Support "x bits in bytes" in phutil_units()

Summary: Ref T11140. I need to generate some encryption keys to do at-rest encryption, and the code can be a little more readable as `256 bits in bytes`, etc.

Test Plan: Unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11140

Differential Revision: https://secure.phabricator.com/D16123

Author: epriestley

src/utils/__tests__/PhutilUtilsTestCase.php

@@ -471,6 +471,9 @@ public function testPhutilUnits() {
       '1 hour in seconds' => 3600,
       '1 day in seconds' => 86400,
       '3 days in seconds' => 259200,
+      '128 bits in bytes' => 16,
+      '1 byte in bytes' => 1,
+      '8 bits in bytes' => 1,
     );
 
     foreach ($cases as $input => $expect) {
@@ -487,6 +490,10 @@ public function testPhutilUnits() {
       '1 day in days',
       '-1 minutes in seconds',
       '1.5 minutes in seconds',
+      '7 bits in bytes',
+      '2 hours in bytes',
+      '1 dram in bytes',
+      '24 bits in seconds',
     );
 
     foreach ($bad_cases as $input) {

src/utils/utils.php

@@ -1070,6 +1070,8 @@ function phutil_units($description) {
   $src_unit = $matches[2];
   $dst_unit = $matches[3];
 
+  $is_divisor = false;
+
   switch ($dst_unit) {
     case 'seconds':
       switch ($src_unit) {
@@ -1096,14 +1098,42 @@ function phutil_units($description) {
               $src_unit));
       }
       break;
+    case 'bytes':
+      switch ($src_unit) {
+        case 'byte':
+        case 'bytes':
+          $factor = 1;
+          break;
+        case 'bit':
+        case 'bits':
+          $factor = 8;
+          $is_divisor = true;
+          break;
+        default:
+          throw new InvalidArgumentException(
+            pht(
+              'This function can not convert from the unit "%s".',
+              $src_unit));
+      }
+      break;
     default:
       throw new InvalidArgumentException(
         pht(
           'This function can not convert into the unit "%s".',
           $dst_unit));
   }
 
-  return $quantity * $factor;
+  if ($is_divisor) {
+    if ($quantity % $factor) {
+      throw new InvalidArgumentException(
+        pht(
+          '"%s" is not an exact quantity.',
+          $description));
+    }
+    return (int)($quantity / $factor);
+  } else {
+    return $quantity * $factor;
+  }
 }