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
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191

use std::fmt;
#[macro_use]
extern crate lazy_static;

/// A probability that can be expressed in a user-friendly form.
/// The `friendly_string` property is a textual representation of
/// the probability (i.e. "5 out of 7")
#[derive(PartialEq, Eq, Debug)]
pub struct FriendlyProbability {
    numerator: u8,
    denominator: u8,
    friendly_string: String
}

impl fmt::Display for FriendlyProbability {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}", self.friendly_string)
    }
}
impl FriendlyProbability {
    // http://xion.io/post/code/rust-optional-args.html
    /// Create a new FriendlyProbability.  Most of the time you will want to use `from_probability()` instead.
    pub fn new<T: Into<Option<String>>>(numerator: u8, denominator: u8, friendly_string: T) -> FriendlyProbability {
        let real_friendly_string = friendly_string.into().unwrap_or_else(|| format!("{} in {}", numerator, denominator));
        FriendlyProbability {
            numerator,
            denominator,
            friendly_string: real_friendly_string
        }
    }
    /// Gets the numerator of the FriendlyProbability.
    pub fn numerator(self: &FriendlyProbability) -> u8 {
        self.numerator
    }
    /// Gets the denominator of the FriendlyProbability.
    pub fn denominator(self: &FriendlyProbability) -> u8 {
        self.denominator
    }
    /// Gets the friendly string of the FriendlyProbability.
    /// Usually this is the same as "{numerator} in {denominator}",
    /// but if the probability is very small it will instead be "<1 in 100",
    /// and if it's very large it will be ">99 in 100"
    pub fn friendly_string(self: &FriendlyProbability) -> &str {
        &self.friendly_string
    }
    /// Create a FriendlyProbability from an f32.
    /// # Examples
    /// 
    /// ```
    /// use probability_to_friendly_string::FriendlyProbability;
    /// 
    /// let friendly = FriendlyProbability::from_probability(0.723);
    /// assert_eq!(5, friendly.numerator());
    /// assert_eq!(7, friendly.denominator());
    /// assert_eq!("5 in 7", friendly.friendly_string());
    /// 
    /// let friendly = FriendlyProbability::from_probability(0.999);
    /// assert_eq!(">99 in 100", friendly.friendly_string());
    /// 
    /// let friendly = FriendlyProbability::from_probability(0.001);
    /// assert_eq!("<1 in 100", friendly.friendly_string());
    /// ```
    /// # Panics
    /// If probability is less than 0.0 or greater than 1.0.
    pub fn from_probability(probability: f32) -> FriendlyProbability {
        if probability < 0.0 || probability > 1.0 {
            panic!("probability is less than 0 or greater than 1!")
        }
        if probability == 0.0 {
            return FriendlyProbability::new(0, 1, None)
        }
        if probability == 1.0 {
            return FriendlyProbability::new(1, 1, None)
        }
        if probability > 0.99 {
            return FriendlyProbability::new(99, 100, String::from(">99 in 100"))
        }
        if probability < 0.01 {
            return FriendlyProbability::new(1, 100, String::from("<1 in 100"))
        }
        let data = &FRACTION_DATA;
        let fraction_to_compare = Fraction::new_for_comparison(probability);
        // use slice::binary_search_by since we can't derive Ord
        // because f32's are not orderable
        // https://stackoverflow.com/questions/28247990/how-to-do-a-binary-search-on-a-vec-of-floats
        let location = data.binary_search_by(|f| {
            f.partial_cmp(&fraction_to_compare).expect("Couldn't compare values?")
        });
        fn friendly_probability_from_fraction(fraction: &Fraction) -> FriendlyProbability {
            FriendlyProbability::new(fraction.numerator, fraction.denominator, None)
        }
        let data_len = data.len();
        match location {
            Ok(i) => friendly_probability_from_fraction(&data[i]),
            Err(i) => {
                // This means it could be inserted at index i
                if i == 0 {
                    return friendly_probability_from_fraction(&data[0]);
                }
                if i == data_len {
                    return friendly_probability_from_fraction(&data[data_len - 1]);
                }
                if probability - (&data[i - 1]).value < (&data[i]).value - probability {
                    return friendly_probability_from_fraction(&data[i - 1]);
                }
                else {
                    return friendly_probability_from_fraction(&data[i]);
                }
            }
        }
    }
}

#[derive(PartialOrd, PartialEq, Debug)]
struct Fraction {
    value: f32,
    numerator: u8,
    denominator: u8,
}
impl Fraction {
    pub fn new(numerator: u8, denominator: u8) -> Fraction {
        Fraction {
            numerator: numerator,
            denominator: denominator,
            value: numerator as f32/denominator as f32
        }
    }
    fn new_for_comparison(value: f32) -> Fraction {
        Fraction {
            numerator: 0,
            denominator: 0,
            value
        }
    }
}

lazy_static! {
    static ref FRACTION_DATA: Vec<Fraction> = {
        let mut fractions : Vec<Fraction> = Vec::new();
        fn gcd(x: u8, y: u8) -> u8 {
            let mut x = x;
            let mut y = y;
            while y != 0 {
                let t = y;
                y = x % y;
                x = t;
            }
            x
        }
        for d in 2..11 {
            for n in 1..d {
                if gcd(n, d) == 1 {
                    fractions.push(Fraction::new(n, d));
                }
            }
        }
        for &d in [12, 15, 20, 30, 40, 50, 60, 80, 100].iter() {
            fractions.push(Fraction::new(1, d));
            fractions.push(Fraction::new(d - 1, d));
        }
        fractions.sort_unstable_by(|a,b| a.partial_cmp(b).unwrap());
        fractions
    };
}

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

    #[test]
    fn friendly_probability_string_matches_numeric_inputs() {
        let fp = FriendlyProbability::new(1, 2, None);
        assert_eq!("1 in 2", fp.friendly_string);
    }
    #[test]
    fn friendly_probability_string_matches_string_input() {
        let s = String::from("something weird");
        let fp = FriendlyProbability::new(1, 2, s.clone());
        assert_eq!(s, fp.friendly_string);
    }
    #[test]
    #[should_panic]
    fn friendly_probability_from_fraction_less_than_0() {
        FriendlyProbability::from_probability(-0.01);
    }
    #[test]
    #[should_panic]
    fn friendly_probability_from_fraction_greater_than_1() {
        FriendlyProbability::from_probability(1.01);
    }
}