diff --git a/2022/15.cpp b/2022/15.cpp
index d2595de..dd4c288 100644
--- a/2022/15.cpp
+++ b/2022/15.cpp
@@ -1,13 +1,12 @@
 #include <array>
 #include <cstdint>
 #include <iostream>
-#include <cmath>
+#include <optional>
 #include <set>
 #include <sstream>
 #include <vector>
 
 #include <boost/phoenix.hpp>
-#include <boost/range/irange.hpp>
 #include <boost/spirit/include/qi.hpp>
 
 #include <doctest.h>
@@ -70,12 +69,15 @@ auto Parse(std::istream & in) -> Input
 struct Range {
   std::int64_t lo, hi;
   auto operator<=>(Range const& rhs) const = default;
+  auto size() const -> std::size_t {
+    return hi - lo;
+  }
 };
 
 template <std::size_t N>
-auto subtract(
+auto subtract_to(
   std::array<Range, N> x,
-  std::array<Range, N> y,
+  std::array<Range, N> const& y,
   std::vector<std::array<Range, N>> & out
 ) -> void
 {
@@ -98,6 +100,17 @@ auto subtract(
   }
 }
 
+template <std::size_t N>
+auto subtract(
+  std::array<Range, N> const& x,
+  std::array<Range, N> const& y 
+) -> std::vector<std::array<Range, N>>
+{
+  std::vector<std::array<Range, N>> result;
+  subtract_to(x, y, result);
+  return result;
+}
+
 auto Segment(std::int64_t center, std::int64_t radius) -> Range {
   return { center - radius, center + radius + 1};
 }
@@ -111,16 +124,59 @@ auto Corner(std::array<Range, 2> const& s) -> aocpp::Coord
   return { (s[0].lo + s[1].lo) / 2, (s[0].lo - s[1].lo) / 2};
 }
 
-auto Part2(Input const& input) -> void {
+auto RowSlice(aocpp::Coord sensor, std::int64_t radius, std::int64_t row)
+-> std::optional<std::array<Range, 1>>
+{
+  auto dy = std::abs(row - sensor.y);
+  auto dx = radius - dy;
+  if (dx >= 0) {
+    return {{Segment(sensor.x, dx)}};
+  } else {
+    return {};
+  }
+} 
 
-  auto region = std::vector<std::array<Range, 2>>{Square({2'000'000, 2'000'000}, 2'000'000)};
+auto Part1(Input const& input, std::int64_t search) -> std::size_t {
+  auto region = std::vector<std::array<Range, 1>>{};
+  auto region_next = std::vector<std::array<Range, 1>>{};
+
+  for (auto const& entry : input) {
+    auto radius = aocpp::Norm1(entry.beacon - entry.sensor);
+    if (auto s = RowSlice(entry.sensor, radius, search)) {
+      region_next.push_back(*s);
+      for (auto const& x : region) {
+        subtract_to(x, *s, region_next);
+      }
+      std::swap(region, region_next);
+      region_next.clear();
+    }
+  }
+
+  std::size_t result {};
+  for (auto const& x : region) {
+    result += x[0].hi - x[0].lo;
+  }
+
+  std::set<std::int64_t> seen;
+  for (auto const& entry : input) {
+    if (entry.beacon.y == search && seen.insert(entry.beacon.x).second) {
+      result--;
+    }
+  }
+
+  return result;
+}
+
+auto Part2(Input const& input, std::int64_t search) -> std::int64_t {
+
+  auto region = std::vector<std::array<Range, 2>>{Square({search/2, search/2}, search)};
   auto region_next = std::vector<std::array<Range, 2>>{};
 
   for (auto const& entry : input) {
     auto radius = aocpp::Norm1(entry.beacon - entry.sensor);
     auto s = Square(entry.sensor, radius);
     for (auto const& x : region) {
-      subtract(x, s, region_next);
+      subtract_to(x, s, region_next);
     }
     std::swap(region, region_next);
     region_next.clear();
@@ -128,19 +184,20 @@ auto Part2(Input const& input) -> void {
 
   for (auto const& entry : region) {
     auto corner = Corner(entry);
-    //if (0 <= corner.x && corner.x <= 4000000 &&
-    //    0 <= corner.y && corner.y <= 4000000)
-    //{
-      std::cout << corner << " " << 4'000'000 * corner.x + corner.y << std::endl;
-    //}
+    if (0 <= corner.x && corner.x <= search &&
+        0 <= corner.y && corner.y <= search)
+    {
+      return 4'000'000 * corner.x + corner.y;
+    }
   }
+
+  throw std::runtime_error{"no solution to part 2"};
 }
 
 } // namespace
 
 TEST_SUITE("2022-15 examples") {
-  /*
-  TEST_CASE("1D subtraction") {
+  TEST_CASE("subtraction") {
     CHECK(subtract<1>({1,3}, {4,5}) == std::vector<std::array<Range, 1>>{{1,3}});
     CHECK(subtract<1>({4,5}, {1,3}) == std::vector<std::array<Range, 1>>{{4,5}});
     CHECK(subtract<1>({1,3}, {2,4}) == std::vector<std::array<Range, 1>>{{1,2}});
@@ -149,8 +206,17 @@ TEST_SUITE("2022-15 examples") {
     CHECK(subtract<1>({1,4}, {2,3}) == std::vector<std::array<Range, 1>>{{1,2}, {3,4}});
     CHECK(subtract<1>({1,4}, {2,4}) == std::vector<std::array<Range, 1>>{{1,2}});
     CHECK(subtract<1>({1,4}, {1,3}) == std::vector<std::array<Range, 1>>{{3,4}});
+
+    CHECK(subtract<2>(
+      std::array<Range,2>{Range{0,3}, Range{0,4}}, std::array<Range,2>{Range{1,2},Range{2,3}})
+      ==
+      std::vector<std::array<Range, 2>>{
+        std::array<Range, 2>{Range{0,1}, Range{0,4}},
+        std::array<Range, 2>{Range{2,3}, Range{0,4}},
+        std::array<Range, 2>{Range{1,2}, Range{0,2}},
+        std::array<Range, 2>{Range{1,2}, Range{3,4}}
+      });
   }
-  */
 
   TEST_CASE("documented example") {
     auto in = std::istringstream{
@@ -170,12 +236,14 @@ TEST_SUITE("2022-15 examples") {
       "Sensor at x=20, y=1: closest beacon is at x=15, y=3\n"
     };
     auto const input = Parse(in);
+    CHECK(26 == Part1(input, 10));
+    CHECK(56000011 == Part2(input, 20));
   }
 }
 
 auto Main(std::istream & in, std::ostream & out) -> void
 {
   auto const input = Parse(in);
-  //out << "Part 1: " << Part1(bottom, world) << std::endl;
-  Part2(input);
+  out << "Part 1: " << Part1(input, 2'000'000) << std::endl;
+  out << "Part 2: " << Part2(input, 4'000'000) << std::endl;
 }