import { randomUUID } from "node:crypto";
import { access, link, mkdir, rename, unlink } from "node:fs/promises";
import { join } from "node:path";

import { writeTextFileAtomic } from "../utils/atomic-write.ts";
import { getUserTemplatesDir, isValidTemplateName } from "./load-template.ts";

export interface SaveUserTemplateOptions {
  /** 是否允许替换现有同名模板。 */
  overwrite?: boolean;
}

export interface StagedTemplateRemoval {
  /** 用户看到的原模板路径。 */
  path: string;
  /** 暂存删除期间保存原内容的隐藏路径。 */
  stagedPath: string;
  /** 永久删除已暂存的模板。 */
  commit(): Promise<void>;
  /** 将已暂存的模板恢复到原路径。 */
  rollback(): Promise<void>;
}

/** 判断未知错误是否具有指定 Node.js 文件系统错误码。 */
function hasErrorCode(error: unknown, code: string): boolean {
  return error instanceof Error && (error as NodeJS.ErrnoException).code === code;
}

/** 返回用户模板文件的完整路径。 */
function getUserTemplatePath(name: string): string {
  return join(getUserTemplatesDir(), `${name}.md`);
}

/** 判断用户模板目录中是否已存在同名模板。 */
export async function userTemplateExists(name: string): Promise<boolean> {
  try {
    await access(getUserTemplatePath(name));
    return true;
  } catch (error) {
    if (hasErrorCode(error, "ENOENT")) {
      return false;
    }
    const detail = error instanceof Error ? error.message : String(error);
    throw new Error(`Failed to inspect template "${name}": ${detail}`);
  }
}

/** 将模板内容原子写入用户模板目录，必要时创建目录。 */
export async function saveUserTemplate(
  name: string,
  content: string,
  options: SaveUserTemplateOptions = {},
): Promise<string> {
  if (!isValidTemplateName(name)) {
    throw new Error(
      `Template name "${name}" is invalid. Use letters, digits, dots, hyphens, or underscores, starting with a letter or digit.`,
    );
  }
  if (!content.trim()) {
    throw new Error("Template content must not be empty.");
  }

  const path = getUserTemplatePath(name);
  await mkdir(getUserTemplatesDir(), { recursive: true });
  try {
    await writeTextFileAtomic(path, content.endsWith("\n") ? content : `${content}\n`, {
      overwrite: options.overwrite ?? true,
      mode: 0o600,
    });
  } catch (error) {
    if (hasErrorCode(error, "EEXIST")) {
      throw new Error(
        `Template "${name}" was created by another process. Review it before choosing whether to overwrite it.`,
      );
    }
    const detail = error instanceof Error ? error.message : String(error);
    throw new Error(`Failed to save template file ${path}: ${detail}`);
  }
  return path;
}

/**
 * 通过同目录原子重命名暂存模板删除；调用方完成配置更新后再 commit，失败时 rollback。
 */
export async function stageUserTemplateRemoval(name: string): Promise<StagedTemplateRemoval> {
  if (!isValidTemplateName(name)) {
    throw new Error(`Template name "${name}" is invalid.`);
  }

  const path = getUserTemplatePath(name);
  const stagedPath = `${path}.${process.pid}.${randomUUID()}.pending-delete`;
  try {
    await rename(path, stagedPath);
  } catch (error) {
    if (hasErrorCode(error, "ENOENT")) {
      throw new Error(`Template "${name}" was not found in your templates directory.`);
    }
    const detail = error instanceof Error ? error.message : String(error);
    throw new Error(`Failed to stage template file ${path} for removal: ${detail}`);
  }

  let active = true;
  return {
    path,
    stagedPath,
    /** 永久删除暂存文件。 */
    async commit(): Promise<void> {
      if (!active) {
        return;
      }
      await unlink(stagedPath);
      active = false;
    },
    /** 通过独占硬链接恢复暂存文件，不覆盖并发创建的同名模板。 */
    async rollback(): Promise<void> {
      if (!active) {
        return;
      }
      try {
        await link(stagedPath, path);
      } catch (error) {
        if (hasErrorCode(error, "EEXIST")) {
          throw new Error(
            `Template "${name}" was recreated by another process, so rollback did not overwrite it. The staged content remains at ${stagedPath}.`,
          );
        }
        throw error;
      }

      active = false;
      try {
        await unlink(stagedPath);
      } catch (error) {
        const detail = error instanceof Error ? error.message : String(error);
        throw new Error(
          `Template "${name}" was restored to ${path}, but the staged file ${stagedPath} could not be removed: ${detail}`,
        );
      }
    },
  };
}

/** 删除用户模板；内部使用可回滚的重命名，提交失败时恢复原文件。 */
export async function removeUserTemplate(name: string): Promise<string> {
  const removal = await stageUserTemplateRemoval(name);
  try {
    await removal.commit();
  } catch (error) {
    try {
      await removal.rollback();
    } catch (rollbackError) {
      const detail = rollbackError instanceof Error ? rollbackError.message : String(rollbackError);
      throw new Error(
        `Failed to remove template file ${removal.path}; rollback also failed: ${detail}`,
        {
          cause: error,
        },
      );
    }
    const detail = error instanceof Error ? error.message : String(error);
    throw new Error(`Failed to remove template file ${removal.path}: ${detail}`);
  }
  return removal.path;
}