/**
 * Deterministic graph utilities for situation graph operations.
 * These functions perform safe, validated operations on the graph.
 * The LLM should never directly modify the graph — it proposes changes,
 * and these utilities apply them safely.
 */

import {
  situationNodeSchema,
  situationEdgeSchema,
  situationGraphSchema,
} from "./schema.js";

function normaliseText(value) {
  return String(value || "")
    .toLowerCase()
    .replace(/[^a-z0-9]+/g, " ")
    .trim();
}

function collectNodeText(node) {
  return `${node?.label || ""} ${node?.description || ""}`.trim();
}

function countIncomingUnknownDependencies(graph, nodeId, resolvedNodeIds) {
  const resolvedSet = new Set(resolvedNodeIds || []);
  const nodesById = new Map(graph.nodes.map((node) => [node.id, node]));
  const incoming = new Set();

  for (const dependencyId of nodesById.get(nodeId)?.dependsOn || []) {
    const dependencyNode = nodesById.get(dependencyId);
    if (dependencyNode?.kind === "unknown" && !resolvedSet.has(dependencyId)) {
      incoming.add(dependencyId);
    }
  }

  for (const edge of graph.edges) {
    if (edge.toNodeId !== nodeId) continue;
    const dependencyNode = nodesById.get(edge.fromNodeId);
    if (
      dependencyNode?.kind === "unknown" &&
      !resolvedSet.has(edge.fromNodeId)
    ) {
      incoming.add(edge.fromNodeId);
    }
  }

  return incoming.size;
}

function classifyUnknownPriority(text) {
  const normalised = normaliseText(text);

  const matches = {
    objective:
      /\b(objective|goal|outcome|value|problem|job to be done|benefit|commercial value)\b/.test(
        normalised,
      ),
    actor:
      /\b(customer|user|buyer|actor|stakeholder|audience|recipient)\b/.test(
        normalised,
      ),
    criteria:
      /\b(success criteria|success threshold|threshold|decision criteria|criterion|justify|sufficient)\b/.test(
        normalised,
      ),
    measure:
      /\b(metric|measure|measurable|roi|demand|evidence|signal|proof)\b/.test(
        normalised,
      ),
    terminology: /\b(define|definition|meaning|means|term|terminology)\b/.test(
      normalised,
    ),
    constraint:
      /\b(constraint|limit|budget|deadline|requirement|regulation)\b/.test(
        normalised,
      ),
    pricing: /\b(price|pricing|price point|subscription|charge|pay for)\b/.test(
      normalised,
    ),
    implementation:
      /\b(implementation|build approach|architecture|stack|feature|technical design)\b/.test(
        normalised,
      ),
    optimisation:
      /\b(optimisation|optimi[sz]ation|improve|efficiency|performance|scale)\b/.test(
        normalised,
      ),
    speculative:
      /\b(maybe|possible|optional|future branch|nice to have|slogan|colour|color|ui)\b/.test(
        normalised,
      ),
  };

  return matches;
}

function buildScoreContributions(
  matches,
  downstreamCount,
  unresolvedParentUnknownCount,
) {
  const contributions = [
    {
      rule: "downstream_dependencies",
      value: downstreamCount,
      weight: 4,
      delta: downstreamCount * 4,
    },
  ];

  if (matches.objective) {
    contributions.push({
      rule: "objective_match",
      value: true,
      weight: 12,
      delta: 12,
    });
  }
  if (matches.actor) {
    contributions.push({
      rule: "actor_match",
      value: true,
      weight: 10,
      delta: 10,
    });
  }
  if (matches.criteria) {
    contributions.push({
      rule: "criteria_match",
      value: true,
      weight: 11,
      delta: 11,
    });
  }
  if (matches.measure) {
    contributions.push({
      rule: "measure_match",
      value: true,
      weight: 8,
      delta: 8,
    });
  }
  if (matches.terminology) {
    contributions.push({
      rule: "terminology_match",
      value: true,
      weight: 7,
      delta: 7,
    });
  }
  if (matches.constraint) {
    contributions.push({
      rule: "constraint_match",
      value: true,
      weight: 9,
      delta: 9,
    });
  }
  if (matches.pricing) {
    contributions.push({
      rule: "pricing_penalty",
      value: true,
      weight: -8,
      delta: -8,
    });
  }
  if (matches.implementation) {
    contributions.push({
      rule: "implementation_penalty",
      value: true,
      weight: -10,
      delta: -10,
    });
  }
  if (matches.optimisation) {
    contributions.push({
      rule: "optimisation_penalty",
      value: true,
      weight: -9,
      delta: -9,
    });
  }
  if (matches.speculative) {
    contributions.push({
      rule: "speculative_penalty",
      value: true,
      weight: -12,
      delta: -12,
    });
  }

  if (
    matches.pricing &&
    !matches.objective &&
    !matches.criteria &&
    !matches.actor
  ) {
    contributions.push({
      rule: "isolated_pricing_penalty",
      value: true,
      weight: -6,
      delta: -6,
    });
  }

  if (unresolvedParentUnknownCount > 0) {
    contributions.push({
      rule: "unresolved_prerequisite_penalty",
      value: unresolvedParentUnknownCount,
      weight: -7,
      delta: unresolvedParentUnknownCount * -7,
    });
  }

  return contributions;
}

function getMeaningfulSemanticContributions(contributions = []) {
  return contributions
    .filter(
      (contribution) =>
        contribution.rule !== "downstream_dependencies" &&
        contribution.rule !== "unresolved_prerequisite_penalty" &&
        contribution.delta !== 0,
    )
    .map((contribution) => ({
      rule: contribution.rule,
      delta: contribution.delta,
    }));
}

function buildCandidateDisplayOrder(candidates) {
  return [...candidates].sort((a, b) => {
    if (b.score !== a.score) return b.score - a.score;
    if (b.downstreamCount !== a.downstreamCount) {
      return b.downstreamCount - a.downstreamCount;
    }
    if (a.unresolvedParentUnknownCount !== b.unresolvedParentUnknownCount) {
      return a.unresolvedParentUnknownCount - b.unresolvedParentUnknownCount;
    }
    return a.label.localeCompare(b.label);
  });
}

function semanticSignature(candidate) {
  return JSON.stringify(
    getMeaningfulSemanticContributions(candidate.contributions),
  );
}

function classifyCandidateOrdering(candidates) {
  const displayOrder = buildCandidateDisplayOrder(candidates);
  const best = displayOrder[0] ?? null;
  if (!best) {
    return {
      displayOrder,
      best: null,
      leadingCandidates: [],
      status: "no_candidates",
      tieType: "none",
      usedAlphabeticalOrdering: false,
      reason: "No unresolved unknown candidates remain.",
    };
  }

  const topScoreCandidates = displayOrder.filter(
    (candidate) => candidate.score === best.score,
  );

  if (topScoreCandidates.length === 1) {
    return {
      displayOrder,
      best,
      leadingCandidates: [best],
      status: "selected",
      tieType: "none",
      usedAlphabeticalOrdering: false,
      reason: `Clear winner by total score (${best.score}).`,
    };
  }

  const topStructuralCandidates = topScoreCandidates.filter(
    (candidate) =>
      candidate.downstreamCount === best.downstreamCount &&
      candidate.unresolvedParentUnknownCount ===
        best.unresolvedParentUnknownCount,
  );

  if (topStructuralCandidates.length === 1) {
    return {
      displayOrder,
      best,
      leadingCandidates: [best],
      status: "selected",
      tieType: "structural_tie",
      usedAlphabeticalOrdering: false,
      reason:
        "Score tie was resolved by downstream dependency count or prerequisite ordering.",
    };
  }

  const topSemanticSignature = semanticSignature(best);
  const semanticPeers = topStructuralCandidates.filter(
    (candidate) => semanticSignature(candidate) === topSemanticSignature,
  );

  if (semanticPeers.length !== topStructuralCandidates.length) {
    return {
      displayOrder,
      best: null,
      leadingCandidates: topStructuralCandidates,
      status: "ambiguous",
      tieType: "semantic_tie",
      usedAlphabeticalOrdering: false,
      reason:
        "Leading candidates remain tied after score and structural checks, but differ in semantic contribution patterns.",
    };
  }

  return {
    displayOrder,
    best: null,
    leadingCandidates: topStructuralCandidates,
    status: "ambiguous",
    tieType: "complete_unresolved_tie",
    usedAlphabeticalOrdering: false,
    reason: "No justified distinction between leading unknowns.",
  };
}

export function scoreUnknownCandidate(graph, node, resolvedNodeIds = []) {
  const text = collectNodeText(node);
  const matches = classifyUnknownPriority(text);
  const downstreamCount = findDependentNodes(graph, node.id).length;
  const unresolvedParentUnknownCount = countIncomingUnknownDependencies(
    graph,
    node.id,
    resolvedNodeIds,
  );

  const contributions = buildScoreContributions(
    matches,
    downstreamCount,
    unresolvedParentUnknownCount,
  );
  const score = contributions.reduce(
    (total, contribution) => total + contribution.delta,
    0,
  );

  return {
    nodeId: node.id,
    label: node.label,
    score,
    downstreamCount,
    unresolvedParentUnknownCount,
    matches,
    contributions,
  };
}

export function buildDeterministicQuestionForUnknown(node) {
  const text = normaliseText(collectNodeText(node));

  if (
    /\b(success criteria|success threshold|threshold|decision criteria|criterion)\b/.test(
      text,
    )
  ) {
    return `What outcome would define success for ${node.label}?`;
  }
  if (
    /\b(customer|user|buyer|actor|stakeholder|audience|recipient)\b/.test(text)
  ) {
    return `Who is the key actor or customer for ${node.label}?`;
  }
  if (
    /\b(define|definition|meaning|means|term|terminology|value)\b/.test(text)
  ) {
    return `How should ${node.label} be defined for this decision?`;
  }
  if (
    /\b(metric|measure|measurable|roi|demand|evidence|signal|proof)\b/.test(
      text,
    )
  ) {
    return `What evidence or measure would resolve ${node.label}?`;
  }

  return `What would resolve ${node.label}?`;
}

// ── Validate that all edge references point to existing nodes ──

export function validateGraphReferences(graph) {
  const errors = [];
  const nodeIds = new Set(graph.nodes.map((n) => n.id));

  for (const node of graph.nodes) {
    if (node.parentId !== null && !nodeIds.has(node.parentId)) {
      errors.push(
        `Node "${node.id}" references parentId "${node.parentId}" which does not exist`,
      );
    }
    for (const cid of node.childIds) {
      if (!nodeIds.has(cid)) {
        errors.push(
          `Node "${node.id}" references childIds "${cid}" which does not exist`,
        );
      }
    }
    for (const dep of node.dependsOn) {
      if (!nodeIds.has(dep)) {
        errors.push(
          `Node "${node.id}" depends on "${dep}" which does not exist`,
        );
      }
    }
    for (const aff of node.affects) {
      if (!nodeIds.has(aff)) {
        errors.push(`Node "${node.id}" affects "${aff}" which does not exist`);
      }
    }
  }

  for (const edge of graph.edges) {
    if (!nodeIds.has(edge.fromNodeId)) {
      errors.push(
        `Edge "${edge.id}" references non-existent fromNodeId "${edge.fromNodeId}"`,
      );
    }
    if (!nodeIds.has(edge.toNodeId)) {
      errors.push(
        `Edge "${edge.id}" references non-existent toNodeId "${edge.toNodeId}"`,
      );
    }
  }

  return { valid: errors.length === 0, errors };
}

// ── Detect duplicate node IDs ──

export function detectDuplicateNodeIds(nodes) {
  const countMap = new Map();
  const seen = new Set();

  for (const node of nodes) {
    if (countMap.has(node.id)) {
      countMap.set(node.id, countMap.get(node.id) + 1);
    } else {
      countMap.set(node.id, 1);
    }
  }

  const duplicates = [];
  for (const [id, count] of countMap.entries()) {
    if (count > 1 && !seen.has(id)) {
      duplicates.push({ nodeId: id, count });
      seen.add(id);
    }
  }

  return duplicates;
}

// ── Detect duplicate edges ──

export function detectDuplicateEdges(edges) {
  const seen = new Set();
  const duplicates = [];

  for (const edge of edges) {
    const key = `${edge.fromNodeId}->${edge.toNodeId}:${edge.relationship}`;
    if (seen.has(key)) {
      duplicates.push({
        edgeId: edge.id,
        fromNodeId: edge.fromNodeId,
        toNodeId: edge.toNodeId,
        relationship: edge.relationship,
      });
    }
    seen.add(key);
  }

  return duplicates;
}

// ── Find all nodes that depend on a given node (transitive) ──

export function findDependentNodes(graph, nodeId) {
  const direct = graph.nodes
    .filter((n) => n.dependsOn.includes(nodeId))
    .map((n) => n.id);
  const affected = new Set(direct);

  // Also propagate through edges where the relationship is depends_on
  for (const edge of graph.edges) {
    if (edge.toNodeId === nodeId && !affected.has(edge.fromNodeId)) {
      direct.push(edge.fromNodeId);
      affected.add(edge.fromNodeId);
    }
  }

  // Transitive propagation — BFS
  const queue = [...direct];
  while (queue.length > 0) {
    const current = queue.shift();
    if (!current || !affected.has(current)) continue;

    for (const node of graph.nodes) {
      if (node.dependsOn.includes(current) && !affected.has(node.id)) {
        affected.add(node.id);
        queue.push(node.id);
      }
    }
  }

  return [...affected];
}

// ── Find all nodes that are directly or indirectly affected by a change in nodeId ──

export function findAffectedNodes(graph, nodeId) {
  // Direct effects: two sources
  // 1. Nodes that depend on this node (they list it in their dependsOn)
  const directFromDepends = graph.nodes
    .filter((n) => n.id !== nodeId && n.dependsOn.includes(nodeId))
    .map((n) => n.id);

  // 2. Targets of the node's affects relationships (this node directly affects them)
  const myAffectedTargets = new Set(
    graph.nodes.find((n) => n.id === nodeId)?.affects || [],
  );

  // Merge: also add edge targets where this node is the source
  for (const edge of graph.edges) {
    if (edge.fromNodeId === nodeId && !myAffectedTargets.has(edge.toNodeId)) {
      myAffectedTargets.add(edge.toNodeId);
    }
  }

  // Combine both sources
  const direct = [...new Set([...directFromDepends, ...myAffectedTargets])];

  // Transitive propagation — BFS through dependsOn and affects of affected nodes
  const affected = new Set(direct);
  const queue = [...direct];
  while (queue.length > 0) {
    const current = queue.shift();
    if (!current || !affected.has(current)) continue;

    for (const node of graph.nodes) {
      if (
        node.id !== nodeId &&
        !affected.has(node.id) &&
        (node.dependsOn.includes(current) || node.affects.includes(current))
      ) {
        affected.add(node.id);
        queue.push(node.id);
      }
    }
  }

  return [...affected];
}

// ── Resolve an unknown node ──

export function resolveUnknownNode(graph, nodeId, newStatus, newValue, reason) {
  const nodeIdx = graph.nodes.findIndex((n) => n.id === nodeId);
  if (nodeIdx === -1) {
    return { success: false, error: `Node "${nodeId}" not found in graph` };
  }

  const previousStatus = graph.nodes[nodeIdx].status;
  const previousValue = graph.nodes[nodeIdx].value;

  return {
    success: true,
    previousStatus,
    newStatus,
    previousValue,
    newValue,
    reason,
    affectedNodes: findAffectedNodes(graph, nodeId),
  };
}

// ── Select the next highest-value active unknown candidate ──

export function selectActiveUnknownCandidate(graph, resolvedNodeIds) {
  // Skip already resolved nodes
  const unresolved = graph.nodes.filter(
    (n) =>
      n.kind === "unknown" &&
      !["known", "resolved", "contradicted"].includes(n.status) &&
      !resolvedNodeIds.includes(n.id),
  );

  if (unresolved.length === 0) return null;

  const scoredCandidates = unresolved.map((node) => ({
    node,
    ...scoreUnknownCandidate(graph, node, resolvedNodeIds),
  }));

  const selection = classifyCandidateOrdering(
    scoredCandidates.map(({ node, ...candidate }) => ({
      ...candidate,
      node,
    })),
  );

  if (selection.status === "ambiguous") {
    return {
      selectedNode: null,
      status: "ambiguous",
      tieType: selection.tieType,
      tiedCandidateIds: selection.leadingCandidates.map(
        (candidate) => candidate.nodeId,
      ),
      displayOrder: selection.displayOrder.map((candidate) => candidate.nodeId),
      reason: selection.reason,
    };
  }

  const best = selection.best;
  if (!best) return null;

  return {
    selectedNode: {
      nodeId: best.node.id,
      label: best.node.label,
    },
    status: "selected",
    tieType: selection.tieType,
    nodeId: best.node.id,
    label: best.node.label,
    score: best.score,
    question: buildDeterministicQuestionForUnknown(best.node),
    reason: `Selected for highest information value (score ${best.score}) with ${best.downstreamCount} downstream dependency node(s) and ${best.unresolvedParentUnknownCount} unresolved prerequisite unknown(s).`,
  };
}

export function explainUnknownSelection(graph, resolvedNodeIds = []) {
  const unresolved = graph.nodes.filter(
    (n) => n.kind === "unknown" && !resolvedNodeIds.includes(n.id),
  );

  if (unresolved.length === 0) {
    return {
      selectedNodeId: null,
      selectedNodeLabel: null,
      status: "no_candidates",
      tieType: "none",
      resolvedNodeIds: [...resolvedNodeIds],
      tiedCandidateIds: [],
      candidates: [],
      competitors: [],
      tieBreakOrder: [
        "score_desc",
        "downstreamCount_desc",
        "unresolvedParentUnknownCount_asc",
        "label_asc",
      ],
      summary: {
        candidateCount: 0,
      },
    };
  }

  const candidates = unresolved.map((node) => ({
    nodeId: node.id,
    label: node.label,
    ...scoreUnknownCandidate(graph, node, resolvedNodeIds),
  }));

  const selection = classifyCandidateOrdering(candidates);
  const orderedCandidates = selection.displayOrder;
  const selected = selection.best;
  const competitors = orderedCandidates
    .filter((candidate) => candidate.nodeId !== selected?.nodeId)
    .map((candidate) => ({
      nodeId: candidate.nodeId,
      label: candidate.label,
      score: candidate.score,
      downstreamCount: candidate.downstreamCount,
      unresolvedParentUnknownCount: candidate.unresolvedParentUnknownCount,
      matches: candidate.matches,
      contributions: candidate.contributions,
      outrankedBy: {
        scoreDelta: (selected?.score ?? candidate.score) - candidate.score,
        downstreamDelta:
          (selected?.downstreamCount ?? candidate.downstreamCount) -
          candidate.downstreamCount,
        unresolvedPrerequisiteDelta:
          candidate.unresolvedParentUnknownCount -
          (selected?.unresolvedParentUnknownCount ??
            candidate.unresolvedParentUnknownCount),
        labelOrderWinner:
          selected &&
          selected.score === candidate.score &&
          selected.downstreamCount === candidate.downstreamCount &&
          selected.unresolvedParentUnknownCount ===
            candidate.unresolvedParentUnknownCount
            ? selected.label.localeCompare(candidate.label) <= 0
              ? selected.label
              : candidate.label
            : null,
      },
    }));

  return {
    selectedNodeId: selected?.nodeId ?? null,
    selectedNodeLabel: selected?.label ?? null,
    status: selection.status,
    tieType: selection.tieType,
    resolvedNodeIds: [...resolvedNodeIds],
    tiedCandidateIds: selection.leadingCandidates.map(
      (candidate) => candidate.nodeId,
    ),
    tieBreakOrder: [
      "score_desc",
      "downstreamCount_desc",
      "unresolvedParentUnknownCount_asc",
      "label_asc",
    ],
    alphabeticalUsedAsReasoning: false,
    candidates: orderedCandidates,
    selected: selected
      ? {
          nodeId: selected.nodeId,
          label: selected.label,
          score: selected.score,
          downstreamCount: selected.downstreamCount,
          unresolvedParentUnknownCount: selected.unresolvedParentUnknownCount,
          matches: selected.matches,
          contributions: selected.contributions,
        }
      : null,
    competitors,
    summary: {
      candidateCount: orderedCandidates.length,
      selectedReason: selected
        ? `highest_score=${selected.score}; downstream=${selected.downstreamCount}; unresolved_prerequisites=${selected.unresolvedParentUnknownCount}`
        : selection.reason,
    },
  };
}

// ── Apply a graph update deterministically ──

export function applyGraphUpdate(graph, update) {
  const errors = [];
  const updatedNodesMap = new Map();

  // Validate that update references existing nodes or newly added ones
  const allNodeIds = new Set(graph.nodes.map((n) => n.id));
  for (const added of update.addedNodes) {
    if (allNodeIds.has(added.id)) {
      errors.push(`Cannot add node with duplicate ID: "${added.id}"`);
      continue;
    }
    allNodeIds.add(added.id);
  }

  // Validate updated nodes exist
  for (const upd of update.updatedNodes) {
    if (!allNodeIds.has(upd.nodeId)) {
      errors.push(`Cannot update non-existent node: "${upd.nodeId}"`);
    }
  }

  // Validate added edges reference existing or new nodes
  for (const edge of update.addedEdges) {
    if (!allNodeIds.has(edge.fromNodeId)) {
      errors.push(
        `Added edge references non-existent fromNodeId: "${edge.fromNodeId}"`,
      );
    }
    if (!allNodeIds.has(edge.toNodeId)) {
      errors.push(
        `Added edge references non-existent toNodeId: "${edge.toNodeId}"`,
      );
    }
  }

  if (errors.length > 0) return { success: false, errors };

  // Build the new nodes list — start with a deep copy of existing
  const newNodes = graph.nodes.map((n) => ({ ...n }));

  // Apply updated nodes
  for (const upd of update.updatedNodes) {
    const idx = newNodes.findIndex((n) => n.id === upd.nodeId);
    if (idx === -1) continue; // already validated above

    if (upd.newStatus !== undefined && upd.newStatus !== null) {
      newNodes[idx].status = upd.newStatus;
    }
    if (upd.newValue !== undefined) {
      newNodes[idx].value = upd.newValue;
    }
    updatedNodesMap.set(upd.nodeId, newNodes[idx]);
  }

  // Add new nodes
  for (const newNode of update.addedNodes) {
    if (!allNodeIds.has(newNode.id)) continue;
    allNodeIds.add(newNode.id);
    newNodes.push({ ...newNode });
  }

  // Remove edges if requested
  const removedEdgeSet = new Set(update.removedEdgeIds);
  const newEdges = graph.edges.filter((e) => !removedEdgeSet.has(e.id));

  // Add new edges
  for (const newEdge of update.addedEdges) {
    newEdges.push({ ...newEdge });

    // Update dependsOn / affects on the nodes
    const fromNode = newNodes.find((n) => n.id === newEdge.fromNodeId);
    const toNode = newNodes.find((n) => n.id === newEdge.toNodeId);
    if (fromNode && !fromNode.childIds.includes(newEdge.toNodeId)) {
      fromNode.childIds.push(newEdge.toNodeId);
    }
    if (toNode && !toNode.dependsOn.includes(newEdge.fromNodeId)) {
      toNode.dependsOn.push(newEdge.fromNodeId);
    }
  }

  // Add resolved node IDs
  const newResolved = [
    ...new Set([...graph.resolvedNodeIds, ...update.resolvedUnknownNodeIds]),
  ];

  return {
    success: true,
    nodes: newNodes,
    edges: newEdges,
    resolvedNodeIds: newResolved,
  };
}

// ── Validate a proposed graph update before application ──

export function validateGraphUpdate(graph, update) {
  const errors = [];

  // Check for duplicate node IDs against existing and newly added nodes
  const extendedIds = new Set(graph.nodes.map((n) => n.id));
  for (const newNode of update.addedNodes) {
    if (extendedIds.has(newNode.id)) {
      errors.push(`Cannot add node with duplicate ID: "${newNode.id}"`);
    } else {
      extendedIds.add(newNode.id);
    }
  }

  // Check updated nodes exist (in original graph, not newly added ones)
  const existingIds = new Set(graph.nodes.map((n) => n.id));
  for (const upd of update.updatedNodes) {
    if (!existingIds.has(upd.nodeId)) {
      errors.push(`Cannot update non-existent node: "${upd.nodeId}"`);
    }
  }

  // ── structuralActionRequired contract (57J.67) ───────────

  const meaningPopulated = !!update.answerMeaning?.userSupportedMeaning;
  const statusChanged = update.updatedNodes.some(
    (u) => u.previousStatus !== null && u.newStatus !== u.previousStatus,
  );
  const valueChanged = update.updatedNodes.some(
    (u) => (u.previousValue ?? null) !== (u.newValue ?? null),
  );

  const hasMeaningfulChange =
    update.addedNodes.length > 0 ||
    statusChanged ||
    valueChanged ||
    update.addedEdges.length > 0 ||
    update.removedEdgeIds.length > 0;

  // Missing/null transition rule: must be present when userSupportedMeaning is populated
  if (
    (update.structuralActionRequired === null || update.structuralActionRequired === undefined) &&
    meaningPopulated
  ) {
    errors.push(
      "structuralActionRequired must be present when userSupportedMeaning is populated",
    );
  }

  // Exact structural claim — four contradiction pairs
  if (update.structuralActionRequired === true && !hasMeaningfulChange) {
    errors.push("structuralActionRequired is true but proposal contains no graph mutation");
  }
  if (update.structuralActionRequired === false && hasMeaningfulChange) {
    errors.push("structuralActionRequired is false but proposal contains meaningful mutations");
  }

  // Legacy no-op guard: only fires when structuralActionRequired is absent (null/undefined).
  // When true or false → the new contract owns no-op/mutation consistency.
  // The contract checks above already produced authoritative errors for those cases.

  const fieldAbsent =
    update.structuralActionRequired === null ||
    update.structuralActionRequired === undefined;

  if (!hasMeaningfulChange && fieldAbsent) {
    if (meaningPopulated) {
      // structuralActionRequired was missing while userSupportedMeaning exists.
      // Missing-field rejection already added above; skip semantic-only guard to avoid duplicate errors on the same proposal.
    } else if (!meaningPopulated) {
      errors.push("Update contains no meaningful change");
    }
  }

  // Legacy guard when structuralActionRequired=false: false declares no action needed,
  // but userSupportedMeaning populated implies semantic intent for change. However the
  // new-contract checks above already produced an error if there IS mutation. If there's
  // zero mutation with false + meaning, treat as intentional no-op (contract PASS).

  // Reject oversized input
  const totalSize = JSON.stringify(update).length;
  if (totalSize > 100000) {
    errors.push(`Proposed graph update exceeds 100KB (${totalSize} bytes)`);
  }

  return { valid: errors.length === 0, errors };
}