import strformat
import tables

import value
import chunk
import config

type
  Frame = object
    stackBottom: int # the absolute index of where 0 inside the frame is
    ii: int

  VM* = ref object
    chunk: Chunk
    ii: int
    stack: seq[KonValue]
    line: int
    hadError: bool
    globals: Table[string, KonValue]
    frames: seq[Frame]

  InterpretResult* = enum
    irOK, irRuntimeError


proc newVM*(ch: Chunk): VM =
  VM(chunk: ch, ii: 0, stack: @[], frames: @[Frame()])

proc push(vm: VM, val: KonValue) =
  vm.stack.add(val)

proc pop(vm: VM): KonValue =
  vm.stack.pop()

proc peek(vm: VM): KonValue =
  vm.stack[vm.stack.high]

proc runtimeError(vm: VM, msg: string) =
  write stderr, &"[line: {vm.line}] {msg}\n"
  vm.hadError = true

proc pushSafe(vm: VM, val: KonValue): bool =
  ## returns if the value is not a runtime error
  ## prints the error if it is a runtime error
  ## pushes it to the stack if it is not an error
  if val.isError():
    vm.runtimeError($val)
    return false
  else:
    vm.push(val)
    return true

proc advance(vm: VM): uint8 =
  vm.ii.inc
  vm.line = vm.chunk.lines[vm.ii-1]
  vm.chunk.code[vm.ii-1]

proc readDU8(vm: VM): int =
  [vm.advance, vm.advance].toInt

proc readConstant(vm: VM): KonValue =
  let index = vm.readDU8()
  vm.chunk.constants[index]

proc binary(op: OpCode, left: KonValue, right: KonValue): KonValue =
  case op:
    of opAdd:
      return left.add(right)
    of opSubtract:
      return left.subtract(right)
    of opMultiply:
      return left.multiply(right)
    of opDivide:
      return left.divide(right)
    else:
      discard #unreachable

proc run*(vm: VM): InterpretResult =

  template frameBottom: int = vm.frames[vm.frames.high].stackBottom

  while true:
    let ins = vm.advance.OpCode

    when debugVM:
      let opname = ($ins)
      var msg = &"[{vm.ii:4}] {opname}"
      msg &= " Stack: [ "
      for i in 0 .. vm.stack.high:
        let e = vm.stack[i]
        if i == frameBottom:
          msg &= &"<{e}> "
        else:
          msg &= &"{e} "
      msg &= "]"
      echo msg
    {.computedgoto.} # See https://nim-lang.org/docs/manual.html#pragmas-computedgoto-pragma
    case ins:
      of opConstant:
        let val: KonValue = vm.readConstant()
        vm.push(val)
      of opNegate:
        let val = vm.pop.negate
        if not vm.pushSafe(val):
          break
      of opAdd, opSubtract, opMultiply, opDivide:
        let right = vm.pop()
        let left = vm.pop()
        if not vm.pushSafe(binary(ins, left, right)):
          break
      of opReturn:
        if frameBottom == 0:
          break
        else:
          discard vm.frames.pop() # remove frame that's over
          vm.ii = vm.frames[vm.frames.high].ii # get backed up ii
      of opTrue:
        vm.push(toKonValue(true))
      of opFalse:
        vm.push(toKonValue(false))
      of opNil:
        vm.push(toKonValue())
      of opNot:
        vm.push(toKonValue(vm.pop.isFalsey))
      of opEqual:
        vm.push(toKonValue(vm.pop.equal(vm.pop())))
      of opLess:
        if not vm.pushSafe(vm.pop() > vm.pop()):
          break
      of opGreater:
        if not vm.pushSafe(vm.pop() < vm.pop()):
          break
      of opPrint:
        echo $vm.peek()
      of opDefineGlobal:
        let name = vm.readConstant().stringValue
        if not vm.globals.hasKey(name):
          vm.globals[name] = vm.pop()
        else:
          vm.runtimeError("Attempt to redefine an existing global variable.")
          break
      of opGetGlobal:
        let name = vm.readConstant().stringValue
        if vm.globals.hasKey(name):
          vm.push(vm.globals[name])
        else:
          vm.runtimeError(&"Undefined global variable {name}.")
          break
      of opSetGlobal:
        let name = vm.readConstant().stringValue
        if vm.globals.hasKey(name):
          vm.globals[name] = vm.peek()
        else:
          vm.runtimeError(&"Attempt to set undefined global variable {name}.")
          break
      of opPop:
        discard vm.pop()
      of opGetLocal:
        let slot = vm.readDU8()
        vm.push(vm.stack[slot + frameBottom])
      of opSetLocal:
        let slot = vm.readDU8()
        vm.stack[slot + frameBottom] = vm.peek()
      of opJumpIfFalse:
        let offset = vm.readDU8()
        if vm.peek.isFalsey:
          vm.ii += offset
      of opJumpIfFalsePop:
        let offset = vm.readDU8()
        if vm.pop.isFalsey:
          vm.ii += offset
      of opJump:
        let offset = vm.readDU8()
        vm.ii += offset
      of opLoop:
        let offset = vm.readDU8()
        vm.ii -= offset
      of opCall:
        # create the call env
        # current stack before opCall:
        # ... <funct obj> <arg1> <arg2> <arg3>
        # opCall converts it to this
        # ... <ret val> <arg1> <arg2> <arg3>
        let argcount = vm.readDU8()
        let funct = vm.stack[vm.stack.high - argcount]
        if funct.konType != ktFunct:
          vm.runtimeError("Attempt to call a non-funct (a defunct?).") # here is a bad defunct joke
          break
        if funct.arity != argcount:
          vm.runtimeError(&"Wrong number of arguments, expected {funct.arity}, got {argcount}.")
          break
        # if funct.module ... TODO

        vm.stack[vm.stack.high - argcount] = toKonValue() # replace the function with nil: this is the return value slot

        # save ii
        vm.frames[vm.frames.high].ii = vm.ii

        # create new frame
        vm.frames.add(Frame(stackBottom: vm.stack.high - argcount))

        vm.ii = funct.entryII # jump to the entry point



  when assertionsVM:
    if not vm.hadError and vm.stack.len > 0:
      vm.runtimeError(&"VM Assertion failed: stack is of non-zero length {vm.stack.len} after execution has finished.")
  if vm.hadError:
    irRuntimeError
  else:
    irOK

proc interpretChunk*(ch: Chunk): InterpretResult =
  let vm = newVM(ch)
  return vm.run()