nondescript/vm.nim

import strformat
import tables

import value
import chunk
import config
import pointerutils

import types/stack
import types/ndstring

when profileInstructions:
  import times
  import std/monotimes

  # not a perfect profiling approach, doing random stacktrace dumps
  # x amount of times per second to get the current source line
  # would be better
  var durations: array[OpCode, Duration]
  var runcounts: array[OpCode, float64]

type
  Frame = object
    stackBottom: int # the absolute index of where 0 inside the frame is
    returnIp: ptr uint8

  InterpretResult* = enum
    irOK, irRuntimeError

proc run*(chunk: Chunk): InterpretResult =

  var
    ip: ptr uint8 = chunk.code[0].unsafeAddr
    stack: Stack[NdValue] = newStack[NdValue](256)
    hadError: bool
    globals: Table[string, NdValue]
    frames: Stack[Frame] = newStack[Frame](4)

  proc runtimeError(msg: string) =
    let ii = ip.pdiff(chunk.code[0].unsafeAddr)
    let line = chunk.lines[ii]
    write stderr, &"[line: {line}] {msg}\n"
    hadError = true

  frames.add(Frame(stackBottom: 0))

  #template peek(stack: seq[NdValue]): NdValue =
  #  stack[stack.high]

  #proc popn(stack: var seq[NdValue], amt: int) {.inline.} =
  #  for i in countup(1, amt):
  #    discard stack.pop()
  template popn(stack: var Stack[NdValue], amt: int) =
    stack.deleteTopN(amt)

  proc readUI8(): int =
    result = ip[].int
    ip = ip.padd(1)
  
  proc readDU8(): int =
    result = ip.DU8ptrToInt
    ip = ip.padd(argSize)

  proc readConstant(): NdValue =
    let index = readDU8()
    chunk.constants[index]
  
  #template frameBottom: int = frames.peek().stackBottom
  template frameBottom: int = frames[frames.high].stackBottom

  while true:
    {.computedgoto.} # See https://nim-lang.org/docs/manual.html#pragmas-computedgoto-pragma

    let ins = ip[].OpCode
    ip = ip.padd(1)

    when debugVM:
      let ii = ip.pdiff(chunk.code[0].unsafeAddr)
      let opname = ($ins)
      var msg = &"[{ii:4}] {opname}"
      msg &= " Stack: [ "
      for i in 0 .. stack.high():
        let e = stack[i]
        if i == frameBottom:
          msg &= &"<{e}> "
        else:
          msg &= &"{e} "
      msg &= "]"
      echo msg
    
    when profileInstructions:
      let startTime = getMonoTime()
      runtimes[ins].inc

    case ins:
      of opPop:
        discard stack.pop()
      of opPopA:
        let amt = readDU8()
        stack.popn(amt)
      of opPopSA:
        let amt = readUI8()
        stack.popn(amt)
      of opConstant:
        let val: NdValue = readConstant()
        stack.add(val)
      of opNegate:
        let res = stack.peek().negate()
        if not res.ok:
          runtimeError(res.msg)
          break
      of opAdd:
        let right = stack.pop()
        let res = stack.peek().add(right)
        if not res.ok:
          runtimeError(res.msg)
          break
      of opSubtract:
        let right = stack.pop()
        let res = stack.peek().subtract(right)
        if not res.ok:
          runtimeError(res.msg)
          break
      of opMultiply:
        let right = stack.pop()
        let res = stack.peek().multiply(right)
        if not res.ok:
          runtimeError(res.msg)
          break
      of opDivide:
        let right = stack.pop()
        let res = stack.peek().divide(right)
        if not res.ok:
          runtimeError(res.msg)
          break
      of opReturn:
        if frames.len() == 1:
          break
        else:
          ip = frames.pop().returnIp # remove frame that's over
      of opTrue:
        stack.add(toNdValue(true))
      of opFalse:
        stack.add(toNdValue(false))
      of opNil:
        stack.add(toNdValue())
      of opNot:
        stack.add(toNdValue(stack.pop().isFalsey))
      of opEqual:
        stack.add(toNdValue(stack.pop().equal(stack.pop())))
      of opLess:
        let right = stack.pop()
        let res = stack.peek().less(right)
        if not res.ok:
          runtimeError(res.msg)
          break
      of opGreater:
        let right = stack.pop()
        let res = stack.peek().greater(right)
        if not res.ok:
          runtimeError(res.msg)
          break
      of opPrint:
        echo $stack.peek()
      of opDefineGlobal:
        let name = $readConstant().stringValue
        if not globals.hasKey(name):
          globals[name] = stack.pop()
        else:
          runtimeError("Attempt to redefine an existing global variable.")
          break
      of opGetGlobal:
        let name = $readConstant().stringValue
        if globals.hasKey(name):
          stack.add(globals[name])
        else:
          runtimeError(&"Undefined global variable {name}.")
          break
      of opSetGlobal:
        let name = $readConstant().stringValue
        if globals.hasKey(name):
          globals[name] = stack.peek()
        else:
          runtimeError(&"Attempt to set undefined global variable {name}.")
          break
      of opGetLocal:
        let slot = readDU8()
        stack.add(stack[slot + frameBottom])
      of opSetLocal:
        let slot = readDU8()
        stack[slot + frameBottom] = stack.peek()
      of opJumpIfFalse:
        let offset = readDU8()
        if stack.peek().isFalsey:
          ip = ip.padd(offset)
      of opJumpIfFalsePop:
        let offset = readDU8()
        if stack.pop().isFalsey:
          ip = ip.padd(offset)
      of opJump:
        let offset = readDU8()
        ip = ip.padd(offset)
      of opLoop:
        let offset = readDU8()
        ip = ip.psub(offset)
      of opCheckArity:
        let arity = readUI8()
        let argcount = stack.high() - frameBottom
        if arity != argcount:
          runtimeError(&"Wrong number of arguments, expected {arity}, got {argcount}.")
          break
      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 = readUI8()
        let funct = stack.getIndexNeg(argcount)
        if funct.ndType != ntFunct:
          runtimeError("Attempt to call a non-funct (a defunct?).") # here is a bad defunct joke
          break

        stack.setIndexNeg(argcount, toNdValue()) # replace the function with nil: this is the return value slot

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

        ip = chunk.code[0].unsafeAddr.padd(funct.entryII) # jump to the entry point

    when profileInstructions:
      durations[ins] += getMonoTime() - startTime

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

  when profileInstructions:
    for op in OpCode:
      let dur = durations[op].inMilliseconds
      let times = runcounts[op]
      echo &"OpCode: {op} total duration {dur} ms {times} times"

  stack.free()
  frames.free()

  if hadError:
    irRuntimeError
  else:
    irOK