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std/ArrayList: Allow ArrayList(u0) to be created #8632

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Jul 28, 2021
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std/ArrayList: Allow ArrayList(u0) to be created #8632

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77 changes: 56 additions & 21 deletions lib/std/array_list.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -65,16 +65,23 @@ pub fn ArrayListAligned(comptime T: type, comptime alignment: ?u29) type {
pub fn initCapacity(allocator: *Allocator, num: usize) !Self {
var self = Self.init(allocator);

const new_memory = try self.allocator.allocAdvanced(T, alignment, num, .at_least);
self.items.ptr = new_memory.ptr;
self.capacity = new_memory.len;
if (@sizeOf(T) > 0) {
const new_memory = try self.allocator.allocAdvanced(T, alignment, num, .at_least);
self.items.ptr = new_memory.ptr;
self.capacity = new_memory.len;
} else {
// If `T` is a zero-sized type, then we do not need to allocate memory.
self.capacity = std.math.maxInt(usize);
}

return self;
}

/// Release all allocated memory.
pub fn deinit(self: Self) void {
self.allocator.free(self.allocatedSlice());
if (@sizeOf(T) > 0) {
self.allocator.free(self.allocatedSlice());
}
}

pub const span = @compileError("deprecated: use `items` field directly");
Expand Down Expand Up @@ -279,13 +286,17 @@ pub fn ArrayListAligned(comptime T: type, comptime alignment: ?u29) type {
pub fn shrinkAndFree(self: *Self, new_len: usize) void {
assert(new_len <= self.items.len);

self.items = self.allocator.realloc(self.allocatedSlice(), new_len) catch |e| switch (e) {
error.OutOfMemory => { // no problem, capacity is still correct then.
self.items.len = new_len;
return;
},
};
self.capacity = new_len;
if (@sizeOf(T) > 0) {
self.items = self.allocator.realloc(self.allocatedSlice(), new_len) catch |e| switch (e) {
error.OutOfMemory => { // no problem, capacity is still correct then.
self.items.len = new_len;
return;
},
};
self.capacity = new_len;
} else {
self.items.len = new_len;
}
}

/// Reduce length to `new_len`.
Expand Down Expand Up @@ -313,18 +324,22 @@ pub fn ArrayListAligned(comptime T: type, comptime alignment: ?u29) type {
/// Modify the array so that it can hold at least `new_capacity` items.
/// Invalidates pointers if additional memory is needed.
pub fn ensureTotalCapacity(self: *Self, new_capacity: usize) !void {
var better_capacity = self.capacity;
if (better_capacity >= new_capacity) return;
if (@sizeOf(T) > 0) {
var better_capacity = self.capacity;
if (better_capacity >= new_capacity) return;

while (true) {
better_capacity += better_capacity / 2 + 8;
if (better_capacity >= new_capacity) break;
}
while (true) {
better_capacity += better_capacity / 2 + 8;
if (better_capacity >= new_capacity) break;
}

// TODO This can be optimized to avoid needlessly copying undefined memory.
const new_memory = try self.allocator.reallocAtLeast(self.allocatedSlice(), better_capacity);
self.items.ptr = new_memory.ptr;
self.capacity = new_memory.len;
// TODO This can be optimized to avoid needlessly copying undefined memory.
const new_memory = try self.allocator.reallocAtLeast(self.allocatedSlice(), better_capacity);
self.items.ptr = new_memory.ptr;
self.capacity = new_memory.len;
} else {
self.capacity = std.math.maxInt(usize);
}
}

/// Modify the array so that it can hold at least `additional_count` **more** items.
Expand Down Expand Up @@ -1299,3 +1314,23 @@ test "ArrayListAligned/ArrayListAlignedUnmanaged accepts unaligned slices" {
try testing.expectEqualSlices(u8, list.items, &.{ 0, 8, 9, 6, 7, 2, 3 });
}
}

test "std.ArrayList(u0)" {
// An ArrayList on zero-sized types should not need to allocate
const a = &testing.FailingAllocator.init(testing.allocator, 0).allocator;

var list = ArrayList(u0).init(a);
defer list.deinit();

try list.append(0);
try list.append(0);
try list.append(0);
try testing.expectEqual(list.items.len, 3);

var count: usize = 0;
for (list.items) |x| {
try testing.expectEqual(x, 0);
count += 1;
}
try testing.expectEqual(count, 3);
}