package vm

import "fmt"

type State struct {
	running bool
	err     error

	program          *Program
	functionIndex    int64
	instructionIndex int64

	comparisonResult bool
	globalMemory     *Memory
	stack            []*stackFrame
}

func NewState(prog *Program) (*State, error) {
	return &State{
		program:      prog,
		globalMemory: NewMemory(16),
	}, nil
}

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

	for state.running {
		state.processInstruction()
	}

	return state.err
}

func (state *State) GlobalMemory() *Memory {
	return state.globalMemory
}

func (state *State) stackFrame() *stackFrame {
	return state.stack[len(state.stack)-1]
}

func (state *State) function() *Function {
	return state.program.Functions[state.functionIndex]
}

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

func (state *State) setHandlers() {
	for _, fnc := range state.program.Functions {
		for _, instr := range fnc.Instructions {
			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.Instructions[state.instructionIndex]
	state.instructionIndex += 1
	instr.opHandler(state, instr)

	if state.functionIndex >= int64(len(state.program.Functions)) {
		state.ret()
	}

	if state.instructionIndex >= int64(len(fnc.Instructions)) {
		state.ret()
	}
}

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

	case FunctionMemoryIndex:
		value, err := state.stackFrame().functionMemory.ReadUnsigned(op.Value)
		if err != nil {
			state.setError(err)
		}
		return value

	case GlobalMemoryIndex:
		value, err := state.globalMemory.ReadUnsigned(op.Value)
		if err != nil {
			state.setError(err)
		}
		return 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:
		err := state.stackFrame().functionMemory.WriteUnsigned(op.Value, value)
		if err != nil {
			state.setError(err)
		}

	case GlobalMemoryIndex:
		err := state.globalMemory.WriteUnsigned(op.Value, value)
		if err != nil {
			state.setError(err)
		}

	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) {
	stackFrame := newStackFrame(state)
	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
	}
}

func (state *State) jump(instructionOffset int64) {
	// -1 accounts for the +1 processInstruction()
	state.instructionIndex += instructionOffset - 1
}