 0x0020d113); // srli x2, x1, 2
    assert(hart.regs[2] == 1);

    execute(&hart, 0x4020d113); // srai x2, x1, 2
    assert(hart.regs[2] == 1);

    hart.regs[1] = (uint32_t)-6;

    execute(&hart, 0x0020d113); // srli x2, x1, 2
    assert(hart.regs[2] == 0x3FFFFFFE);

    execute(&hart, 0x4020d113); // srai x2, x1, 2
    assert(hart.regs[2] == 0xFFFFFFFE);
}

void test_lui_auipc()
{
    struct Hart hart = {0};
    
    hart.pc = 0;
    execute(&hart, 0x0007b0b7); // lui x1, 503808
    assert(hart.regs[1] == 503808);
    
    hart.pc = 0;
    execute(&hart, 0x0007b097); // auipc x1, 503808
    assert(hart.regs[1] == 503808);
    
    hart.pc = 12;
    execute(&hart, 0x0007b097); // auipc x1, 503808
    assert(hart.regs[1] == 503820);
}

void test_op()
{
    struct Hart hart = {0};

    hart.regs[1] = 3;
    hart.regs[2] = 4;
    hart.regs[4] = (uint32_t)-1;

    execute(&hart, 0x002081b3); // add, x3, x1, x2
    assert(hart.regs[3] == 7);

    execute(&hart, 0x402081b3); // sub x3, x1, x2
    assert(hart.regs[3] == 0xFFFFFFFF);

    execute(&hart, 0x0020a1b3); // slt x3, x1, x2
    assert(hart.regs[3] == 1);

    execute(&hart, 0x001121b3); // slt x3, x2, 1
    assert(hart.regs[3] == 0);

    execute(&hart, 0x001131b3); // sltu x3, x2, x1
    assert(hart.regs[3] == 0);

    execute(&hart, 0x0020b1b3); // sltu x3, x1, x2
    assert(hart.regs[3] == 1);

    execute(&hart, 0x0040a1b3); // slt x3, x1, x4
    assert(hart.regs[3] == 0);
    
    hart.regs[1] = 6;
    hart.regs[2] = 12;

    execute(&hart, 0x0020e1b3); // or x3, x1, x2
    assert(hart.regs[3] == 14);

    execute(&hart, 0x0020c1b3); // xor x3, x1, x2
    assert(hart.regs[3] == 10);

    execute(&hart, 0x0020f1b3); // and x3, x1, x2
    assert(hart.regs[3] == 4);
    
    hart.regs[1] = 6;
    hart.regs[2] = 2;

    execute(&hart, 0x002091b3); // sll x3, x1, x2
    assert(hart.regs[3] == 24);

    execute(&hart, 0x0020d1b3); // srl x3, x1, x2
    assert(hart.regs[3] == 1);

    execute(&hart, 0x4020d1b3); // sra x3, x1, x2
    assert(hart.regs[3] == 1);
    
    hart.regs[1] = (uint32_t)-6;
    hart.regs[2] = 2;

    execute(&hart, 0x4020d1b3); // sra x3, x1, x2
    assert(hart.regs[3] == 0xFFFFFFFE);
}

void test_jal()
{
    struct Hart hart = {0};

    hart.pc = 12;

    execute(&hart, 0x12c000ef); // jal x1, 300
    assert(hart.regs[1] == 16);
    assert(hart.pc == 312);

    execute(&hart, 0xed5ff0ef); // jal x1, -300
    assert(hart.regs[1] == 316);
    assert(hart.pc == 12);
}

void test_jalr()
{
    struct Hart hart = {0};

    hart.pc = 12;
    hart.regs[1] = 300;

    execute(&hart, 0x00a08167); // jalr x2, 10(x1)
    assert(hart.regs[2] == 16);
    assert(hart.pc == 310);

    execute(&hart, 0xff608167); // jalr x2, -10(x1)
    assert(hart.regs[2] == 314);
    assert(hart.pc == 290);
}

void test_branch()
{
    struct Hart hart = {0};

    hart.pc = 100;
    hart.regs[1] = 2;
    hart.regs[2] = 0xFFFFFFFC;

    execute(&hart, 0x00208c63); // beq x1, x2, 24
    assert(hart.pc == 104);

    hart.pc = 100;
    execute(&hart, 0x00209c63); // bne x1, x2, 24
    assert(hart.pc == 124);

    hart.pc = 100;
    execute(&hart, 0x0020cc63); // blt x1, x2, 24
    assert(hart.pc == 124);

    hart.pc = 100;
    execute(&hart, 0x0020dc63); // bge x1, x2, 24
    assert(hart.pc == 104);

    hart.pc = 100;
    execute(&hart, 0x0020ec63); // bltu x1, x2, 24
    assert(hart.pc == 104);

    hart.pc = 100;
    execute(&hart, 0x0020fc63); // bgeu x1, x2, 24
    assert(hart.pc == 124);
}

void test()
{
    test_addi();
    test_slti_sltiu();
    test_andi_ori_xori();
    test_slli_srli_srai();
    test_lui_auipc();
    test_op();
    test_jal();
    test_jalr();
    test_branch();
}

int main(int argc, char* argv[])
{
    test();

    if (argc < 2)
    {
        printf("Usage: %s <input elf>\n", argv[0]);
        return EXIT_FAILURE;
    }

    uint32_t mem_size = 16 * 1024 * 1024;
    char* mem = (char*)malloc(mem_size);
    uint32_t start_address = 0;

    if (!load_elf(argv[1], mem, mem_size, &start_address))
    {
        printf("Error loading ELF into memory\n");
        return EXIT_FAILURE;
    }

    struct Hart hart = {0};
    hart.pc = start_address;

    while (true)
    {
        uint32_t instruction = *(uint32_t*)(mem + hart.pc);
        execute(&hart, instruction);
        printf("pc=%x x1=%d x2=%d x3=%d x4=%d\n",
            hart.pc, hart.regs[1], hart.regs[2], hart.regs[3], hart.regs[4]);
        fgetc(stdout);
    }
    
    return EXIT_SUCCESS;
}