mirror of
https://github.com/wassname/segpy.git
synced 2026-06-27 20:06:49 +08:00
Robert Smallshire
92 lines
2.6 KiB
Python
92 lines
2.6 KiB
Python
from __future__ import print_function
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import sys
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from math import frexp, isnan, isinf
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from portability import long_int, byte_string, four_bytes
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_IBM_FLOAT32_BITS_PRECISION = 24
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_L24 = long_int(2) ** _IBM_FLOAT32_BITS_PRECISION
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_F24 = float(pow(2, _IBM_FLOAT32_BITS_PRECISION))
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def ibm2ieee(big_endian_bytes):
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"""Interpret a byte string as a big-endian IBM float.
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Args:
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big_endian_bytes (str): A string containing at least four bytes.
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Returns:
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The floating point value.
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"""
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a, b, c, d = four_bytes(big_endian_bytes)
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if a == b == c == c == 0:
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return 0.0
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sign = -1 if (a & 0x80) else 1
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exponent = a & 0x7f
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mantissa = ((b << 16) | (c << 8) | d) / _F24
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value = sign * mantissa * pow(16, exponent - 64)
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return value
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def ieee2ibm(f):
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"""Covert a float to four big-endian bytes representing an IBM float.
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Args:
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f (float): The value to be converted.
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Returns:
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A bytes object (Python 3) or a string (Python 2) containing four
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bytes representing a big-endian IBM float.
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"""
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if f == 0:
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# There are many potential representations of zero - this is the standard one
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return b'\x00\x00\x00\x00'
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if isnan(f):
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raise ValueError("NaN cannot be represented in IBM floating point")
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if isinf(f):
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raise ValueError("Infinities cannot be represented in IBM floating point")
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# Now compute m and e to satisfy:
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#
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# f = m * 2^e
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#
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# where 0.5 <= abs(m) < 1
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# except when f == 0 in which case m == 0 and e == 0, which we've already
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# dealt with.
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m, e = frexp(f)
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# Convert the fraction (m) into an integer representation. IEEE float32
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# numbers have 23 explicit (24 implicit) bits of precision.
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mantissa = abs(long_int(m * _L24))
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exponent = e
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sign = 0x80 if f < 0 else 0x00
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# IBM single precision floats are of the form
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# (-1)^sign * 0.significand * 16^(exponent-64)
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# Adjust the exponent, and the mantissa in sympathy so it is
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# a multiple of four, so it can be expressed in base 16
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remainder = exponent % 4
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if remainder != 0:
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shift = 4 - remainder
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mantissa >>= shift
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exponent += shift
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exponent_16 = exponent >> 2 # Divide by four to convert to base 16
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exponent_16_biased = exponent_16 + 64 # Add the exponent bias of 64
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# TODO: I'm not entirely sure we're producing properly normalised representations.
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a = sign | exponent_16_biased
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b = (mantissa >> 16) & 0xff
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c = (mantissa >> 8) & 0xff
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d = mantissa & 0xff
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return byte_string((a, b, c, d))
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