wager 0.1.1

Primitive types and functionality for betting odds
Documentation 
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131

// This file is a Rust port of the C code here:
// https://github.com/google/audio-to-tactile/blob/main/src/dsp/number_util.c:
//
// Copyright 2020 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

const DEFAULT_MAX_TERMS: i32 = 47;
const DEFAULT_CONVERGENCE_TOLERANCE: f64 = 1e-9;
const DEFAULT_MAX_DENOMINATOR: i32 = 100;

pub fn rational_approximation(value: f64) -> (i32, i32) {
    approximate_rational(value, DEFAULT_MAX_DENOMINATOR)
}

fn approximate_rational(value: f64, max_denominator: i32) -> (i32, i32) {
    if max_denominator <= 0 {
        return (0, 0);
    } else if value > f64::MAX - 0.5 {
        return (i32::MAX, 1);
    } else if value < f64::MIN + 0.5 {
        return (i32::MIN, 1);
    }

    let sign: i32 = if value < 0.0 { -1 } else { 1 };

    let value = value.abs();

    if !value.is_finite() {
        return (0, 0);
    }

    let mut reciprocal_residual = value;
    let mut continued_fraction_term = value.floor() as i32;
    let mut prev_convergent = (1, 0);
    let mut convergent = (continued_fraction_term, 1);

    let mut n = 0;

    for term in 2.. {
        let next_residual = reciprocal_residual - (continued_fraction_term as f64);
        if next_residual.abs() <= DEFAULT_CONVERGENCE_TOLERANCE {
            return (sign * convergent.0, convergent.1);
        }

        reciprocal_residual = 1.0 / next_residual;
        continued_fraction_term = reciprocal_residual.floor() as i32;

        n = (max_denominator - prev_convergent.1) / convergent.1;
        if convergent.0 > 0 {
            let upper_bound = (i32::MAX - prev_convergent.0) / convergent.0;
            if n > upper_bound {
                n = upper_bound;
            }
        }

        if term >= DEFAULT_MAX_TERMS || continued_fraction_term >= n {
            break;
        }

        let next_convergent =
            append_continued_fraction_term(convergent, prev_convergent, continued_fraction_term);
        prev_convergent = convergent;
        convergent = next_convergent;
    }

    let mut best_approximation = convergent;
    let lower_bound = continued_fraction_term / 2;

    if n >= lower_bound {
        if n > continued_fraction_term {
            n = continued_fraction_term;
        }

        let semiconvergent = append_continued_fraction_term(convergent, prev_convergent, n);

        if (n > lower_bound)
            || (value - fraction_to_double(semiconvergent).abs()
                < (value - fraction_to_double(convergent)).abs())
        {
            best_approximation = semiconvergent;
        }
    }

    (sign * best_approximation.0, best_approximation.1)
}

fn append_continued_fraction_term(
    fraction: (i32, i32),
    prev_convergent: (i32, i32),
    term: i32,
) -> (i32, i32) {
    (
        term * fraction.0 + prev_convergent.0,
        term * fraction.1 + prev_convergent.1,
    )
}

fn fraction_to_double(fraction: (i32, i32)) -> f64 {
    (fraction.0 as f64) / (fraction.1 as f64)
}

#[cfg(test)]
mod tests {
    use super::*;
    use test_case::test_case;

    #[test_case(0.0, (0, 1))]
    #[test_case(1.0, (1, 1))]
    #[test_case(0.5, (1, 2))]
    #[test_case(0.3333333333333333, (1, 3))]
    #[test_case(0.14285714285714285, (1, 7))]
    #[test_case(0.125, (1, 8))]
    #[test_case(0.1111111111111111, (1, 9))]
    #[test_case(0.1, (1, 10))]
    #[test_case(0.01123595506, (1, 89))]
    #[test_case(0.5280898876, (47, 89))]
    fn test_rational_approximation(value: f64, expected: (i32, i32)) {
        assert_eq!(rational_approximation(value), expected);
    }
}