Count ones in a segment

count-ones-in-a-segment/index.js

@@ -0,0 +1,48 @@
+//  0  0 0 0 0
+//  1  0 0 0 1
+//  2  0 0 1 0
+//  3  0 0 1 1
+//  4  0 1 0 0
+//  5  0 1 0 1
+//  6  0 1 1 0
+//  7  0 1 1 1
+//  8  1 0 0 0
+//  9  1 0 0 1
+// 10  1 0 1 0
+// 11  1 0 1 1
+// 12  1 1 0 0
+// 13  1 1 0 1
+// 14  1 1 1 0
+// 15  1 1 1 1
+
+
+// count column bits
+function ccb(col, n) {
+  let div = (n + 1) / (2 << col);
+  let sum = Math.floor(div);
+  let rest = Math.max(0, (div - sum) * (2 << col) - (1 << col));
+  return sum * (1 << col) + rest;
+}
+
+// count all bits
+function cab(n) {
+  // n < 1e9 => log2(1e9) ~= 30
+  let bits = 0;
+  for (let i = 0; i < 30; ++i) {
+    bits += ccb(i, n);
+  }
+  return bits;
+}
+
+// calc between range
+function cbr(a, b) {
+  return cab(b) - cab(a-1)
+}
+
+// for (let i = 0; i < 16; ++i) {
+//   console.log(i, cab(i));
+// }
+
+function countOnes(left, right) {
+  return cbr(left, right);
+}