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std.HashMap: make ensureCapacity optimize for the expected count, add ensureCapacityExact

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Ryan Liptak 2019-05-02 17:01:30 -07:00
parent 0afa2d040a
commit 4d42275d03
1 changed files with 29 additions and 11 deletions
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40
std/hash_map.zig
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@ -135,21 +135,39 @@ pub fn HashMap(comptime K: type, comptime V: type, comptime hash: fn (key: K) u3
return res.kv;
}
fn optimizedCapacity(expected_count: usize) usize {
// ensure that the hash map will be at most 60% full if
// new_capacity items are put into the hash map
var optimized_capacity = expected_count * 5 / 3;
// round capacity to the next power of two
const is_power_of_two = optimized_capacity & (optimized_capacity-1) == 0;
if (!is_power_of_two) {
const pow = math.log2_int_ceil(usize, optimized_capacity);
optimized_capacity = math.pow(usize, 2, pow);
}
return optimized_capacity;
}
/// Increase capacity so that the hash map will be at most
/// 60% full when expected_count items are put into it
pub fn ensureCapacity(self: *Self, expected_count: usize) !void {
const optimized_capacity = optimizedCapacity(expected_count);
return self.ensureCapacityExact(optimized_capacity);
}
/// Sets the capacity to the new capacity if the new
/// capacity is greater than the current capacity.
pub fn ensureCapacity(self: *Self, new_capacity: usize) !void {
/// New capacity must be a power of two.
pub fn ensureCapacityExact(self: *Self, new_capacity: usize) !void {
const is_power_of_two = new_capacity & (new_capacity-1) == 0;
assert(is_power_of_two);
if (new_capacity <= self.entries.len) {
return;
}
// make sure capacity is a power of two
var capacity = new_capacity;
const is_power_of_two = capacity & (capacity-1) == 0;
if (!is_power_of_two) {
const pow = math.log2_int_ceil(usize, capacity);
capacity = math.pow(usize, 2, pow);
}
const old_entries = self.entries;
try self.initCapacity(capacity);
try self.initCapacity(new_capacity);
if (old_entries.len > 0) {
// dump all of the old elements into the new table
for (old_entries) |*old_entry| {
@ -236,11 +254,11 @@ pub fn HashMap(comptime K: type, comptime V: type, comptime hash: fn (key: K) u3
fn autoCapacity(self: *Self) !void {
if (self.entries.len == 0) {
return self.ensureCapacity(16);
return self.ensureCapacityExact(16);
}
// if we get too full (60%), double the capacity
if (self.size * 5 >= self.entries.len * 3) {
return self.ensureCapacity(self.entries.len * 2);
return self.ensureCapacityExact(self.entries.len * 2);
}
}