Skip the indentation after a newline 8 bytes at a time

Pretty-printed JSON indents every line with a run of spaces, which
skip_whitespace() read one character at a time. For contiguous input
(bulk_scan) the lexer now skips the spaces after a newline 8 bytes at a
time directly in the input buffer, including the leading spaces of the
last, partial word. This bypasses get(), which is safe because
bulk_scan implies the lazy position from the previous commit: nothing
is counted per character.

Compared to lazy positions alone, pretty-printed input (twitter,
citm_catalog with indent 2 and 4) parses up to 13% faster and accept()
gets up to 30% faster (GCC 13 and clang 18 on x86-64, Apple clang 21 on
arm64). Minified input is unchanged within noise.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/input/lexer.hpp

@@ -17,6 +17,7 @@
 #include <utility> // move
 #include <vector> // vector
 
+#include <nlohmann/detail/bit_ops.hpp>
 #include <nlohmann/detail/input/input_adapters.hpp>
 #include <nlohmann/detail/input/number_parse.hpp>
 #include <nlohmann/detail/input/position_t.hpp>
@@ -2079,11 +2080,53 @@ scan_number_done:
         // into one that reloads the pointer from memory every character
         do
         {
+            if (current == '\n')
+            {
+                skip_indentation(std::integral_constant<bool, bulk_scan> {});
+            }
             get_ignoring_pending_unget();
         }
         while (current_is_whitespace());
     }
 
+    /*!
+    @brief contiguous input: skip the space indentation that follows a newline
+           8 bytes at a time
+
+    Pretty-printed JSON indents every line with a run of spaces, which the loop
+    in skip_whitespace() would otherwise read one character at a time. The
+    spaces are skipped 8 bytes at a time; the last 7 bytes of the input and any
+    other whitespace are left to that loop. Skipping bypasses get(), which is
+    fine because bulk_scan implies lazy_position: the position is derived from
+    the adapter, not counted per character.
+    */
+    void skip_indentation(std::true_type /*bulk*/)
+    {
+        JSON_ASSERT(!next_unget);
+        const std::size_t remaining = ia.bulk_remaining();
+        if (remaining < 8)
+        {
+            return;
+        }
+        const char* const data = reinterpret_cast<const char*>(ia.bulk_data());
+        std::size_t pos = 0;
+        for (; pos + 8 <= remaining; pos += 8)
+        {
+            // little-endian, so the first byte is the lowest
+            const std::uint64_t non_space = read_eight_bytes(data + pos) ^ 0x2020202020202020ull;
+            if (non_space != 0)
+            {
+                // skip the spaces before the first other byte of this word
+                pos += static_cast<std::size_t>(count_trailing_zeros(non_space)) / 8;
+                break;
+            }
+        }
+        ia.bulk_skip(pos);
+    }
+
+    /// streaming input: no bulk fast path
+    void skip_indentation(std::false_type /*bulk*/) const noexcept {}
+
     token_type scan()
     {
         // initially, skip the BOM