feat: implement first flow control (if)

Author: aymaneelmaini

src/chunk.h

@@ -26,6 +26,7 @@ typedef enum
     OP_NOT,
     OP_NEGATE,
     OP_PRINT,
+    OP_JUMP_IF_FALSE,
     OP_RETURN,
 } OpCode;
 

src/common.h

@@ -8,6 +8,7 @@
 #define UINT8_COUNT (UINT8_MAX + 1)
 
 typedef uint8_t  u8;
+typedef uint16_t u16;
 typedef uint32_t u32;
 
 #define DEBUG_PRINT_CODE

src/compiler.c

@@ -64,9 +64,9 @@ typedef struct
 
 static bool immutable_globals[UINT8_MAX];
 
-Parser      parser;
-Compiler*   current;
-Chunk*      compiling_chunk;
+Parser    parser;
+Compiler* current;
+Chunk*    compiling_chunk;
 
 static Chunk* current_chunk()
 {
@@ -153,6 +153,14 @@ static void emit_bytes(u8 byte_1, u8 byte_2)
     emit_byte(byte_2);
 }
 
+static int emit_jump(u8 instruction)
+{
+    emit_byte(instruction);
+    emit_byte(0xff);
+    emit_byte(0xff);
+    return current_chunk()->count - 2;
+}
+
 static void emit_return()
 {
     emit_byte(OP_RETURN);
@@ -174,6 +182,15 @@ static void emit_constant(Value value)
     emit_bytes(OP_CONSTANT, make_constant(value));
 }
 
+static void patch_jump(int offset)
+{
+    int jump = current_chunk()->count - offset - 2;
+    if (jump > UINT16_MAX)
+        error("Too much code to jump over");
+
+    current_chunk()->code[offset] = (jump >> 8) & 0xff;
+    current_chunk()->code[offset + 1] = jump & 0xff;
+}
 static void init_compiler(Compiler* compiler)
 {
     compiler->local_count = 0;
@@ -566,6 +583,17 @@ static void expression_statement()
     emit_byte(OP_POP);
 }
 
+static void if_statement()
+{
+    consume(TOKEN_LEFT_PAREN, "Expect '(' after if statement");
+    expression();
+    consume(TOKEN_RIGHT_PAREN, "Expect ')' after condition");
+
+    int then_jump = emit_jump(OP_JUMP_IF_FALSE);
+    statement();
+    patch_jump(then_jump);
+}
+
 static void print_statement()
 {
     expression();
@@ -619,6 +647,10 @@ static void statement()
     {
         print_statement();
     }
+    else if (TOKEN_IF)
+    {
+        if_statement();
+    }
     else if (match(TOKEN_LEFT_BRACE))
     {
         begin_scope();

src/vm.c

@@ -90,6 +90,7 @@ static InterpretResult run()
 {
 #define READ_BYTE() (*vm.ip++)
 #define READ_CONSTANT() (vm.chunk->constants.values[READ_BYTE()])
+#define READ_SHORT() (vm.ip += 2, (u16)(vm.ip[-2] << 8) | vm.ip[-1])
 #define READ_STRING() AS_STRING(READ_CONSTANT())
 #define BINARY_OP(value_type, op)                                              \
     do                                                                         \
@@ -237,13 +238,21 @@ static InterpretResult run()
             printf("\n");
             break;
         }
+        case OP_JUMP_IF_FALSE:
+        {
+            u16 offset = READ_SHORT();
+            if (is_falsey(peek(0)))
+                vm.ip += offset;
+            break;
+        }
         case OP_RETURN:
             return INTERPRET_OK;
         }
     }
 
 #undef READ_BYTE
 #undef READ_CONSTANT
+#undef READ_SHORT
 #undef READ_STRING
 #undef BINARY_OP
 }