add a basic filesystem library and use it for path concat

src/lua/fs.zig

@@ -0,0 +1,43 @@
+const Fs = @This();
+
+const std = @import("std");
+const zlua = @import("zlua");
+
+const Lua = @import("lua.zig");
+
+const gpa = std.heap.c_allocator;
+
+pub fn joinpath(L: *zlua.Lua) i32 {
+  const nargs: i32 = L.getTop();
+  if (nargs < 2) {
+    L.raiseErrorStr("Expected at least two paths to join", .{});
+    return 0;
+  }
+
+  var paths = std.ArrayList([:0]const u8).initCapacity(gpa, @intCast(nargs)) catch {
+    return 0;
+  };
+  defer paths.deinit(gpa);
+
+  var i: u8 = 1;
+  while (i <= nargs) : (i += 1) {
+    if (!L.isString(i)) {
+      L.raiseErrorStr("Expected string at argument %d", .{i});
+      return 0;
+    }
+
+    const partial_path = L.toString(i) catch unreachable;
+    paths.append(gpa, partial_path) catch {
+      // TODO: tell lua?
+      return 0;
+    };
+  }
+
+  const final_path: []const u8 = std.fs.path.join(gpa, paths.items) catch {
+    // TODO: tell lua?
+    return 0;
+  };
+  _ = L.pushString(final_path);
+
+  return 1;
+}

src/lua/lua.zig

@@ -5,6 +5,7 @@ const config = @import("config");
 const zlua = @import("zlua");
 
 const Bridge = @import("bridge.zig");
+const Fs = @import("fs.zig");
 
 const gpa = std.heap.c_allocator;
 
@@ -51,6 +52,11 @@ pub fn init(self: *Lua) !void {
       self.state.newTable();
       defer _ = self.state.setField(-2, "path");
     }
+    {
+      const funcs = zlua.fnRegsFromType(Fs);
+      self.state.newLib(funcs);
+      self.state.setField(-2, "fs");
+    }
   }
 
   loadRuntimeDir(self) catch |err| {