src: fast deinit feature.

.gitignore

@@ -2,3 +2,4 @@
 /zig-out
 /.vscode
 /.zig-cache
+*.code-workspace

README.md

@@ -10,11 +10,19 @@ To use this library, you need at least Zig 0.13.x.
 
 ## API
 ```zig
+// create tree type:
+pub const Options = struct {
+    countChildren: bool = false,
+};
 pub fn TreeWithOptions(comptime K: type, comptime V: type, comptime Cmp: fn (a: K, b: K) math.Order, comptime options: Options) type
 pub fn Tree(comptime K: type, comptime V: type, comptime Cmp: fn (a: K, b: K) math.Order) type
 
 // init/deinit:
+pub const InitOptions = struct {
+    allowFastDeinit: enum { always, auto, never } = .never,
+};
 pub fn init(a: std.mem.Allocator) Self
+pub fn initWithOptions(a: std.mem.Allocator, io: InitOptions) Self
 pub fn deinit()
 
 // insert:

build.zig.zon

@@ -1,6 +1,6 @@
 .{
     .name = "zigavl",
-    .version = "0.6.1",
+    .version = "0.7.0",
     .dependencies = .{},
     .paths = .{""},
 }

src/avl.zig

@@ -113,6 +113,25 @@ fn locationCache(comptime K: type, comptime V: type, comptime Tags: type) type {
             };
         }
 
+        const allocAddrs = blk: {
+            var aa: std.heap.ArenaAllocator = undefined;
+            const aaa = aa.allocator();
+            var fba: std.heap.FixedBufferAllocator = undefined;
+            const fbaa = fba.allocator();
+            break :blk [_]*const anyopaque{
+                @ptrCast(@alignCast(aaa.vtable.alloc)),
+                @ptrCast(@alignCast(fbaa.vtable.alloc)),
+            };
+        };
+
+        fn fastDeinitAllowed(self: *Self) bool {
+            const ourAllocAddr: *const anyopaque = @ptrCast(@alignCast(self.a.vtable.alloc));
+            inline for (allocAddrs) |ptr| {
+                if (ourAllocAddr == ptr) return true;
+            }
+            return false;
+        }
+
         fn create(self: *Self) !Location {
             const node = try self.a.create(Location.Node);
             node.* = Location.Node.init();
@@ -125,13 +144,27 @@ fn locationCache(comptime K: type, comptime V: type, comptime Tags: type) type {
     };
 }
 
-// Options defines some parameters of the tree.
+// Options defines some comptime parameters of the tree type.
 pub const Options = struct {
     // countChildren, if set, enables children counts for every node of the tree.
     // the number of children allows to locate a node by its position with a guaranteed complexity O(logn).
     countChildren: bool = false,
 };
 
+// InitOptions defines some runtime parameters of the tree instance.
+pub const InitOptions = struct {
+    // allowFastDeinit speeds up deinit() call by making it a no-op in cases
+    // where all the memory can be freed on the allocator level.
+    // normally, deinit() traverses the tree removing each node, however,
+    // this might not be necessary, if certain types of allocators are used.
+    // enum values:
+    //  always - deinit() never deletes the nodes.
+    //  auto - deinit() does not delete the nodes,
+    //    if std.heap.ArenaAllocator or std.heap.FixedBufferAllocator are for allocations.
+    //  never[default] - deinit() always deletes the nodes.
+    allowFastDeinit: enum { always, auto, never } = .never,
+};
+
 // Tree is a generic avl tree.
 // AVL tree (https://en.wikipedia.org/wiki/AVL_tree) is a self-balancing binary search tree.
 // For each node of the tree the heights of the left and right sub-trees differ by at most one.
@@ -353,6 +386,7 @@ pub fn TreeWithOptions(comptime K: type, comptime V: type, comptime Cmp: fn (a:
             return b;
         }
 
+        // Iterator traverses the tree.
         pub const Iterator = struct {
             tree: *Self,
             loc: ?Location,
@@ -387,6 +421,7 @@ pub fn TreeWithOptions(comptime K: type, comptime V: type, comptime Cmp: fn (a:
             }
         };
 
+        io: InitOptions,
         lc: Cache,
         length: usize,
         root: ?Location,
@@ -395,16 +430,29 @@ pub fn TreeWithOptions(comptime K: type, comptime V: type, comptime Cmp: fn (a:
 
         // init initializes the tree.
         pub fn init(a: std.mem.Allocator) Self {
+            return Self.initWithOptions(a, .{});
+        }
+
+        // initWithOptions initializes the tree.
+        pub fn initWithOptions(a: std.mem.Allocator, io: InitOptions) Self {
             return Self{
                 .lc = Cache.init(a),
                 .length = 0,
                 .root = null,
                 .min = null,
                 .max = null,
+                .io = io,
             };
         }
 
+        // deinit releases the memory taken by all the nodes.
+        // Time complexity:
+        //  O(1) - if fast deinit is enabled (see InitOptions.allowFastDeinit).
+        //  O(n) - otherwise.
         pub fn deinit(self: *Self) void {
+            if (self.io.allowFastDeinit == .always or self.io.allowFastDeinit == .auto and self.lc.fastDeinitAllowed()) {
+                return;
+            }
             const min = self.min orelse return;
             var loc = goLeftRight(min);
             while (true) {
@@ -1407,6 +1455,58 @@ test "tree random" {
     }
 }
 
+const failingFreeAllocator = struct {
+    ptr: *anyopaque,
+    vtable: std.mem.Allocator.VTable,
+
+    fn free(_: *anyopaque, _: []u8, _: u8, _: usize) void {
+        @panic("should not happen");
+    }
+
+    fn init(a: std.mem.Allocator) failingFreeAllocator {
+        return failingFreeAllocator{ .ptr = a.ptr, .vtable = .{
+            .alloc = a.vtable.alloc,
+            .free = free,
+            .resize = a.vtable.resize,
+        } };
+    }
+
+    fn allocator(self: *failingFreeAllocator) std.mem.Allocator {
+        return std.mem.Allocator{
+            .ptr = self.ptr,
+            .vtable = &self.vtable,
+        };
+    }
+};
+
+test "arena allocator: auto fast deinit" {
+    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
+    defer arena.deinit();
+    try testFastDeinit(.{ .allowFastDeinit = .auto }, arena.allocator());
+}
+
+test "arena allocator: always fast deinit" {
+    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
+    defer arena.deinit();
+    try testFastDeinit(.{ .allowFastDeinit = .always }, arena.allocator());
+}
+
+test "fixed buffer allocator: auto fast deinit" {
+    var buff: [512]u8 = undefined;
+    var fb = std.heap.FixedBufferAllocator.init(&buff);
+    try testFastDeinit(.{ .allowFastDeinit = .auto }, fb.allocator());
+}
+
+fn testFastDeinit(io: InitOptions, a: std.mem.Allocator) !void {
+    const TreeType = TreeWithOptions(i64, i64, i64Cmp, .{});
+    var ta: failingFreeAllocator = failingFreeAllocator.init(a);
+    var t = TreeType.initWithOptions(ta.allocator(), io);
+    defer t.deinit();
+    _ = try t.insert(0, 0);
+    _ = try t.insert(1, 1);
+    _ = try t.insert(2, 2);
+}
+
 fn checkHeightAndBalance(comptime T: type, loc: ?T.Location) !void {
     _ = try recalcHeightAndBalance(T, loc);
 }

src/lib.zig

@@ -2,3 +2,4 @@ const avl = @import("./avl.zig");
 pub const Tree = avl.Tree;
 pub const TreeWithOptions = avl.TreeWithOptions;
 pub const Options = avl.Options;
+pub const InitOptions = avl.InitOptions;

src/tests.zig

@@ -9,7 +9,12 @@ fn i64Cmp(a: i64, b: i64) math.Order {
 test "test pub decls" {
     const a = std.testing.allocator;
     const TreeType = lib.Tree(i64, i64, i64Cmp);
-    var t = TreeType.init(a);
+    const options = lib.InitOptions{
+        .allowFastDeinit = .auto,
+    };
+    var aa = std.heap.ArenaAllocator.init(a);
+    defer aa.deinit();
+    var t = TreeType.initWithOptions(aa.allocator(), options);
     defer t.deinit();
     _ = try t.insert(0, 0);
     var it = t.ascendFromStart();
@@ -29,7 +34,7 @@ test "tree example usage" {
     defer _ = gpa.detectLeaks();
     // first, create an i64-->i64 tree
     const TreeType = lib.TreeWithOptions(i64, i64, i64Cmp, .{ .countChildren = true });
-    var t = TreeType.init(gpa.allocator());
+    var t = TreeType.initWithOptions(gpa.allocator(), .{ .allowFastDeinit = .auto });
     defer t.deinit();
     // add some elements
     var i: i64 = 10;