From 61e463713c8b5ddc8fa7922cefae3d93b53d83d3 Mon Sep 17 00:00:00 2001
From: Luna <git@l4.pm>
Date: Sat, 1 Jun 2019 15:23:23 -0300
Subject: [PATCH] vm: add stack

---
 src/vm.zig | 56 ++++++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 48 insertions(+), 8 deletions(-)

diff --git a/src/vm.zig b/src/vm.zig
index 1765c6c..b4243b8 100644
--- a/src/vm.zig
+++ b/src/vm.zig
@@ -5,6 +5,8 @@ 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,
@@ -13,17 +15,31 @@ pub const InterpretResult = enum {
 
 pub const VM = struct {
     chk: *Chunk,
-    ip: usize,
+    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 {
-        return VM{
-            .stdout = stdout,
+        var self = VM{
             .chk = chk,
-            .ip = 0,
+
+            // 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 {
@@ -53,9 +69,22 @@ pub const VM = struct {
         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);
             }
 
@@ -64,17 +93,18 @@ pub const VM = struct {
             switch (instruction) {
                 chunk.OpCode.Constant => blk: {
                     var constant = self.readConst();
-                    try value.printValue(self.stdout, constant);
-                    try self.stdout.write("\n");
+                    self.push(constant);
                     break :blk;
                 },
                 chunk.OpCode.ConstantLong => blk: {
                     var constant = self.readConstLong();
-                    try value.printValue(self.stdout, constant);
-                    try self.stdout.write("\n");
+                    self.push(constant);
                     break :blk;
                 },
+
                 chunk.OpCode.Return => blk: {
+                    try value.printValue(self.stdout, self.pop());
+                    try self.stdout.print("\n");
                     return InterpretResult.Ok;
                 },
                 else => blk: {
@@ -93,4 +123,14 @@ pub const VM = struct {
         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];
+    }
 };