Better CSS scaling for deviceRixelRatio != 1.

web/ui_utils.js

@@ -237,6 +237,55 @@ function binarySearchFirstItem(items, condition) {
   return minIndex; /* === maxIndex */
 }
 
+/**
+ *  Approximates float number as a fraction using Farey sequence (max order
+ *  of 8).
+ *  @param {number} x - Positive float number.
+ *  @returns {Array} Estimated fraction: the first array item is a numerator,
+ *                   the second one is a denominator.
+ */
+function approximateFraction(x) {
+  // Fast paths for int numbers or their inversions.
+  if (Math.floor(x) === x) {
+    return [x, 1];
+  }
+  var xinv = 1 / x;
+  var limit = 8;
+  if (xinv > limit) {
+    return [1, limit];
+  } else  if (Math.floor(xinv) === xinv) {
+    return [1, xinv];
+  }
+
+  var x_ = x > 1 ? xinv : x;
+  // a/b and c/d are neighbours in Farey sequence.
+  var a = 0, b = 1, c = 1, d = 1;
+  // Limiting search to order 8.
+  while (true) {
+    // Generating next term in sequence (order of q).
+    var p = a + c, q = b + d;
+    if (q > limit) {
+      break;
+    }
+    if (x_ <= p / q) {
+      c = p; d = q;
+    } else {
+      a = p; b = q;
+    }
+  }
+  // Select closest of the neighbours to x.
+  if (x_ - a / b < c / d - x_) {
+    return x_ === x ? [a, b] : [b, a];
+  } else {
+    return x_ === x ? [c, d] : [d, c];
+  }
+}
+
+function roundToDivide(x, div) {
+  var r = x % div;
+  return r === 0 ? x : Math.round(x - r + div);
+}
+
 /**
  * Generic helper to find out what elements are visible within a scroll pane.
  */