Merge "Precalculate the required number of characters for path shortening"

#include "optimize/ResourcePathShortener.h"

#include <math.h>

#include <unordered_set>

#include "androidfw/StringPiece.h"

}

// Calculate the optimal hash length such that an average of 10% of resources

// collide in their shortened path.

// Return the optimal hash length such that at most 10% of resources collide in

// their shortened path.

// Reference: http://matt.might.net/articles/counting-hash-collisions/

int OptimalShortenedLength(int num_resources) {

  int num_chars = 2;

  double N = 64*64; // hash space when hash is 2 chars long

  double max_collisions = num_resources * 0.1;

  while (num_resources - N + N * pow((N - 1) / N, num_resources) > max_collisions) {

    N *= 64;

    num_chars++;

  if (num_resources > 4000) {

    return 3;

  } else {

    return 2;

  }

  return num_chars;

}

std::string GetShortenedPath(const android::StringPiece& shortened_filename,