74 lines
2.4 KiB
Markdown
74 lines
2.4 KiB
Markdown
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The code below deals with expansion of single-precision IEEE 754 floating point values to
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double-precision (and vice-versa) without losing detail of NaN bit-patterns.
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```python
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def to_bytes(self):
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"""Converts this 32-bit single-precision floating point value to its binary32 format,
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taking care to preserve the quiet/signalling bit-pattern of NaN values, unlike its
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`struct.pack('>f', ...)` equivalent.
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```python
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>>> Float.from_bytes(b'\\x7f\\x80\\x00{')
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Float(nan)
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>>> Float.from_bytes(b'\\x7f\\x80\\x00{').to_bytes()
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b'\\x7f\\x80\\x00{'
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>>> struct.unpack('>f', b'\\x7f\\x80\\x00{')[0]
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nan
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>>> Float(struct.unpack('>f', b'\\x7f\\x80\\x00{')[0]).to_bytes()
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b'\\x7f\\xc0\\x00{'
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>>> struct.pack('>f', struct.unpack('>f', b'\\x7f\\x80\\x00{')[0])
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b'\\x7f\\xc0\\x00{'
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```
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(Note well the difference between `7f80007b` and `7fc0007b`!)
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"""
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if math.isnan(self.value) or math.isinf(self.value):
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dbs = struct.pack('>d', self.value)
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vd = struct.unpack('>Q', dbs)[0]
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sign = vd >> 63
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payload = (vd >> 29) & 0x007fffff
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vf = (sign << 31) | 0x7f800000 | payload
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return struct.pack('>I', vf)
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else:
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return struct.pack('>f', self.value)
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@staticmethod
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def from_bytes(bs):
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"""Converts a 4-byte-long byte string to a 32-bit single-precision floating point value
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wrapped in a [Float][preserves.values.Float] instance. Takes care to preserve the
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quiet/signalling bit-pattern of NaN values, unlike its `struct.unpack('>f', ...)`
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equivalent.
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```python
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>>> Float.from_bytes(b'\\x7f\\x80\\x00{')
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Float(nan)
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>>> Float.from_bytes(b'\\x7f\\x80\\x00{').to_bytes()
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b'\\x7f\\x80\\x00{'
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>>> struct.unpack('>f', b'\\x7f\\x80\\x00{')[0]
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nan
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>>> Float(struct.unpack('>f', b'\\x7f\\x80\\x00{')[0]).to_bytes()
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b'\\x7f\\xc0\\x00{'
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>>> struct.pack('>f', struct.unpack('>f', b'\\x7f\\x80\\x00{')[0])
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b'\\x7f\\xc0\\x00{'
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```
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(Note well the difference between `7f80007b` and `7fc0007b`!)
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"""
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vf = struct.unpack('>I', bs)[0]
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if (vf & 0x7f800000) == 0x7f800000:
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# NaN or inf. Preserve quiet/signalling bit by manually expanding to double-precision.
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sign = vf >> 31
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payload = vf & 0x007fffff
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dbs = struct.pack('>Q', (sign << 63) | 0x7ff0000000000000 | (payload << 29))
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return Float(struct.unpack('>d', dbs)[0])
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else:
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return Float(struct.unpack('>f', bs)[0])
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```
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