Add count/radix sort

Author: adamant-pwn

cp-algo/util/compress_coords.hpp

@@ -1,13 +1,13 @@
 #ifndef CP_ALGO_UTIL_COMPRESS_COORDS_HPP
 #define CP_ALGO_UTIL_COMPRESS_COORDS_HPP
-#include <algorithm>
+#include "sort.hpp"
 #include <vector>
 namespace cp_algo {
     // coords is a range of reference_wrapper<T>
     auto compress_coords(auto &coords) {
         std::vector<int> original;
         original.reserve(size(coords));
-        std::ranges::sort(coords);
+        radix_sort(coords);
         int idx = -1, prev = -1;
         for(auto &x: coords) {
             if(x != prev) {

cp-algo/util/sort.hpp

@@ -0,0 +1,34 @@
+#ifndef CP_ALGO_UTIL_SORT_HPP
+#define CP_ALGO_UTIL_SORT_HPP
+#include <algorithm>
+#include <numeric>
+#include <ranges>
+#include <vector>
+namespace cp_algo {
+    void count_sort(auto &a, size_t maxc, auto &&proj = std::identity{}) {
+        std::vector<int> cnt(maxc);
+        for(auto &x: a) {
+            cnt[proj(x)]++;
+        }
+        std::partial_sum(begin(cnt), end(cnt), begin(cnt));
+        auto res = a;
+        for(auto const& it: a | std::views::reverse) {
+            res[--cnt[proj(it)]] = it;
+        }
+        a = std::move(res);
+    }
+
+    void radix_sort(auto &a) {
+        if(empty(a)) {
+            return;
+        }
+        int base = std::bit_floor(size(a));
+        auto mx = std::ranges::max(a);
+        for(int64_t i = 1; i <= mx; i *= base) {
+            count_sort(a, base, [&](auto x) {
+                return x / i % base;
+            });
+        }
+    }
+}
+#endif // CP_ALGO_UTIL_SORT_HPP