A HAMT is a hashmap/set stored as trie, which provides update and lookup performance similarly to a normal hashmap/set, but needs no rehashing and also allows one to copy the entire map/set in constant time. This implementation uses a 32 bit hash and trie nodes with 32 children, so 5 bits of the hash are consumed to choose the next child. When there are no hash collisions, this HAMT can store 2^32 items with a maximal tree depth of (log_32 2^32) = 6, i.e., you need 6 memory accesses to find any key/value which is practically O(1). Although when there are hash collisions, they are put in an array.
Each HAMT node carries a reference counter, since FreePascal has no garbage collector. If the reference count is 1, the node can mutate, otherwise it is immutable with a copy-on-write semantic like strings. The counter is updated atomically, so the map could be shared across threads. This might lead to a large number of memory writes when a path of a full tree is copied (6 levels of up 32 children), but still less than copying a full hash table.
Everything is implemented using generics, so it can be used with all types.
Mutable Map:
type TMutableMapStringString = specialize TMutableMap<string, string, THAMTTypeInfo>;
var map: TMutableMapStringString;
p: TMutableMapStringString.PPair;
begin
map := TMutableMapStringString.create;
map.Insert('hello', 'world');
map.insert('foo', 'bar');
map['abc'] := 'def';
writeln(map['hello']); // world
writeln(map.get('foo')); // bar
writeln(map.get('abc', 'default')); // def
//enumerate all
for p in map do
writeln(p^.key, ': ', p^.value);
map.free;
end.Immutable Map:
type TImmutableMapStringString = specialize TImmutableMap<string, string, THAMTTypeInfo>;
var map, map2, map3: TImmutableMapStringString;
p: TImmutableMapStringString.PPair;
begin
map := TImmutableMapStringString.create;
map2 := map.Insert('hello', 'world');
map3 := map2.insert('foo', 'bar');
writeln(map.get('hello', 'default')); // default
writeln(map.get('foo', 'default')); // default
writeln(map2.get('hello')); // world
writeln(map2.get('foo', 'default')); // default
writeln(map3['hello']); // world
writeln(map3['foo']); // bar
//enumerate all
for p in map3 do
writeln(p^.key, ': ', p^.value);
map.free;
map2.free;
map3.free;
end.Mutable Set:
type TMutableSetString = specialize TMutableSet<string, THAMTTypeInfo>;
var stringSet: TMutableSetString;
p: TMutableSetString.PItem;
begin
stringSet := TMutableSetString.create;
stringSet.Insert('hello');
stringSet.insert('foo');
writeln(stringSet['hello']); // true
writeln(stringSet.contains('foo')); // true
writeln(stringSet.contains('abc')); // false
//enumerate all
for p in stringSet do
writeln(p^);
stringSet.free;
end.Immutable Set:
type TImmutableSetString = specialize TImmutableSet<string, THAMTTypeInfo>;
var set1, set2, set3: TImmutableSetString;
p: TImmutableSetString.PItem;
begin
set1 := TImmutableSetString.create;
set2 := set1.Insert('hello');
set3 := set2.insert('foo');
writeln(set1.contains('hello')); // false
writeln(set1['foo']); // false
writeln(set2.contains('hello')); // true
writeln(set2['foo']); // false
writeln(set3.contains('hello')); // true
writeln(set3['foo']); // true
//enumerate all
for p in set3 do
writeln(p^);
set1.free;
set2.free;
set3.free;
end.Manual:
This library requires no installation, there are no dependencies besides the FreePascal compiler. Just copy the hamt.* files in the unit search path of FreePascal. Then you can use the maps with uses hamt.maps and the sets with uses hamt.sets.
However, beware that the last stable release of FreePascal, 3.0.4, cannot compile the complex generics used here. You need to have at least FreePascal 3.1.1, preferably 3.3.1 with revision r39690.
To run the tests cases in tests/hamt_tests.pas, you also need to have bbutils in the search path.
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