// Pure SysML model validator.
//
// 13 rules, grouped:
//   - Structural (S1–S5) — referential integrity. Errors.
//   - Semantic (M1–M5) — SysML metamodel rules. Mostly errors.
//   - Traceability (T1–T2) — soft warnings on coverage.
//
// All rules are described in docs/sysml-modeling.md §7.
// Pure function; safe to call repeatedly. Output is element-anchored so the
// UI can dot/highlight the offending block, association, etc.

import type {
  SysMLModel,
  Block,
  Association,
  Constraint,
  Requirement,
} from "./model";

export type Severity = "error" | "warning" | "soft";

export type IssueAnchor =
  | { kind: "block"; id: string }
  | { kind: "association"; id: string }
  | { kind: "constraint"; id: string }
  | { kind: "requirement"; id: string }
  | { kind: "property"; blockId: string; propertyId: string }
  | { kind: "term"; id: string }
  | { kind: "model" };

export interface ValidationIssue {
  /** Stable rule code: S1, M2, T1, etc. */
  code: string;
  severity: Severity;
  message: string;
  anchor: IssueAnchor;
}

export function validate(model: SysMLModel): ValidationIssue[] {
  const issues: ValidationIssue[] = [];

  const blockIds = new Set(model.blocks.map(b => b.id));
  const reqIds = new Set(model.requirements.map(r => r.id));

  // Lookups
  const blockById = new Map<string, Block>(model.blocks.map(b => [b.id, b]));

  // ─── Structural ────────────────────────────────────────────────────────

  // S1 — every association.fromBlockId / toBlockId resolves to a block
  for (const a of model.associations) {
    if (!blockIds.has(a.fromBlockId)) {
      issues.push(s1IssueFor(a, "from", a.fromBlockId));
    }
    if (!blockIds.has(a.toBlockId)) {
      issues.push(s1IssueFor(a, "to", a.toBlockId));
    }
  }

  // S2 — every constraint.appliesTo entry resolves to a block
  for (const c of model.constraints) {
    for (const target of c.appliesTo) {
      if (!blockIds.has(target)) {
        issues.push({
          code: "S2",
          severity: "error",
          message: `Constraint "${c.label}" applies to unknown block "${target}".`,
          anchor: { kind: "constraint", id: c.id },
        });
      }
    }
  }

  // S3 — every requirement satisfy.blockId resolves to a block
  // S4 — every requirement derive.fromReqId resolves to a requirement
  for (const r of model.requirements) {
    for (const rel of r.relations) {
      if (rel.kind === "satisfy" && !blockIds.has(rel.blockId)) {
        issues.push({
          code: "S3",
          severity: "error",
          message: `Requirement ${r.tag} is satisfied by unknown block "${rel.blockId}".`,
          anchor: { kind: "requirement", id: r.id },
        });
      }
      if (rel.kind === "derive" && !reqIds.has(rel.fromReqId)) {
        issues.push({
          code: "S4",
          severity: "error",
          message: `Requirement ${r.tag} derives from unknown requirement "${rel.fromReqId}".`,
          anchor: { kind: "requirement", id: r.id },
        });
      }
    }
  }

  // S5 — block ids unique; requirement tags unique; association ids unique
  uniquenessIssues(model.blocks, b => b.id, "S5", "block id", b => ({ kind: "block", id: b.id })).forEach(i => issues.push(i));
  uniquenessIssues(model.associations, a => a.id, "S5", "association id", a => ({ kind: "association", id: a.id })).forEach(i => issues.push(i));
  uniquenessIssues(model.constraints, c => c.id, "S5", "constraint id", c => ({ kind: "constraint", id: c.id })).forEach(i => issues.push(i));
  uniquenessIssues(model.requirements, r => r.tag, "S5", "requirement tag", r => ({ kind: "requirement", id: r.id })).forEach(i => issues.push(i));

  // ─── Semantic (SysML metamodel) ────────────────────────────────────────

  // M1 — generalization is acyclic
  cycles(
    model.blocks,
    block => model.associations.filter(a => a.kind === "generalization" && a.fromBlockId === block.id).map(a => a.toBlockId),
    blockById
  ).forEach(cycle =>
    issues.push({
      code: "M1",
      severity: "error",
      message: `Cyclic generalization: ${cycle.join(" → ")} → ${cycle[0]}`,
      anchor: { kind: "block", id: cycle[0]! },
    })
  );

  // M2 — composition is acyclic
  cycles(
    model.blocks,
    block => model.associations.filter(a => a.kind === "composition" && a.fromBlockId === block.id).map(a => a.toBlockId),
    blockById
  ).forEach(cycle =>
    issues.push({
      code: "M2",
      severity: "error",
      message: `Cyclic composition: ${cycle.join(" → ")} → ${cycle[0]}`,
      anchor: { kind: "block", id: cycle[0]! },
    })
  );

  // M3 — exactly zero or one block has kind "system"
  const systems = model.blocks.filter(b => b.kind === "system");
  if (systems.length > 1) {
    for (const s of systems) {
      issues.push({
        code: "M3",
        severity: "error",
        message: `Multiple System-of-Interest blocks: ${systems.map(x => x.label).join(", ")}.`,
        anchor: { kind: "block", id: s.id },
      });
    }
  }

  // M4 — constraint blocks may not participate in composition
  for (const a of model.associations) {
    if (a.kind !== "composition") continue;
    const from = blockById.get(a.fromBlockId);
    const to = blockById.get(a.toBlockId);
    if ((from && from.kind === "constraint") || (to && to.kind === "constraint")) {
      issues.push({
        code: "M4",
        severity: "error",
        message: `Composition involves a constraint block (${from?.kind === "constraint" ? from.label : to?.label}).`,
        anchor: { kind: "association", id: a.id },
      });
    }
  }

  // M5 — property names unique within a block
  for (const b of model.blocks) {
    const seen = new Map<string, number>();
    for (const p of b.properties) {
      seen.set(p.name, (seen.get(p.name) ?? 0) + 1);
    }
    for (const [name, count] of seen) {
      if (count > 1) {
        issues.push({
          code: "M5",
          severity: "error",
          message: `Block "${b.label}" has duplicate property "${name}" (×${count}).`,
          anchor: { kind: "block", id: b.id },
        });
      }
    }
  }

  // ─── Traceability ──────────────────────────────────────────────────────

  // T1 — every requirement has at least one satisfy relation
  for (const r of model.requirements) {
    const hasSatisfier = r.relations.some(rel => rel.kind === "satisfy");
    if (!hasSatisfier) {
      issues.push({
        code: "T1",
        severity: "warning",
        message: `Requirement ${r.tag} has no satisfying block.`,
        anchor: { kind: "requirement", id: r.id },
      });
    }
  }

  // T2 — every block is referenced by ≥1 association OR satisfies ≥1 requirement
  // (constraint blocks are exempt — they don't need to be referenced via association)
  const referencedBlockIds = new Set<string>();
  for (const a of model.associations) {
    referencedBlockIds.add(a.fromBlockId);
    referencedBlockIds.add(a.toBlockId);
  }
  for (const r of model.requirements) {
    for (const rel of r.relations) {
      if (rel.kind === "satisfy") referencedBlockIds.add(rel.blockId);
    }
  }
  for (const c of model.constraints) {
    for (const target of c.appliesTo) referencedBlockIds.add(target);
  }
  for (const b of model.blocks) {
    if (b.kind === "constraint") continue;
    if (!referencedBlockIds.has(b.id)) {
      issues.push({
        code: "T2",
        severity: "soft",
        message: `Block "${b.label}" is not referenced by any association, constraint, or requirement.`,
        anchor: { kind: "block", id: b.id },
      });
    }
  }

  return issues;
}

// ─── Helpers ─────────────────────────────────────────────────────────────

function s1IssueFor(a: Association, side: "from" | "to", missingId: string): ValidationIssue {
  return {
    code: "S1",
    severity: "error",
    message: `Association "${a.label || a.id}" ${side} unknown block "${missingId}".`,
    anchor: { kind: "association", id: a.id },
  };
}

function uniquenessIssues<T>(
  items: T[],
  keyOf: (item: T) => string,
  code: string,
  what: string,
  anchorOf: (item: T) => IssueAnchor
): ValidationIssue[] {
  const counts = new Map<string, number>();
  for (const it of items) counts.set(keyOf(it), (counts.get(keyOf(it)) ?? 0) + 1);
  const issues: ValidationIssue[] = [];
  for (const it of items) {
    const k = keyOf(it);
    if ((counts.get(k) ?? 0) > 1) {
      issues.push({
        code,
        severity: "error",
        message: `Duplicate ${what} "${k}".`,
        anchor: anchorOf(it),
      });
    }
  }
  return issues;
}

/**
 * Find all simple cycles in a directed graph, returning each cycle as an
 * array of node ids. Uses DFS with a recursion stack (Tarjan-style).
 * Each cycle is reported once, starting from its lexicographically smallest
 * node (so duplicates from different start positions collapse).
 */
function cycles<T extends { id: string }>(
  nodes: T[],
  outgoing: (node: T) => string[],
  byId: Map<string, T>
): string[][] {
  const result: string[][] = [];
  const seenCycles = new Set<string>();

  function dfs(currentId: string, path: string[], onPath: Set<string>) {
    const node = byId.get(currentId);
    if (!node) return;
    for (const nextId of outgoing(node)) {
      if (onPath.has(nextId)) {
        // Found a cycle from nextId back to itself through the path
        const startIdx = path.indexOf(nextId);
        if (startIdx >= 0) {
          const cycle = path.slice(startIdx);
          const key = canonicalCycleKey(cycle);
          if (!seenCycles.has(key)) {
            seenCycles.add(key);
            result.push(cycle);
          }
        }
        continue;
      }
      onPath.add(nextId);
      path.push(nextId);
      dfs(nextId, path, onPath);
      path.pop();
      onPath.delete(nextId);
    }
  }

  for (const n of nodes) {
    dfs(n.id, [n.id], new Set([n.id]));
  }
  return result;
}

function canonicalCycleKey(cycle: string[]): string {
  // Rotate so the smallest id is first; then join.
  let minIdx = 0;
  for (let i = 1; i < cycle.length; i++) {
    if (cycle[i]! < cycle[minIdx]!) minIdx = i;
  }
  return [...cycle.slice(minIdx), ...cycle.slice(0, minIdx)].join("→");
}

// ─── Convenience accessors used by the UI ────────────────────────────────

export function severityRank(s: Severity): number {
  return s === "error" ? 3 : s === "warning" ? 2 : 1;
}

export function maxSeverity(issues: ValidationIssue[]): Severity | null {
  let best: Severity | null = null;
  for (const i of issues) {
    if (best === null || severityRank(i.severity) > severityRank(best)) best = i.severity;
  }
  return best;
}