[wpiutil] Add compile-time string utilities (ct_string) (#5462)

wpiutil/src/main/native/include/wpi/ct_string.h

@@ -0,0 +1,168 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+#include <stdint.h>
+
+#include <array>
+#include <limits>
+#include <stdexcept>
+#include <string>
+#include <string_view>
+
+namespace wpi {
+
+// derived from:
+// https://codereview.stackexchange.com/questions/282514/string-literals-concatenation-with-support-for-dynamic-strings
+
+/**
+ * Fixed length string (array of character) for compile time use.
+ *
+ * @tparam N number of characters
+ * @tparam Char character type
+ * @tparam Traits character traits
+ */
+template <typename Char, typename Traits, size_t N>
+  requires(N < (std::numeric_limits<size_t>::max)())
+struct ct_string {
+  std::array<Char, N + 1> chars;
+
+  template <size_t M>
+    requires(M <= (N + 1))
+  consteval ct_string(Char const (&s)[M]) {  // NOLINT
+    if constexpr (M == (N + 1)) {
+      if (s[N] != Char{}) {
+        throw std::logic_error{"char array not null terminated"};
+      }
+    }
+
+    // avoid dependency on <algorithm>
+    // auto p = std::ranges::copy(s, chars.begin()).out;
+    auto p = chars.begin();
+    for (auto c : s) {
+      *p++ = c;
+    }
+    // std::ranges::fill() isn't constexpr on GCC 11
+    while (p != chars.end()) {
+      *p++ = Char{};
+    }
+  }
+
+  explicit consteval ct_string(std::basic_string_view<Char, Traits> s) {
+    // avoid dependency on <algorithm>
+    // auto p = std::ranges::copy(s, chars.begin()).out;
+    auto p = chars.begin();
+    for (auto c : s) {
+      *p++ = c;
+    }
+    // std::ranges::fill() isn't constexpr on GCC 11
+    while (p != chars.end()) {
+      *p++ = Char{};
+    }
+  }
+
+  constexpr auto size() const noexcept { return N; }
+
+  constexpr auto begin() const noexcept { return chars.begin(); }
+  constexpr auto end() const noexcept { return chars.begin() + N; }
+
+  constexpr auto data() const noexcept { return chars.data(); }
+  constexpr auto c_str() const noexcept { return chars.data(); }
+
+  constexpr operator std::basic_string_view<Char, Traits>()  // NOLINT
+      const noexcept {
+    return std::basic_string_view<Char, Traits>{chars.data(), N};
+  }
+};
+
+template <typename Char, size_t M>
+ct_string(Char const (&s)[M]) -> ct_string<Char, std::char_traits<Char>, M - 1>;
+
+namespace literals {
+template <ct_string S>
+consteval auto operator""_ct_string() {
+  return S;
+}
+}  // namespace literals
+
+template <typename Char, typename Traits, size_t N1, size_t N2>
+consteval auto operator+(ct_string<Char, Traits, N1> const& s1,
+                         ct_string<Char, Traits, N2> const& s2) noexcept {
+  return Concat(s1, s2);
+}
+
+/**
+ * Concatenates multiple fixed_strings into a larger fixed_string at compile
+ * time.
+ *
+ * @param s1 first string
+ * @param s second and later strings
+ * @return concatenated string
+ */
+template <typename Char, typename Traits, size_t N1, size_t... N>
+consteval auto Concat(ct_string<Char, Traits, N1> const& s1,
+                      ct_string<Char, Traits, N> const&... s) {
+  // Need a dummy array to instantiate a ct_string.
+  constexpr Char dummy[1] = {};
+  auto res = ct_string<Char, Traits, (N1 + ... + N)>{dummy};
+
+  auto p = res.chars.begin();
+  auto append = [&p](auto&& s) {
+    // avoid dependency on <algorithm>
+    // p = std::ranges::copy(s, p).out;
+    for (auto c : s) {
+      *p++ = c;
+    }
+  };
+
+  (append(s1), ..., append(s));
+
+  return res;
+}
+
+// derived from:
+// https://github.com/tcsullivan/constexpr-to-string/blob/master/to_string.hpp
+
+/**
+ * Converts any integral to a ct_string at compile-time.
+ *
+ * @tparam N number to convert
+ * @tparam Base desired base, can be from 2 to 36
+ * @tparam Char character type
+ * @tparam Traits character traits
+ */
+template <intmax_t N, int Base = 10, typename Char = char,
+          typename Traits = std::char_traits<Char>>
+  requires(Base >= 2 && Base <= 36)
+consteval auto NumToCtString() {
+  constexpr char digits[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+
+  auto buflen = [] {
+    size_t len = N > 0 ? 0 : 1;
+    for (auto n = N; n; len++, n /= Base) {
+    }
+    return len;
+  };
+  constexpr size_t size = buflen();
+
+  constexpr Char dummy[1] = {};
+  auto res = ct_string<Char, Traits, size>{dummy};
+
+  auto ptr = res.chars.data() + size;
+  if (N != 0) {
+    for (auto n = N; n; n /= Base) {
+      *--ptr = digits[(N < 0 ? -1 : 1) * (n % Base)];
+    }
+    if (N < 0) {
+      *--ptr = '-';
+    }
+  } else {
+    res.chars[0] = '0';
+  }
+
+  return res;
+}
+
+}  // namespace wpi