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solver: Extract compute_scc into a function
Signed-off-by: Niels Thykier <niels@thykier.net>
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Niels Thykier
parent
bd375fdd85
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69473eefca
@ -21,6 +21,115 @@ from britney2.utils import (ifilter_only, iter_except)
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from britney2.installability.tester import InstallabilityTester
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def compute_scc(graph):
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"""Iterative algorithm for strongly-connected components
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Iterative variant of Tarjan's algorithm for finding strongly-connected
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components.
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:param graph: Table of all nodes along which their edges (in "before" and "after")
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:return: List of components (each component is a list of items)
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"""
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result = []
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low = {}
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node_stack = []
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def _handle_succ(parent, parent_num, successors_remaining):
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while successors_remaining:
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succ = successors_remaining.pop()
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succ_num = low.get(succ, None)
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if succ_num is not None:
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if succ_num < parent_num:
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# These two nodes are part of the probably
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# same SSC (or succ is isolated
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low[parent] = parent_num = succ_num
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continue
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# It cannot be a part of a SCC if it does not have depends
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# or reverse depends.
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if not graph[succ]['before'] or not graph[succ]['after']:
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# Short-cut obviously isolated component
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result.append((succ,))
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# Set the item number so high that no other item might
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# mistakenly assume that they can form a component via
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# this item.
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# (Replaces the "is w on the stack check" for us from
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# the original algorithm)
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low[succ] = len(graph) + 1
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continue
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succ_num = len(low)
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low[succ] = succ_num
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work_stack.append((succ, len(node_stack), succ_num, graph[succ]['before']))
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node_stack.append(succ)
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# "Recurse" into the child node first
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return True
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return False
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for n in graph:
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if n in low:
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continue
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# It cannot be a part of a SCC if it does not have depends
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# or reverse depends.
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if not graph[n]['before'] or not graph[n]['after']:
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# Short-cut obviously isolated component
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result.append((n,))
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# Set the item number so high that no other item might
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# mistakenly assume that they can form a component via
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# this item.
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# (Replaces the "is w on the stack check" for us from
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# the original algorithm)
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low[n] = len(graph) + 1
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continue
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root_num = len(low)
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low[n] = root_num
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# DFS work-stack needed to avoid call recursion. It (more or less)
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# replaces the variables on the call stack in Tarjan's algorithm
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work_stack = [(n, len(node_stack), root_num, graph[n]['before'])]
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node_stack.append(n)
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while work_stack:
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node, stack_idx, orig_node_num, successors = work_stack[-1]
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if successors and _handle_succ(node, low[node], successors):
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# _handle_succ has pushed a new node on to work_stack
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# and we need to "restart" the loop to handle that first
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continue
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# This node is done; remove it from the work stack
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work_stack.pop()
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# This node is out of successor. Push up the "low" value
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# (Exception: root node has no parent)
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node_num = low[node]
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if work_stack:
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parent = work_stack[-1][0]
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parent_num = low[parent]
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if node_num <= parent_num:
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# This node is a part of a component with its parent.
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# We update the parent's node number and push the
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# responsibility of building the component unto the
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# parent.
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low[parent] = node_num
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continue
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if node_num != orig_node_num:
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# The node is a part of an SCC with a ancestor (and parent)
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continue
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# We got a component
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component = tuple(node_stack[stack_idx:])
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del node_stack[stack_idx:]
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result.append(component)
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# Re-number all items, so no other item might
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# mistakenly assume that they can form a component via
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# one of these items.
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# (Replaces the "is w on the stack check" for us from
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# the original algorithm)
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new_num = len(graph) + 1
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for item in component:
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low[item] = new_num
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assert not node_stack
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return result
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class InstallabilitySolver(InstallabilityTester):
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def __init__(self, universe, revuniverse, testing, broken, essentials,
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@ -182,7 +291,7 @@ class InstallabilitySolver(InstallabilityTester):
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#
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# Each one of those components will become an "easy" hint.
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comps = self._compute_scc(order)
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comps = compute_scc(order)
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merged = {}
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scc = {}
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# Now that we got the SSCs (in comps), we select on item from
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@ -269,93 +378,6 @@ class InstallabilitySolver(InstallabilityTester):
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return result
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def _compute_scc(self, order):
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"""Iterative algorithm for strongly-connected components
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Iterative variant of Tarjan's algorithm for finding strongly-connected
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components.
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:param order: Table of all nodes along which their ordering constraints
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:return: List of components (each component is a list of items)
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"""
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result = []
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low = {}
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node_stack = []
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def _handle_succ(parent, parent_num, successors_remaining):
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while successors_remaining:
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succ = successors_remaining.pop()
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succ_num = low.get(succ, None)
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if succ_num is not None:
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if succ_num < parent_num:
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low[parent] = parent_num = succ_num
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continue
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succ_num = len(low)
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low[succ] = succ_num
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# It cannot be a part of a SCC if it does not have depends
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# or reverse depends.
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if not order[succ]['before'] or not order[succ]['after']:
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# Short-cut obviously isolated component
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result.append((succ,))
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continue
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work_stack.append((succ, len(node_stack), succ_num, order[succ]['before']))
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node_stack.append(succ)
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# "Recurse" into the child node first
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return True
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return False
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for n in order:
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if n in low:
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continue
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root_num = len(low)
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low[n] = root_num
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# It cannot be a part of a SCC if it does not have depends
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# or reverse depends.
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if not order[n]['before'] or not order[n]['after']:
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# Short-cut obviously isolated component
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result.append((n,))
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continue
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# DFS work-stack needed to avoid call recursion. It (more or less)
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# replaces the variables on the call stack in Tarjan's algorithm
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work_stack = [(n, len(node_stack), root_num, order[n]['before'])]
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node_stack.append(n)
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while work_stack:
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node, stack_idx, orig_node_num, successors = work_stack[-1]
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if successors and _handle_succ(node, low[node], successors):
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# _handle_succ has pushed a new node on to work_stack
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# and we need to "restart" the loop to handle that first
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continue
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# This node is done; remove it from the work stack
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work_stack.pop()
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# This node is out of successor. Push up the "low" value
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# (Exception: root node has no parent)
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node_num = low[node]
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if work_stack:
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parent = work_stack[-1][0]
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parent_num = low[parent]
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if node_num <= parent_num:
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# This node is a part of a component with its parent.
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# We update the parent's node number and push the
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# responsibility of building the component unto the
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# parent.
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low[parent] = node_num
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continue
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if node_num != orig_node_num:
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# The node is a part of an SCC with a ancestor (and parent)
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continue
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# We got a component
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component = tuple(node_stack[stack_idx:])
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del node_stack[stack_idx:]
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result.append(component)
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for item in component:
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low[item] = node_num
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assert not node_stack
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return result
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def _dump_groups(self, groups): # pragma: no cover
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print("N: === Groups ===")
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for (item, adds, rms) in groups:
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