Function weightedIndex [src]

Randomly selects an index into proportions, where the likelihood of each index is weighted by that proportion. It is more likely for the index of the last proportion to be returned than the index of the first proportion in the slice, and vice versa. This is useful for selecting an item from a slice where weights are not equal. T must be a numeric type capable of holding the sum of proportions.

Prototype

pub fn weightedIndex(r: Random, comptime T: type, proportions: []const T) usize

Parameters

r: RandomT: typeproportions: []const T

Source

  pub fn weightedIndex(r: Random, comptime T: type, proportions: []const T) usize {
    // This implementation works by summing the proportions and picking a
    // random point in [0, sum).  We then loop over the proportions,
    // accumulating until our accumulator is greater than the random point.

    const sum = s: {
        var sum: T = 0;
        for (proportions) |v| sum += v;
        break :s sum;
    };

    const point = switch (@typeInfo(T)) {
        .int => |int_info| switch (int_info.signedness) {
            .signed => r.intRangeLessThan(T, 0, sum),
            .unsigned => r.uintLessThan(T, sum),
        },
        // take care that imprecision doesn't lead to a value slightly greater than sum
        .float => @min(r.float(T) * sum, sum - std.math.floatEps(T)),
        else => @compileError("weightedIndex does not support proportions of type " ++
            @typeName(T)),
    };

    assert(point < sum);

    var accumulator: T = 0;
    for (proportions, 0..) |p, index| {
        accumulator += p;
        if (point < accumulator) return index;
    } else unreachable;
}  