feat: formatting strings at runtime

src/Story.zig

@@ -31,6 +31,7 @@ stack: []Value = &.{},
 call_stack: []CallFrame = &.{},
 code_chunks: std.ArrayListUnmanaged(*Object.Code) = .empty,
 /// Linked list of all tracked runtime objects.
+/// We don't currently have a garbage collector, though this will necessary.
 gc_objects: std.SinglyLinkedList = .{},
 /// Global constants pool.
 constants_pool: []const Value = &.{},
@@ -110,6 +111,9 @@ pub const Opcode = enum(u8) {
     br_table,
     br_dispatch,
     br_select_index,
+    string_builder,
+    string_append,
+    string_freeze,
     _,
 };
 
@@ -138,6 +142,13 @@ pub const Value = union(enum) {
         };
     }
 
+    pub fn castObject(v: Value, comptime T: type) *T {
+        return switch (v) {
+            .object => |object| return @ptrCast(object),
+            else => unreachable,
+        };
+    }
+
     pub fn isNumeric(v: Value) bool {
         return v == .int or v == .float;
     }
@@ -230,24 +241,20 @@ pub const Value = union(enum) {
     pub fn eql(lhs: Value, rhs: Value) bool {
         return switch (lhs) {
             .nil => rhs == .nil,
-
             .bool => |l| switch (rhs) {
                 .bool => |r| l == r,
                 else => false,
             },
-
             .int => |l| switch (rhs) {
                 .int => |r| l == r,
                 .float => |r| @as(f64, @floatFromInt(l)) == r,
                 else => false,
             },
-
             .float => |l| switch (rhs) {
                 .int => |r| l == @as(f64, @floatFromInt(r)),
                 .float => |r| l == r,
                 else => false,
             },
-
             .object => |lobj| switch (rhs) {
                 .object => |robj| Object.eql(lobj, robj),
                 else => false,
@@ -771,6 +778,32 @@ fn step(vm: *Story, variables: *VariablesState) !StepSignal {
                     return error.InvalidArgument;
                 }
             },
+            .string_builder => {
+                const builder_object = try Object.StringBuilder.create(vm);
+                try pushStack(vm, .{ .object = &builder_object.base });
+                frame.ip += 1;
+            },
+            .string_append => {
+                if (popStack(vm)) |value| {
+                    if (peekStack(vm, 0)) |builder| {
+                        const string_builder = builder.castObject(Object.StringBuilder);
+                        try string_builder.append(value);
+                        frame.ip += 1;
+                        continue;
+                    }
+                }
+                return error.InvalidArgument;
+            },
+            .string_freeze => {
+                if (popStack(vm)) |value| {
+                    const string_builder = value.castObject(Object.StringBuilder);
+                    const frozen = try string_builder.freeze(vm);
+                    try pushStack(vm, .{ .object = &frozen.base });
+                    frame.ip += 1;
+                    continue;
+                }
+                return error.InvalidArgument;
+            },
             else => return error.InvalidInstruction,
         }
     }