//-
// Copyright 2017 Jason Lingle
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! Strategies for generating `char` values.
//!
//! Unlike most strategies in Proptest, character generation is by default
//! biased to particular values known to be difficult to handle in various
//! circumstances.
//!
//! The main things of interest are `any()` to generate truly arbitrary
//! characters, and `range()` and `ranges()` to select characters from
//! inclusive ranges.

use core::ops::RangeInclusive;
use crate::std_facade::Cow;

use rand::Rng;

use crate::num;
use crate::strategy::*;
use crate::test_runner::*;

/// An inclusive char range from fst to snd.
type CharRange = RangeInclusive<char>;

/// A default set of characters to consider as "special" during character
/// generation.
///
/// Most of the characters here were chosen specifically because they are
/// difficult to handle in particular contexts.
pub const DEFAULT_SPECIAL_CHARS: &[char] = &[
    // Things to give shell scripts and filesystem logic difficulties
    '/', '\\', '$', '.', '*', '{', '\'', '"', '`', ':',
    // Characters with special significance in URLs and elsewhere
    '?', '%', '=', '&', '<',
    // Interesting ASCII control characters
    // NUL, HT,   CR,   LF,   VT      ESC     DEL
    '\x00', '\t', '\r', '\n', '\x0B', '\x1B', '\x7F',
    // ¥ both to test simple Unicode handling and because it has interesting
    // properties on MS Shift-JIS systems.
    '¥',
    // No non-Unicode encoding has both ¥ and Ѩ
    'Ѩ',
    // In UTF-8, Ⱥ increases in length from 2 to 3 bytes when lowercased
    'Ⱥ',
    // More Unicode edge-cases: BOM, replacement character, RTL override, and non-BMP
    '\u{FEFF}', '\u{FFFD}', '\u{202E}', '🕴',
];

/// A default sequence of ranges used preferentially when generating random
/// characters.
pub const DEFAULT_PREFERRED_RANGES: &[CharRange] = &[
    // ASCII printable
    ' '..='~', ' '..='~', ' '..='~', ' '..='~', ' '..='~',
    // Latin-1
    '\u{0040}'..='\u{00ff}',
];

/// Selects a random character the way `CharStrategy` does.
///
/// If `special` is non-empty, there is a 50% chance that a character from this
/// array is chosen randomly, and will be returned if that character falls
/// within `ranges`.
///
/// If `preferred` is non-empty, there is a 50% chance that any generation
/// which gets past the `special` step picks a random element from this list,
/// then a random character from within that range (both endpoints inclusive).
/// That character will be returned if it falls within `ranges`.
///
/// In all other cases, an element is picked randomly from `ranges` and a
/// random character within the range (both endpoints inclusive) is chosen and
/// returned.
///
/// Notice that in all cases, `ranges` completely defines the set of characters
/// that can possibly be defined.
///
/// It is legal for ranges in all cases to contain non-characters.
///
/// Both `preferred` and `ranges` bias selection towards characters in smaller
/// ranges. This is deliberate. `preferred` is usually tuned to select
/// particular characters anyway. `ranges` is usually derived from some
/// external property, and the fact that a range is small often means it is
/// more interesting.
pub fn select_char(rnd: &mut impl Rng,
                   special: &[char],
                   preferred: &[CharRange],
                   ranges: &[CharRange]) -> char {
    let (base, offset) = select_range_index(rnd, special, preferred, ranges);
    ::core::char::from_u32(base + offset).expect("bad character selected")
}

fn select_range_index(rnd: &mut impl Rng,
                      special: &[char],
                      preferred: &[CharRange],
                      ranges: &[CharRange])
                      -> (u32, u32) {
    fn in_range(ranges: &[CharRange], ch: char) -> Option<(u32, u32)> {
        ranges.iter().find(|r| ch >= *r.start() && ch <= *r.end()).map(
            |r| (*r.start() as u32, ch as u32 - *r.start() as u32))
    }

    if !special.is_empty() && rnd.gen() {
        let s = special[rnd.gen_range(0, special.len())];
        if let Some(ret) = in_range(ranges, s) { return ret; }
    }

    if !preferred.is_empty() && rnd.gen() {
        let range = preferred[rnd.gen_range(0, preferred.len())].clone();
        if let Some(ch) = ::core::char::from_u32(
            rnd.gen_range(*range.start() as u32, *range.end() as u32 + 1))
        {
            if let Some(ret) = in_range(ranges, ch) { return ret; }
        }
    }

    for _ in 0..65_536 {
        let range = ranges[rnd.gen_range(0, ranges.len())].clone();
        if let Some(ch) = ::core::char::from_u32(
            rnd.gen_range(*range.start() as u32, *range.end() as u32 + 1))
        { return (*range.start() as u32, ch as u32 - *range.start() as u32); }
    }

    // Give up and return a character we at least know is valid.
    (*ranges[0].start() as u32, 0)
}

/// Strategy for generating `char`s.
///
/// Character selection is more sophisticated than integer selection. Naïve
/// selection (particularly in the larger context of generating strings) would
/// result in starting inputs like `ꂡ螧轎ቶᢹ糦狥芹ᘆ㶏曊ᒀ踔虙ჲ` and "simplified"
/// inputs consisting mostly of control characters. It also has difficulty
/// locating edge cases, since the vast majority of code points (such as the
/// enormous CJK regions) don't cause problems for anything with even basic
/// Unicode support.
///
/// Instead, character selection is always based on explicit ranges, and is
/// designed to bias to specifically chosen characters and character ranges to
/// produce inputs that are both more useful and easier for humans to
/// understand. There are also hard-wired simplification targets based on ASCII
/// instead of simply simplifying towards NUL to avoid problematic inputs being
/// reduced to a bunch of NUL characters.
///
/// Shrinking never crosses ranges. If you have a complex range like `[A-Za-z]`
/// and the starting point `x` is chosen, it will not shrink to the first `A-Z`
/// group, but rather simply to `a`.
///
/// The usual way to get instances of this class is with the module-level `ANY`
/// constant or `range` function. Directly constructing a `CharStrategy` is
/// only necessary for complex ranges or to override the default biases.
#[derive(Debug, Clone)]
#[must_use = "strategies do nothing unless used"]
pub struct CharStrategy<'a> {
    special: Cow<'a, [char]>,
    preferred: Cow<'a, [CharRange]>,
    ranges: Cow<'a, [CharRange]>,
}

impl<'a> CharStrategy<'a> {
    /// Construct a new `CharStrategy` with the parameters it will pass to the
    /// function underlying `select_char()`.
    ///
    /// All arguments as per `select_char()`.
    pub fn new(special: Cow<'a, [char]>,
               preferred: Cow<'a, [CharRange]>,
               ranges: Cow<'a, [CharRange]>) -> Self {
        CharStrategy { special, preferred, ranges }
    }

    /// Same as `CharStrategy::new()` but using `Cow::Borrowed` for all parts.
    pub fn new_borrowed(special: &'a [char],
                        preferred: &'a [CharRange],
                        ranges: &'a [CharRange]) -> Self {
        CharStrategy::new(
            Cow::Borrowed(special),
            Cow::Borrowed(preferred),
            Cow::Borrowed(ranges),
        )
    }
}

const WHOLE_RANGE: &[CharRange] = &[
    '\x00' ..= ::core::char::MAX
];

/// Creates a `CharStrategy` which picks from literally any character, with the
/// default biases.
pub fn any() -> CharStrategy<'static> {
    CharStrategy {
        special: Cow::Borrowed(DEFAULT_SPECIAL_CHARS),
        preferred: Cow::Borrowed(DEFAULT_PREFERRED_RANGES),
        ranges: Cow::Borrowed(WHOLE_RANGE),
    }
}

/// Creates a `CharStrategy` which selects characters within the given
/// endpoints, inclusive, using the default biases.
pub fn range(start: char, end: char) -> CharStrategy<'static> {
    CharStrategy {
        special: Cow::Borrowed(DEFAULT_SPECIAL_CHARS),
        preferred: Cow::Borrowed(DEFAULT_PREFERRED_RANGES),
        ranges: Cow::Owned(vec![start..=end]),
    }
}

/// Creates a `CharStrategy` which selects characters within the given ranges,
/// all inclusive, using the default biases.
pub fn ranges(ranges: Cow<[CharRange]>) -> CharStrategy {
    CharStrategy {
        special: Cow::Borrowed(DEFAULT_SPECIAL_CHARS),
        preferred: Cow::Borrowed(DEFAULT_PREFERRED_RANGES),
        ranges,
    }
}

/// The `ValueTree` corresponding to `CharStrategy`.
#[derive(Debug, Clone, Copy)]
pub struct CharValueTree {
    value: num::u32::BinarySearch,
}

impl<'a> Strategy for CharStrategy<'a> {
    type Tree = CharValueTree;
    type Value = char;

    fn new_tree(&self, runner: &mut TestRunner) -> NewTree<Self> {
        let (base, offset) = select_range_index(
            runner.rng(), &self.special, &self.preferred, &self.ranges);

        // Select a minimum point more convenient than 0
        let start = base + offset;
        let bottom = if start >= '¡' as u32 && base < '¡' as u32 {
            '¡' as u32
        } else if start >= 'a' as u32 && base < 'a' as u32 {
            'a' as u32
        } else if start >= 'A' as u32 && base < 'A' as u32 {
            'A' as u32
        } else if start >= '0' as u32 && base < '0' as u32 {
            '0' as u32
        } else if start >= ' ' as u32 && base < ' ' as u32 {
            ' ' as u32
        } else {
            base
        };

        Ok(CharValueTree {
            value: num::u32::BinarySearch::new_above(bottom, start)
        })
    }
}

impl CharValueTree {
    fn reposition(&mut self) {
        while ::core::char::from_u32(self.value.current()).is_none() {
            if !self.value.complicate() {
                panic!("Converged to non-char value");
            }
        }
    }
}

impl ValueTree for CharValueTree {
    type Value = char;

    fn current(&self) -> char {
        ::core::char::from_u32(self.value.current()).expect(
            "Generated non-char value")
    }

    fn simplify(&mut self) -> bool {
        if self.value.simplify() {
            self.reposition();
            true
        } else {
            false
        }
    }

    fn complicate(&mut self) -> bool {
        if self.value.complicate() {
            self.reposition();
            true
        } else {
            false
        }
    }
}

#[cfg(test)]
mod test {
    use std::cmp::{min, max};
    use std::vec::Vec;

    use super::*;
    use crate::collection;

    proptest! {
        #[test]
        fn stays_in_range(input_ranges in collection::vec(
            (0..::std::char::MAX as u32,
             0..::std::char::MAX as u32),
            1..5))
        {
            let input = ranges(Cow::Owned(input_ranges.iter().map(
                |&(lo, hi)| ::std::char::from_u32(lo).and_then(
                    |lo| ::std::char::from_u32(hi).map(
                        |hi| min(lo, hi) ..= max(lo, hi)))
                    .ok_or_else(|| TestCaseError::reject("non-char")))
                                          .collect::<Result<Vec<CharRange>,_>>()?));

            let mut runner = TestRunner::default();
            for _ in 0..256 {
                let mut value = input.new_tree(&mut runner).unwrap();
                loop {
                    let ch = value.current() as u32;
                    assert!(input_ranges.iter().any(
                        |&(lo, hi)| ch >= min(lo, hi) &&
                            ch <= max(lo, hi)));

                    if !value.simplify() { break; }
                }
            }
        }
    }

    #[test]
    fn applies_desired_bias() {
        let mut men_in_business_suits_levitating = 0;
        let mut ascii_printable = 0;
        let mut runner = TestRunner::deterministic();

        for _ in 0..1024 {
            let ch = any().new_tree(&mut runner).unwrap().current();
            if '🕴' == ch {
                men_in_business_suits_levitating += 1;
            } else if ch >= ' ' && ch <= '~' {
                ascii_printable += 1;
            }
        }

        assert!(ascii_printable >= 256);
        assert!(men_in_business_suits_levitating >= 1);
    }

    #[test]
    fn doesnt_shrink_to_ascii_control() {
        let mut accepted = 0;
        let mut runner = TestRunner::deterministic();

        for _ in 0..256 {
            let mut value = any().new_tree(&mut runner).unwrap();

            if value.current() <= ' ' { continue; }

            while value.simplify() { }

            assert!(value.current() >= ' ');
            accepted += 1;
        }

        assert!(accepted >= 200);
    }

    #[test]
    fn test_sanity() {
        check_strategy_sanity(any(), Some(CheckStrategySanityOptions {
            // `simplify()` can itself `complicate()` back to the starting
            // position, so the overly strict complicate-after-simplify check
            // must be disabled.
            strict_complicate_after_simplify: false,
            .. CheckStrategySanityOptions::default()
        }));
    }
}