package riscvemu

import (
	"math"
	"os"
)

type VirtualEEI struct {
	memPages map[uint32]*[4096]byte
}

func NewVirtualEEI() VirtualEEI {
	return VirtualEEI{
		memPages: make(map[uint32]*[4096]byte),
	}
}

func (ve *VirtualEEI) ReadByte(addr uint32) (byte, error) {
	page, ok := ve.memPages[addr>>12]
	if !ok {
		return 0, nil
	}

	return page[addr&0b111111111111], nil
}

func (ve *VirtualEEI) Read16(addr uint32) (uint16, error) {
	// n.b. will panic on overflow

	page, ok := ve.memPages[addr>>12]
	if !ok {
		return 0, nil
	}

	inPageAddr := addr & 0b111111111111

	return (uint16(page[inPageAddr+1]) << 8) | uint16(page[inPageAddr]), nil
}

func (ve *VirtualEEI) Read32(addr uint32) (uint32, error) {
	// n.b. will panic on overflow
	// RISC-V allows 32-bit load/stores to be implemented as either little-endian
	// or big-endian. This is the little-endian version

	page, ok := ve.memPages[addr>>12]
	if !ok {
		return 0, nil
	}

	inPageAddr := addr & 0b111111111111

	return (uint32(page[inPageAddr+3]) << 24) | (uint32(page[inPageAddr+2]) << 16) | (uint32(page[inPageAddr+1]) << 8) | uint32(page[inPageAddr]), nil
}

func (ve *VirtualEEI) WriteByte(addr uint32, value byte) error {
	page, ok := ve.memPages[addr>>12]
	if !ok {
		page = new([4096]byte)
		ve.memPages[addr>>12] = page
	}

	// The pointer-to-page-array is a reference type, this will write-through into ve.memPages
	page[addr&0b111111111111] = value
	return nil
}

func (ve *VirtualEEI) Write16(addr uint32, value uint16) error {

	page, ok := ve.memPages[addr>>12]
	if !ok {
		page = new([4096]byte)
		ve.memPages[addr>>12] = page
	}

	inPageAddr := addr & 0b111111111111

	// The pointer-to-page-array is a reference type, this will write-through into ve.memPages
	// n.b. will panic on split page boundary
	page[inPageAddr+1] = byte((value >> 8) & 0b11111111)
	page[inPageAddr] = byte(value & 0b11111111)

	return nil
}

func (ve *VirtualEEI) Write32(addr, value uint32) error {

	page, ok := ve.memPages[addr>>12]
	if !ok {
		page = new([4096]byte)
		ve.memPages[addr>>12] = page
	}
	inPageAddr := addr & 0b111111111111

	// The pointer-to-page-array is a reference type, this will write-through into ve.memPages
	// n.b. will panic on split page boundary
	page[inPageAddr+3] = byte((value >> 24) & 0b11111111)
	page[inPageAddr+2] = byte((value >> 16) & 0b11111111)
	page[inPageAddr+1] = byte((value >> 8) & 0b11111111)
	page[inPageAddr] = byte(value & 0b11111111)

	return nil
}

// WriteAt implements the io.WriterAt interface on top of the EEI's virtual memory.
func (ve *VirtualEEI) WriteAt(p []byte, off int64) (n int, err error) {
	if off < 0 || off >= math.MaxUint32 {
		return 0, os.ErrInvalid
	}

	addr := uint32(off) // bounds-checked

	// TODO do something more optimized
	for i, bb := range p {
		err := ve.WriteByte(addr+uint32(i), bb)
		if err != nil {
			return i, err
		}
	}

	return len(p), nil
}

func (ve *VirtualEEI) Syscall() error {
	panic("syscall")
}

func (ve *VirtualEEI) MemFence(opcode uint32) error {
	// In the virtual EEI, a memory fence is a no-op.
	// It matters for synchronizing multiple cores or for peripherals.
	return nil
}

var _ EEI = &VirtualEEI{} // interface assertion