# Copyright 2022 Mattia Giambirtone & All Contributors
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#    http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

## The Peon runtime environment
import types
import ../config
import ../frontend/meta/bytecode


type 
    PeonVM* = ref object
        ## The Peon Virtual Machine
        stack: seq[PeonObject]
        ip: int   # Instruction pointer
        sp: int   # Stack pointer
        cache: array[6, PeonObject]  # Singletons cache
        chunk: Chunk   # Piece of bytecode to execute


proc newPeonVM*: PeonVM =
    ## Initializes a new, blank VM
    ## for executing Peon bytecode
    new(result)
    result.ip = 0
    result.sp = 0
    result.stack = newSeqOfCap[PeonObject](INITIAL_STACK_SIZE)
    result.cache[0] = newNil()
    result.cache[1] = newBool(true)
    result.cache[2] = newBool(false)
    result.cache[3] = newInf(true)
    result.cache[4] = newInf(false)
    result.cache[5] = newNan()
    for _ in 0..<INITIAL_STACK_SIZE:
        result.stack.add(result.cache[0])


proc getNil*(self: PeonVM): Nil = Nil(self.cache[0])

proc getBool*(self: PeonVM, value: bool): Bool =
    if value:
        return Bool(self.cache[1])
    return Bool(self.cache[2])

proc getInf*(self: PeonVM, positive: bool): Inf =
    if positive:
        return types.Inf(self.cache[3])
    return types.Inf(self.cache[4])

proc getNan*(self: PeonVM): Nan = types.Nan(self.cache[5])


proc push(self: PeonVM, obj: PeonObject) =
    if self.sp >= self.stack.high():
        for _ in 0..self.stack.len():
            self.stack.add(newNil())
    self.stack[self.sp] = obj
    inc(self.sp)


proc pop(self: PeonVM): PeonObject =
    dec(self.sp)
    return self.stack[self.sp]


proc readByte(self: PeonVM, chunk: Chunk): uint8 =
    inc(self.ip)
    return chunk.code[self.ip - 1]


proc dispatch*(self: PeonVM) =
    ## Main bytecode dispatch loop
    var instruction: OpCode
    while true:
        stdout.write("[")
        for i, e in self.stack:
            stdout.write($(e[]))
            if i < self.stack.high():
                stdout.write(", ")
        echo "]"
        instruction = OpCode(self.readByte(self.chunk))
        echo instruction
        case instruction:
            of OpCode.True:
                self.push(self.getBool(true))
            of OpCode.False:
                self.push(self.getBool(false))
            of OpCode.Nan:
                self.push(self.getNan())
            of OpCode.Nil:
                self.push(self.getNil())
            of OpCode.Inf:
                self.push(self.getInf(true))
            of UnaryPlus:
                self.push(self.pop())
            of OpCode.Return:
                return
            else:
                discard


proc run*(self: PeonVM, chunk: Chunk) =
    ## Executes a piece of Peon bytecode.
    self.chunk = chunk
    self.sp = 0
    self.ip = 0
    self.dispatch()