'use strict';
var Promise = require('../utils').Promise;

// this is essentially the "update sugar" function from daleharvey/pouchdb#1388
// the diffFun tells us what delta to apply to the doc.  it either returns
// the doc, or false if it doesn't need to do an update after all
function upsert(db, docId, diffFun) {
  return new Promise(function (fulfill, reject) {
    if (docId && typeof docId === 'object') {
      docId = docId._id;
    }
    if (typeof docId !== 'string') {
      return reject(new Error('doc id is required'));
    }

    db.get(docId, function (err, doc) {
      if (err) {
        if (err.status !== 404) {
          return reject(err);
        }
        return fulfill(tryAndPut(db, diffFun({_id : docId}), diffFun));
      }
      var newDoc = diffFun(doc);
      if (!newDoc) {
        return fulfill(doc);
      }
      fulfill(tryAndPut(db, newDoc, diffFun));
    });
  });
}

function tryAndPut(db, doc, diffFun) {
  return db.put(doc).catch(function (err) {
    if (err.status !== 409) {
      throw err;
    }
    return upsert(db, doc, diffFun);
  });
}

module.exports = function (db, docId, diffFun, cb) {
  if (typeof cb === 'function') {
    upsert(db, docId, diffFun).then(function (resp) {
      cb(null, resp);
    }, cb);
  } else {
    return upsert(db, docId, diffFun);
  }
};