package vm

import (
	"encoding/binary"
	"errors"

	"github.com/CityOfZion/neo-go/pkg/crypto/hash"
	"github.com/CityOfZion/neo-go/pkg/util"
	"github.com/CityOfZion/neo-go/pkg/vm/opcode"
)

// Context represents the current execution context of the VM.
type Context struct {
	// Instruction pointer.
	ip int

	// The next instruction pointer.
	nextip int

	// The raw program script.
	prog []byte

	// Breakpoints.
	breakPoints []int

	// Return value count, -1 is unspecified.
	rvcount int

	// Evaluation stack pointer.
	estack *Stack

	// Alt stack pointer.
	astack *Stack

	// Script hash of the prog.
	scriptHash util.Uint160
}

var errNoInstParam = errors.New("failed to read instruction parameter")

// NewContext returns a new Context object.
func NewContext(b []byte) *Context {
	return &Context{
		prog:        b,
		breakPoints: []int{},
		rvcount:     -1,
	}
}

// Next returns the next instruction to execute with its parameter if any. After
// its invocation the instruction pointer points to the instruction being
// returned.
func (c *Context) Next() (opcode.Opcode, []byte, error) {
	var err error

	c.ip = c.nextip
	if c.ip >= len(c.prog) {
		return opcode.RET, nil, nil
	}

	var instrbyte = c.prog[c.ip]
	instr := opcode.Opcode(instrbyte)
	c.nextip++

	var numtoread int
	switch instr {
	case opcode.PUSHDATA1, opcode.SYSCALL:
		if c.nextip >= len(c.prog) {
			err = errNoInstParam
		} else {
			numtoread = int(c.prog[c.nextip])
			c.nextip++
		}
	case opcode.PUSHDATA2:
		if c.nextip+1 >= len(c.prog) {
			err = errNoInstParam
		} else {
			numtoread = int(binary.LittleEndian.Uint16(c.prog[c.nextip : c.nextip+2]))
			c.nextip += 2
		}
	case opcode.PUSHDATA4:
		if c.nextip+3 >= len(c.prog) {
			err = errNoInstParam
		} else {
			var n = binary.LittleEndian.Uint32(c.prog[c.nextip : c.nextip+4])
			if n > MaxItemSize {
				return instr, nil, errors.New("parameter is too big")
			}
			numtoread = int(n)
			c.nextip += 4
		}
	case opcode.JMP, opcode.JMPIF, opcode.JMPIFNOT, opcode.CALL, opcode.CALLED, opcode.CALLEDT:
		numtoread = 2
	case opcode.CALLI:
		numtoread = 4
	case opcode.APPCALL, opcode.TAILCALL:
		numtoread = 20
	case opcode.CALLE, opcode.CALLET:
		numtoread = 22
	default:
		if instr >= opcode.PUSHBYTES1 && instr <= opcode.PUSHBYTES75 {
			numtoread = int(instr)
		} else {
			// No parameters, can just return.
			return instr, nil, nil
		}
	}
	if c.nextip+numtoread-1 >= len(c.prog) {
		err = errNoInstParam
	}
	if err != nil {
		return instr, nil, err
	}
	parameter := make([]byte, numtoread)
	copy(parameter, c.prog[c.nextip:c.nextip+numtoread])
	c.nextip += numtoread
	return instr, parameter, nil
}

// IP returns the absolute instruction without taking 0 into account.
// If that program starts the ip = 0 but IP() will return 1, cause its
// the first instruction.
func (c *Context) IP() int {
	return c.ip + 1
}

// LenInstr returns the number of instructions loaded.
func (c *Context) LenInstr() int {
	return len(c.prog)
}

// CurrInstr returns the current instruction and opcode.
func (c *Context) CurrInstr() (int, opcode.Opcode) {
	return c.ip, opcode.Opcode(c.prog[c.ip])
}

// Copy returns an new exact copy of c.
func (c *Context) Copy() *Context {
	ctx := new(Context)
	*ctx = *c
	return ctx
}

// Program returns the loaded program.
func (c *Context) Program() []byte {
	return c.prog
}

// ScriptHash returns a hash of the script in the current context.
func (c *Context) ScriptHash() util.Uint160 {
	if c.scriptHash.Equals(util.Uint160{}) {
		c.scriptHash = hash.Hash160(c.prog)
	}
	return c.scriptHash
}

// Value implements StackItem interface.
func (c *Context) Value() interface{} {
	return c
}

func (c *Context) atBreakPoint() bool {
	for _, n := range c.breakPoints {
		if n == c.ip {
			return true
		}
	}
	return false
}

func (c *Context) String() string {
	return "execution context"
}