readme.md

Various Functions to Calculate the Number of Days in a Month (Non-Leap Year)

After reading Curtis McEnroe's article about creating a formula to calculate the number of days in a month, I got to thinking about a way to make it more compact. The first method I came up with (lotm.bitwise) shaved a few bytes of code but I figured it would also be a bit faster. I put together this jsPerf and found that in fact it was significantly quicker. I then came up with a few other approaches for comparison.

It turns out the lotm.lookupBitwise is both the most compact and the fastest (Chrome 39.0.2171.71 32-bit on Windows Server 2008 R2 / 7 64-bit).

lotm.js

/**
 * Length of the Month  
 */
 
var lotm = {
  
  lookupTable: [0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31],
  
  curtisMcEnroe: function(x) {
    return 28 + (x + Math.floor(x / 8)) % 2 + 2 % x + 2 * Math.floor(1 / x);
  },
  
  bitwise: function(x) {
    var y;
    return (y = 30 | (x >> 3 ^ x) & 1) ^ (~(x >> 2 | x | y) & 1) << 1
  },
  
  bitwiseEqual: function(x) {
    return 28 | (x >> 3 ^ x) & 1 | (x!==2)<<1; 
  },
  
  lookup: function(x) {
    return this.lookupTable[x];
  },
  
  lookupBitwise: function(x) {
    return 28 | (0x3BBEECC >> x >> x) & 3;
  },
  
  bigSwitch: function(x) {
    switch (x) {
      case 1:
        return 31;
      case 2:
        return 28;
      case 3:
        return 31;
      case 4:
        return 30;
      case 5:
        return 31;
      case 6:
        return 30;
      case 7:
        return 31;
      case 8:
        return 31;
      case 9:
        return 30;
      case 10:
        return 31;
      case 11:
        return 30;
      case 12:
        return 31;
    }
  },
  
  bigIf: function(x) {
    if (x === 1) x = 31;
    else if (x === 2) x = 28;
    else if (x === 3) x = 31;
    else if (x === 4) x = 30;
    else if (x === 5) x = 31;
    else if (x === 6) x = 30;
    else if (x === 7) x = 31;
    else if (x === 8) x = 31;
    else if (x === 9) x = 30;
    else if (x === 10) x = 31;
    else if (x === 11) x = 30;
    else if (x === 12) x = 31;
    return x;
  }
};