Write compact dumps of json_view without a library call per token

The default dump() (no indentation, no ensure_ascii) gets its own
writer: the same walk and output, with the write position in a local
variable (stores through char pointers would otherwise force a reload
of the buffer's members after each one), strings and number tokens of
the source copied by fixed-size moves of 32 bytes where the source has
that many bytes left (the buffer keeps 64 bytes of slack), and decoded
strings copied in runs up to the next quote, backslash, or control
character. Documents that are not edited are walked through the node
array in order, so that a frame only needs the end of its container,
and integer tokens are read from the source directly. The innermost
open container is kept in local variables, and the stack holds only
the ones around it; the stack starts in a local array of 32 and moves
to the heap only for deeper nesting (its address does not escape, so
its pointers stay in registers). Dumps of shallow documents thus
allocate only the output, whose first size includes the slack, so it
does not grow just before the end.

The long copies are out of line: otherwise, the compiler merges the
fixed-size moves into the same library call.

These techniques come from the prototype; the writer lost them when the
view was split into pull requests, which made dump() 2 to 3 times
slower.

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

include/nlohmann/detail/view/serializer.hpp

@@ -75,6 +75,23 @@ class output_buffer
         m_pos += n;
     }
 
+    /// the write position and the end of the writable space, for a writer
+    /// that keeps the position in a local variable (set_cursor() hands it back)
+    char* cursor() const noexcept
+    {
+        return m_pos;
+    }
+
+    char* limit() const noexcept
+    {
+        return m_end;
+    }
+
+    void set_cursor(char* p) noexcept
+    {
+        m_pos = p;
+    }
+
   private:
     static StringType& sized(StringType& out, std::size_t estimate)
     {
@@ -95,6 +112,105 @@ class output_buffer
     char* m_end;
 };
 
+/// The length of the run at s that dump() writes unchanged without
+/// ensure_ascii: all bytes but quotes, backslashes, and control characters.
+/// Unlike detail::string_bulk_run(), non-ASCII bytes are not validated: the
+/// strings of a document are valid UTF-8 (a damaged image loaded with
+/// image_check::bounds can have others, which are then written unchanged).
+inline std::size_t plain_output_run(const unsigned char* s, std::size_t n) noexcept
+{
+    constexpr std::uint64_t ones = 0x0101010101010101ull;
+    constexpr std::uint64_t high = 0x8080808080808080ull;
+    std::size_t i = 0;
+    for (; i + 8 <= n; i += 8)
+    {
+        const std::uint64_t v = read_eight_bytes(s + i);
+        const std::uint64_t q = v ^ 0x2222222222222222ull; // '"'
+        const std::uint64_t b = v ^ 0x5C5C5C5C5C5C5C5Cull; // '\\'
+        const std::uint64_t stop = (((q - ones) & ~q) | ((b - ones) & ~b) | ((v - 0x2020202020202020ull) & ~v)) & high;
+        if (stop != 0)
+        {
+            // the lowest flagged byte is the first stop: borrows only flag bytes above a true one
+            return i + (static_cast<std::size_t>(count_trailing_zeros(stop)) / 8);
+        }
+    }
+    for (; i < n; ++i)
+    {
+        if (s[i] == '"' || s[i] == '\\' || s[i] < 0x20)
+        {
+            return i;
+        }
+    }
+    return n;
+}
+
+/// A stack that starts in a buffer of the caller (a local array) and moves to
+/// the heap (a vector of the caller) only when that is full, so that dumps of
+/// shallow documents need no allocation. The top is a pointer, as in
+/// std::vector. The address of the stack never escapes (the growth gets the
+/// vector and returns the new storage), so its pointers stay in registers.
+template<typename T>
+class small_stack
+{
+  public:
+    small_stack(T* buffer, std::size_t capacity, std::vector<T>& heap) noexcept
+        : m_begin(buffer), m_top(buffer), m_end(buffer + capacity), m_heap(&heap)
+    {}
+    small_stack(const small_stack&) = delete;
+    small_stack(small_stack&&) = delete;
+    small_stack& operator=(const small_stack&) = delete;
+    small_stack& operator=(small_stack&&) = delete;
+    ~small_stack() = default;
+
+    NLOHMANN_VIEW_ALWAYS_INLINE void push_back(const T& x)
+    {
+        if (NLOHMANN_VIEW_UNLIKELY(m_top == m_end))
+        {
+            const std::size_t used = size();
+            const std::size_t capacity = 2 * static_cast<std::size_t>(m_end - m_begin);
+            m_begin = grow(*m_heap, m_begin, used, capacity);
+            m_top = m_begin + used;
+            m_end = m_begin + capacity;
+        }
+        *m_top++ = x;
+    }
+
+    NLOHMANN_VIEW_ALWAYS_INLINE T& back() noexcept
+    {
+        return m_top[-1];
+    }
+
+    NLOHMANN_VIEW_ALWAYS_INLINE void pop_back() noexcept
+    {
+        --m_top;
+    }
+
+    NLOHMANN_VIEW_ALWAYS_INLINE bool empty() const noexcept
+    {
+        return m_top == m_begin;
+    }
+
+    NLOHMANN_VIEW_ALWAYS_INLINE std::size_t size() const noexcept
+    {
+        return static_cast<std::size_t>(m_top - m_begin);
+    }
+
+  private:
+    /// the used entries moved to heap storage of the given capacity
+    NLOHMANN_VIEW_NOINLINE static T* grow(std::vector<T>& heap, const T* begin, std::size_t used, std::size_t capacity)
+    {
+        std::vector<T> bigger(capacity);
+        std::copy(begin, begin + used, bigger.begin());
+        heap.swap(bigger);
+        return heap.data();
+    }
+
+    T* m_begin;
+    T* m_top;
+    T* m_end;
+    std::vector<T>* m_heap;
+};
+
 /// how the view's dump() writes a value
 struct dump_style
 {
@@ -129,6 +245,18 @@ class view_serializer
 
     void dump(const node* root)
     {
+        if (!m_style.pretty && !m_style.ensure_ascii)
+        {
+            if (m_style.source_numbers)
+            {
+                dump_compact<true>(root);
+            }
+            else
+            {
+                dump_compact<false>(root);
+            }
+            return;
+        }
         struct frame
         {
             const node* pos; ///< next element, or key of the next member
@@ -136,7 +264,9 @@ class view_serializer
             bool object;
             bool first;  ///< nothing written yet
         };
-        std::vector<frame> stack;
+        std::array<frame, 32> buffer; // NOLINT(cppcoreguidelines-pro-type-member-init,hicpp-member-init): written before read
+        std::vector<frame> heap;
+        small_stack<frame> stack(buffer.data(), buffer.size(), heap);
         const node* n = root;
         for (;;)
         {
@@ -207,6 +337,284 @@ class view_serializer
     }
 
   private:
+    /*!
+    @brief the compact output without ensure_ascii (the default dump())
+
+    The same walk as dump(), with the write position in a local variable
+    (stores through char pointers would otherwise force a reload of the
+    buffer's members after each one), and with strings and number tokens of
+    the source copied by fixed-size moves of 32 bytes where the source has
+    that many bytes left, instead of a library call per token. The buffer
+    keeps 64 bytes of slack for the overshoot.
+    */
+    /// a string that is not a plain string of the source (decoded, or written
+    /// by an edit), without ensure_ascii: runs without characters to escape
+    /// are copied
+    NLOHMANN_VIEW_NOINLINE void write_decoded(const node& n)
+    {
+        const auto* const s = reinterpret_cast<const unsigned char*>(m_doc.str(n)); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+        m_out.put('"');
+        for (std::size_t i = 0; i < n.len;)
+        {
+            const std::size_t run = plain_output_run(s + i, n.len - i);
+            if (run != 0)
+            {
+                m_out.put(reinterpret_cast<const char*>(s + i), run); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+                i += run;
+                continue;
+            }
+            write_codepoint<false>(s[i], s + i, 1); // a quote, a backslash, or a control character
+            ++i;
+        }
+        m_out.put('"');
+    }
+
+    /// the copies of dump_compact() that are not fixed-size moves (long
+    /// strings, or near the end of the source); out of line, so that the
+    /// compiler does not merge the fixed-size moves into this call
+    NLOHMANN_VIEW_NOINLINE static void copy_long(char* to, const char* from, std::size_t n) noexcept
+    {
+        std::memcpy(to, from, n);
+    }
+
+    template<bool SourceNumbers>
+    void dump_compact(const node* root)
+    {
+        struct frame
+        {
+            const node* pos; ///< (editable documents) next element, or key of the next member
+            const node* end;
+            bool object;
+        };
+        std::array<frame, 32> buffer; // NOLINT(cppcoreguidelines-pro-type-member-init,hicpp-member-init): written before read
+        std::vector<frame> heap;
+        small_stack<frame> stack(buffer.data(), buffer.size(), heap);
+        const char* const src = m_doc.src;
+        const char* const src_end = src + m_doc.size;
+        char* w = m_out.cursor();
+        char* lim = m_out.limit();
+        // room for n bytes and the slack
+        const auto room = [&](std::size_t n)
+        {
+            if (NLOHMANN_VIEW_UNLIKELY(static_cast<std::size_t>(lim - w) < n + 64))
+            {
+                m_out.set_cursor(w);
+                m_out.reserve(n + 64);
+                w = m_out.cursor();
+                lim = m_out.limit();
+            }
+        };
+        // copy n bytes of the source (after room(n))
+        const auto copy = [&](const char* from, std::size_t n)
+        {
+            if (n <= 32 && src_end - from >= 32)
+            {
+                std::memcpy(w, from, 32);
+            }
+            else if (n <= 256 && src_end - from >= static_cast<std::ptrdiff_t>(n) + 32)
+            {
+                for (std::size_t i = 0; i < n; i += 32)
+                {
+                    std::memcpy(w + i, from + i, 32);
+                }
+            }
+            else
+            {
+                copy_long(w, from, n);
+            }
+            w += n;
+        };
+        // a literal of n bytes (after room(n))
+        const auto literal = [&](const char* text, std::size_t n)
+        {
+            std::memcpy(w, text, n);
+            w += n;
+        };
+        // a string that is not a plain string of the source (out of line, so
+        // that the cursor stays in a register here)
+        const auto escaped = [&](const node & n)
+        {
+            m_out.set_cursor(w);
+            write_decoded(n);
+            w = m_out.cursor();
+            lim = m_out.limit();
+        };
+
+        // Read-only documents: the elements of a container follow it in the
+        // node array, so the walk goes through the array in order, and a
+        // frame only needs the end of its container. Editable documents: the
+        // elements of a moved container live elsewhere, so a frame keeps the
+        // position of the next element (see navigation).
+        // The innermost open container is kept in registers (cur; end ==
+        // nullptr: none), the stack holds the ones around it.
+        frame cur{nullptr, nullptr, false};
+        const node* n = root;
+        for (;;)
+        {
+            // write the value at n (read-only documents: and advance n)
+            bool opened = false;
+            switch (static_cast<value_t>(n->kind))
+            {
+                case value_t::string:
+                    if ((n->flags & node_flags::storage) == 0)
+                    {
+                        room(n->len + 2);
+                        *w++ = '"';
+                        copy(src + n->off, n->len);
+                        *w++ = '"';
+                    }
+                    else
+                    {
+                        escaped(*n);
+                    }
+                    break;
+                case value_t::number_integer:
+                case value_t::number_unsigned:
+                {
+                    const std::uint32_t len = number_length(*n);
+                    room(len);
+                    if (Editable && (n->flags & node_flags::storage) != 0)
+                    {
+                        copy_long(w, m_doc.str(*n), len); // a canonical token written by an edit
+                        w += len;
+                        break;
+                    }
+                    const char* const token = src + n->off;
+                    if (!SourceNumbers && NLOHMANN_VIEW_UNLIKELY(len == 2 && token[0] == '-' && token[1] == '0'))
+                    {
+                        *w++ = '0'; // parse() reads -0 as the integer 0
+                    }
+                    else
+                    {
+                        copy(token, len);
+                    }
+                    break;
+                }
+                case value_t::number_float:
+                    if (SourceNumbers && (n->flags & node_flags::storage) != node_flags::edited)
+                    {
+                        room(n->len);
+                        copy(src + n->off, n->len);
+                    }
+                    else
+                    {
+                        m_out.set_cursor(w);
+                        write_float(float_value<number_float_t>(m_doc, *n));
+                        w = m_out.cursor();
+                        lim = m_out.limit();
+                    }
+                    break;
+                case value_t::boolean:
+                    room(8);
+                    if ((n->flags & node_flags::is_true) != 0)
+                    {
+                        literal("true", 4);
+                    }
+                    else
+                    {
+                        literal("false", 5);
+                    }
+                    break;
+                case value_t::object:
+                case value_t::array:
+                {
+                    const bool object = n->kind == static_cast<std::uint8_t>(value_t::object);
+                    room(8);
+                    if (n->len == 0)
+                    {
+                        literal(object ? "{}" : "[]", 2);
+                    }
+                    else
+                    {
+                        *w++ = object ? '{' : '[';
+                        stack.push_back(cur);
+                        if (Editable)
+                        {
+                            cur = frame{nav::first(m_doc, n), nav::end(m_doc, n), object};
+                        }
+                        else
+                        {
+                            cur = frame{nullptr, n + n->next, object};
+                        }
+                        opened = true;
+                    }
+                    break;
+                }
+                case value_t::null:
+                    room(8);
+                    literal("null", 4);
+                    break;
+                case value_t::binary:    // LCOV_EXCL_LINE (not in a document)
+                case value_t::discarded: // LCOV_EXCL_LINE
+                default:                 // LCOV_EXCL_LINE
+                    break;               // LCOV_EXCL_LINE
+            }
+            if (!Editable)
+            {
+                ++n; // the next node: the first element of an opened container, or the node after a scalar
+            }
+
+            // go to the next value: close finished containers, then separate
+            // (a container just opened has an element)
+            if (!opened)
+            {
+                for (;;)
+                {
+                    if (cur.end == nullptr)
+                    {
+                        m_out.set_cursor(w);
+                        m_out.finish();
+                        return;
+                    }
+                    if ((Editable ? cur.pos : n) != cur.end)
+                    {
+                        break;
+                    }
+                    room(1);
+                    *w++ = cur.object ? '}' : ']';
+                    cur = stack.back();
+                    stack.pop_back();
+                }
+                room(1);
+                *w++ = ',';
+            }
+            const node* const at = Editable ? cur.pos : n;
+            if (cur.object)
+            {
+                const node& key = *at;
+                if ((key.flags & node_flags::storage) == 0)
+                {
+                    room(key.len + 3);
+                    *w++ = '"';
+                    copy(src + key.off, key.len);
+                    w[0] = '"';
+                    w[1] = ':';
+                    w += 2;
+                }
+                else
+                {
+                    escaped(key);
+                    room(1);
+                    *w++ = ':';
+                }
+                if (Editable)
+                {
+                    n = nav::value(at + 1);
+                    cur.pos = document_data::after(at + 1);
+                }
+                else
+                {
+                    ++n;
+                }
+            }
+            else if (Editable)
+            {
+                n = nav::value(at);
+                cur.pos = document_data::after(at);
+            }
+        }
+    }
+
     void newline(std::size_t level)
     {
         if (m_style.pretty)

include/nlohmann/json_view.hpp

@@ -592,8 +592,10 @@ class basic_json_view
         style.indent_char = indent_char;
         style.ensure_ascii = ensure_ascii;
         style.source_numbers = numbers == number_format::source;
-        // the compact text is about as long as the source text of the value
-        const std::size_t estimate = source_extent() + (style.pretty ? source_extent() / 2 : 0) + 64;
+        // the compact text is about as long as the source text of the value;
+        // the compact writer keeps 64 bytes of slack, so that it does not grow
+        // the buffer just before the end
+        const std::size_t estimate = source_extent() + (style.pretty ? source_extent() / 2 : 0) + 160;
         detail::view::view_serializer<BasicJsonType, Editable>(*m_doc, out, estimate, style).dump(m_node);
         return out;
     }

single_include/nlohmann/json_view.hpp

@@ -5369,6 +5369,23 @@ class output_buffer
         m_pos += n;
     }
 
+    /// the write position and the end of the writable space, for a writer
+    /// that keeps the position in a local variable (set_cursor() hands it back)
+    char* cursor() const noexcept
+    {
+        return m_pos;
+    }
+
+    char* limit() const noexcept
+    {
+        return m_end;
+    }
+
+    void set_cursor(char* p) noexcept
+    {
+        m_pos = p;
+    }
+
   private:
     static StringType& sized(StringType& out, std::size_t estimate)
     {
@@ -5389,6 +5406,105 @@ class output_buffer
     char* m_end;
 };
 
+/// The length of the run at s that dump() writes unchanged without
+/// ensure_ascii: all bytes but quotes, backslashes, and control characters.
+/// Unlike detail::string_bulk_run(), non-ASCII bytes are not validated: the
+/// strings of a document are valid UTF-8 (a damaged image loaded with
+/// image_check::bounds can have others, which are then written unchanged).
+inline std::size_t plain_output_run(const unsigned char* s, std::size_t n) noexcept
+{
+    constexpr std::uint64_t ones = 0x0101010101010101ull;
+    constexpr std::uint64_t high = 0x8080808080808080ull;
+    std::size_t i = 0;
+    for (; i + 8 <= n; i += 8)
+    {
+        const std::uint64_t v = read_eight_bytes(s + i);
+        const std::uint64_t q = v ^ 0x2222222222222222ull; // '"'
+        const std::uint64_t b = v ^ 0x5C5C5C5C5C5C5C5Cull; // '\\'
+        const std::uint64_t stop = (((q - ones) & ~q) | ((b - ones) & ~b) | ((v - 0x2020202020202020ull) & ~v)) & high;
+        if (stop != 0)
+        {
+            // the lowest flagged byte is the first stop: borrows only flag bytes above a true one
+            return i + (static_cast<std::size_t>(count_trailing_zeros(stop)) / 8);
+        }
+    }
+    for (; i < n; ++i)
+    {
+        if (s[i] == '"' || s[i] == '\\' || s[i] < 0x20)
+        {
+            return i;
+        }
+    }
+    return n;
+}
+
+/// A stack that starts in a buffer of the caller (a local array) and moves to
+/// the heap (a vector of the caller) only when that is full, so that dumps of
+/// shallow documents need no allocation. The top is a pointer, as in
+/// std::vector. The address of the stack never escapes (the growth gets the
+/// vector and returns the new storage), so its pointers stay in registers.
+template<typename T>
+class small_stack
+{
+  public:
+    small_stack(T* buffer, std::size_t capacity, std::vector<T>& heap) noexcept
+        : m_begin(buffer), m_top(buffer), m_end(buffer + capacity), m_heap(&heap)
+    {}
+    small_stack(const small_stack&) = delete;
+    small_stack(small_stack&&) = delete;
+    small_stack& operator=(const small_stack&) = delete;
+    small_stack& operator=(small_stack&&) = delete;
+    ~small_stack() = default;
+
+    NLOHMANN_VIEW_ALWAYS_INLINE void push_back(const T& x)
+    {
+        if (NLOHMANN_VIEW_UNLIKELY(m_top == m_end))
+        {
+            const std::size_t used = size();
+            const std::size_t capacity = 2 * static_cast<std::size_t>(m_end - m_begin);
+            m_begin = grow(*m_heap, m_begin, used, capacity);
+            m_top = m_begin + used;
+            m_end = m_begin + capacity;
+        }
+        *m_top++ = x;
+    }
+
+    NLOHMANN_VIEW_ALWAYS_INLINE T& back() noexcept
+    {
+        return m_top[-1];
+    }
+
+    NLOHMANN_VIEW_ALWAYS_INLINE void pop_back() noexcept
+    {
+        --m_top;
+    }
+
+    NLOHMANN_VIEW_ALWAYS_INLINE bool empty() const noexcept
+    {
+        return m_top == m_begin;
+    }
+
+    NLOHMANN_VIEW_ALWAYS_INLINE std::size_t size() const noexcept
+    {
+        return static_cast<std::size_t>(m_top - m_begin);
+    }
+
+  private:
+    /// the used entries moved to heap storage of the given capacity
+    NLOHMANN_VIEW_NOINLINE static T* grow(std::vector<T>& heap, const T* begin, std::size_t used, std::size_t capacity)
+    {
+        std::vector<T> bigger(capacity);
+        std::copy(begin, begin + used, bigger.begin());
+        heap.swap(bigger);
+        return heap.data();
+    }
+
+    T* m_begin;
+    T* m_top;
+    T* m_end;
+    std::vector<T>* m_heap;
+};
+
 /// how the view's dump() writes a value
 struct dump_style
 {
@@ -5423,6 +5539,18 @@ class view_serializer
 
     void dump(const node* root)
     {
+        if (!m_style.pretty && !m_style.ensure_ascii)
+        {
+            if (m_style.source_numbers)
+            {
+                dump_compact<true>(root);
+            }
+            else
+            {
+                dump_compact<false>(root);
+            }
+            return;
+        }
         struct frame
         {
             const node* pos; ///< next element, or key of the next member
@@ -5430,7 +5558,9 @@ class view_serializer
             bool object;
             bool first;  ///< nothing written yet
         };
-        std::vector<frame> stack;
+        std::array<frame, 32> buffer; // NOLINT(cppcoreguidelines-pro-type-member-init,hicpp-member-init): written before read
+        std::vector<frame> heap;
+        small_stack<frame> stack(buffer.data(), buffer.size(), heap);
         const node* n = root;
         for (;;)
         {
@@ -5501,6 +5631,284 @@ class view_serializer
     }
 
   private:
+    /*!
+    @brief the compact output without ensure_ascii (the default dump())
+
+    The same walk as dump(), with the write position in a local variable
+    (stores through char pointers would otherwise force a reload of the
+    buffer's members after each one), and with strings and number tokens of
+    the source copied by fixed-size moves of 32 bytes where the source has
+    that many bytes left, instead of a library call per token. The buffer
+    keeps 64 bytes of slack for the overshoot.
+    */
+    /// a string that is not a plain string of the source (decoded, or written
+    /// by an edit), without ensure_ascii: runs without characters to escape
+    /// are copied
+    NLOHMANN_VIEW_NOINLINE void write_decoded(const node& n)
+    {
+        const auto* const s = reinterpret_cast<const unsigned char*>(m_doc.str(n)); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+        m_out.put('"');
+        for (std::size_t i = 0; i < n.len;)
+        {
+            const std::size_t run = plain_output_run(s + i, n.len - i);
+            if (run != 0)
+            {
+                m_out.put(reinterpret_cast<const char*>(s + i), run); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+                i += run;
+                continue;
+            }
+            write_codepoint<false>(s[i], s + i, 1); // a quote, a backslash, or a control character
+            ++i;
+        }
+        m_out.put('"');
+    }
+
+    /// the copies of dump_compact() that are not fixed-size moves (long
+    /// strings, or near the end of the source); out of line, so that the
+    /// compiler does not merge the fixed-size moves into this call
+    NLOHMANN_VIEW_NOINLINE static void copy_long(char* to, const char* from, std::size_t n) noexcept
+    {
+        std::memcpy(to, from, n);
+    }
+
+    template<bool SourceNumbers>
+    void dump_compact(const node* root)
+    {
+        struct frame
+        {
+            const node* pos; ///< (editable documents) next element, or key of the next member
+            const node* end;
+            bool object;
+        };
+        std::array<frame, 32> buffer; // NOLINT(cppcoreguidelines-pro-type-member-init,hicpp-member-init): written before read
+        std::vector<frame> heap;
+        small_stack<frame> stack(buffer.data(), buffer.size(), heap);
+        const char* const src = m_doc.src;
+        const char* const src_end = src + m_doc.size;
+        char* w = m_out.cursor();
+        char* lim = m_out.limit();
+        // room for n bytes and the slack
+        const auto room = [&](std::size_t n)
+        {
+            if (NLOHMANN_VIEW_UNLIKELY(static_cast<std::size_t>(lim - w) < n + 64))
+            {
+                m_out.set_cursor(w);
+                m_out.reserve(n + 64);
+                w = m_out.cursor();
+                lim = m_out.limit();
+            }
+        };
+        // copy n bytes of the source (after room(n))
+        const auto copy = [&](const char* from, std::size_t n)
+        {
+            if (n <= 32 && src_end - from >= 32)
+            {
+                std::memcpy(w, from, 32);
+            }
+            else if (n <= 256 && src_end - from >= static_cast<std::ptrdiff_t>(n) + 32)
+            {
+                for (std::size_t i = 0; i < n; i += 32)
+                {
+                    std::memcpy(w + i, from + i, 32);
+                }
+            }
+            else
+            {
+                copy_long(w, from, n);
+            }
+            w += n;
+        };
+        // a literal of n bytes (after room(n))
+        const auto literal = [&](const char* text, std::size_t n)
+        {
+            std::memcpy(w, text, n);
+            w += n;
+        };
+        // a string that is not a plain string of the source (out of line, so
+        // that the cursor stays in a register here)
+        const auto escaped = [&](const node & n)
+        {
+            m_out.set_cursor(w);
+            write_decoded(n);
+            w = m_out.cursor();
+            lim = m_out.limit();
+        };
+
+        // Read-only documents: the elements of a container follow it in the
+        // node array, so the walk goes through the array in order, and a
+        // frame only needs the end of its container. Editable documents: the
+        // elements of a moved container live elsewhere, so a frame keeps the
+        // position of the next element (see navigation).
+        // The innermost open container is kept in registers (cur; end ==
+        // nullptr: none), the stack holds the ones around it.
+        frame cur{nullptr, nullptr, false};
+        const node* n = root;
+        for (;;)
+        {
+            // write the value at n (read-only documents: and advance n)
+            bool opened = false;
+            switch (static_cast<value_t>(n->kind))
+            {
+                case value_t::string:
+                    if ((n->flags & node_flags::storage) == 0)
+                    {
+                        room(n->len + 2);
+                        *w++ = '"';
+                        copy(src + n->off, n->len);
+                        *w++ = '"';
+                    }
+                    else
+                    {
+                        escaped(*n);
+                    }
+                    break;
+                case value_t::number_integer:
+                case value_t::number_unsigned:
+                {
+                    const std::uint32_t len = number_length(*n);
+                    room(len);
+                    if (Editable && (n->flags & node_flags::storage) != 0)
+                    {
+                        copy_long(w, m_doc.str(*n), len); // a canonical token written by an edit
+                        w += len;
+                        break;
+                    }
+                    const char* const token = src + n->off;
+                    if (!SourceNumbers && NLOHMANN_VIEW_UNLIKELY(len == 2 && token[0] == '-' && token[1] == '0'))
+                    {
+                        *w++ = '0'; // parse() reads -0 as the integer 0
+                    }
+                    else
+                    {
+                        copy(token, len);
+                    }
+                    break;
+                }
+                case value_t::number_float:
+                    if (SourceNumbers && (n->flags & node_flags::storage) != node_flags::edited)
+                    {
+                        room(n->len);
+                        copy(src + n->off, n->len);
+                    }
+                    else
+                    {
+                        m_out.set_cursor(w);
+                        write_float(float_value<number_float_t>(m_doc, *n));
+                        w = m_out.cursor();
+                        lim = m_out.limit();
+                    }
+                    break;
+                case value_t::boolean:
+                    room(8);
+                    if ((n->flags & node_flags::is_true) != 0)
+                    {
+                        literal("true", 4);
+                    }
+                    else
+                    {
+                        literal("false", 5);
+                    }
+                    break;
+                case value_t::object:
+                case value_t::array:
+                {
+                    const bool object = n->kind == static_cast<std::uint8_t>(value_t::object);
+                    room(8);
+                    if (n->len == 0)
+                    {
+                        literal(object ? "{}" : "[]", 2);
+                    }
+                    else
+                    {
+                        *w++ = object ? '{' : '[';
+                        stack.push_back(cur);
+                        if (Editable)
+                        {
+                            cur = frame{nav::first(m_doc, n), nav::end(m_doc, n), object};
+                        }
+                        else
+                        {
+                            cur = frame{nullptr, n + n->next, object};
+                        }
+                        opened = true;
+                    }
+                    break;
+                }
+                case value_t::null:
+                    room(8);
+                    literal("null", 4);
+                    break;
+                case value_t::binary:    // LCOV_EXCL_LINE (not in a document)
+                case value_t::discarded: // LCOV_EXCL_LINE
+                default:                 // LCOV_EXCL_LINE
+                    break;               // LCOV_EXCL_LINE
+            }
+            if (!Editable)
+            {
+                ++n; // the next node: the first element of an opened container, or the node after a scalar
+            }
+
+            // go to the next value: close finished containers, then separate
+            // (a container just opened has an element)
+            if (!opened)
+            {
+                for (;;)
+                {
+                    if (cur.end == nullptr)
+                    {
+                        m_out.set_cursor(w);
+                        m_out.finish();
+                        return;
+                    }
+                    if ((Editable ? cur.pos : n) != cur.end)
+                    {
+                        break;
+                    }
+                    room(1);
+                    *w++ = cur.object ? '}' : ']';
+                    cur = stack.back();
+                    stack.pop_back();
+                }
+                room(1);
+                *w++ = ',';
+            }
+            const node* const at = Editable ? cur.pos : n;
+            if (cur.object)
+            {
+                const node& key = *at;
+                if ((key.flags & node_flags::storage) == 0)
+                {
+                    room(key.len + 3);
+                    *w++ = '"';
+                    copy(src + key.off, key.len);
+                    w[0] = '"';
+                    w[1] = ':';
+                    w += 2;
+                }
+                else
+                {
+                    escaped(key);
+                    room(1);
+                    *w++ = ':';
+                }
+                if (Editable)
+                {
+                    n = nav::value(at + 1);
+                    cur.pos = document_data::after(at + 1);
+                }
+                else
+                {
+                    ++n;
+                }
+            }
+            else if (Editable)
+            {
+                n = nav::value(at);
+                cur.pos = document_data::after(at);
+            }
+        }
+    }
+
     void newline(std::size_t level)
     {
         if (m_style.pretty)
@@ -6480,8 +6888,10 @@ class basic_json_view
         style.indent_char = indent_char;
         style.ensure_ascii = ensure_ascii;
         style.source_numbers = numbers == number_format::source;
-        // the compact text is about as long as the source text of the value
-        const std::size_t estimate = source_extent() + (style.pretty ? source_extent() / 2 : 0) + 64;
+        // the compact text is about as long as the source text of the value;
+        // the compact writer keeps 64 bytes of slack, so that it does not grow
+        // the buffer just before the end
+        const std::size_t estimate = source_extent() + (style.pretty ? source_extent() / 2 : 0) + 160;
         detail::view::view_serializer<BasicJsonType, Editable>(*m_doc, out, estimate, style).dump(m_node);
         return out;
     }

tests/src/unit-json_view.cpp

@@ -1089,6 +1089,9 @@ TEST_CASE("json_view dump")
         CHECK(d.root().dump() == json::parse(text).dump());
         CHECK(d.root().dump() == "[1.5,100.0,0,-0.0,1.2345678901234568e+29,18446744073709551615,-9223372036854775808,0.1,1e-07,5e-324]");
         CHECK(d.root().dump(-1, ' ', false, json_view::number_format::source) == "[1.50,1E2,-0,-0.0,123456789012345678901234567890,18446744073709551615,-9223372036854775808,0.1,1e-7,5e-324]");
+        // also indented, and with ensure_ascii
+        CHECK(d.root().dump(0, ' ', false, json_view::number_format::source) == "[\n1.50,\n1E2,\n-0,\n-0.0,\n123456789012345678901234567890,\n18446744073709551615,\n-9223372036854775808,\n0.1,\n1e-7,\n5e-324\n]");
+        CHECK(d.root().dump(-1, ' ', true, json_view::number_format::source) == "[1.50,1E2,-0,-0.0,123456789012345678901234567890,18446744073709551615,-9223372036854775808,0.1,1e-7,5e-324]");
 
         // random doubles, written as parse() and dump() would
         std::mt19937_64 rng(1170); // NOLINT(cert-msc32-c,cert-msc51-cpp,bugprone-random-generator-seed)