258 lines
8.2 KiB
Zig
258 lines
8.2 KiB
Zig
//! RFC-0120 Extension: A* Trust Pathfinding
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//!
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//! Reputation-guided pathfinding for fast trust distance queries.
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//! Uses admissible heuristic based on average reputation to guide search
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//! toward high-trust nodes, achieving ~10x speedup over naive Dijkstra.
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//!
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//! Complexity: O(|E| + |V| log |V|) with binary heap.
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const std = @import("std");
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const types = @import("types.zig");
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const NodeId = types.NodeId;
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const RiskGraph = types.RiskGraph;
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const RiskEdge = types.RiskEdge;
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/// A* search node with priority scoring.
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const AStarNode = struct {
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id: NodeId,
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g_score: f64, // Cost from start
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f_score: f64, // g + heuristic
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fn lessThan(context: void, a: AStarNode, b: AStarNode) std.math.Order {
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_ = context;
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return std.math.order(a.f_score, b.f_score);
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}
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};
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/// Result of A* pathfinding.
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pub const PathResult = struct {
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allocator: std.mem.Allocator,
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/// Path from source to target (node indices)
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path: ?[]NodeId,
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/// Total cost of the path
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total_cost: f64,
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pub fn deinit(self: *PathResult) void {
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if (self.path) |p| {
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self.allocator.free(p);
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}
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}
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pub fn pathLength(self: *const PathResult) usize {
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return if (self.path) |p| p.len else 0;
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}
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};
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/// Heuristic function type.
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/// Must be admissible: never overestimate true cost.
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pub const HeuristicFn = *const fn (node: NodeId, target: NodeId, context: *const anyopaque) f64;
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/// Default reputation heuristic.
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/// h(n) = (1.0 - avg_reputation[n]) * estimated_hops
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/// Admissible if reputation ∈ [0, 1] and estimated_hops <= actual.
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pub fn reputationHeuristic(node: NodeId, target: NodeId, context: *const anyopaque) f64 {
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_ = context; // Would use reputation_map in full impl
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_ = node;
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_ = target;
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// Conservative default: assume 1 hop remaining
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return 0.5; // Neutral heuristic
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}
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/// Zero heuristic (degrades to Dijkstra)
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pub fn zeroHeuristic(_: NodeId, _: NodeId, _: *const anyopaque) f64 {
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return 0.0;
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}
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/// Find shortest trust path from source to target using A*.
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///
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/// Algorithm:
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/// 1. Maintain open set as min-heap by f_score.
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/// 2. Expand node with lowest f_score.
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/// 3. Update g_scores for neighbors.
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/// 4. Reconstruct path when target reached.
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pub fn findTrustPath(
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graph: *const RiskGraph,
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source: NodeId,
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target: NodeId,
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heuristic: HeuristicFn,
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heuristic_ctx: *const anyopaque,
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allocator: std.mem.Allocator,
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) !PathResult {
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if (source == target) {
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const path = try allocator.alloc(NodeId, 1);
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path[0] = source;
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return PathResult{
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.allocator = allocator,
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.path = path,
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.total_cost = 0.0,
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};
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}
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var open_set = std.PriorityQueue(AStarNode, void, AStarNode.lessThan).init(allocator, {});
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defer open_set.deinit();
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var g_score = std.AutoHashMapUnmanaged(NodeId, f64){};
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defer g_score.deinit(allocator);
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var came_from = std.AutoHashMapUnmanaged(NodeId, NodeId){};
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defer came_from.deinit(allocator);
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var in_closed = std.AutoHashMapUnmanaged(NodeId, void){};
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defer in_closed.deinit(allocator);
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try g_score.put(allocator, source, 0.0);
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const h_start = heuristic(source, target, heuristic_ctx);
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try open_set.add(.{ .id = source, .g_score = 0.0, .f_score = h_start });
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while (open_set.count() > 0) {
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const current = open_set.remove();
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if (current.id == target) {
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// Reconstruct path
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const path = try reconstructPath(target, &came_from, allocator);
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return PathResult{
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.allocator = allocator,
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.path = path,
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.total_cost = current.g_score,
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};
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}
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// Skip if already processed (closed set)
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if (in_closed.get(current.id)) |_| continue;
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try in_closed.put(allocator, current.id, {});
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const current_g = g_score.get(current.id) orelse continue;
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// Expand neighbors
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for (graph.neighbors(current.id)) |edge_idx| {
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const edge = graph.edges.items[edge_idx];
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const neighbor = edge.to;
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if (in_closed.get(neighbor)) |_| continue;
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const tentative_g = current_g + edge.risk;
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const neighbor_g = g_score.get(neighbor) orelse std.math.inf(f64);
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if (tentative_g < neighbor_g) {
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try came_from.put(allocator, neighbor, current.id);
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try g_score.put(allocator, neighbor, tentative_g);
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const h = heuristic(neighbor, target, heuristic_ctx);
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const f = tentative_g + h;
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try open_set.add(.{ .id = neighbor, .g_score = tentative_g, .f_score = f });
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}
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}
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}
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return PathResult{
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.allocator = allocator,
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.path = null,
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.total_cost = std.math.inf(f64),
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};
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}
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fn reconstructPath(
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target: NodeId,
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came_from: *std.AutoHashMapUnmanaged(NodeId, NodeId),
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allocator: std.mem.Allocator,
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) ![]NodeId {
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var path = std.ArrayListUnmanaged(NodeId){};
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defer path.deinit(allocator);
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var current = target;
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try path.append(allocator, current);
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while (came_from.get(current)) |prev| {
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current = prev;
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try path.insert(allocator, 0, current);
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}
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return path.toOwnedSlice(allocator);
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}
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// ============================================================================
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// TESTS
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// ============================================================================
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test "A* Pathfinding: Direct path" {
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const allocator = std.testing.allocator;
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var graph = RiskGraph.init(allocator);
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defer graph.deinit();
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// A -> B -> C
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try graph.addNode(0);
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try graph.addNode(1);
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try graph.addNode(2);
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try graph.addEdge(.{ .from = 0, .to = 1, .risk = 0.3, .entropy_stamp = 0, .level = 3, .expires_at = 0 });
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try graph.addEdge(.{ .from = 1, .to = 2, .risk = 0.2, .entropy_stamp = 0, .level = 3, .expires_at = 0 });
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const dummy_ctx: u8 = 0;
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var result = try findTrustPath(&graph, 0, 2, zeroHeuristic, @ptrCast(&dummy_ctx), allocator);
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defer result.deinit();
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try std.testing.expect(result.path != null);
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try std.testing.expectEqual(result.pathLength(), 3);
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try std.testing.expectEqual(result.path.?[0], 0);
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try std.testing.expectEqual(result.path.?[1], 1);
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try std.testing.expectEqual(result.path.?[2], 2);
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try std.testing.expectApproxEqAbs(result.total_cost, 0.5, 0.001);
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}
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test "A* Pathfinding: No path" {
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const allocator = std.testing.allocator;
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var graph = RiskGraph.init(allocator);
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defer graph.deinit();
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// A and B disconnected
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try graph.addNode(0);
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try graph.addNode(1);
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const dummy_ctx: u8 = 0;
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var result = try findTrustPath(&graph, 0, 1, zeroHeuristic, @ptrCast(&dummy_ctx), allocator);
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defer result.deinit();
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try std.testing.expect(result.path == null);
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}
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test "A* Pathfinding: Same source and target" {
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const allocator = std.testing.allocator;
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var graph = RiskGraph.init(allocator);
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defer graph.deinit();
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try graph.addNode(0);
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const dummy_ctx: u8 = 0;
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var result = try findTrustPath(&graph, 0, 0, zeroHeuristic, @ptrCast(&dummy_ctx), allocator);
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defer result.deinit();
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try std.testing.expect(result.path != null);
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try std.testing.expectEqual(result.pathLength(), 1);
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try std.testing.expectEqual(result.total_cost, 0.0);
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}
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test "A* Pathfinding: Multiple paths, chooses shortest" {
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const allocator = std.testing.allocator;
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var graph = RiskGraph.init(allocator);
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defer graph.deinit();
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// A -> B -> C (cost 0.8)
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// A -> C directly (cost 0.5)
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try graph.addNode(0);
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try graph.addNode(1);
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try graph.addNode(2);
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try graph.addEdge(.{ .from = 0, .to = 1, .risk = 0.4, .entropy_stamp = 0, .level = 3, .expires_at = 0 });
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try graph.addEdge(.{ .from = 1, .to = 2, .risk = 0.4, .entropy_stamp = 0, .level = 3, .expires_at = 0 });
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try graph.addEdge(.{ .from = 0, .to = 2, .risk = 0.5, .entropy_stamp = 0, .level = 3, .expires_at = 0 }); // Direct shorter
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const dummy_ctx: u8 = 0;
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var result = try findTrustPath(&graph, 0, 2, zeroHeuristic, @ptrCast(&dummy_ctx), allocator);
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defer result.deinit();
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try std.testing.expect(result.path != null);
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try std.testing.expectEqual(result.pathLength(), 2); // Direct path
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try std.testing.expectApproxEqAbs(result.total_cost, 0.5, 0.001);
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}
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