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[EDIT] FILE: _http_writer.pyx

from cpython.bytes cimport PyBytes_FromStringAndSize
from cpython.exc cimport PyErr_NoMemory
from cpython.mem cimport PyMem_Free, PyMem_Malloc, PyMem_Realloc
from cpython.object cimport PyObject_Str
from libc.stdint cimport uint8_t, uint64_t
from libc.string cimport memcpy

from multidict import istr

DEF BUF_SIZE = 16 * 1024  # 16KiB
cdef char BUFFER[BUF_SIZE]

cdef object _istr = istr

# ----------------- writer ---------------------------

cdef struct Writer:
    char *buf
    Py_ssize_t size
    Py_ssize_t pos

cdef inline void _init_writer(Writer* writer):
    writer.buf = &BUFFER[0]
    writer.size = BUF_SIZE
    writer.pos = 0

cdef inline void _release_writer(Writer* writer):
    if writer.buf != BUFFER:
        PyMem_Free(writer.buf)

cdef inline int _write_byte(Writer* writer, uint8_t ch):
    cdef char * buf
    cdef Py_ssize_t size

if writer.pos == writer.size:
        # reallocate
        size = writer.size + BUF_SIZE
        if writer.buf == BUFFER:
            buf = <char*>PyMem_Malloc(size)
            if buf == NULL:
                PyErr_NoMemory()
                return -1
            memcpy(buf, writer.buf, writer.size)
        else:
            buf = <char*>PyMem_Realloc(writer.buf, size)
            if buf == NULL:
                PyErr_NoMemory()
                return -1
        writer.buf = buf
        writer.size = size
    writer.buf[writer.pos] = <char>ch
    writer.pos += 1
    return 0

cdef inline int _write_utf8(Writer* writer, Py_UCS4 symbol):
    cdef uint64_t utf = <uint64_t> symbol

if utf < 0x80:
        return _write_byte(writer, <uint8_t>utf)
    elif utf < 0x800:
        if _write_byte(writer, <uint8_t>(0xc0 | (utf >> 6))) < 0:
            return -1
        return _write_byte(writer,  <uint8_t>(0x80 | (utf & 0x3f)))
    elif 0xD800 <= utf <= 0xDFFF:
        # surogate pair, ignored
        return 0
    elif utf < 0x10000:
        if _write_byte(writer, <uint8_t>(0xe0 | (utf >> 12))) < 0:
            return -1
        if _write_byte(writer, <uint8_t>(0x80 | ((utf >> 6) & 0x3f))) < 0:
            return -1
        return _write_byte(writer, <uint8_t>(0x80 | (utf & 0x3f)))
    elif utf > 0x10FFFF:
        # symbol is too large
        return 0
    else:
        if _write_byte(writer,  <uint8_t>(0xf0 | (utf >> 18))) < 0:
            return -1
        if _write_byte(writer,
                       <uint8_t>(0x80 | ((utf >> 12) & 0x3f))) < 0:
           return -1
        if _write_byte(writer,
                       <uint8_t>(0x80 | ((utf >> 6) & 0x3f))) < 0:
            return -1
        return _write_byte(writer, <uint8_t>(0x80 | (utf & 0x3f)))

cdef inline int _write_str(Writer* writer, str s):
    cdef Py_UCS4 ch
    for ch in s:
        if _write_utf8(writer, ch) < 0:
            return -1

# --------------- _serialize_headers ----------------------

cdef str to_str(object s):
    typ = type(s)
    if typ is str:
        return <str>s
    elif typ is _istr:
        return PyObject_Str(s)
    elif not isinstance(s, str):
        raise TypeError("Cannot serialize non-str key {!r}".format(s))
    else:
        return str(s)

cdef void _safe_header(str string) except *:
    if "\r" in string or "\n" in string:
        raise ValueError(
            "Newline or carriage return character detected in HTTP status message or "
            "header. This is a potential security issue."
        )

def _serialize_headers(str status_line, headers):
    cdef Writer writer
    cdef object key
    cdef object val
    cdef bytes ret

_init_writer(&writer)

for key, val in headers.items():
        _safe_header(to_str(key))
        _safe_header(to_str(val))

try:
        if _write_str(&writer, status_line) < 0:
            raise
        if _write_byte(&writer, b'\r') < 0:
            raise
        if _write_byte(&writer, b'\n') < 0:
            raise

for key, val in headers.items():
            if _write_str(&writer, to_str(key)) < 0:
                raise
            if _write_byte(&writer, b':') < 0:
                raise
            if _write_byte(&writer, b' ') < 0:
                raise
            if _write_str(&writer, to_str(val)) < 0:
                raise
            if _write_byte(&writer, b'\r') < 0:
                raise
            if _write_byte(&writer, b'\n') < 0:
                raise

if _write_byte(&writer, b'\r') < 0:
            raise
        if _write_byte(&writer, b'\n') < 0:
            raise

return PyBytes_FromStringAndSize(writer.buf, writer.pos)
    finally:
        _release_writer(&writer)

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