std::accumulate all the nonsense

src/day7.cpp

@@ -1,11 +1,14 @@
 // :autocmd BufWritePost *.cpp :silent exec "!script/async-build 7"
+#include <algorithm>
 #include <cstdint>
 #include <fstream>
 #include <iostream>
 #include <map>
 #include <numeric>
+#include <ranges>
 #include <set>
 #include <string>
+#include <utility>
 #include <vector>
 
 using Grid = std::vector<std::string>;
@@ -35,14 +38,12 @@ char at_loc(const Grid &grid, size_t row, size_t col) {
 }
 
 Location find_start(const Grid &grid) {
-  for (size_t row{0}; row < grid.size(); ++row) {
-    for (size_t col{0}; col < grid[row].length(); ++col) {
-      if (grid[row][col] == 'S') {
-        return {row, col};
-      }
-    }
+  const auto col{grid[0].find_first_of('S')};
+  if (col != std::string::npos) {
+    return {0, col};
+  } else {
+    std::unreachable();
   }
-  throw "unreachable";
 }
 
 uint64_t part_1(const Grid &grid) {
@@ -69,23 +70,18 @@ uint64_t part_1(const Grid &grid) {
 
 void insert_or_increment(std::map<size_t, uint64_t> &cols, size_t col,
                          uint64_t value) {
-  auto iter{cols.find(col)};
-  if (iter == cols.end()) {
-    cols.insert({col, value});
-  } else {
-
-    iter->second += value;
-  }
+  cols.insert({col, 0}).first->second += value;
 }
 
 uint64_t part_2(const Grid &grid) {
   Location start = find_start(grid);
   const auto [start_row, start_col]{start};
-  std::map<size_t, uint64_t> cols;
-  cols.insert({start_col, 1});
-  for (size_t row{start_row}; row < grid.size(); ++row) {
+  using namespace std::ranges::views;
+  auto view = std::ranges::iota_view{start_row, grid.size()};
+  std::map<size_t, uint64_t> starting_cols{{start_col, 1}};
+  auto cols = std::accumulate(view.begin(), view.end(), starting_cols, [&grid](auto acc, auto row){
     std::map<size_t, uint64_t> next_cols;
-    for (auto [col, cnt] : cols) {
+    for (auto [col, cnt] : acc) {
       if (at_loc(grid, row, col) == '^') {
         insert_or_increment(next_cols, col - 1, cnt);
         insert_or_increment(next_cols, col + 1, cnt);
@@ -93,8 +89,8 @@ uint64_t part_2(const Grid &grid) {
         insert_or_increment(next_cols, col, cnt);
       }
     }
-    cols = std::move(next_cols);
-  }
+    return next_cols;
+    });
   return std::accumulate(
       cols.begin(), cols.end(), 0ull,
       [](const uint64_t acc, const auto &entry) { return acc + entry.second; });