jimzah/zig
1
0
Fork 0
mirror of https://github.com/ziglang/zig.git synced 2024-12-03 18:38:45 +00:00
Code Issues Actions 1 Packages Projects Releases Wiki Activity

factor out and expose biased range limiting function

Browse Source
This commit is contained in:
Josh Wolfe 2018-11-21 19:46:42 -05:00
parent eed7b48fe3
commit 9ae5200bd2
1 changed files with 19 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

30
std/rand/index.zig
View File

@ -63,17 +63,11 @@ pub const Random = struct {
comptime assert(T.is_signed == false);
comptime assert(T.bit_count <= 64); // TODO: workaround: LLVM ERROR: Unsupported library call operation!
assert(0 < less_than);
// Small is typically u32
const Small = @IntType(false, @divTrunc(T.bit_count + 31, 32) * 32);
// Large is typically u64
const Large = @IntType(false, Small.bit_count * 2);
// adapted from:
// http://www.pcg-random.org/posts/bounded-rands.html
// "Integer Multiplication (Biased)"
var x: Small = r.int(Small);
var m: Large = Large(x) * Large(less_than);
return @intCast(T, m >> Small.bit_count);
if (T.bit_count <= 32) {
return @intCast(T, limitRangeBiased(u32, r.int(u32), less_than));
} else {
return @intCast(T, limitRangeBiased(u64, r.int(u64), less_than));
}
}
/// Returns an evenly distributed random unsigned integer `0 <= i < less_than`.
/// This function assumes that the underlying ::fillFn produces evenly distributed values.
@ -276,6 +270,20 @@ pub const Random = struct {
}
};
/// Convert a random integer 0 <= random_int <= maxValue(T),
/// into an integer 0 <= result < less_than.
/// This function introduces a minor bias.
pub fn limitRangeBiased(comptime T: type, random_int: T, less_than: T) T {
comptime assert(T.is_signed == false);
const T2 = @IntType(false, T.bit_count * 2);
// adapted from:
// http://www.pcg-random.org/posts/bounded-rands.html
// "Integer Multiplication (Biased)"
var m: T2 = T2(random_int) * T2(less_than);
return @intCast(T, m >> T.bit_count);
}
const SequentialPrng = struct {
const Self = @This();
random: Random,