Restore prec_words_to_bits (renamed to prec_pari_to_bits) for backwar…

…ds compatibility

cypari2/pari_instance.pxd

@@ -4,6 +4,7 @@ cimport cython
 from .gen cimport Gen
 
 cpdef long prec_bits_to_pari(unsigned long prec_in_bits) noexcept
+cpdef long prec_pari_to_bits(long prec_pari) noexcept
 cpdef long default_bitprec() noexcept
 
 cdef class Pari_auto:

cypari2/pari_instance.pyx

@@ -366,6 +366,25 @@ cpdef long prec_bits_to_pari(unsigned long prec_in_bits) noexcept:
     return nbits2prec(prec_in_bits)
 
 
+cpdef long prec_pari_to_bits(long prec_pari) noexcept:
+    r"""
+    Convert from pari real precision to precision
+    expressed in bits. Note: this adjusts for the two codewords of a
+    pari real, and is architecture-dependent.
+    Examples:
+    >>> from cypari2.pari_instance import prec_words_to_bits
+    >>> import sys
+    >>> bitness = '64' if sys.maxsize > (1 << 32) else '32'
+    >>> prec_words_to_bits(10) == (256 if bitness == '32' else 512)
+    True
+    >>> ans32 = [(3, 32), (4, 64), (5, 96), (6, 128), (7, 160), (8, 192), (9, 224)]
+    >>> ans64 = [(3, 64), (4, 128), (5, 192), (6, 256), (7, 320), (8, 384), (9, 448)] # 64-bit
+    >>> [(n, prec_words_to_bits(n)) for n in range(3, 10)] == (ans32 if bitness == '32' else ans64)
+    True
+    """
+    return prec2nbits(prec_pari)
+
+
 cpdef long default_bitprec() noexcept:
     r"""
     Return the default precision in bits.