add lambda walk to BIT

Author: lrvideckis

library/data_structures_[l,r)/bit.hpp

@@ -15,5 +15,6 @@ struct BIT {
     return ret;
   }
   ll query(int l, int r) { return query(r) - query(l); }
+#include "bit_uncommon/walk_lambda.hpp"
 #include "bit_uncommon/walk.hpp"
 };

library/data_structures_[l,r)/bit_uncommon/walk.hpp

@@ -1,7 +1,7 @@
 //! Requires sum of [i,i] >= 0
 //! Returns min r s.t. sum of [0,r] >= sum
 //! Returns n if sum of [0,n-1] < sum
-int walk(ll sum) {
+int walk2(ll sum) {
   if (sum <= 0) return -1;
   int r = 0;
   for (int i = bit_floor(size(s)); i; i /= 2)

library/data_structures_[l,r)/bit_uncommon/walk_lambda.hpp

@@ -0,0 +1,6 @@
+void walk(const auto& f) {
+  ll sum = 0;
+  for (int i = bit_floor(size(s)), r = 0; i; i /= 2)
+    if (r + i <= sz(s) && f(r + i, sum + s[r + i - 1]))
+      sum += s[(r += i) - 1];
+}

tests/library_checker_aizu_tests/data_structures/bit_walk.test.cpp

@@ -47,14 +47,23 @@ int main() {
         }
         return 1;
       };
-      int res = bit.walk(order + 1);
+      int res = bit.walk2(order + 1);
       assert(res == st.find_first(0, n, f));
       assert(res ==
         st.find_first(k, n,
           [&](int64_t x, int, int) -> bool {
             return x > 0;
           }));
       if (res == n) res = -1;
+      int res_lambda = -1;
+      bit.walk([&](int r, ll sum) {
+        if (sum > order) {
+          res_lambda = r - 1;
+          return 0;
+        }
+        return 1;
+      });
+      assert(res == res_lambda);
       cout << res << '\n';
     } else {
       if (bit.query(k, k + 1) == 1) {
@@ -71,13 +80,25 @@ int main() {
         }
         return 1;
       };
-      int res = bit.walk(order);
+      int res = bit.walk2(order);
       assert(max(res, 0) == st.find_first(0, n, f));
       assert(res ==
         st.find_last(0, k + 1,
           [&](int64_t x, int, int) -> bool {
             return x > 0;
           }));
+      int res_lambda = -1;
+      if (order) {
+        bit.walk([&](int r, ll sum) {
+          if (sum < order) {
+            res_lambda = r;
+            return 1;
+          }
+          res_lambda = r - 1;
+          return 0;
+        });
+      }
+      assert(res == res_lambda);
       cout << res << '\n';
     }
   }