import * as plugins from './plugins.js';
import type { IMigrationStepDefinition } from './interfaces.js';

/**
 * SmartMigrationError — thrown for all user-visible failures (validation,
 * planning, lock acquisition, ledger I/O). The exact error site is encoded
 * in `code` so callers can branch on it.
 */
export class SmartMigrationError extends Error {
  public readonly code: string;
  public readonly details?: Record<string, unknown>;
  constructor(code: string, message: string, details?: Record<string, unknown>) {
    super(message);
    this.name = 'SmartMigrationError';
    this.code = code;
    this.details = details;
  }
}

/**
 * Wraps `@push.rocks/smartversion` to provide simple comparison helpers.
 * All public methods accept and return plain semver strings.
 */
export class VersionResolver {
  /** True iff `a` and `b` represent the same semver version. */
  public static equals(a: string, b: string): boolean {
    // Uses SmartVersion.equalsString (added in @push.rocks/smartversion 3.1.0)
    // which delegates to semver.eq under the hood.
    return new plugins.smartversion.SmartVersion(a).equalsString(b);
  }

  /** True iff `a < b` semver-wise. */
  public static lessThan(a: string, b: string): boolean {
    return new plugins.smartversion.SmartVersion(a).lessThanString(b);
  }

  /** True iff `a > b` semver-wise. */
  public static greaterThan(a: string, b: string): boolean {
    return new plugins.smartversion.SmartVersion(a).greaterThanString(b);
  }

  /** Throws if `version` is not a valid semver string. */
  public static assertValid(version: string, label: string): void {
    try {
      // Constructing SmartVersion throws if the string is invalid.
      // eslint-disable-next-line no-new
      new plugins.smartversion.SmartVersion(version);
    } catch (err) {
      throw new SmartMigrationError(
        'INVALID_VERSION',
        `${label} is not a valid semver string: "${version}"`,
        { version, label, cause: (err as Error).message },
      );
    }
  }

  /**
   * Validate the chain of registered steps:
   *   - ids are unique
   *   - from/to versions are valid semver
   *   - each step's from < to
   *   - each step's `from` strictly equals the previous step's `to`
   */
  public static validateChain(steps: IMigrationStepDefinition[]): void {
    const seenIds = new Set<string>();
    for (let i = 0; i < steps.length; i++) {
      const step = steps[i];

      if (seenIds.has(step.id)) {
        throw new SmartMigrationError(
          'DUPLICATE_STEP_ID',
          `Migration step id "${step.id}" is registered more than once.`,
          { stepId: step.id },
        );
      }
      seenIds.add(step.id);

      this.assertValid(step.fromVersion, `step "${step.id}" fromVersion`);
      this.assertValid(step.toVersion, `step "${step.id}" toVersion`);

      if (!this.lessThan(step.fromVersion, step.toVersion)) {
        throw new SmartMigrationError(
          'NON_INCREASING_STEP',
          `Migration step "${step.id}" has fromVersion "${step.fromVersion}" >= toVersion "${step.toVersion}". Steps must strictly upgrade.`,
          { stepId: step.id, fromVersion: step.fromVersion, toVersion: step.toVersion },
        );
      }

      if (i > 0) {
        const prev = steps[i - 1];
        if (!this.equals(prev.toVersion, step.fromVersion)) {
          throw new SmartMigrationError(
            'CHAIN_GAP',
            `Migration step "${step.id}" starts at "${step.fromVersion}", but the previous step "${prev.id}" ended at "${prev.toVersion}". Steps must form a contiguous chain in registration order.`,
            {
              prevStepId: prev.id,
              prevToVersion: prev.toVersion,
              stepId: step.id,
              fromVersion: step.fromVersion,
            },
          );
        }
      }
    }
  }

  /**
   * Compute which subset of `steps` should be executed to advance the data
   * from `currentVersion` to `targetVersion`.
   *
   * Behavior:
   *   - If currentVersion === targetVersion → returns []
   *   - Otherwise, finds the first step whose `toVersion > currentVersion`
   *     (i.e. the first step that hasn't been fully applied yet) and returns
   *     it plus all subsequent steps up to (and including) the one whose
   *     `toVersion === targetVersion`.
   *   - Supports two resume modes:
   *     1. **Exact resume**: currentVersion === step.fromVersion — the normal
   *        case, where the ledger sits exactly at a step's starting point.
   *     2. **Skip-forward resume**: currentVersion > step.fromVersion but
   *        currentVersion < step.toVersion — the orphan case, where the
   *        ledger was stamped to an intermediate version that no registered
   *        step starts at (e.g. fresh installs using
   *        `freshInstallVersion: targetVersion` across releases that didn't
   *        add migrations). Skip-forward assumes step handlers are
   *        idempotent (safe to re-run against data already partially in the
   *        target shape). This is the documented contract for all step
   *        handlers.
   *   - If a step's `toVersion` overshoots `targetVersion` → throws
   *     TARGET_NOT_REACHABLE.
   *   - If no step can advance `currentVersion` toward `targetVersion`
   *     (currentVersion is past the end of the chain) → throws
   *     TARGET_NOT_REACHABLE.
   *   - If currentVersion > targetVersion → throws DOWNGRADE_NOT_SUPPORTED.
   */
  public static computePlan(
    steps: IMigrationStepDefinition[],
    currentVersion: string,
    targetVersion: string,
  ): IMigrationStepDefinition[] {
    this.assertValid(currentVersion, 'currentVersion');
    this.assertValid(targetVersion, 'targetVersion');

    if (this.equals(currentVersion, targetVersion)) {
      return [];
    }

    if (this.greaterThan(currentVersion, targetVersion)) {
      throw new SmartMigrationError(
        'DOWNGRADE_NOT_SUPPORTED',
        `Current data version "${currentVersion}" is ahead of target version "${targetVersion}". smartmigration v1 does not support rollback.`,
        { currentVersion, targetVersion },
      );
    }

    // Find the first step that hasn't been fully applied yet.
    // A step is "not yet applied" iff its toVersion > currentVersion.
    const startIndex = steps.findIndex((s) => this.greaterThan(s.toVersion, currentVersion));
    if (startIndex === -1) {
      throw new SmartMigrationError(
        'TARGET_NOT_REACHABLE',
        `Current data version "${currentVersion}" is past the end of the registered chain, but target "${targetVersion}" has not been reached. No step can advance. Registered chain: ${this.describeChain(steps)}`,
        { currentVersion, targetVersion, registeredChain: steps.map((s) => `${s.fromVersion}→${s.toVersion}`) },
      );
    }

    const plan: IMigrationStepDefinition[] = [];
    for (let i = startIndex; i < steps.length; i++) {
      const step = steps[i];
      plan.push(step);
      if (this.equals(step.toVersion, targetVersion)) {
        return plan;
      }
      if (this.greaterThan(step.toVersion, targetVersion)) {
        throw new SmartMigrationError(
          'TARGET_NOT_REACHABLE',
          `Migration step "${step.id}" upgrades to "${step.toVersion}", which overshoots target "${targetVersion}". No exact-match step ends at the target.`,
          { stepId: step.id, stepTo: step.toVersion, targetVersion },
        );
      }
    }

    throw new SmartMigrationError(
      'TARGET_NOT_REACHABLE',
      `Walked the entire registered chain from "${currentVersion}" but never reached target "${targetVersion}". Registered chain: ${this.describeChain(steps)}`,
      { currentVersion, targetVersion, registeredChain: steps.map((s) => `${s.fromVersion}→${s.toVersion}`) },
    );
  }

  private static describeChain(steps: IMigrationStepDefinition[]): string {
    if (steps.length === 0) return '(empty)';
    return steps.map((s) => `${s.id}(${s.fromVersion}→${s.toVersion})`).join(' → ');
  }
}