POD => trivial, also clarify tstrlen

ostd/stream.hh

@@ -84,7 +84,7 @@ enum class stream_seek {
 };
 
 
-template<typename T = char, bool = std::is_pod_v<T>>
+template<typename T = char, bool = std::is_trivial_v<T>>
 struct stream_range;
 
 /** @brief Thrown on stream errors. */
@@ -398,7 +398,7 @@ struct stream {
      * By default, it's a `char` range that can be read from if the stream
      * can be read from and written into if the stream can be written into.
      * You can override the type by passing in the template parameter. The
-     * type must always be POD.
+     * type must always be trivial.
      *
      * @tparam T The type to use for reading/writing (`char` by default).
      *
@@ -422,7 +422,7 @@ struct stream {
     /** @brief Writes several values into the stream.
      *
      * Uses write_bytes() to write `count` values from `v` into the stream.
-     * The type must be POD.
+     * The type must be trivial.
      *
      * @throws ostd::stream_error on write failure.
      */
@@ -433,7 +433,7 @@ struct stream {
 
     /** @brief Writes a single value into the stream.
      *
-     * Uses write_bytes() to write the value. The type must be POD.
+     * Uses write_bytes() to write the value. The type must be trivial.
      *
      * @throws ostd::stream_error on write failure.
      */
@@ -447,7 +447,7 @@ struct stream {
      * Uses read_bytes() to read `count` values into `v` from the stream.
      * If an end-of-stream is encountered while reading, this does not
      * throw, but it returns the number of entire values successfully read.
-     * The type must be POD.
+     * The type must be trivial.
      *
      * @returns The number of whole values read.
      *
@@ -462,7 +462,7 @@ struct stream {
     /** @brief Reads a single value from the stream.
      *
      * If the value couldn't be read (reading failed or end-of-stream
-     * occured), this throws ostd::stream_error. The type must be POD.
+     * occured), this throws ostd::stream_error. The type must be trivial.
      *
      * @throws ostd::stream_error on read failure or end-of-stream.
      */
@@ -476,7 +476,7 @@ struct stream {
     /** @brief Reads a single value from the stream.
      *
      * If the value couldn't be read (reading failed or end-of-stream
-     * occured), this throws ostd::stream_error. The type must be POD.
+     * occured), this throws ostd::stream_error. The type must be trivial.
      *
      * @returns The read value.
      *
@@ -548,7 +548,7 @@ private:
  * example).
  *
  * This template is a specialization of undefined base type because stream
- * ranges only work with POD types.
+ * ranges only work with trivial types.
  *
  * @see stream_line_range
  */
@@ -660,7 +660,7 @@ inline stream_range<T> stream::iter() {
  * The lines are read using ostd::stream::get_line().
  *
  * You can provide a custom type for characters, by default it's `char`.
- * It must be POD.
+ * It must be trivial.
  *
  * You can also provide a custom type used to hold the line, which must be a
  * container type over `T` which can append at the end. By default,

src/string.cc

@@ -20,24 +20,38 @@ template<typename C>
 inline std::size_t tstrlen_impl(C const *p) noexcept {
     using SL = std::numeric_limits<std::size_t>;
     using UL = std::numeric_limits<std::make_unsigned_t<C>>;
+    /* low bits of each UL contained in SL (0000000100000001... etc) */
     constexpr std::size_t Lbits = SL::max() / UL::max();
+    /* high bits of each UL contained in SL (1000000010000000... etc) */
     constexpr std::size_t Hbits = Lbits << (UL::digits - 1);
 
     C const *bp = p;
+    /* 1 unit or less per size_t, simple loop */
     if constexpr(sizeof(C) >= sizeof(std::size_t)) {
         goto sloop;
     }
+    /* need a pointer aligned to sizeof(size_t) */
     for (; std::uintptr_t(p) % sizeof(std::size_t); ++p) {
         if (!*p) {
             return (p - bp);
         }
     }
     {
+        /* (e.g. x86_64 => sizeof(size_t) == 8 * sizeof(char) */
         auto *wp = reinterpret_cast<std::size_t const *>(p);
+        /* check if any unit in the size_t is zero, in binary:
+         *
+         * XXX1 - 0001 => XXX0; XXX0 & YYY0 => 0000; 0000 & 1000 => 0000
+         * XX10 - 0001 => XX01; XX01 & YY01 => 0001; 0001 & 1000 => 0000
+         * 0000 - 0001 => 1111; 1111 & 1111 => 1111; 1111 & 1000 => 1000
+         *
+         * if the check passes, a terminating zero is in that size_t, break
+         */
         for (; !(((*wp - Lbits) & ~*wp) & Hbits); ++wp) {}
         p = reinterpret_cast<C const *>(wp);
     }
 sloop:
+    /* either all of the string if goto'd, or contains terminating zero */
     for (; *p; ++p) {}
     return (p - bp);
 }