Chapter 19.4 - 19.5

clox/src/CMakeLists.txt

@@ -16,6 +16,8 @@ add_executable(Lox
   scanner.c
   object.h
   object.c
+  table.h
+  table.c
   main.c)
 
 install(TARGETS Lox)

clox/src/object.c

@@ -3,6 +3,7 @@
 
 #include "memory.h"
 #include "object.h"
+#include "table.h"
 #include "value.h"
 #include "vm.h"
 
@@ -18,22 +19,45 @@ static Obj *allocateObject(size_t size, ObjType type) {
   return object;
 }
 
-static ObjString *allocateString(char *chars, int length) {
+static ObjString *allocateString(char *chars, int length, uint32_t hash) {
   ObjString *string = ALLOCATE_OBJ(ObjString, OBJ_STRING);
   string->length = length;
   string->chars = chars;
+  string->hash = hash;
+  tableSet(&vm.strings, string, NIL_VAL);
   return string;
 }
 
+static uint32_t hashString(const char *key, int length) {
+  uint32_t hash = 2166136261u;
+  for (int i = 0; i < length; i++) {
+    hash ^= (uint32_t)key[i];
+    hash *= 16777619;
+  }
+  return hash;
+}
+
 ObjString *takeString(char *chars, int length) {
-  return allocateString(chars, length);
+  uint32_t hash = hashString(chars, length);
+  ObjString *interned = tableFindString(&vm.strings, chars, length, hash);
+  if (interned != NULL) {
+    FREE_ARRAY(char, chars, length + 1);
+    return interned;
+  }
+
+  return allocateString(chars, length, hash);
 }
 
 ObjString *copyString(const char *chars, int length) {
+  uint32_t hash = hashString(chars, length);
+  ObjString *interned = tableFindString(&vm.strings, chars, length, hash);
+  if (interned != NULL)
+    return interned;
+
   char *heapChars = ALLOCATE(char, length + 1);
   memcpy(heapChars, chars, length);
   heapChars[length] = '\0';
-  return allocateString(heapChars, length);
+  return allocateString(heapChars, length, hash);
 }
 
 void printObject(Value value) {

clox/src/object.h

@@ -24,6 +24,7 @@ struct ObjString {
   Obj obj;
   int length;
   char *chars;
+  uint32_t hash;
 };
 
 ObjString *takeString(char *chars, int length);

clox/src/table.c

@@ -0,0 +1,142 @@
+#include <stdlib.h>
+#include <string.h>
+
+#include "memory.h"
+#include "object.h"
+#include "table.h"
+#include "value.h"
+
+#define TABLE_MAX_LOAD 0.75
+
+void initTable(Table *table) {
+  table->count = 0;
+  table->capacity = 0;
+  table->entries = NULL;
+}
+
+void freeTable(Table *table) {
+  FREE_ARRAY(Entry, table->entries, table->capacity);
+  initTable(table);
+}
+
+static Entry *findEntry(Entry *entries, int capacity, ObjString *key) {
+  uint32_t index = key->hash % capacity;
+  Entry *tombstone = NULL;
+
+  for (;;) {
+    Entry *entry = &entries[index];
+    if (entry->key == NULL) {
+      if (IS_NIL(entry->value)) {
+        // Empty entry.
+        return tombstone != NULL ? tombstone : entry;
+      } else {
+        // We found a tombstone.
+        if (tombstone == NULL)
+          tombstone = entry;
+      }
+    } else if (entry->key == key) {
+      // We found the entry.
+      return entry;
+    }
+
+    index = (index + 1) % capacity;
+  }
+}
+
+bool tableGet(Table *table, ObjString *key, Value *value) {
+  if (table->count == 0)
+    return false;
+
+  Entry *entry = findEntry(table->entries, table->capacity, key);
+  if (entry->key == NULL)
+    return false;
+
+  *value = entry->value;
+  return true;
+}
+
+static void adjustCapacity(Table *table, int capacity) {
+  Entry *entries = ALLOCATE(Entry, capacity);
+  for (int i = 0; i < capacity; i++) {
+    entries[i].key = NULL;
+    entries[i].value = NIL_VAL;
+  }
+
+  table->count = 0;
+  for (int i = 0; i < table->capacity; i++) {
+    Entry *entry = &table->entries[i];
+    if (entry->key == NULL)
+      continue;
+
+    Entry *dest = findEntry(entries, capacity, entry->key);
+    dest->key = entry->key;
+    dest->value = entry->value;
+    table->count++;
+  }
+
+  FREE_ARRAY(Entry, table->entries, table->capacity);
+  table->entries = entries;
+  table->capacity = capacity;
+}
+
+bool tableSet(Table *table, ObjString *key, Value value) {
+  if (table->count + 1 > table->capacity * TABLE_MAX_LOAD) {
+    int capacity = GROW_CAPACITY(table->capacity);
+    adjustCapacity(table, capacity);
+  }
+
+  Entry *entry = findEntry(table->entries, table->capacity, key);
+  bool isNewKey = entry->key == NULL;
+  if (isNewKey && IS_NIL(entry->value))
+    table->count++;
+
+  entry->key = key;
+  entry->value = value;
+  return isNewKey;
+}
+
+bool tableDelete(Table *table, ObjString *key) {
+  if (table->count == 0)
+    return false;
+
+  /* Find the entry. */
+  Entry *entry = findEntry(table->entries, table->capacity, key);
+  if (entry->key == NULL)
+    return false;
+
+  /* Place a tombstone in the entry. */
+  entry->key = NULL;
+  entry->value = BOOL_VAL(true);
+  return true;
+}
+
+void tableAddAll(Table *from, Table *to) {
+  for (int i = 0; i < from->capacity; i++) {
+    Entry *entry = &from->entries[i];
+    if (entry->key != NULL) {
+      tableSet(to, entry->key, entry->value);
+    }
+  }
+}
+
+ObjString *tableFindString(Table *table, const char *chars, int length,
+                           uint32_t hash) {
+  if (table->count == 0)
+    return NULL;
+
+  uint32_t index = hash % table->capacity;
+  for (;;) {
+    Entry *entry = &table->entries[index];
+    if (entry->key == NULL) {
+      /* Stop if we find an empty non-tombstone entry. */
+      if (IS_NIL(entry->value))
+        return NULL;
+    } else if (entry->key->length == length && entry->key->hash == hash &&
+               memcmp(entry->key->chars, chars, length) == 0) {
+      /* We found it. */
+      return entry->key;
+    }
+
+    index = (index + 1) % table->capacity;
+  }
+}

clox/src/table.h

@@ -0,0 +1,27 @@
+#ifndef TABLE_H
+#define TABLE_H
+
+#include "common.h"
+#include "value.h"
+
+typedef struct {
+  ObjString *key;
+  Value value;
+} Entry;
+
+typedef struct {
+  int count;
+  int capacity;
+  Entry *entries;
+} Table;
+
+void initTable(Table *table);
+void freeTable(Table *table);
+bool tableGet(Table *table, ObjString *key, Value *value);
+bool tableSet(Table *table, ObjString *key, Value value);
+bool tableDelete(Table *table, ObjString *key);
+void tableAddAll(Table *from, Table *to);
+ObjString *tableFindString(Table *talbe, const char *chars, int length,
+                           uint32_t hash);
+
+#endif

clox/src/value.c

@@ -56,10 +56,7 @@ bool valuesEqual(Value a, Value b) {
   case VAL_NUMBER:
     return AS_NUMBER(a) == AS_NUMBER(b);
   case VAL_OBJ: {
-    ObjString *aString = AS_STRING(a);
-    ObjString *bString = AS_STRING(b);
-    return aString->length == bString->length &&
-           memcmp(aString->chars, bString->chars, aString->length) == 0;
+    return AS_OBJ(a) == AS_OBJ(b);
   }
   default:
     return false; /* Unreachable */

clox/src/vm.c

@@ -29,9 +29,13 @@ static void runtimeError(const char *format, ...) {
 void initVM() {
   resetStack();
   vm.objects = NULL;
+  initTable(&vm.strings);
 }
 
-void freeVM() { freeObjects(); }
+void freeVM() {
+  freeTable(&vm.strings);
+  freeObjects();
+}
 
 void push(Value value) {
   *vm.stackTop = value;

clox/src/vm.h

@@ -2,6 +2,7 @@
 #define clox_vm_h
 
 #include "chunk.h"
+#include "table.h"
 #include "value.h"
 
 #define STACK_MAX 256
@@ -11,6 +12,7 @@ typedef struct {
   uint8_t *ip;
   Value stack[STACK_MAX];
   Value *stackTop;
+  Table strings;
   Obj *objects;
 } VM;