fixes on describe

src/bigmath/util/describe.cpp

@@ -0,0 +1,71 @@
+#include "describe.hpp"
+
+#include <bigmath/chain/segment.hpp>
+
+#include <sstream>
+#include <iomanip>
+#include <charconv>
+#include <algorithm>
+
+namespace bigmath {
+
+    std::string describe(const segment& s) {
+        std::string buf;
+        buf.reserve(segment::digit_count * 2);
+        u32 group_count = 0;
+
+        isize i = 0;
+        while (i < segment::digit_count) {
+            u8 val = s[static_cast<u8>(i)];
+
+            // Count repetitions
+            isize count = 1;
+            while (i + count < segment::digit_count && s[static_cast<u8>(i + count)] == val) {
+                ++count;
+            }
+
+            if (count > 5) {
+                if (!buf.empty() && buf.back() != ' ') {
+                    buf.push_back(' ');
+                }
+
+                // Append reversed tag directly: "]count x val["
+                buf.push_back(']');
+
+                // Fast reverse-append for count digits without heap allocation
+                isize temp_count = count;
+                do {
+                    buf.push_back(static_cast<char>('0' + (temp_count % 10)));
+                    temp_count /= 10;
+                } while (temp_count > 0);
+
+                buf.append(" x ");
+                buf.push_back(static_cast<char>('0' + val));
+                buf.push_back('[');
+
+                buf.push_back(' ');
+                group_count = 0;
+            } else {
+                char ch = static_cast<char>('0' + val);
+                for (isize j = 0; j < count; ++j) {
+                    if (group_count == 3) {
+                        buf.push_back(' ');
+                        group_count = 0;
+                    }
+                    buf.push_back(ch);
+                    group_count++;
+                }
+            }
+
+            i += count;
+        }
+
+        if (!buf.empty() && buf.back() == ' ') {
+            buf.pop_back();
+        }
+
+        std::reverse(buf.begin(), buf.end());
+        return buf;
+    }
+
+}

src/bigmath/util/describe.hpp

@@ -0,0 +1,9 @@
+#pragma once
+
+#include <bigmath/BigMath.hpp>
+
+namespace bigmath {
+
+    std::string describe(const segment& s);
+
+}