ksys/utils: Add FixedString for light constexpr string manipulation

CMakeLists.txt

@@ -111,6 +111,7 @@ add_executable(uking
   src/KingSystem/Utils/ByamlLocal.h
   src/KingSystem/Utils/ByamlUtil.cpp
   src/KingSystem/Utils/Debug.h
+  src/KingSystem/Utils/FixedString.h
   src/KingSystem/Utils/HashUtil.h
   src/KingSystem/Utils/HeapUtil.cpp
   src/KingSystem/Utils/HeapUtil.h

src/KingSystem/Utils/FixedString.h

@@ -0,0 +1,94 @@
+#pragma once
+
+#include <array>
+#include <cstddef>
+#include <type_traits>
+
+namespace ksys::util {
+
+// https://stackoverflow.com/a/53190503/1636285
+
+struct concat_op {};
+
+template <std::size_t N>
+struct FixedString {
+    constexpr static std::size_t length() { return N; }
+    constexpr static std::size_t capacity() { return N + 1; }
+
+    template <std::size_t L, std::size_t R>
+    constexpr FixedString(concat_op, FixedString<L> l, FixedString<R> r) : FixedString() {
+        static_assert(L + R == N);
+        overwrite(0, l.data(), L);
+        overwrite(L, r.data(), R);
+    }
+
+    constexpr FixedString() : buffer_{} {}
+
+    constexpr FixedString(const char (&source)[N + 1]) : FixedString() {
+        do_copy(source, buffer_.data());
+    }
+
+    constexpr const char* data() const { return buffer_.data(); }
+
+    constexpr char* data() { return buffer_.data(); }
+
+private:
+    static constexpr void do_copy(const char (&source)[N + 1], char* dest) {
+        for (std::size_t i = 0; i < capacity(); ++i)
+            dest[i] = source[i];
+    }
+
+    constexpr void overwrite(std::size_t where, const char* source, std::size_t len) {
+        auto dest = buffer_.data() + where;
+        while (len--)
+            *dest++ = *source++;
+    }
+
+    std::array<char, capacity()> buffer_;
+};
+
+template <std::size_t N>
+constexpr FixedString<N - 1> Str(const char (&c)[N]) {
+    return FixedString<N - 1>(c);
+}
+
+template <auto N, int base = 10>
+constexpr auto Str() {
+    constexpr char digits[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+
+    FixedString<([]() constexpr noexcept {
+        unsigned int len = N > 0 ? 0 : 1;
+        for (auto n = N; n; len++, n /= base)
+            continue;
+        return len;
+    }())>
+        buf{};
+
+    auto ptr = buf.data() + buf.length();
+    if (N == 0) {
+        buf.data()[0] = '0';
+    } else {
+        for (auto n = N; n; n /= base)
+            *--ptr = digits[(N < 0 ? -1 : 1) * (n % base)];
+        if (N < 0)
+            *--ptr = '-';
+    }
+
+    return buf;
+}
+
+template <std::size_t L, std::size_t R>
+constexpr auto operator+(FixedString<L> l, FixedString<R> r) -> FixedString<L + R> {
+    auto result = FixedString<L + R>(concat_op(), l, r);
+    return result;
+}
+
+/// Takes any expression that returns a FixedString<N> and returns a const char*.
+#define KSYS_STR(STR)                                                                              \
+    ([]() -> const char* {                                                                         \
+        using ksys::util::Str;                                                                     \
+        static constexpr auto x = STR;                                                             \
+        return x.data();                                                                           \
+    }())
+
+}  // namespace ksys::util