69 lines
1.5 KiB
Plaintext
69 lines
1.5 KiB
Plaintext
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= Bithacks =
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Evil memory manipulation for speed
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== Bitwise_cast ==
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This works for C/C++. Basically all you do it cast like this
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{{{C
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float cast(int foo){
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return (* (float*)&foo);
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}
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}}}
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This is also known as the "evil floating point bit level hacking" from the
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quake III fast inverse square root. Great for casting structs as memory for
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hashing, or generally treating one set of bits as another type
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== Negating Floats ==
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{{{
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float Negatefloat(float f){
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return cast_float_to_uint(f) ^ 0x80000000;
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}
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}}}
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== Fast log base 2 ==
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IEE 754 is sci notification of a base 2 value, therefore this works very very
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well. Breaks awfully with negtaives so be sure to check
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{{{
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const unsigned int OneAsInt = 0x3F800000;
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const float ScaleUp = float(0x00800000); //2^23
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const float ScaleDown = 1.0f/ScaleUp;
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float Log2(float x){
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return float (cast_float_to_uint(f)-OneAsInt)*ScaleDown
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}
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}}}
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== Fast base 2 exponent ==
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This is the opposite of the last one. Does 2^n fast
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{{{
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const unsigned int OneAsInt = 0x3F800000;
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const float ScaleUp = float(0x00800000); //2^23
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const float ScaleDown = 1.0f/ScaleUp;
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float Log2(float x){
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return asfloat (cast_float_to_uint(f)*ScaleUp)+OneAsInt;
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}
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}}}
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== Not accurate square root ==
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This doesnt do that well but its better really fast. Does a fast taking of the
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float to the power of 0.5
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{{{
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const unsigned int OneAsInt = 0x3F800000;
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float sqrt(float x){
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return asfloat((asint(x)>>1)+(OneAsInt>>1));
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}
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}}}
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[[index]]
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