package vm

import "fmt"

import "github.com/pkg/errors"

const GlobalMemoryEntries = 16

type State struct {
	Running bool
	Error   error

	Functions        [][]*Instruction
	FunctionIndex    int64
	InstructionIndex int64

	ComparisonResult bool
	GlobalMemory     [GlobalMemoryEntries]uint64
	Stack            []*StackFrame
}

func NewState(byteCodes [][]byte) (*State, error) {
	state := &State{}

	for i, byteCode := range byteCodes {
		instrs := []*Instruction{}

		for start := 0; start < len(byteCode); start += InstructionBytes {
			chunk := byteCode[start : start+InstructionBytes]
			instr, err := NewInstruction(chunk)
			if err != nil {
				return nil, errors.Wrapf(err, "At function index %d, byte offset %d", i, start)
			}
			instrs = append(instrs, instr)
		}

		instrs = append(instrs, &Instruction{
			OpCode: OpReturn,
		})

		state.Functions = append(state.Functions, instrs)
	}

	return state, nil
}

func (state *State) StackFrame() *StackFrame {
	return state.Stack[len(state.Stack)-1]
}

func (state *State) Function() []*Instruction {
	return state.Functions[state.FunctionIndex]
}

func (state *State) Execute() {
	state.setHandlers()
	state.call(0)
	state.Running = true

	for state.Running {
		state.ProcessInstruction()
	}
}

func (state *State) setError(err error) {
	state.Error = err
	state.Running = false
}

func (state *State) setHandlers() {
	for _, fnc := range state.Functions {
		for _, instr := range fnc {
			handler, found := OpHandlers[instr.OpCode]
			if !found {
				state.setError(fmt.Errorf("Invalid OpCode: 0x%08x", instr.OpCode))
				return
			}

			instr.opHandler = handler
		}
	}
}

func (state *State) ProcessInstruction() {
	fnc := state.Function()
	instr := fnc[state.InstructionIndex]
	state.InstructionIndex += 1
	instr.opHandler(state, instr)
}

func (state *State) ReadUnsigned(op *Operand) uint64 {
	switch op.Type {
	case Literal:
		return op.Value

	case FunctionMemoryIndex:
		if op.Value >= FunctionMemoryEntries {
			state.setError(fmt.Errorf("Invalid function memory index: %016x", op.Value))
			return 0
		}
		return state.StackFrame().FunctionMemory[op.Value]

	case GlobalMemoryIndex:
		if op.Value >= GlobalMemoryEntries {
			state.setError(fmt.Errorf("Invalid global memory index: %016x", op.Value))
			return 0
		}
		return state.GlobalMemory[op.Value]

	default:
		state.setError(fmt.Errorf("Unknown operand type: 0x%02x", op.Type))
		return 0
	}
}

func (state *State) ReadSigned(op *Operand) int64 {
	return int64(state.ReadUnsigned(op))
}

func (state *State) WriteUnsigned(op *Operand, value uint64) {
	switch op.Type {
	case Literal:
		state.setError(fmt.Errorf("Write to literal operand"))

	case FunctionMemoryIndex:
		if op.Value >= FunctionMemoryEntries {
			state.setError(fmt.Errorf("Invalid function memory index: %016x", op.Value))
			return
		}
		state.StackFrame().FunctionMemory[op.Value] = value

	case GlobalMemoryIndex:
		if op.Value >= GlobalMemoryEntries {
			state.setError(fmt.Errorf("Invalid global memory index: %016x", op.Value))
			return
		}
		state.GlobalMemory[op.Value] = value

	default:
		state.setError(fmt.Errorf("Unknown operand type: 0x%02x", op.Type))
	}
}

func (state *State) WriteSigned(op *Operand, value int64) {
	state.WriteUnsigned(op, uint64(value))
}

func (state *State) call(functionOffset int64) {
	if state.FunctionIndex+functionOffset >= int64(len(state.Functions)) {
		state.setError(fmt.Errorf("Invalid function call index: %d + %d = %d", state.FunctionIndex, functionOffset, state.FunctionIndex+functionOffset))
		return
	}

	stackFrame := &StackFrame{
		PreviousFunctionIndex:    state.FunctionIndex,
		PreviousInstructionIndex: state.InstructionIndex,
	}
	state.Stack = append(state.Stack, stackFrame)
	state.FunctionIndex += functionOffset
	state.InstructionIndex = 0
}

func (state *State) ret() {
	state.FunctionIndex = state.StackFrame().PreviousFunctionIndex
	state.InstructionIndex = state.StackFrame().PreviousInstructionIndex
	state.Stack = state.Stack[:len(state.Stack)-1]
	if len(state.Stack) == 0 {
		state.Running = false
	}
}