Add: Branchless .empty() for small strings

Relates to #288

Author: ashvardanian

include/stringzilla/small_string.h

@@ -95,9 +95,21 @@ typedef union sz_string_t {
 
 } sz_string_t;
 
+/*
+ *  Verify structure layout for branchless length extraction.
+ *  On little-endian: internal.length (8-bit) overlaps with LSB of external.length at offset 8.
+ *  On big-endian: internal.length is at offset 31, external.length at offset 24.
+ */
 #if !SZ_AVOID_LIBC // `offsetof` comes from `stddef.h`, which is part of the C standard library.
 sz_static_assert(offsetof(sz_string_t, internal.start) == offsetof(sz_string_t, external.start),
                  Alignment_confusion_between_internal_and_external_storage);
+#if !SZ_IS_BIG_ENDIAN_
+sz_static_assert(offsetof(sz_string_t, internal.length) == 8, Internal_length_offset_mismatch_on_little_endian);
+sz_static_assert(offsetof(sz_string_t, external.length) == 8, External_length_offset_mismatch_on_little_endian);
+#else
+sz_static_assert(offsetof(sz_string_t, internal.length) == 31, Internal_length_offset_mismatch_on_big_endian);
+sz_static_assert(offsetof(sz_string_t, external.length) == 24, External_length_offset_mismatch_on_big_endian);
+#endif
 #endif
 
 #pragma endregion // Core Structure
@@ -138,6 +150,11 @@ SZ_PUBLIC void sz_string_unpack( //
  */
 SZ_PUBLIC void sz_string_range(sz_string_t const *string, sz_ptr_t *start, sz_size_t *length);
 
+/**
+ *  @brief  Branchless check if the string is empty.
+ */
+SZ_PUBLIC sz_bool_t sz_string_is_empty(sz_string_t const *string);
+
 /**
  *  @brief  Constructs a string of a given ::length with noisy contents.
  *          Use the returned character pointer to populate the string.
@@ -220,6 +237,12 @@ SZ_PUBLIC void sz_string_range(sz_string_t const *string, sz_ptr_t *start, sz_si
     *length = string->external.length & (0x00000000000000FFull | is_big_mask);
 }
 
+SZ_PUBLIC sz_bool_t sz_string_is_empty(sz_string_t const *string) {
+    sz_size_t is_small = (sz_cptr_t)string->internal.start == (sz_cptr_t)&string->internal.chars[0];
+    sz_size_t is_big_mask = is_small - 1ull;
+    return (sz_bool_t)((string->external.length & (0x00000000000000FFull | is_big_mask)) == 0);
+}
+
 SZ_PUBLIC void sz_string_unpack( //
     sz_string_t const *string, sz_ptr_t *start, sz_size_t *length, sz_size_t *space, sz_bool_t *is_external) {
     sz_size_t is_small = (sz_cptr_t)string->internal.start == (sz_cptr_t)&string->internal.chars[0];

include/stringzilla/stringzilla.hpp

@@ -2833,7 +2833,7 @@ class basic_string {
     size_type size() const noexcept { return view().size(); }
     size_type length() const noexcept { return size(); }
     size_type max_size() const noexcept { return npos - 1; }
-    bool empty() const noexcept { return string_.external.length == 0; }
+    bool empty() const noexcept { return sz_string_is_empty(&string_); }
     size_type capacity() const noexcept {
         sz_ptr_t string_start;
         sz_size_t string_length;

include/stringzilla/types.h

@@ -104,10 +104,10 @@
  *  Fall back to legacy macros and known arch tags when unavailable.
  */
 #if !defined(SZ_IS_BIG_ENDIAN_)
-#if (defined(__BYTE_ORDER__) && (__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__)) ||                                           \
-    (defined(__BYTE_ORDER) && (__BYTE_ORDER == __BIG_ENDIAN)) || defined(__BIG_ENDIAN__) || defined(_BIG_ENDIAN) ||    \
-    defined(BIG_ENDIAN) || defined(__ARMEB__) || defined(__THUMBEB__) || defined(__AARCH64EB__) || defined(_MIBSEB) || \
-    defined(__MIBSEB) || defined(__MIBSEB__) || defined(__s390x__) || defined(__s390__)
+#if (defined(__BYTE_ORDER__) && (__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__)) ||                                         \
+    (defined(__BYTE_ORDER) && (__BYTE_ORDER == __BIG_ENDIAN)) || defined(__BIG_ENDIAN__) || defined(_BIG_ENDIAN) ||  \
+    defined(__ARMEB__) || defined(__THUMBEB__) || defined(__AARCH64EB__) || defined(_MIBSEB) || defined(__MIBSEB) || \
+    defined(__MIBSEB__) || defined(__s390x__) || defined(__s390__)
 #define SZ_IS_BIG_ENDIAN_ (1) //< It's a big-endian target architecture
 #else
 #define SZ_IS_BIG_ENDIAN_ (0) //< It's a little-endian target architecture