/// [`Strategy`] generators for property tests.
///
/// most importantly, this provides a [`TestInput`] type and an accompanying [`input_strategy()`]
/// that can be used to generate inputs for a property test.
///
/// use [`values_that_need_truncation()`] and [`values_that_fit()`] for values that do or do not
/// need to be limited, respectively.
use {
    proptest::{prop_compose, strategy::Strategy},
    std::ops::Not,
};

/// an input to a property test.
#[derive(Debug)]
pub struct TestInput {
    // the value whose contents we will limit.
    pub value: String,
    // the length we will limit to.
    pub length: usize,
}

prop_compose! {
    /// a strategy that generates [`TestInput`] values.
    pub fn input_strategy()
    (
        value in value_strategy(),
        length in length_strategy()
    ) -> TestInput {
        TestInput { value, length }
    }
}

fn length_strategy() -> impl Strategy<Value = usize> {
    4..2048_usize
}

/// returns a [`Strategy`] for strings of lowercase alphabetic characters.
///
/// NB: this string may be empty.
pub fn value_strategy() -> impl Strategy<Value = String> {
    "[a-z]{0,1024}"
}

/// returns a [`Strategy`] for *non-empty* string of lowercase alphabetic characters.
#[allow(dead_code)] // this is used by property tests outside of this file.
pub fn value_strategy_non_empty() -> impl Strategy<Value = String> {
    const REASON: &'static str = "value should be non-empty";
    let is_not_empty = |s: &String| s.is_empty().not();
    value_strategy().prop_filter(REASON, is_not_empty)
}

/// returns a [`Strategy`] that creates values that fit into the target width.
///
/// inputs generated by this function will always have a value that fits in the target length.
#[allow(dead_code)] // this is used by property tests outside of this file.
pub fn values_that_fit() -> impl Strategy<Value = TestInput> {
    const REASON: &'static str = "test input should be shorter than its target length";
    let fits = |input: &TestInput| match input {
        TestInput { value, length } if value.len() <= *length => true,
        _ => false,
    };

    input_strategy().prop_filter(REASON, fits)
}

/// returns a [`Strategy`] that creates values that will need to be truncated.
///
/// inputs generated by this function will always have a value longer than the target length.
pub fn values_that_need_truncation() -> impl Strategy<Value = TestInput> {
    const REASON: &'static str = "test input should be longer than its target length";
    let needs_truncation = |input: &TestInput| match input {
        TestInput { value, length } if value.len() > *length => true,
        _ => false,
    };

    input_strategy().prop_filter(REASON, needs_truncation)
}