//  * This file is part of the uutils coreutils package.
//  *
//  * (c) Michael Debertol <michael.debertol..AT..gmail.com>
//  *
//  * For the full copyright and license information, please view the LICENSE
//  * file that was distributed with this source code.

//! Fast comparison for strings representing a base 10 number without precision loss.
//!
//! To be able to short-circuit when comparing, [NumInfo] must be passed along with each number
//! to [numeric_str_cmp]. [NumInfo] is generally obtained by calling [NumInfo::parse] and should be cached.
//! It is allowed to arbitrarily modify the exponent afterwards, which is equivalent to shifting the decimal point.
//!
//! More specifically, exponent can be understood so that the original number is in (1..10)*10^exponent.
//! From that follows the constraints of this algorithm: It is able to compare numbers in ±(1*10^[i64::MIN]..10*10^[i64::MAX]).

use std::{cmp::Ordering, ops::Range};

#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Clone)]
enum Sign {
    Negative,
    Positive,
}

#[derive(Debug, PartialEq, Clone)]
pub struct NumInfo {
    exponent: i64,
    sign: Sign,
}
#[derive(Debug, PartialEq, Clone)]
pub struct NumInfoParseSettings {
    pub accept_si_units: bool,
    pub thousands_separator: Option<char>,
    pub decimal_pt: Option<char>,
}

impl Default for NumInfoParseSettings {
    fn default() -> Self {
        Self {
            accept_si_units: false,
            thousands_separator: None,
            decimal_pt: Some('.'),
        }
    }
}

impl NumInfo {
    /// Parse NumInfo for this number.
    /// Also returns the range of num that should be passed to numeric_str_cmp later
    pub fn parse(num: &str, parse_settings: NumInfoParseSettings) -> (Self, Range<usize>) {
        let mut exponent = -1;
        let mut had_decimal_pt = false;
        let mut had_digit = false;
        let mut start = None;
        let mut sign = Sign::Positive;

        let mut first_char = true;

        for (idx, char) in num.char_indices() {
            if first_char && char.is_whitespace() {
                continue;
            }

            if first_char && char == '-' {
                sign = Sign::Negative;
                first_char = false;
                continue;
            }
            first_char = false;

            if parse_settings
                .thousands_separator
                .map_or(false, |c| c == char)
            {
                continue;
            }

            if Self::is_invalid_char(char, &mut had_decimal_pt, &parse_settings) {
                let si_unit = if parse_settings.accept_si_units {
                    match char {
                        'K' | 'k' => 3,
                        'M' => 6,
                        'G' => 9,
                        'T' => 12,
                        'P' => 15,
                        'E' => 18,
                        'Z' => 21,
                        'Y' => 24,
                        _ => 0,
                    }
                } else {
                    0
                };
                return if let Some(start) = start {
                    (
                        NumInfo {
                            exponent: exponent + si_unit,
                            sign,
                        },
                        start..idx,
                    )
                } else {
                    (
                        NumInfo {
                            sign: if had_digit { sign } else { Sign::Positive },
                            exponent: 0,
                        },
                        0..0,
                    )
                };
            }
            if Some(char) == parse_settings.decimal_pt {
                continue;
            }
            had_digit = true;
            if start.is_none() && char == '0' {
                if had_decimal_pt {
                    // We're parsing a number whose first nonzero digit is after the decimal point.
                    exponent -= 1;
                } else {
                    // Skip leading zeroes
                    continue;
                }
            }
            if !had_decimal_pt {
                exponent += 1;
            }
            if start.is_none() && char != '0' {
                start = Some(idx);
            }
        }
        if let Some(start) = start {
            (NumInfo { exponent, sign }, start..num.len())
        } else {
            (
                NumInfo {
                    sign: if had_digit { sign } else { Sign::Positive },
                    exponent: 0,
                },
                if had_digit {
                    // In this case there were only zeroes.
                    // For debug output to work properly, we have to claim to match the end of the number.
                    num.len()..num.len()
                } else {
                    // This was no number at all.
                    // For debug output to work properly, we have to claim to match the start of the number.
                    0..0
                },
            )
        }
    }

    fn is_invalid_char(
        c: char,
        had_decimal_pt: &mut bool,
        parse_settings: &NumInfoParseSettings,
    ) -> bool {
        if Some(c) == parse_settings.decimal_pt {
            if *had_decimal_pt {
                // this is a decimal pt but we already had one, so it is invalid
                true
            } else {
                *had_decimal_pt = true;
                false
            }
        } else {
            !c.is_ascii_digit()
        }
    }
}

/// compare two numbers as strings without parsing them as a number first. This should be more performant and can handle numbers more precisely.
/// NumInfo is needed to provide a fast path for most numbers.
pub fn numeric_str_cmp((a, a_info): (&str, &NumInfo), (b, b_info): (&str, &NumInfo)) -> Ordering {
    // check for a difference in the sign
    if a_info.sign != b_info.sign {
        return a_info.sign.cmp(&b_info.sign);
    }

    // check for a difference in the exponent
    let ordering = if a_info.exponent != b_info.exponent && !a.is_empty() && !b.is_empty() {
        a_info.exponent.cmp(&b_info.exponent)
    } else {
        // walk the characters from the front until we find a difference
        let mut a_chars = a.chars().filter(|c| c.is_ascii_digit());
        let mut b_chars = b.chars().filter(|c| c.is_ascii_digit());
        loop {
            let a_next = a_chars.next();
            let b_next = b_chars.next();
            match (a_next, b_next) {
                (None, None) => break Ordering::Equal,
                (Some(c), None) => {
                    break if c == '0' && a_chars.all(|c| c == '0') {
                        Ordering::Equal
                    } else {
                        Ordering::Greater
                    }
                }
                (None, Some(c)) => {
                    break if c == '0' && b_chars.all(|c| c == '0') {
                        Ordering::Equal
                    } else {
                        Ordering::Less
                    }
                }
                (Some(a_char), Some(b_char)) => {
                    let ord = a_char.cmp(&b_char);
                    if ord != Ordering::Equal {
                        break ord;
                    }
                }
            }
        }
    };

    if a_info.sign == Sign::Negative {
        ordering.reverse()
    } else {
        ordering
    }
}

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

    #[test]
    fn parses_exp() {
        let n = "1";
        assert_eq!(
            NumInfo::parse(n, Default::default()),
            (
                NumInfo {
                    exponent: 0,
                    sign: Sign::Positive
                },
                0..1
            )
        );
        let n = "100";
        assert_eq!(
            NumInfo::parse(n, Default::default()),
            (
                NumInfo {
                    exponent: 2,
                    sign: Sign::Positive
                },
                0..3
            )
        );
        let n = "1,000";
        assert_eq!(
            NumInfo::parse(
                n,
                NumInfoParseSettings {
                    thousands_separator: Some(','),
                    ..Default::default()
                }
            ),
            (
                NumInfo {
                    exponent: 3,
                    sign: Sign::Positive
                },
                0..5
            )
        );
        let n = "1,000";
        assert_eq!(
            NumInfo::parse(n, Default::default()),
            (
                NumInfo {
                    exponent: 0,
                    sign: Sign::Positive
                },
                0..1
            )
        );
        let n = "1000.00";
        assert_eq!(
            NumInfo::parse(n, Default::default()),
            (
                NumInfo {
                    exponent: 3,
                    sign: Sign::Positive
                },
                0..7
            )
        );
    }
    #[test]
    fn parses_negative_exp() {
        let n = "0.00005";
        assert_eq!(
            NumInfo::parse(n, Default::default()),
            (
                NumInfo {
                    exponent: -5,
                    sign: Sign::Positive
                },
                6..7
            )
        );
        let n = "00000.00005";
        assert_eq!(
            NumInfo::parse(n, Default::default()),
            (
                NumInfo {
                    exponent: -5,
                    sign: Sign::Positive
                },
                10..11
            )
        );
    }

    #[test]
    fn parses_sign() {
        let n = "5";
        assert_eq!(
            NumInfo::parse(n, Default::default()),
            (
                NumInfo {
                    exponent: 0,
                    sign: Sign::Positive
                },
                0..1
            )
        );
        let n = "-5";
        assert_eq!(
            NumInfo::parse(n, Default::default()),
            (
                NumInfo {
                    exponent: 0,
                    sign: Sign::Negative
                },
                1..2
            )
        );
        let n = "    -5";
        assert_eq!(
            NumInfo::parse(n, Default::default()),
            (
                NumInfo {
                    exponent: 0,
                    sign: Sign::Negative
                },
                5..6
            )
        );
    }

    fn test_helper(a: &str, b: &str, expected: Ordering) {
        let (a_info, a_range) = NumInfo::parse(a, Default::default());
        let (b_info, b_range) = NumInfo::parse(b, Default::default());
        let ordering = numeric_str_cmp(
            (&a[a_range.to_owned()], &a_info),
            (&b[b_range.to_owned()], &b_info),
        );
        assert_eq!(ordering, expected);
        let ordering = numeric_str_cmp((&b[b_range], &b_info), (&a[a_range], &a_info));
        assert_eq!(ordering, expected.reverse());
    }
    #[test]
    fn test_single_digit() {
        test_helper("1", "2", Ordering::Less);
        test_helper("0", "0", Ordering::Equal);
    }
    #[test]
    fn test_minus() {
        test_helper("-1", "-2", Ordering::Greater);
        test_helper("-0", "-0", Ordering::Equal);
    }
    #[test]
    fn test_different_len() {
        test_helper("-20", "-100", Ordering::Greater);
        test_helper("10.0", "2.000000", Ordering::Greater);
    }
    #[test]
    fn test_decimal_digits() {
        test_helper("20.1", "20.2", Ordering::Less);
        test_helper("20.1", "20.15", Ordering::Less);
        test_helper("-20.1", "+20.15", Ordering::Less);
        test_helper("-20.1", "-20", Ordering::Less);
    }
    #[test]
    fn test_trailing_zeroes() {
        test_helper("20.00000", "20.1", Ordering::Less);
        test_helper("20.00000", "20.0", Ordering::Equal);
    }
    #[test]
    fn test_invalid_digits() {
        test_helper("foo", "bar", Ordering::Equal);
        test_helper("20.1", "a", Ordering::Greater);
        test_helper("-20.1", "a", Ordering::Less);
        test_helper("a", "0.15", Ordering::Less);
    }
    #[test]
    fn test_multiple_decimal_pts() {
        test_helper("10.0.0", "50.0.0", Ordering::Less);
        test_helper("0.1.", "0.2.0", Ordering::Less);
        test_helper("1.1.", "0", Ordering::Greater);
        test_helper("1.1.", "-0", Ordering::Greater);
    }
    #[test]
    fn test_leading_decimal_pts() {
        test_helper(".0", ".0", Ordering::Equal);
        test_helper(".1", ".0", Ordering::Greater);
        test_helper(".02", "0", Ordering::Greater);
    }
    #[test]
    fn test_leading_zeroes() {
        test_helper("000000.0", ".0", Ordering::Equal);
        test_helper("0.1", "0000000000000.0", Ordering::Greater);
        test_helper("-01", "-2", Ordering::Greater);
    }

    #[test]
    fn minus_zero() {
        // This matches GNU sort behavior.
        test_helper("-0", "0", Ordering::Less);
        test_helper("-0x", "0", Ordering::Less);
    }
    #[test]
    fn double_minus() {
        test_helper("--1", "0", Ordering::Equal);
    }
    #[test]
    fn single_minus() {
        let info = NumInfo::parse("-", Default::default());
        assert_eq!(
            info,
            (
                NumInfo {
                    exponent: 0,
                    sign: Sign::Positive
                },
                0..0
            )
        );
    }
    #[test]
    fn invalid_with_unit() {
        let info = NumInfo::parse(
            "-K",
            NumInfoParseSettings {
                accept_si_units: true,
                ..Default::default()
            },
        );
        assert_eq!(
            info,
            (
                NumInfo {
                    exponent: 0,
                    sign: Sign::Positive
                },
                0..0
            )
        );
    }
}
