package main

import (
	"fmt"
)

type ErrInvalidOpcode struct{}

func (e ErrInvalidOpcode) Error() string { return "invalid opcode" }

type CPUState struct {
	Registers [32]uint32
	Pc        uint32
	w         World
}

func opcode_rd(opcode uint32) uint32  { return (opcode >> 7) & 0b11111 }
func opcode_rs1(opcode uint32) uint32 { return (opcode >> 15) & 0b11111 } // 2^5 is 32 for 32 registers
func opcode_rs2(opcode uint32) uint32 { return (opcode >> 19) & 0b11111 }

func (c *CPUState) ReadRegister(r uint32) uint32 {
	if r == 0 {
		return 0 // Read from the zero register always returns 0
	}

	return c.Registers[r]
}

func (c *CPUState) Step() error {
	opcode, err := c.w.ReadMemory(c.Pc)
	if err != nil {
		return fmt.Errorf("ReadMemory: %w", err)
	}

	switch opcode & 0b1111111 {
	case 0b0110111:
		// LUI
		panic("todo")

	case 0b0010111:
		// AUIPC
		panic("todo")

	case 0b01101111:
		// JAL
		panic("todo")

	case 0b1100111:
		// JALR
		panic("todo")

	case 0b1100011:
		// BEQ/BNE/BLT/BGE/BLTU/BGEU
		panic("todo")

	case 0b0000011:
		// LB/LH/LW/LBU/LHU
		panic("todo")

	case 0b0100011:
		// SB/SH/SW
		panic("todo")

	case 0b0010011:
		// ADDI/SLTI/SLTIU/XORI/ORI/ANDI/SLLI/SLRI/SRAI
		funct3 := (opcode >> 12) & 0b111

		var imm uint32 = (opcode >> 20) & 0b111111111111
		if imm&0b100000000000 != 0 {
			imm |= 0b11111111111111111111000000000000 // sign extension: if MSB is set in imm, extend with 1's instead of 0's
		}

		switch funct3 {
		case 0b000:
			// ADDI
			// ADDI adds the sign-extended 12-bit immediate to register rs1. Arithmetic overflow is ignored and
			// the result is simply the low XLEN bits of the result. ADDI rd, rs1, 0 is used to implement the MV
			// rd, rs1 assembler pseudoinstruction.
			c.Registers[opcode_rd(opcode)] = c.ReadRegister(opcode_rs1(opcode)) + imm

		case 0b010:
			// SLTI
			// SLTI (set less than immediate) places the value 1 in register rd if register rs1 is less than the
			// signextended immediate when both are treated as signed numbers, else 0 is written to rd. SLTIU is
			// similar but compares the values as unsigned numbers (i.e., the immediate is first sign-extended to
			// XLEN bits then treated as an unsigned number). Note, SLTIU rd, rs1, 1 sets rd to 1 if rs1 equals
			// zero, otherwise sets rd to 0 (assembler pseudoinstruction SEQZ rd, rs).
			if int32(c.ReadRegister(opcode_rs1(opcode))) < int32(imm) {
				c.Registers[opcode_rd(opcode)] = 1
			} else {
				c.Registers[opcode_rd(opcode)] = 0
			}

		case 0b011:
			// SLTIU
			if c.ReadRegister(opcode_rs1(opcode)) < imm {
				c.Registers[opcode_rd(opcode)] = 1
			} else {
				c.Registers[opcode_rd(opcode)] = 0
			}

		case 0b100:
			// XORI
			// ANDI, ORI, XORI are logical operations that perform bitwise AND, OR, and XOR on register rs1
			// and the sign-extended 12-bit immediate and place the result in rd. Note, XORI rd, rs1, -1 performs
			// a bitwise logical inversion of register rs1 (assembler pseudoinstruction NOT rd, rs).
			c.Registers[opcode_rd(opcode)] = c.ReadRegister(opcode_rs1(opcode)) ^ imm

		case 0b110:
			// ORI
			c.Registers[opcode_rd(opcode)] = c.ReadRegister(opcode_rs1(opcode)) | imm

		case 0b111:
			// ANDI
			c.Registers[opcode_rd(opcode)] = c.ReadRegister(opcode_rs1(opcode)) & imm

		case 0b001:
			// SLLI
			// Shifts by a constant are encoded as a specialization of the I-type format. The operand to be shifted
			// is in rs1, and the shift amount is encoded in the lower 5 bits of the I-immediate field. The right
			// shift type is encoded in bit 30. SLLI is a logical left shift (zeros are shifted into the lower bits);
			// SRLI is a logical right shift (zeros are shifted into the upper bits); and SRAI is an arithmetic right
			// shift (the original sign bit is copied into the vacated upper bits).

			shamt := (opcode >> 20) & 0b1111
			c.Registers[opcode_rd(opcode)] = c.ReadRegister(opcode_rs1(opcode)) << shamt

		case 0b101:
			// SRLI/SRAI share the same `funct3`
			shamt := (opcode >> 20) & 0b1111
			distinguish := (opcode >> 25) & 0b1111111
			if distinguish == 0b0000000 {
				// SRLI
				c.Registers[opcode_rd(opcode)] = c.ReadRegister(opcode_rs1(opcode)) >> shamt

			} else if distinguish == 0b0100000 {
				// SRAI
				// Go's shift operator implements arithmetic shifts if the left operand is a signed integer and logical shifts if it is an unsigned integer.
				c.Registers[opcode_rd(opcode)] = uint32(int32(c.ReadRegister(opcode_rs1(opcode))) >> shamt)

			} else {
				return ErrInvalidOpcode{}
			}

		default:
			return ErrInvalidOpcode{}
		}

	case 0b0110011:
		// ADD/SUB/SLL/SLT/SLTU/XOR/SRL/SRA/OR/AND
		panic("todo")

	case 0b0001111:
		// FENCE/FENCE.TSO/PAUSE
		panic("todo")

	case 0b1110011:
		// ECALL/EBREAK
		panic("todo")

	default:
		return ErrInvalidOpcode{}
	}

	return nil
}

type World interface {
	ReadMemory(addr uint32) (uint32, error)
	WriteMemory(addr, value uint32) error
	Syscall()
	Trap() (bool, error)
}

type SampleWorld struct {
	ram [1024]uint32
}

func (sw *SampleWorld) ReadMemory(addr uint32) (uint32, error) {
	if addr >= uint32(len(sw.ram)) {
		panic("out of bounds")
	}

	return sw.ram[addr], nil
}

func (sw *SampleWorld) WriteMemory(addr, value uint32) error {
	if addr >= uint32(len(sw.ram)) {
		panic("out of bounds")
	}

	sw.ram[addr] = value
	return nil
}

func (sw *SampleWorld) Syscall() {
	panic("syscall")
}

func (sw *SampleWorld) Trap() (bool, error) {
	panic("trap")
}

func main() {
	sw := SampleWorld{}
	sw.ram[0] = 0b00000000000100000000000001110011 // EBREAK

	c := CPUState{
		w: &sw,
	}

	err := c.Step()
	panic(err)
}