Haibo Huang | 5eba2b4 | 2021-01-22 11:22:02 -0800 | [diff] [blame] | 1 | __all__ = ["TopologicalSorter", "CycleError"] |
| 2 | |
| 3 | _NODE_OUT = -1 |
| 4 | _NODE_DONE = -2 |
| 5 | |
| 6 | |
| 7 | class _NodeInfo: |
| 8 | __slots__ = "node", "npredecessors", "successors" |
| 9 | |
| 10 | def __init__(self, node): |
| 11 | # The node this class is augmenting. |
| 12 | self.node = node |
| 13 | |
| 14 | # Number of predecessors, generally >= 0. When this value falls to 0, |
| 15 | # and is returned by get_ready(), this is set to _NODE_OUT and when the |
| 16 | # node is marked done by a call to done(), set to _NODE_DONE. |
| 17 | self.npredecessors = 0 |
| 18 | |
| 19 | # List of successor nodes. The list can contain duplicated elements as |
Yi Kong | 7119932 | 2022-08-30 15:53:45 +0800 | [diff] [blame] | 20 | # long as they're all reflected in the successor's npredecessors attribute. |
Haibo Huang | 5eba2b4 | 2021-01-22 11:22:02 -0800 | [diff] [blame] | 21 | self.successors = [] |
| 22 | |
| 23 | |
| 24 | class CycleError(ValueError): |
| 25 | """Subclass of ValueError raised by TopologicalSorter.prepare if cycles |
| 26 | exist in the working graph. |
| 27 | |
| 28 | If multiple cycles exist, only one undefined choice among them will be reported |
| 29 | and included in the exception. The detected cycle can be accessed via the second |
| 30 | element in the *args* attribute of the exception instance and consists in a list |
| 31 | of nodes, such that each node is, in the graph, an immediate predecessor of the |
| 32 | next node in the list. In the reported list, the first and the last node will be |
| 33 | the same, to make it clear that it is cyclic. |
| 34 | """ |
| 35 | |
| 36 | pass |
| 37 | |
| 38 | |
| 39 | class TopologicalSorter: |
| 40 | """Provides functionality to topologically sort a graph of hashable nodes""" |
| 41 | |
| 42 | def __init__(self, graph=None): |
| 43 | self._node2info = {} |
| 44 | self._ready_nodes = None |
| 45 | self._npassedout = 0 |
| 46 | self._nfinished = 0 |
| 47 | |
| 48 | if graph is not None: |
| 49 | for node, predecessors in graph.items(): |
| 50 | self.add(node, *predecessors) |
| 51 | |
| 52 | def _get_nodeinfo(self, node): |
| 53 | if (result := self._node2info.get(node)) is None: |
| 54 | self._node2info[node] = result = _NodeInfo(node) |
| 55 | return result |
| 56 | |
| 57 | def add(self, node, *predecessors): |
| 58 | """Add a new node and its predecessors to the graph. |
| 59 | |
| 60 | Both the *node* and all elements in *predecessors* must be hashable. |
| 61 | |
| 62 | If called multiple times with the same node argument, the set of dependencies |
| 63 | will be the union of all dependencies passed in. |
| 64 | |
| 65 | It is possible to add a node with no dependencies (*predecessors* is not provided) |
| 66 | as well as provide a dependency twice. If a node that has not been provided before |
| 67 | is included among *predecessors* it will be automatically added to the graph with |
| 68 | no predecessors of its own. |
| 69 | |
| 70 | Raises ValueError if called after "prepare". |
| 71 | """ |
| 72 | if self._ready_nodes is not None: |
| 73 | raise ValueError("Nodes cannot be added after a call to prepare()") |
| 74 | |
| 75 | # Create the node -> predecessor edges |
| 76 | nodeinfo = self._get_nodeinfo(node) |
| 77 | nodeinfo.npredecessors += len(predecessors) |
| 78 | |
| 79 | # Create the predecessor -> node edges |
| 80 | for pred in predecessors: |
| 81 | pred_info = self._get_nodeinfo(pred) |
| 82 | pred_info.successors.append(node) |
| 83 | |
| 84 | def prepare(self): |
| 85 | """Mark the graph as finished and check for cycles in the graph. |
| 86 | |
| 87 | If any cycle is detected, "CycleError" will be raised, but "get_ready" can |
| 88 | still be used to obtain as many nodes as possible until cycles block more |
| 89 | progress. After a call to this function, the graph cannot be modified and |
| 90 | therefore no more nodes can be added using "add". |
| 91 | """ |
| 92 | if self._ready_nodes is not None: |
| 93 | raise ValueError("cannot prepare() more than once") |
| 94 | |
| 95 | self._ready_nodes = [ |
| 96 | i.node for i in self._node2info.values() if i.npredecessors == 0 |
| 97 | ] |
| 98 | # ready_nodes is set before we look for cycles on purpose: |
| 99 | # if the user wants to catch the CycleError, that's fine, |
| 100 | # they can continue using the instance to grab as many |
| 101 | # nodes as possible before cycles block more progress |
| 102 | cycle = self._find_cycle() |
| 103 | if cycle: |
| 104 | raise CycleError(f"nodes are in a cycle", cycle) |
| 105 | |
| 106 | def get_ready(self): |
| 107 | """Return a tuple of all the nodes that are ready. |
| 108 | |
| 109 | Initially it returns all nodes with no predecessors; once those are marked |
| 110 | as processed by calling "done", further calls will return all new nodes that |
| 111 | have all their predecessors already processed. Once no more progress can be made, |
| 112 | empty tuples are returned. |
| 113 | |
| 114 | Raises ValueError if called without calling "prepare" previously. |
| 115 | """ |
| 116 | if self._ready_nodes is None: |
| 117 | raise ValueError("prepare() must be called first") |
| 118 | |
| 119 | # Get the nodes that are ready and mark them |
| 120 | result = tuple(self._ready_nodes) |
| 121 | n2i = self._node2info |
| 122 | for node in result: |
| 123 | n2i[node].npredecessors = _NODE_OUT |
| 124 | |
| 125 | # Clean the list of nodes that are ready and update |
| 126 | # the counter of nodes that we have returned. |
| 127 | self._ready_nodes.clear() |
| 128 | self._npassedout += len(result) |
| 129 | |
| 130 | return result |
| 131 | |
| 132 | def is_active(self): |
| 133 | """Return ``True`` if more progress can be made and ``False`` otherwise. |
| 134 | |
| 135 | Progress can be made if cycles do not block the resolution and either there |
| 136 | are still nodes ready that haven't yet been returned by "get_ready" or the |
| 137 | number of nodes marked "done" is less than the number that have been returned |
| 138 | by "get_ready". |
| 139 | |
| 140 | Raises ValueError if called without calling "prepare" previously. |
| 141 | """ |
| 142 | if self._ready_nodes is None: |
| 143 | raise ValueError("prepare() must be called first") |
| 144 | return self._nfinished < self._npassedout or bool(self._ready_nodes) |
| 145 | |
| 146 | def __bool__(self): |
| 147 | return self.is_active() |
| 148 | |
| 149 | def done(self, *nodes): |
| 150 | """Marks a set of nodes returned by "get_ready" as processed. |
| 151 | |
| 152 | This method unblocks any successor of each node in *nodes* for being returned |
| 153 | in the future by a call to "get_ready". |
| 154 | |
| 155 | Raises :exec:`ValueError` if any node in *nodes* has already been marked as |
| 156 | processed by a previous call to this method, if a node was not added to the |
| 157 | graph by using "add" or if called without calling "prepare" previously or if |
| 158 | node has not yet been returned by "get_ready". |
| 159 | """ |
| 160 | |
| 161 | if self._ready_nodes is None: |
| 162 | raise ValueError("prepare() must be called first") |
| 163 | |
| 164 | n2i = self._node2info |
| 165 | |
| 166 | for node in nodes: |
| 167 | |
| 168 | # Check if we know about this node (it was added previously using add() |
| 169 | if (nodeinfo := n2i.get(node)) is None: |
| 170 | raise ValueError(f"node {node!r} was not added using add()") |
| 171 | |
| 172 | # If the node has not being returned (marked as ready) previously, inform the user. |
| 173 | stat = nodeinfo.npredecessors |
| 174 | if stat != _NODE_OUT: |
| 175 | if stat >= 0: |
| 176 | raise ValueError( |
| 177 | f"node {node!r} was not passed out (still not ready)" |
| 178 | ) |
| 179 | elif stat == _NODE_DONE: |
| 180 | raise ValueError(f"node {node!r} was already marked done") |
| 181 | else: |
| 182 | assert False, f"node {node!r}: unknown status {stat}" |
| 183 | |
| 184 | # Mark the node as processed |
| 185 | nodeinfo.npredecessors = _NODE_DONE |
| 186 | |
| 187 | # Go to all the successors and reduce the number of predecessors, collecting all the ones |
| 188 | # that are ready to be returned in the next get_ready() call. |
| 189 | for successor in nodeinfo.successors: |
| 190 | successor_info = n2i[successor] |
| 191 | successor_info.npredecessors -= 1 |
| 192 | if successor_info.npredecessors == 0: |
| 193 | self._ready_nodes.append(successor) |
| 194 | self._nfinished += 1 |
| 195 | |
| 196 | def _find_cycle(self): |
| 197 | n2i = self._node2info |
| 198 | stack = [] |
| 199 | itstack = [] |
| 200 | seen = set() |
| 201 | node2stacki = {} |
| 202 | |
| 203 | for node in n2i: |
| 204 | if node in seen: |
| 205 | continue |
| 206 | |
| 207 | while True: |
| 208 | if node in seen: |
| 209 | # If we have seen already the node and is in the |
| 210 | # current stack we have found a cycle. |
| 211 | if node in node2stacki: |
| 212 | return stack[node2stacki[node] :] + [node] |
| 213 | # else go on to get next successor |
| 214 | else: |
| 215 | seen.add(node) |
| 216 | itstack.append(iter(n2i[node].successors).__next__) |
| 217 | node2stacki[node] = len(stack) |
| 218 | stack.append(node) |
| 219 | |
| 220 | # Backtrack to the topmost stack entry with |
| 221 | # at least another successor. |
| 222 | while stack: |
| 223 | try: |
| 224 | node = itstack[-1]() |
| 225 | break |
| 226 | except StopIteration: |
| 227 | del node2stacki[stack.pop()] |
| 228 | itstack.pop() |
| 229 | else: |
| 230 | break |
| 231 | return None |
| 232 | |
| 233 | def static_order(self): |
| 234 | """Returns an iterable of nodes in a topological order. |
| 235 | |
| 236 | The particular order that is returned may depend on the specific |
| 237 | order in which the items were inserted in the graph. |
| 238 | |
| 239 | Using this method does not require to call "prepare" or "done". If any |
| 240 | cycle is detected, :exc:`CycleError` will be raised. |
| 241 | """ |
| 242 | self.prepare() |
| 243 | while self.is_active(): |
| 244 | node_group = self.get_ready() |
| 245 | yield from node_group |
| 246 | self.done(*node_group) |