use crate::solvers::Solver;
use primes::{PrimeSet, Sieve};
use std::collections::HashMap;
use std::io::BufRead;

pub struct GiftShop {}

impl GiftShop {
    const RANGES_SPLIT_CHAR: char = ',';
}

impl Solver for GiftShop {
    const PUZZLE_INDEX: u8 = 2;
    const PUZZLE_NAME: &'static str = "Gift Shop";

    fn process_data<R: BufRead>(&self, reader: R) -> (u64, u64) {
        // Cache of part configuration data for number of digits.
        let mut cache = Cache::new();

        let mut result = (0, 0);
        for line in reader.lines().map_while(Result::ok) {
            let ranges = line.split(Self::RANGES_SPLIT_CHAR);
            for range in ranges {
                for range in Range::from_str(range) {
                    let invalid_id_sums = range.sum_invalid_ids(&mut cache);
                    result.0 += invalid_id_sums.0;
                    result.1 += invalid_id_sums.1;
                }
            }
        }
        result
    }
}

struct Range {
    first: u64,
    last: u64,
    digits: u32,
}

impl Range {
    const SPLIT_CHAR: char = '-';

    // Returns a vector of Range from a string, such that each element describes an interval of
    // integers with the same number of digits (base 10), and the intervals combined describe the
    // full range.
    fn from_str(s: &str) -> Vec<Self> {
        // Parses first and last IDs from range string.
        let ids: Vec<u64> = s
            .split(Self::SPLIT_CHAR)
            .map(|s| {
                s.parse()
                    .expect("could not parse id range, must be integers")
            })
            .collect();
        assert_eq!(ids.len(), 2, "could not parse id range, must be two values");

        // Calculates the logs of first and last IDs and checks the size of the range.
        let (a_log, b_log) = (ids[0].ilog10(), ids[1].ilog10());
        assert!(b_log < a_log + 2, "id range is unexpectedly large");

        if a_log != 0 {
            if a_log == b_log {
                vec![Range {
                    first: ids[0],
                    last: ids[1],
                    digits: a_log + 1,
                }]
            } else {
                vec![
                    // Last ID becomes the largest ID with a number of digits equal to 'a_log + 1'.
                    Range {
                        first: ids[0],
                        last: 10_u64.pow(b_log) - 1,
                        digits: b_log,
                    },
                    // First ID becomes the smallest ID with a number of digits equal to 'a_log + 2'.
                    Range {
                        first: 10_u64.pow(b_log),
                        last: ids[1],
                        digits: b_log + 1,
                    },
                ]
            }
        } else {
            // There are no invalid IDs with one digit.
            if a_log == b_log {
                vec![]
            } else {
                vec![Range {
                    first: 10,
                    last: ids[1],
                    digits: 2,
                }]
            }
        }
    }

    // Calculates the sum of invalid IDs contained in this Range. The first number in the returned
    // tuple is the sum of invalid IDs made from some sequence of digits repeated exactly twice,
    // while the second number results from considering sequences repeated twice or more.
    fn sum_invalid_ids(&self, cache: &mut Cache) -> (u64, u64) {
        let parts_configs = cache.entry(self.digits);

        // Determines sum of invalid IDs.
        let mut result = (0, 0);
        let mut duplicate_correction = 0;
        for config in parts_configs {
            let invalid_id_sum = self.sum_invalid_ids_fixed_parts(&config);
            if config.parts == 2 {
                result.0 += invalid_id_sum;
            }
            result.1 += invalid_id_sum;
            if config.parts == self.digits {
                duplicate_correction = invalid_id_sum;
            }
        }
        // This correction ensures that invalid IDs that are composed of a repetition of a single
        // digit, e.g. '111111' are counted only once.
        result.1 -= duplicate_correction * (parts_configs.len() as u64 - 1);
        result
    }

    // Calculates the sum of invalid IDs contained in this Range made from a fixed, given number of
    // repetitions of a sequence of digits.
    fn sum_invalid_ids_fixed_parts(&self, config: &PartsConfig) -> u64 {
        let mut sum = 0;
        let first_left = self.first / config.base;
        let last_left = self.last / config.base;
        let first_right = self.first % config.base;
        let last_right = self.last % config.base;

        // Checks, for a given range 'ABC...-DEF...', whether invalid ID 'AAA...' is in the range.
        let target_right = first_left * config.part_base_repeat;
        if target_right >= first_right && (first_left < last_left || target_right <= last_right) {
            sum += first_left;
        }

        // Calculates the sum of the in-between invalid IDs directly without loop, but we need to
        // explicitly check whether there are any values.
        if first_left + 1 < last_left {
            sum += (first_left + last_left) * (last_left - first_left - 1) / 2;
        }

        // Checks, for a given range 'ABC...-DEF...', whether invalid ID 'DDD...' is in the range.
        let target_right = last_left * config.part_base_repeat;
        if first_left != last_left && target_right <= last_right {
            sum += last_left;
        }
        sum * (config.base + config.part_base_repeat)
    }
}

struct Cache {
    map: HashMap<u32, Vec<PartsConfig>>,
    primes: Sieve,
}

impl Cache {
    fn new() -> Self {
        Cache {
            map: HashMap::new(),
            primes: Sieve::new(),
        }
    }

    // Gets cached or inserts new parts configurations for number of digits.
    fn entry(&mut self, digits: u32) -> &Vec<PartsConfig> {
        self.map
            .entry(digits)
            .or_insert_with(|| Self::build_cache_entry(digits, &mut self.primes))
    }

    fn build_cache_entry(digits: u32, primes: &mut Sieve) -> Vec<PartsConfig> {
        let mut result = Vec::new();
        result.push(PartsConfig::new(digits, digits));
        let max = digits / 2;
        for i in primes.iter().take_while(|&p| p <= u64::from(max)) {
            let i = u32::try_from(i).unwrap();
            if digits % i == 0 {
                result.push(PartsConfig::new(digits, i));
            }
        }
        result
    }
}

struct PartsConfig {
    parts: u32,
    base: u64,
    part_base_repeat: u64,
}

impl PartsConfig {
    fn new(digits: u32, parts: u32) -> Self {
        let part_digits = digits / parts;
        PartsConfig {
            parts,
            base: 10_u64.pow(digits - part_digits),
            part_base_repeat: Self::repeat_base(part_digits, parts - 1),
        }
    }

    fn repeat_base(part_digits: u32, repeat: u32) -> u64 {
        let base = 10_u64.pow(part_digits);
        let mut result = 1;
        for _ in 1..repeat {
            result = result * base + 1;
        }
        result
    }
}

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

    #[test]
    fn part1() {
        test_fixed_parts(11, 22, 2, 2, 33);
        test_fixed_parts(11, 21, 2, 2, 11);
        test_fixed_parts(12, 22, 2, 2, 22);
        test_fixed_parts(12, 32, 2, 2, 22);
    }

    #[test]
    fn part2() {
        test_fixed_parts(111, 333, 3, 3, 666);
        test_fixed_parts(565653, 565659, 6, 3, 565656);
        test_fixed_parts(56565300, 56565900, 8, 4, 56565656);
    }

    fn test_fixed_parts(first: u64, last: u64, digits: u32, parts: u32, expected: u64) {
        assert_eq!(
            Range {
                first,
                last,
                digits
            }
            .sum_invalid_ids_fixed_parts(&PartsConfig::new(digits, parts)),
            expected
        );
    }

    #[test]
    fn ignore_duplicates() {
        assert_eq!(
            Range {
                first: 222222,
                last: 222222,
                digits: 6
            }
            .sum_invalid_ids(&mut Cache::new()),
            (222222, 222222)
        );
    }
}