34 lines
941 B
Plaintext
34 lines
941 B
Plaintext
import "banks";
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// Random Number Library
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//
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// Uses a Galois Linear Feedback Shift Register to generate random numbers.
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// http://en.wikipedia.org/wiki/Linear_feedback_shift_register#Galois_LFSRs
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//
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// [random.lo] and [random.hi] must be defined in RAM somewhere.
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namespace random {
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in ram {
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var lo : u8;
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var hi : u8;
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}
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// The polynomial value is taken from here:
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// http://users.ece.cmu.edu/~koopman/lfsr/16.txt
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let LSFR_POLYNOMIAL = 0x845F;
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// Get a pseudorandom 16-bit number.
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// Precondition:
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// random.lo, random.hi are currently seeded to some non-zero value.
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// Return:
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// random.lo, random.hi = result
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func next() {
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random.hi = random.hi >>> 1;
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random.lo = random.lo >>>># 1;
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if carry {
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random.lo = a = random.lo ^ LSFR_POLYNOMIAL as u8;
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random.hi = a = random.hi ^ (LSFR_POLYNOMIAL >> 8) as u8;
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}
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}
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}
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