// https://docs.riscv.org/reference/isa/_attachments/riscv-unprivileged.pdf
// https://riscv.org/wp-content/uploads/2024/12/riscv-calling.pdf
#include <gelf.h>
#include <libelf.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define PARSE_I_INS(ins)                                                       \
  uint8_t rd = (ins >> 7) & 0b11111;                                           \
  uint8_t funct3 = (ins >> 12) & 0b111;                                        \
  uint8_t rs1 = (ins >> 15) & 0b11111;                                         \
  int32_t imm = ((int32_t)ins) >> 20

#define PARSE_U_INS(ins)                                                       \
  uint8_t rd = (ins >> 7) & 0b11111;                                           \
  int32_t imm = ((int32_t)ins) >> 20

static const char *REGS[32] = {
    "zero", "ra", "sp", "gp", "tp",  "t0",  "t1", "t2", "fp", "s1", "a0",
    "a1",   "a2", "a3", "a4", "a5",  "a6",  "a7", "s2", "s3", "s4", "s5",
    "s6",   "s7", "s8", "s9", "s10", "s11", "t3", "t4", "t5", "t6"};

typedef struct {
  size_t offset;
  size_t size;
} Section;

typedef struct {
  uint8_t *memory;
  size_t pc;
  uint64_t regs[32];
  Section code_section;
} RISCV64;

Section riscv64_get_code_section(uint8_t *exe_bytes, size_t exe_size) {
  Elf *elf = elf_memory((char *)exe_bytes, exe_size);
  if (!elf) {
    fprintf(stderr, "elf_begin failed: %s\n", elf_errmsg(-1));
    exit(1);
  }

  size_t str_table_index;
  if (elf_getshdrstrndx(elf, &str_table_index) != 0) {
    fprintf(stderr, "elf_getshdrstrndx failed: %s\n", elf_errmsg(-1));
    exit(1);
  }

  Elf_Scn *section = NULL;
  while ((section = elf_nextscn(elf, section)) != NULL) {
    GElf_Shdr header;
    if (gelf_getshdr(section, &header) != &header)
      continue;

    const char *name = elf_strptr(elf, str_table_index, header.sh_name);
    if (name && strcmp(name, ".text") == 0) {
      elf_end(elf);
      return (Section){.offset = header.sh_offset, .size = header.sh_size};
    }
  }

  fprintf(stderr, "Failed to locate .text\n");
  exit(1);
}

RISCV64 riscv64_load(uint8_t *exe_bytes, size_t exe_size) {
  Section section = riscv64_get_code_section(exe_bytes, exe_size);

  return (RISCV64){
      .memory = exe_bytes,
      .pc = section.offset,
      .regs = {0},
      .code_section = section,
  };
}

void riscv64_dump(RISCV64 *r) {
  printf("REGS:");
  for (size_t i = 0; i < 32; i++) {
    printf(" %lu", r->regs[i]);
  }
  printf("\n");
}

void riscv64_disassemble(RISCV64 *r) {
  r->pc = r->code_section.offset;

  while (r->pc < r->code_section.offset + r->code_section.size) {
    uint32_t ins;
    memcpy(&ins, r->memory + r->pc, 4);

    uint16_t opcode = ins & 0b1111111;

    // https://stackoverflow.com/questions/62939410/how-can-i-find-out-the-instruction-format-of-a-risc-v-instruction
    switch (opcode) {
    case 0b1100011:
      printf("B-type\n");
      break;
    case 0b0010011: {
      PARSE_I_INS(ins);

      switch (funct3) {
      case 0b000:
        printf("addi %s, %s, %d\n", REGS[rd], REGS[rs1], imm);
        break;
      default:
        fprintf(stderr, "I-type 1: unrecognized funct3: %03b\n", funct3);
        exit(1);
      }
    }; break;
    case 0b0000011:
      printf("I-type 2\n");
      break;
    case 0b1100111:
      printf("I-type 3\n");
      break;
    case 0b1110011: {
      PARSE_I_INS(ins);

      switch (funct3) {
      case 0b000:
        switch (imm) {
        case 0:
          printf("ecall\n");
          break;
        default:
          fprintf(stderr, "I-type 4: funct3=000 unrecognized imm: %b\n", imm);
          exit(1);
        };
        break;
      default:
        fprintf(stderr, "I-type 4: unrecognized funct3: %03b\n", funct3);
        exit(1);
      }
    }; break;
    case 0b1101111:
      printf("J-type\n");
      break;
    case 0b0110011:
      printf("R-type 1\n");
      break;
    case 0b0101111:
      printf("R-type 2\n");
      break;
    case 0b0111011:
      printf("R-type 3\n");
      break;
    case 0b0100011:
      printf("S-type\n");
      break;
    case 0b0110111:
      printf("U-type 1\n");
      break;
    case 0b0010111: {
      PARSE_U_INS(ins);
      printf("auipc %s, %d\n", REGS[rd], imm);
    }; break;
    default:
      fprintf(stderr, "Unrecognized opcode: %07b\n", opcode);
      exit(1);
    }

    r->pc += 4;
  }
}

void riscv64_execute(RISCV64 *r) {
  r->pc = r->code_section.offset;

  while (r->pc < r->code_section.offset + r->code_section.size) {
    r->regs[0] = 0; // clear the zero register

    uint32_t ins;
    memcpy(&ins, r->memory + r->pc, 4);

    uint16_t opcode = ins & 0b1111111;

    // https://stackoverflow.com/questions/62939410/how-can-i-find-out-the-instruction-format-of-a-risc-v-instruction
    switch (opcode) {
    case 0b0010011: {
      PARSE_I_INS(ins);

      switch (funct3) {
      case 0b000:
        r->regs[rd] = r->regs[rs1] + imm;
        break;
      default:
        fprintf(stderr, "I-type 1: unrecognized funct3: %03b\n", funct3);
        exit(1);
      }
    }; break;
    case 0b1110011: {
      PARSE_I_INS(ins);

      switch (funct3) {
      case 0b000:
        switch (imm) {
        case 0:
          // https://jborza.com/post/2021-05-11-riscv-linux-syscalls/
          switch (r->regs[17]) {
          case 64: { // write
            if (r->regs[10] != 1) {
              fprintf(stderr, "write syscall is implemented only for stdout\n");
              exit(1);
            }

            size_t start = r->regs[11];
            size_t end = r->regs[11] + r->regs[12];

            for (size_t i = start; i < end; i++) {
              putchar(r->memory[i]);
            }
          }; break;
          case 93: { // exit
            printf("Program exited with code %lu.\n", r->regs[11]);
            return;
          }; break;
          default:
            fprintf(stderr, "Unimplemented syscall: %lu\n", r->regs[17]);
            exit(1);
          }
          break;
        default:
          fprintf(stderr, "I-type 4: funct3=000 unrecognized imm: %b\n", imm);
          exit(1);
        };
        break;
      default:
        fprintf(stderr, "I-type 4: unrecognized funct3: %03b\n", funct3);
        exit(1);
      }
    }; break;
    case 0b0010111: {
      PARSE_U_INS(ins);
      r->regs[rd] = r->pc + (imm << 12);
    }; break;
    default:
      fprintf(stderr, "Unrecognized opcode: %07b\n", opcode);
      exit(1);
    }

    r->pc += 4;
  }
}

int main(int argc, char *argv[]) {
  if (elf_version(EV_CURRENT) == EV_NONE) {
    fprintf(stderr, "Failed to initialize libelf: %s\n", elf_errmsg(-1));
    return 1;
  }

  const char *path = NULL;
  for (int i = 1; i < argc; i++) {
    path = argv[i];
  }

  if (path == NULL) {
    fprintf(stderr, "Usage: %s <path>\n", argv[0]);
    return 1;
  }

  FILE *file = fopen(path, "rb");
  if (!file) {
    fprintf(stderr, "Failed to open %s\n", path);
    return 1;
  }

  // probably enough for anything we can handle
  uint8_t *exe_bytes = malloc(20 * 1024 * 1024);
  size_t exe_size = fread(exe_bytes, 1, 20 * 1024 * 1024, file);
  fclose(file);

  RISCV64 r = riscv64_load(exe_bytes, exe_size);
  riscv64_disassemble(&r);
  riscv64_execute(&r);
}