import defined from "../Core/defined.js";
import DeveloperError from "../Core/DeveloperError.js";
import getTimestamp from "../Core/getTimestamp.js";
import JobType from "./JobType.js";

/**
 *
 * @private
 * @constructor
 */
function JobTypeBudget(total) {
  /**
   * Total budget, in milliseconds, allowed for one frame
   */
  this._total = total;

  /**
   * Time, in milliseconds, used so far during this frame
   */
  this.usedThisFrame = 0.0;

  /**
   * Time, in milliseconds, that other job types stole this frame
   */
  this.stolenFromMeThisFrame = 0.0;

  /**
   * Indicates if this job type was starved this frame, i.e., a job
   * tried to run but didn't have budget
   */
  this.starvedThisFrame = false;

  /**
   * Indicates if this job was starved last frame.  This prevents it
   * from being stolen from this frame.
   */
  this.starvedLastFrame = false;
}

Object.defineProperties(JobTypeBudget.prototype, {
  total: {
    get: function () {
      return this._total;
    },
  },
});

/**
 * Engine for time slicing jobs during a frame to amortize work over multiple frames.  This supports:
 * <ul>
 *   <li>
 *     Separate budgets for different job types, e.g., texture, shader program, and buffer creation.  This
 *     allows all job types to make progress each frame.
 *   </li>
 *   <li>
 *     Stealing from other jobs type budgets if they were not exhausted in the previous frame.  This allows
 *     using the entire budget for all job types each frame even if, for example, all the jobs are the same type.
 *   </li>
 *   <li>
 *     Guaranteed progress on all job types each frame, even if it means exceeding the total budget for the frame.
 *     This prevents, for example, several expensive texture uploads over many frames from prevent a shader compile.
 *   </li>
 * </ul>
 *
 * @private
 */
function JobScheduler(budgets) {
  //>>includeStart('debug', pragmas.debug);
  if (defined(budgets) && budgets.length !== JobType.NUMBER_OF_JOB_TYPES) {
    throw new DeveloperError(
      "A budget must be specified for each job type; budgets.length should equal JobType.NUMBER_OF_JOB_TYPES."
    );
  }
  //>>includeEnd('debug');

  // Total for defaults is half of of one frame at 10 fps
  const jobBudgets = new Array(JobType.NUMBER_OF_JOB_TYPES);
  jobBudgets[JobType.TEXTURE] = new JobTypeBudget(
    defined(budgets) ? budgets[JobType.TEXTURE] : 10.0
  );
  // On cache miss, this most likely only allows one shader compile per frame
  jobBudgets[JobType.PROGRAM] = new JobTypeBudget(
    defined(budgets) ? budgets[JobType.PROGRAM] : 10.0
  );
  jobBudgets[JobType.BUFFER] = new JobTypeBudget(
    defined(budgets) ? budgets[JobType.BUFFER] : 30.0
  );

  const length = jobBudgets.length;
  let i;

  let totalBudget = 0.0;
  for (i = 0; i < length; ++i) {
    totalBudget += jobBudgets[i].total;
  }

  const executedThisFrame = new Array(length);
  for (i = 0; i < length; ++i) {
    executedThisFrame[i] = false;
  }

  this._totalBudget = totalBudget;
  this._totalUsedThisFrame = 0.0;
  this._budgets = jobBudgets;
  this._executedThisFrame = executedThisFrame;
}

// For unit testing
JobScheduler.getTimestamp = getTimestamp;

Object.defineProperties(JobScheduler.prototype, {
  totalBudget: {
    get: function () {
      return this._totalBudget;
    },
  },
});

JobScheduler.prototype.disableThisFrame = function () {
  // Prevent jobs from running this frame
  this._totalUsedThisFrame = this._totalBudget;
};

JobScheduler.prototype.resetBudgets = function () {
  const budgets = this._budgets;
  const length = budgets.length;
  for (let i = 0; i < length; ++i) {
    const budget = budgets[i];
    budget.starvedLastFrame = budget.starvedThisFrame;
    budget.starvedThisFrame = false;
    budget.usedThisFrame = 0.0;
    budget.stolenFromMeThisFrame = 0.0;
  }
  this._totalUsedThisFrame = 0.0;
};

JobScheduler.prototype.execute = function (job, jobType) {
  const budgets = this._budgets;
  const budget = budgets[jobType];

  // This ensures each job type makes progress each frame by executing at least once
  const progressThisFrame = this._executedThisFrame[jobType];

  if (this._totalUsedThisFrame >= this._totalBudget && progressThisFrame) {
    // No budget left this frame for jobs of any type
    budget.starvedThisFrame = true;
    return false;
  }

  let stolenBudget;

  if (budget.usedThisFrame + budget.stolenFromMeThisFrame >= budget.total) {
    // No budget remaining for jobs of this type. Try to steal from other job types.
    const length = budgets.length;
    let i;
    for (i = 0; i < length; ++i) {
      stolenBudget = budgets[i];

      // Steal from this budget if it has time left and it wasn't starved last fame
      if (
        stolenBudget.usedThisFrame + stolenBudget.stolenFromMeThisFrame <
          stolenBudget.total &&
        !stolenBudget.starvedLastFrame
      ) {
        break;
      }
    }

    if (i === length && progressThisFrame) {
      // No other job types can give up their budget this frame, and
      // this job type already progressed this frame
      return false;
    }

    if (progressThisFrame) {
      // It is considered "starved" even if it executes using stolen time so that
      // next frame, no other job types can steal time from it.
      budget.starvedThisFrame = true;
    }
  }

  const startTime = JobScheduler.getTimestamp();
  job.execute();
  const duration = JobScheduler.getTimestamp() - startTime;

  // Track both time remaining for this job type and all jobs
  // so budget stealing does send us way over the total budget.
  this._totalUsedThisFrame += duration;

  if (stolenBudget) {
    stolenBudget.stolenFromMeThisFrame += duration;
  } else {
    budget.usedThisFrame += duration;
  }
  this._executedThisFrame[jobType] = true;

  return true;
};
export default JobScheduler;