diff --git a/checker.c b/checker.c
index c1dfa96..531d2b8 100644
--- a/checker.c
+++ b/checker.c
@@ -1 +1,76 @@
-#include "parser.h"
+#include "compiler.h"
+#include <stdlib.h>
+
+size_t common_string_hash_djb2(const unsigned char* value, size_t length)
+{
+    size_t hash = 5381;
+    for (size_t i = 0; i < length && value[i] != '\0'; ++i)
+        hash = ((hash << 5) + hash) + value[i];
+    return hash;
+}
+
+int string_symbol_map_construct(StringSymbolMap* map)
+{
+    const size_t start_capacity = 4;
+    *map = (StringSymbolMap) {
+        .data = malloc(sizeof(StringSymbolMap) * start_capacity),
+        .length = 0,
+        .capacity = start_capacity,
+    };
+    if (map->data == NULL) {
+        return -1;
+    }
+    return 0;
+}
+
+void string_symbol_map_destroy(StringSymbolMap* map)
+{
+    if (map->data != NULL) {
+        free(map->data);
+    }
+}
+
+Symbol* string_symbol_map_get(const StringSymbolMap* map, size_t key_hash)
+{
+    for (size_t i = 0; i < map->length; ++i) {
+        if (map->data[i].key_hash == key_hash) {
+            return &map->data[i].value;
+        }
+    }
+    return NULL;
+}
+
+int string_symbol_map_set(StringSymbolMap* map, size_t key_hash, Symbol value)
+{
+    Symbol* existing = string_symbol_map_get(map, key_hash);
+    if (existing != NULL) {
+        *existing = value;
+        return 0;
+    }
+    if (map->length + 1 >= map->capacity) {
+        map->capacity *= 2;
+        StringSymbolMapEntry* data
+            = realloc(map->data, sizeof(StringSymbolMapEntry) * map->capacity);
+        if (data == NULL) {
+            return -1;
+        }
+        map->data = data;
+    }
+    map->data[map->length] = (StringSymbolMapEntry) { key_hash, value };
+    map->length += 1;
+    return 0;
+}
+
+void checker_construct(Checker* checker)
+{
+    *checker = (Checker) {
+        .failed = false,
+    };
+}
+
+bool checker_failed(const Checker* checker) { return checker->failed; }
+
+void checker_check_statements(Checker* checker, ASTNode* node)
+{
+    for (size_t i = 0; i < node->statements.length; ++i) { }
+}
diff --git a/parser.h b/compiler.h
similarity index 87%
rename from parser.h
rename to compiler.h
index 813db4a..5cc0a01 100644
--- a/parser.h
+++ b/compiler.h
@@ -1,5 +1,5 @@
-#ifndef PARSER_H
-#define PARSER_H
+#ifndef COMPILER_H
+#define COMPILER_H
 
 #include <stdbool.h>
 #include <stddef.h>
@@ -249,11 +249,40 @@ ASTNode* parser_parse_block(Parser* parser);
 ASTNode* parser_parse_if(Parser* parser);
 ASTNode* parser_parse_loop(Parser* parser);
 
+typedef enum {
+    SymbolType_Local,
+    SymbolType_Global,
+    SymbolType_Builtin,
+} SymbolType;
+
 typedef struct {
-    int _a;
+    SymbolType type;
+} Symbol;
+
+size_t common_string_hash_djb2(const unsigned char* value, size_t length);
+
+typedef struct {
+    size_t key_hash;
+    Symbol value;
+} StringSymbolMapEntry;
+
+typedef struct {
+    StringSymbolMapEntry* data;
+    size_t length;
+    size_t capacity;
+} StringSymbolMap;
+
+int string_symbol_map_construct(StringSymbolMap* map);
+void string_symbol_map_destroy(StringSymbolMap* map);
+Symbol* string_symbol_map_get(const StringSymbolMap* map, size_t key_hash);
+int string_symbol_map_set(StringSymbolMap* map, size_t key_hash, Symbol value);
+
+typedef struct {
+    bool failed;
 } Checker;
 
 void checker_construct(Checker* checker);
-void checker_(Checker* checker);
+bool checker_failed(const Checker* checker);
+void checker_check_statements(Checker* checker, ASTNode* node);
 
 #endif
diff --git a/main.c b/main.c
index 61ce2e2..71fa9c4 100644
--- a/main.c
+++ b/main.c
@@ -1,4 +1,4 @@
-#include "parser.h"
+#include "compiler.h"
 #include <stddef.h>
 #include <stdio.h>
 #include <string.h>
diff --git a/parser.c b/parser.c
index be9a96e..0c84c3c 100644
--- a/parser.c
+++ b/parser.c
@@ -1,4 +1,4 @@
-#include "parser.h"
+#include "compiler.h"
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
diff --git a/vm.c b/vm.c
index d3561cc..d7c5194 100644
--- a/vm.c
+++ b/vm.c
@@ -1,4 +1,4 @@
-#include "vm.h"
+#include "compiler.h"
 #include <stdlib.h>
 
 void vm_construct(