# frozen_string_literal: true
# https://www.intel.com/content/dam/develop/public/us/en/documents/325383-sdm-vol-2abcd.pdf
module RubyVM::MJIT
  class X86Assembler
    class Label < Data.define(:id, :name); end

    ByteWriter = CType::Immediate.parse('char')

    ### prefix ###
    # REX =   0100WR0B
    REX_W = 0b01001000

    attr_reader :comments

    def initialize
      @bytes = []
      @labels = {}
      @label_id = 0
      @comments = Hash.new { |h, k| h[k] = [] }
    end

    def compile(addr)
      link_labels
      writer = ByteWriter.new(addr)
      # If you pack bytes containing \x00, Ruby fails to recognize bytes after \x00.
      # So writing byte by byte to avoid hitting that situation.
      @bytes.each_with_index do |byte, index|
        writer[index] = byte
      end
      @bytes.size
    ensure
      @bytes.clear
    end

    def add(dst, src)
      case [dst, src]
      # ADD r/m64, imm8 (Mod 11)
      in [Symbol => dst_reg, Integer => src_imm] if r64?(dst_reg) && imm8?(src_imm)
        # REX.W + 83 /0 ib
        # MI: Operand 1: ModRM:r/m (r, w), Operand 2: imm8/16/32
        insn(
          prefix: REX_W,
          opcode: 0x83,
          mod_rm: mod_rm(mod: 0b11, rm: reg_code(dst_reg)),
          imm: imm8(src_imm),
        )
      # ADD r/m64, imm8 (Mod 00)
      in [[Symbol => dst_reg], Integer => src_imm] if r64?(dst_reg) && imm8?(src_imm)
        # REX.W + 83 /0 ib
        # MI: Operand 1: ModRM:r/m (r, w), Operand 2: imm8/16/32
        insn(
          prefix: REX_W,
          opcode: 0x83,
          mod_rm: mod_rm(mod: 0b00, rm: reg_code(dst_reg)), # Mod 00: [reg]
          imm: imm8(src_imm),
        )
      else
        raise NotImplementedError, "add: not-implemented operands: #{dst.inspect}, #{src.inspect}"
      end
    end

    # JZ rel8
    # @param [RubyVM::MJIT::X86Assembler::Label] label
    def jz(label)
      # 74 cb
      insn(opcode: 0x74)
      @bytes.push(label)
    end

    def mov(dst, src)
      case [dst, src]
      # MOV r32 r/m32 (Mod 01)
      in [Symbol => dst_reg, [Symbol => src_reg, Integer => src_disp]] if r32?(dst_reg) && imm8?(src_disp)
        # 8B /r
        # RM: Operand 1: ModRM:reg (w), Operand 2: ModRM:r/m (r)
        insn(
          opcode: 0x8b,
          mod_rm: mod_rm(mod: 0b01, reg: reg_code(dst_reg), rm: reg_code(src_reg)), # Mod 01: [reg]+disp8
          disp: src_disp,
        )
      # MOV r/m64, imm32 (Mod 00)
      in [[Symbol => dst_reg], Integer => src_imm] if r64?(dst_reg)
        # REX.W + C7 /0 id
        # MI: Operand 1: ModRM:r/m (w), Operand 2: imm8/16/32/64
        insn(
          prefix: REX_W,
          opcode: 0xc7,
          mod_rm: mod_rm(mod: 0b00, rm: reg_code(dst_reg)), # Mod 00: [reg]
          imm: imm32(src_imm),
        )
      # MOV r/m64, imm32 (Mod 11)
      in [Symbol => dst_reg, Integer => src_imm] if r64?(dst_reg) && imm32?(src_imm)
        # REX.W + C7 /0 id
        # MI: Operand 1: ModRM:r/m (w), Operand 2: imm8/16/32/64
        insn(
          prefix: REX_W,
          opcode: 0xc7,
          mod_rm: mod_rm(mod: 0b11, rm: reg_code(dst_reg)), # Mod 11: reg
          imm: imm32(src_imm),
        )
      # MOV r64, imm64
      in [Symbol => dst_reg, Integer => src_imm] if r64?(dst_reg) && imm64?(src_imm)
        # REX.W + B8+ rd io
        # OI: Operand 1: opcode + rd (w), Operand 2: imm8/16/32/64
        insn(
          prefix: REX_W,
          opcode: 0xb8 + reg_code(dst_reg),
          imm: imm64(src_imm),
        )
      # MOV r/m64, r64
      in [[Symbol => dst_reg, Integer => dst_disp], Symbol => src_reg] if r64?(dst_reg) && r64?(src_reg) && imm8?(dst_disp)
        # REX.W + 89 /r
        # MR: Operand 1: ModRM:r/m (w), Operand 2: ModRM:reg (r)
        insn(
          prefix: REX_W,
          opcode: 0x89,
          mod_rm: mod_rm(mod: 0b01, reg: reg_code(src_reg), rm: reg_code(dst_reg)), # Mod 01: [reg]+disp8
          disp: dst_disp,
        )
      # MOV r64, r/m64 (Mod 00)
      in [Symbol => dst_reg, [Symbol => src_reg]] if r64?(dst_reg) && r64?(src_reg)
        # REX.W + 8B /r
        # RM: Operand 1: ModRM:reg (w), Operand 2: ModRM:r/m (r)
        insn(
          prefix: REX_W,
          opcode: 0x8b,
          mod_rm: mod_rm(mod: 0b00, reg: reg_code(dst_reg), rm: reg_code(src_reg)), # Mod 00: [reg]
        )
      # MOV r64, r/m64 (Mod 01)
      in [Symbol => dst_reg, [Symbol => src_reg, Integer => src_offset]] if r64?(dst_reg) && r64?(src_reg) && imm8?(src_offset)
        # REX.W + 8B /r
        # RM: Operand 1: ModRM:reg (w), Operand 2: ModRM:r/m (r)
        insn(
          prefix: REX_W,
          opcode: 0x8b,
          mod_rm: mod_rm(mod: 0b01, reg: reg_code(dst_reg), rm: reg_code(src_reg)), # Mod 01: [reg]+disp8
          disp: src_offset,
        )
      else
        raise NotImplementedError, "mov: not-implemented operands: #{dst.inspect}, #{src.inspect}"
      end
    end

    def push(src)
      case src
      # PUSH r64
      in Symbol => src_reg if r64?(src_reg)
        # 50+rd
        # O: Operand 1: opcode + rd (r)
        insn(opcode: 0x50 + reg_code(src_reg))
      else
        raise NotImplementedError, "push: not-implemented operands: #{src.inspect}"
      end
    end

    def pop(dst)
      case dst
      # POP r64
      in Symbol => dst_reg if r64?(dst_reg)
        # 58+ rd
        # O: Operand 1: opcode + rd (r)
        insn(opcode: 0x58 + reg_code(dst_reg))
      else
        raise NotImplementedError, "pop: not-implemented operands: #{dst.inspect}"
      end
    end

    # RET
    def ret
      # Near return: A return to a procedure within the current code segment
      insn(opcode: 0xc3)
    end

    def test(left, right)
      case [left, right]
      # TEST r/m32, r32 (Mod 11)
      in [Symbol => left_reg, Symbol => right_reg] if r32?(left_reg) && r32?(right_reg)
        # 85 /r
        # MR: Operand 1: ModRM:r/m (r), Operand 2: ModRM:reg (r)
        insn(
          opcode: 0x85,
          mod_rm: mod_rm(mod: 0b11, reg: reg_code(right_reg), rm: reg_code(left_reg)), # Mod 11: reg
        )
      else
        raise NotImplementedError, "pop: not-implemented operands: #{dst.inspect}"
      end
    end

    def comment(message)
      @comments[@bytes.size] << message
    end

    def new_label(name)
      Label.new(id: @label_id += 1, name:)
    end

    # @param [RubyVM::MJIT::X86Assembler::Label] label
    def write_label(label)
      @labels[label] = @bytes.size
    end

    def incr_counter(name)
      if C.mjit_opts.stats
        comment("increment counter #{name}")
        mov(:rax, C.rb_mjit_counters[name].to_i)
        add([:rax], 1) # TODO: lock
      end
    end

    private

    def insn(prefix: nil, opcode:, mod_rm: nil, disp: nil, imm: nil)
      if prefix
        @bytes.push(prefix)
      end
      @bytes.push(opcode)
      if mod_rm
        @bytes.push(mod_rm)
      end
      if disp
        if disp < 0 || disp > 0xff # TODO: support displacement in 2 or 4 bytes as well
          raise NotImplementedError, "not-implemented disp: #{disp}"
        end
        @bytes.push(disp)
      end
      if imm
        @bytes.push(*imm)
      end
    end

    def reg_code(reg)
      case reg
      when :al, :ax, :eax, :rax then 0
      when :cl, :cx, :ecx, :rcx then 1
      when :dl, :dx, :edx, :rdx then 2
      when :bl, :bx, :ebx, :rbx then 3
      when :ah, :sp, :esp, :rsp then 4
      when :ch, :bp, :ebp, :rbp then 5
      when :dh, :si, :esi, :rsi then 6
      when :bh, :di, :edi, :rdi then 7
      else raise ArgumentError, "unexpected reg: #{reg.inspect}"
      end
    end

    # Table 2-2. 32-Bit Addressing Forms with the ModR/M Byte
    #
    #  7  6  5  4  3  2  1  0
    # +--+--+--+--+--+--+--+--+
    # | Mod | Reg/   | R/M    |
    # |     | Opcode |        |
    # +--+--+--+--+--+--+--+--+
    #
    # The r/m field can specify a register as an operand or it can be combined
    # with the mod field to encode an addressing mode.
    #
    # /0: R/M is 0 (not used)
    # /r: R/M is a register
    def mod_rm(mod:, reg: 0, rm: 0)
      if mod > 0b11
        raise ArgumentError, "too large Mod: #{mod}"
      end
      if reg > 0b111
        raise ArgumentError, "too large Reg/Opcode: #{reg}"
      end
      if rm > 0b111
        raise ArgumentError, "too large R/M: #{rm}"
      end
      (mod << 6) + (reg << 3) + rm
    end

    # ib: 1 byte
    def imm8(imm)
      unless imm8?(imm)
        raise ArgumentError, "unexpected imm8: #{imm}"
      end
      imm_bytes(imm, 1)
    end

    # id: 4 bytes
    def imm32(imm)
      unless imm32?(imm)
        raise ArgumentError, "unexpected imm32: #{imm}"
      end
      imm_bytes(imm, 4)
    end

    # io: 8 bytes
    def imm64(imm)
      unless imm64?(imm)
        raise ArgumentError, "unexpected imm64: #{imm}"
      end
      imm_bytes(imm, 8)
    end

    def imm_bytes(imm, num_bytes)
      bytes = []
      bits = imm
      num_bytes.times do
        bytes << (bits & 0xff)
        bits >>= 8
      end
      if bits != 0
        raise ArgumentError, "unexpected imm with #{num_bytes} bytes: #{imm}"
      end
      bytes
    end

    def imm8?(imm)
      raise "negative imm not supported: #{imm}" if imm.negative? # TODO: support this
      imm <= 0x7f # TODO: consider uimm
    end

    def imm32?(imm)
      raise "negative imm not supported: #{imm}" if imm.negative? # TODO: support this
      # TODO: consider rejecting small values
      imm <= 0x7fff_ffff # TODO: consider uimm
    end

    def imm64?(imm)
      raise "negative imm not supported: #{imm}" if imm.negative? # TODO: support this
      # TODO: consider rejecting small values
      imm <= 0x7fff_ffff_ffff_ffff # TODO: consider uimm
    end

    def r32?(reg)
      reg.start_with?('e')
    end

    def r64?(reg)
      reg.start_with?('r')
    end

    def link_labels
      @bytes.each_with_index do |byte, index|
        if byte.is_a?(Label)
          src_index = index + 1 # offset 1 byte for rel8 itself
          dst_index = @labels.fetch(byte)
          rel8 = dst_index - src_index
          raise "unexpected offset: #{rel8}" unless imm8?(rel8)
          @bytes[index] = rel8
        end
      end
    end
  end
end