const std = @import("std");
const chunk = @import("chunk.zig");
const value = @import("value.zig");

const Chunk = chunk.Chunk;
const StdOut = *std.io.OutStream(std.fs.File.WriteError);

pub const STACK_MAX = 256;

pub const InterpretResult = enum {
    Ok,
    CompileError,
    RuntimeError,
};

pub const VM = struct {
    chk: *Chunk,
    ip: usize = 0,

    stack: [STACK_MAX]value.Value,
    stackTop: usize = 0,

    stdout: StdOut,
    debug_flag: bool,

    fn resetStack(self: *VM) void {
        self.stackTop = 0;
    }

    pub fn init(stdout: StdOut, chk: *Chunk, debug_flag: bool) VM {
        var self = VM{
            .chk = chk,

            // TODO move this to a nil value or something.
            .stack = []value.Value{0} ** STACK_MAX,
            .stdout = stdout,
            .debug_flag = debug_flag,
        };

        self.resetStack();

        return self;
    }

    pub fn debug(self: *VM, comptime fmt: []const u8, args: ...) void {
        if (self.debug_flag) {
            std.debug.warn(fmt, args);
        }
    }

    fn readByte(self: *VM) u8 {
        var byte: u8 = self.chk.code[self.ip];
        self.ip += 1;
        return byte;
    }

    fn readConst(self: *VM) value.Value {
        return self.chk.constants.values[self.readByte()];
    }

    fn readConstLong(self: *VM) value.Value {
        const v3 = self.readByte();
        const v2 = self.readByte();
        const v1 = self.readByte();
        const const_idx = (@intCast(u24, v3) << 16) |
            (@intCast(u24, v2) << 8) |
            v1;

        return self.chk.constants.values[const_idx];
    }

    fn debugStack(self: *VM) !void {
        try self.stdout.print("          ");
        for (self.stack) |val, idx| {
            if (idx >= self.stackTop) break;

            try self.stdout.print("[ ");
            try value.printValue(self.stdout, val);
            try self.stdout.print(" ]");
        }
        try self.stdout.print("\n");
    }

    fn run(self: *VM) !InterpretResult {
        while (true) {
            if (self.debug_flag) {
                try self.debugStack();
                _ = try self.chk.disassembleInstruction(self.stdout, self.ip);
            }

            var instruction = self.readByte();

            switch (instruction) {
                chunk.OpCode.Constant => blk: {
                    var constant = self.readConst();
                    self.push(constant);
                    break :blk;
                },
                chunk.OpCode.ConstantLong => blk: {
                    var constant = self.readConstLong();
                    self.push(constant);
                    break :blk;
                },

                chunk.OpCode.Return => blk: {
                    try value.printValue(self.stdout, self.pop());
                    try self.stdout.print("\n");
                    return InterpretResult.Ok;
                },

                chunk.OpCode.Negate => self.push(-self.pop()),
                else => blk: {
                    std.debug.warn("Unknown instruction: {x}\n", instruction);
                    return InterpretResult.RuntimeError;
                },
            }
        }
    }

    pub fn interpret(self: *VM) !InterpretResult {
        self.ip = 0;

        self.debug("VM start\n");
        var res = try self.run();
        self.debug("VM end\n");
        return res;
    }

    pub fn push(self: *VM, val: value.Value) void {
        self.stack[self.stackTop] = val;
        self.stackTop += 1;
    }

    pub fn pop(self: *VM) value.Value {
        self.stackTop -= 1;
        return self.stack[self.stackTop];
    }
};