From b39dfd2df0383760ef22d887e5444b15bbabd2b2 Mon Sep 17 00:00:00 2001 From: Rekai Musuka Date: Fri, 21 Apr 2023 00:42:17 -0500 Subject: [PATCH] feat: implement free lists --- src/bench.zig | 4 +- src/trie.zig | 138 +++++++++++++++++++++++++++++++++++++++++++++++--- 2 files changed, 132 insertions(+), 10 deletions(-) diff --git a/src/bench.zig b/src/bench.zig index 164814d..c5cd7a3 100644 --- a/src/bench.zig +++ b/src/bench.zig @@ -18,12 +18,12 @@ pub fn main() !void { defer std.debug.assert(!gpa.deinit()); const allocator = gpa.allocator(); - const elem_count = 10; + const elem_count = 1000; const keys = try allocator.alloc([32]u8, elem_count); defer allocator.free(keys); - var rand = std.rand.DefaultPrng.init(0); + var rand = std.rand.DefaultPrng.init(1337); for (keys) |*key| rand.fill(key); var trie = try HashArrayMappedTrie([]const u8, void, StringContext).init(allocator); diff --git a/src/trie.zig b/src/trie.zig index baea3fd..1b446c3 100644 --- a/src/trie.zig +++ b/src/trie.zig @@ -17,18 +17,139 @@ pub fn HashArrayMappedTrie(comptime K: type, comptime V: type, comptime Context: const table_size = @typeInfo(Digest).Int.bits; const t = @intCast(Log2Int(Digest), @typeInfo(Log2Int(Digest)).Int.bits); + free_list: FreeList, root: []?*Node, const Node = union(enum) { kv: Pair, table: Table }; const Table = struct { map: Digest = 0, base: [*]Node }; pub const Pair = struct { key: K, value: V }; + /// Responsible for managing HAMT Memory + const FreeList = struct { + list: *[table_size]?FreeList.Node, + + // The index of the array of the linked list any given node belongs to + // informs us of how many elements there are in the [*]Node ptr. + const Node = struct { + inner: [*]Self.Node, + next: ?*FreeList.Node = null, + + pub fn deinit(self: *const FreeList.Node, allocator: Allocator, len: usize) void { + switch (len) { + 0 => unreachable, + 1 => allocator.destroy(@ptrCast(*Self.Node, self.inner)), + else => allocator.free(self.inner[0..len]), + } + } + }; + + pub fn init(allocator: Allocator) !FreeList { + const list = try allocator.create([table_size]?FreeList.Node); + std.mem.set(?FreeList.Node, list, null); + + return .{ .list = list }; + } + + pub fn deinit(self: *FreeList, allocator: Allocator) void { + for (self.list, 0..) |maybe_node, i| { + // the nodes that exist within the array `self.list` are freed outside + // of this `for` loop, so if any given `maybe_node` is a linked list that is + // 0 or 1 elements long, there is no thing to do here. + const len = i + 1; + + var current: *FreeList.Node = blk: { + const head = maybe_node orelse continue; // skip if list is 0 elements long + + head.deinit(allocator, len); // while we know the head exists, free the memory it points to + break :blk head.next orelse continue; // skip if list is 1 element long (see above comment) + }; + + while (current.next) |next| { + const next_ptr = next; // copy the pointer 'cause we're about to deallocate it's owner + + current.deinit(allocator, len); + allocator.destroy(current); + + current = next_ptr; + } + + current.deinit(allocator, len); // free the tail of the list + allocator.destroy(current); + } + + allocator.destroy(self.list); + self.* = undefined; + } + + pub fn alloc(self: *FreeList, allocator: Allocator, comptime T: type, len: usize) ![]T { + if (len == 0 or len > table_size) return error.unexpected_table_length; + + // If head is null, (head is self.list[len - 1]) then there was nothing in the free list + // therefore we should use the backup allocator + var current: *FreeList.Node = &(self.list[len - 1] orelse return try allocator.alloc(T, len)); + var prev: ?*FreeList.Node = null; + + while (current.next) |next| { + prev = current; + current = next; + } + + const ret_ptr = current.inner; + + if (current == &self.list[len - 1].?) { + // The current node is also the head, meaning that there's only one + // element in this linked list. Nodes in self.list are deallocated by another + // part of the program, so we just want to set the ?FreeList.Node to null + self.list[len - 1] = null; + } else { + std.debug.assert(prev != null); // this is invaraibly true if current != the head node + std.debug.assert(prev.?.next == current); // FIXME: is this ptr comparison even valuable? + + prev.?.next = null; // remove node from linked list + allocator.destroy(current); + } + + // this is safe because we've grabbed this many-ptr from the linked list of AMTs that have this size + return ret_ptr[0..len]; + } + + pub fn create(self: *FreeList, allocator: Allocator, comptime T: type) !*T { + return @ptrCast(*T, try self.alloc(allocator, T, 1)); + } + + /// Free'd nodes aren't deallocated, but instead are tracked by a free list where they + /// may be reused in the future + /// + /// We may allocate to append a new FreeList Node to the end of the Linked List + pub fn free(self: *FreeList, allocator: Allocator, ptr: []Self.Node) !void { + if (ptr.len == 0 or ptr.len > table_size) return error.unexpected_table_length; + + var current: *FreeList.Node = &(self.list[ptr.len - 1] orelse { + // There were no nodes present so start off the linked list + self.list[ptr.len - 1] = .{ .inner = ptr.ptr }; + return; + }); + + // traverse the linked list + while (current.next) |next| current = next; + + const tail = try allocator.create(FreeList.Node); + tail.* = .{ .inner = ptr.ptr }; + + current.next = tail; + } + + pub fn destroy(self: *FreeList, allocator: Allocator, node: *Self.Node) !void { + self.free(allocator, @ptrCast([*]Self.Node, node)[0..1]); + } + }; + pub fn init(allocator: Allocator) !Self { // TODO: Add ability to have a larger root node (for quicker lookup times) const root = try allocator.alloc(?*Node, table_size); std.mem.set(?*Node, root, null); - return Self{ .root = root }; + return Self{ .root = root, .free_list = try FreeList.init(allocator) }; } pub fn deinit(self: *Self, allocator: Allocator) void { @@ -40,6 +161,7 @@ pub fn HashArrayMappedTrie(comptime K: type, comptime V: type, comptime Context: } allocator.free(self.root); + self.free_list.deinit(allocator); } fn _deinit(allocator: Allocator, node: *Node) void { @@ -101,7 +223,7 @@ pub fn HashArrayMappedTrie(comptime K: type, comptime V: type, comptime Context: var current: *Node = self.root[root_idx] orelse { // node in root table is empty, place the KV here - const node = try allocator.create(Node); + const node = try self.free_list.create(allocator, Node); node.* = .{ .kv = .{ .key = key, .value = value } }; self.root[root_idx] = node; @@ -116,7 +238,7 @@ pub fn HashArrayMappedTrie(comptime K: type, comptime V: type, comptime Context: if (table.map & mask == 0) { // Empty const old_len = @popCount(table.map); - const new_base = try allocator.alloc(Node, old_len + 1); + const new_base = try self.free_list.alloc(allocator, Node, old_len + 1); const new_map = table.map | mask; var i: Log2Int(Digest) = 0; @@ -132,7 +254,7 @@ pub fn HashArrayMappedTrie(comptime K: type, comptime V: type, comptime Context: } } - allocator.free(table.base[0..old_len]); + try self.free_list.free(allocator, table.base[0..old_len]); table.base = new_base.ptr; table.map = new_map; @@ -152,7 +274,7 @@ pub fn HashArrayMappedTrie(comptime K: type, comptime V: type, comptime Context: switch (std.math.order(mask, prev_mask)) { .lt, .gt => { // there are no collisions between the two hash subsets. - const pairs = try allocator.alloc(Node, 2); + const pairs = try self.free_list.alloc(allocator, Node, 2); const map = mask | prev_mask; pairs[@popCount(map & (prev_mask - 1))] = .{ .kv = prev_pair }; @@ -162,10 +284,10 @@ pub fn HashArrayMappedTrie(comptime K: type, comptime V: type, comptime Context: return; }, .eq => { - const copied_pair = try allocator.alloc(Node, 1); - copied_pair[0] = .{ .kv = prev_pair }; + const copied_pair = try self.free_list.create(allocator, Node); + copied_pair.* = .{ .kv = prev_pair }; - current.* = .{ .table = .{ .map = mask, .base = copied_pair.ptr } }; + current.* = .{ .table = .{ .map = mask, .base = @ptrCast([*]Node, copied_pair) } }; }, } },