implementing hash tables (ch20)

src/compile.zig

@@ -290,7 +290,8 @@ const Parser = struct {
 
     fn string(self: *Parser) ParsingError!void {
         const str = self.previous.?.start[1 .. self.previous.?.length - 1];
-        var string_obj = Obj.String.new(self.chunk.allocator, str);
+
+        var string_obj = self.vm.copy_string(str);
 
         self.vm.add_reference(&string_obj.obj);
 

src/main.zig

@@ -7,6 +7,9 @@ const OpCode = @import("./opcode.zig").OpCode;
 const VM = @import("./vm.zig").VM;
 const InterpretResult = @import("./vm.zig").InterpretResult;
 
+// XXX imported to run tests.
+const Table = @import("./table.zig");
+
 pub const DEBUG_TRACE_EXECUTION = true;
 
 pub fn repl(allocator: Allocator, vm: *VM) !void {

src/object.zig

@@ -2,6 +2,8 @@ const std = @import("std");
 const debug = std.debug;
 const Allocator = std.mem.Allocator;
 
+const compute_hash = @import("./utils.zig").compute_hash;
+
 pub const ObjType = enum {
     String,
 };
@@ -13,8 +15,9 @@ pub const Obj = struct {
     pub const String = struct {
         chars: []const u8,
         obj: Obj,
+        hash: u32,
 
-        pub fn new(allocator: std.mem.Allocator, str: []const u8) *String {
+        pub fn new(allocator: std.mem.Allocator, chars: []const u8) *String {
             const obj = Obj{
                 .kind = ObjType.String,
                 .allocator = allocator,
@@ -22,7 +25,9 @@ pub const Obj = struct {
 
             const str_obj = allocator.create(String) catch unreachable;
             str_obj.obj = obj;
-            str_obj.chars = allocator.dupe(u8, str) catch unreachable;
+
+            str_obj.chars = chars;
+            str_obj.hash = compute_hash(str_obj.chars);
 
             return str_obj;
         }

src/table.zig

@@ -0,0 +1,302 @@
+const std = @import("std");
+const debug = std.debug;
+const Allocator = std.mem.Allocator;
+
+const Obj = @import("./object.zig").Obj;
+const Value = @import("./values.zig").Value;
+
+const grow_capacity = @import("./utils.zig").grow_capacity;
+const compute_hash = @import("./utils.zig").compute_hash;
+
+const TABLE_MAX_LOAD = 0.75;
+
+const Entry = struct {
+    key: ?*Obj.String,
+    value: Value,
+};
+
+pub const Table = struct {
+    allocator: Allocator,
+    count: usize,
+    capacity: usize,
+    entries: []Entry,
+
+    pub fn new(allocator: Allocator) Table {
+        return Table{
+            .allocator = allocator,
+            .count = 0,
+            .capacity = 0,
+            .entries = &.{},
+        };
+    }
+
+    pub fn deinit(self: *Table) void {
+        if (self.capacity == 0) {
+            return;
+        }
+
+        self.allocator.free(self.entries);
+    }
+
+    pub fn set(self: *Table, key: *Obj.String, value: Value) bool {
+        const current_count: f32 = @floatFromInt(self.count + 1);
+        const current_capacity: f32 = @floatFromInt(self.capacity);
+
+        if (current_count > current_capacity * TABLE_MAX_LOAD) {
+            const capacity = grow_capacity(self.capacity);
+            self.adjust_capacity(capacity);
+        }
+
+        const entry = Table.find_entry(self.entries, key);
+        const is_new = entry.?.key == null;
+        if (is_new and entry.?.value.is_nil()) {
+            self.count += 1;
+        }
+
+        entry.?.key = key;
+        entry.?.value = value;
+
+        return is_new;
+    }
+
+    pub fn find_entry(entries: []Entry, key: *Obj.String) ?*Entry {
+        var tombstone: ?*Entry = null;
+        var index = key.hash % entries.len;
+
+        while (true) {
+            const entry = &entries[index];
+
+            if (entry.key == null) {
+                if (entry.value.is_nil()) {
+                    // Empty entry.
+                    if (tombstone != null) {
+                        return tombstone;
+                    } else {
+                        return entry;
+                    }
+                } else {
+                    // We found a tombestone
+                    if (tombstone == null) {
+                        tombstone = entry;
+                    }
+                }
+            } else if (entry.key == key) {
+                // We found the key
+                return entry;
+            }
+
+            index = (index + 1) % entries.len;
+        }
+    }
+
+    pub fn adjust_capacity(self: *Table, capacity: usize) void {
+        var entries = self.allocator.alloc(Entry, capacity) catch unreachable;
+
+        for (0..entries.len) |idx| {
+            entries[idx].key = null;
+            entries[idx].value = Value.nil_val();
+        }
+
+        self.count = 0;
+        for (0..self.capacity) |idx| {
+            const entry = self.entries[idx];
+            if (entry.key == null) {
+                continue;
+            }
+
+            const dest_entry = Table.find_entry(entries, entry.key.?);
+            dest_entry.?.key = entry.key;
+            dest_entry.?.value = entry.value;
+
+            self.count += 1;
+        }
+
+        self.capacity = capacity;
+        if (entries.len > 0) {
+            self.allocator.free(self.entries);
+        }
+        self.entries = entries;
+    }
+
+    pub fn dump(self: Table) void {
+        std.debug.print("== Hash table count:{} capacity:{} ==\n", .{ self.count, self.capacity });
+        for (self.entries, 0..) |entry, idx| {
+            if (entry.key != null) {
+                std.debug.print("{d} ({d}) - {s}: ", .{ idx, entry.key.?.hash, entry.key.?.chars });
+                entry.value.print();
+                std.debug.print("\n", .{});
+            }
+
+            if (entry.key == null and entry.value.as_bool()) {
+                std.debug.print("{d} - tombstone\n", .{idx});
+            }
+        }
+        std.debug.print("== End of hash table ==\n\n", .{});
+    }
+
+    pub fn add_all(self: *Table, from: Table) void {
+        for (from.entries) |entry| {
+            if (entry.key == null) {
+                continue;
+            }
+            _ = self.set(entry.key.?, entry.value);
+        }
+    }
+
+    pub fn get(self: Table, key: *Obj.String, value: *Value) bool {
+        if (self.count == 0) {
+            return false;
+        }
+
+        const entry = Table.find_entry(self.entries, key);
+        if (entry.?.key == null) {
+            return false;
+        }
+
+        value.* = entry.?.value;
+
+        return true;
+    }
+
+    pub fn del(self: *Table, key: *Obj.String) bool {
+        if (self.count == 0) {
+            return false;
+        }
+
+        // Find the entry
+        const entry = Table.find_entry(self.entries, key);
+        if (entry.?.key == null) {
+            return false;
+        }
+
+        // Place a tombstone in the entry
+        entry.?.key = null;
+        entry.?.value = Value.bool_val(true);
+
+        return true;
+    }
+
+    pub fn find_string(self: *Table, chars: []const u8, hash: u32) ?*Obj.String {
+        if (self.count == 0) {
+            return null;
+        }
+
+        var index = hash % self.capacity;
+        while (true) {
+            const entry = &self.entries[index];
+            if (entry.key == null) {
+                // Stop if we find an empty non-tombstone entry.
+                if (entry.value.is_nil()) {
+                    return null;
+                }
+            } else if (entry.key.?.chars.len == chars.len and entry.key.?.hash == hash and std.mem.eql(u8, chars, entry.key.?.chars)) {
+                return entry.key;
+            }
+
+            index = (index + 1) % self.capacity;
+        }
+    }
+};
+
+test "initialize an hash table" {
+    const allocator = std.testing.allocator;
+
+    var table = Table.new(allocator);
+    defer table.deinit();
+    try std.testing.expectEqual(0, table.count);
+    try std.testing.expectEqual(0, table.capacity);
+}
+
+test "adding values" {
+    const allocator = std.testing.allocator;
+
+    const key = Obj.String.new(allocator, "hello world");
+    defer key.destroy();
+
+    var table = Table.new(allocator);
+    defer table.deinit();
+
+    var res = table.set(key, Value.nil_val());
+    try std.testing.expectEqual(true, res);
+    try std.testing.expectEqual(1, table.count);
+    try std.testing.expectEqual(8, table.capacity);
+
+    res = table.set(key, Value.nil_val());
+    try std.testing.expectEqual(false, res);
+    try std.testing.expectEqual(1, table.count);
+    try std.testing.expectEqual(8, table.capacity);
+}
+
+test "adding tables" {
+    const allocator = std.testing.allocator;
+
+    const key = Obj.String.new(allocator, "hello world");
+    defer key.destroy();
+
+    var table = Table.new(allocator);
+    defer table.deinit();
+
+    const res = table.set(key, Value.nil_val());
+    try std.testing.expectEqual(true, res);
+    try std.testing.expectEqual(8, table.capacity);
+    try std.testing.expectEqual(1, table.count);
+
+    var table2 = Table.new(allocator);
+    defer table2.deinit();
+
+    try std.testing.expectEqual(0, table2.capacity);
+    try std.testing.expectEqual(0, table2.count);
+
+    table2.add_all(table);
+    try std.testing.expectEqual(8, table2.capacity);
+    try std.testing.expectEqual(1, table2.count);
+}
+
+test "deleting from table" {
+    const allocator = std.testing.allocator;
+
+    const key = Obj.String.new(allocator, "hello world");
+    defer key.destroy();
+
+    var table = Table.new(allocator);
+    defer table.deinit();
+
+    var res = table.set(key, Value.nil_val());
+    try std.testing.expectEqual(true, res);
+
+    // table.dump();
+
+    res = table.del(key);
+    try std.testing.expectEqual(true, res);
+
+    // table.dump();
+}
+
+test "find" {
+    const allocator = std.testing.allocator;
+
+    const key = Obj.String.new(allocator, "hello world");
+    defer key.destroy();
+
+    var table = Table.new(allocator);
+    defer table.deinit();
+
+    const value = Value.number_val(42.0);
+
+    var res = table.set(key, value);
+    try std.testing.expectEqual(true, res);
+
+    var entry = table.find_string("bye world", compute_hash("bye world"));
+    try std.testing.expectEqual(entry, null);
+
+    entry = table.find_string("hello world", compute_hash("hello world"));
+    // std.debug.print("{any}\n", .{entry});
+    try std.testing.expect(entry != null);
+
+    var value_obj = Value.nil_val();
+    res = table.get(entry.?, &value_obj);
+    try std.testing.expect(res);
+    // std.debug.print("{any}\n", .{value_obj});
+
+    try std.testing.expectEqual(value_obj.as_number(), value.as_number());
+}

src/utils.zig

@@ -24,3 +24,14 @@ pub fn constant_instruction(opcode_name: []const u8, chunk: Chunk, offset: usize
     debug.print("'\n", .{});
     return offset + 2;
 }
+
+pub fn compute_hash(str: []const u8) u32 {
+    var res_hash: u32 = 2166136261;
+
+    for (str) |c| {
+        res_hash ^= c;
+        res_hash *%= 16777619;
+    }
+
+    return res_hash;
+}

src/values.zig

@@ -114,14 +114,18 @@ pub const Value = struct {
             ValueType.Nil => true,
             ValueType.Bool => self.as_bool() == other.as_bool(),
             ValueType.Number => self.as_number() == other.as_number(),
-            ValueType.Obj => {
-                const obj_string0 = self.as_cstring();
-                const obj_string1 = other.as_cstring();
-
-                return std.mem.eql(u8, obj_string0, obj_string1);
-            },
+            ValueType.Obj => self.as_obj() == other.as_obj(),
         };
     }
+
+    pub fn print(self: Value) void {
+        switch (self.value_type) {
+            ValueType.Nil => debug.print("nil", .{}),
+            ValueType.Bool => debug.print("{any}", .{self.as_bool()}),
+            ValueType.Number => debug.print("{d}", .{self.as_number()}),
+            ValueType.Obj => self.as_obj().print(),
+        }
+    }
 };
 
 pub const ValueArray = struct {
@@ -156,10 +160,5 @@ pub const ValueArray = struct {
 };
 
 pub fn print_value(value: Value) void {
-    switch (value.value_type) {
-        ValueType.Nil => debug.print("nil", .{}),
-        ValueType.Bool => debug.print("{any}", .{value.as_bool()}),
-        ValueType.Number => debug.print("{d}", .{value.as_number()}),
-        ValueType.Obj => value.as_obj().print(),
-    }
+    value.print();
 }

src/vm.zig

@@ -6,8 +6,10 @@ const Chunk = @import("./chunk.zig").Chunk;
 const OpCode = @import("./opcode.zig").OpCode;
 const Value = @import("./values.zig").Value;
 const Obj = @import("./object.zig").Obj;
+const Table = @import("./table.zig").Table;
 
 const compile = @import("./compile.zig").compile;
+const compute_hash = @import("./utils.zig").compute_hash;
 
 const DEBUG_TRACE_EXECUTION = @import("./main.zig").DEBUG_TRACE_EXECUTION;
 
@@ -22,24 +24,31 @@ pub const InterpretResult = enum {
 };
 
 pub const VM = struct {
+    allocator: Allocator,
     chunk: ?*Chunk,
     ip: ?usize,
     stack: std.ArrayList(Value),
     // Keeping creating objects in references to destroy objects on cleaning.
     // In the book, a linked list between objects is used to handle this.
     references: std.ArrayList(*Obj),
+    strings: Table,
 
     pub fn new(allocator: Allocator) VM {
         return VM{
+            .allocator = allocator,
             .chunk = null,
             .ip = null,
             .stack = std.ArrayList(Value).init(allocator),
             .references = std.ArrayList(*Obj).init(allocator),
+            .strings = Table.new(allocator),
         };
     }
 
     pub fn free(self: *VM) void {
         self.stack.deinit();
+
+        self.strings.dump();
+        self.strings.deinit();
         self.clean_references();
         self.references.deinit();
     }
@@ -179,9 +188,8 @@ pub const VM = struct {
         const a = self.pop().as_cstring();
 
         const concat_str = try std.mem.concat(self.chunk.?.allocator, u8, &.{ a, b });
-        defer self.chunk.?.allocator.free(concat_str);
 
-        var string_obj = Obj.String.new(self.chunk.?.allocator, concat_str);
+        var string_obj = self.take_string(concat_str);
 
         self.add_reference(&string_obj.obj);
 
@@ -201,6 +209,12 @@ pub const VM = struct {
     }
 
     pub fn add_reference(self: *VM, obj: *Obj) void {
+        // do not add duplicate references
+        for (self.references.items) |item| {
+            if (item == obj) {
+                return;
+            }
+        }
         // XXX TODO catch unreachable to prevents
         self.references.append(obj) catch unreachable;
     }
@@ -210,4 +224,36 @@ pub const VM = struct {
             item.destroy();
         }
     }
+
+    pub fn copy_string(self: *VM, source: []const u8) *Obj.String {
+        const hash = compute_hash(source);
+        const obj_string = self.strings.find_string(source, hash);
+
+        if (obj_string != null) {
+            return obj_string.?;
+        }
+
+        const copy: []const u8 = self.allocator.dupe(u8, source) catch unreachable;
+        return self.allocate_string(copy);
+    }
+
+    pub fn take_string(self: *VM, source: []const u8) *Obj.String {
+        const hash = compute_hash(source);
+        const obj_string = self.strings.find_string(source, hash);
+
+        if (obj_string != null) {
+            // free given string
+            self.allocator.free(source);
+            return obj_string.?;
+        }
+
+        return self.allocate_string(source);
+    }
+
+    pub fn allocate_string(self: *VM, source: []const u8) *Obj.String {
+        const obj_string = Obj.String.new(self.allocator, source);
+        _ = self.strings.set(obj_string, Value.nil_val());
+
+        return obj_string;
+    }
 };