EPI: Primitive divide (py, cpp)

Author: jsgoller1

elements-of-programming-interviews/cpp/primitive_divide.cc

@@ -1,12 +1,41 @@
 #include "test_framework/generic_test.h"
-int Divide(int x, int y) {
-  // TODO - you fill in here.
-  return 0;
+int Divide(int a, int b) {
+  /*
+  Divide x by y, using only subtraction, addition, and shifting.
+
+  Brute force would be to iteratively subtract until a remainder occurs, but
+  this could take up to 2^n (e.g. (2^31 - 1) / 1).
+
+  Could we use the same strategy for multiply, but with subtraction?
+  */
+  if (a < b || b == 0) {
+    return 0;
+  }
+
+  long long x = a;
+  long long y = b;
+  long long amount = 1;
+  long long quotient = 0;
+  while (x > y) {
+    y <<= 1;
+    amount <<= 1;
+  }
+  while (x) {
+    if (x >= y) {
+      x -= y;
+      quotient += amount;
+    } else {
+      y >>= 1;
+      amount >>= 1;
+    }
+  }
+  return quotient;
 }
 
 int main(int argc, char* argv[]) {
   std::vector<std::string> args{argv + 1, argv + argc};
   std::vector<std::string> param_names{"x", "y"};
   return GenericTestMain(args, "primitive_divide.cc", "primitive_divide.tsv",
                          &Divide, DefaultComparator{}, param_names);
+  // Divide(2097428739, 186);
 }

elements-of-programming-interviews/problem_mapping.js

@@ -86,15 +86,15 @@ problem_mapping = {
         },
         "4.06 Compute quotient without arithmetical operators": {
             "C++: primitive_divide.cc": {
-                "passed": 0,
+                "passed": 10000,
                 "total": 10000
             },
             "Java: PrimitiveDivide.java": {
                 "passed": 0,
                 "total": 10000
             },
             "Python: primitive_divide.py": {
-                "passed": 0,
+                "passed": 10000,
                 "total": 10000
             }
         },

elements-of-programming-interviews/python/primitive_divide.py

@@ -1,9 +1,22 @@
 from test_framework import generic_test
 
 
-def divide(x: int, y: int) -> int:
-    # TODO - you fill in here.
-    return 0
+def divide(x, y):
+    if (x < y or y == 0):
+        return 0
+    amount = 1
+    quotient = 0
+    while x > y:
+        y <<= 1
+        amount <<= 1
+    while x:
+        if x >= y:
+            x -= y
+            quotient += amount
+        else:
+            y >>= 1
+            amount >>= 1
+    return quotient
 
 
 if __name__ == '__main__':