/*  OpenTally: Open-source election vote counting
 *  Copyright © 2021 Lee Yingtong Li (RunasSudo)
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU Affero General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU Affero General Public License for more details.
 *
 *  You should have received a copy of the GNU Affero General Public License
 *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

use super::{NextPreferencesEntry, NextPreferencesResult, STVError, STVOptions, SumSurplusTransfersMode, SurplusMethod, SurplusOrder};

use crate::election::{Candidate, CountCard, CountState, Parcel, Vote};
use crate::numbers::Number;

use itertools::Itertools;

use std::cmp::max;
use std::ops;

/// Distribute the largest surplus according to the Gregory method, based on [STVOptions::surplus]
pub fn distribute_surpluses<N: Number>(state: &mut CountState<N>, opts: &STVOptions) -> Result<bool, STVError>
where
	for<'r> &'r N: ops::Sub<&'r N, Output=N>,
	for<'r> &'r N: ops::Div<&'r N, Output=N>,
	for<'r> &'r N: ops::Neg<Output=N>
{
	let quota = state.quota.as_ref().unwrap();
	let mut has_surplus: Vec<(&Candidate, &CountCard<N>)> = state.election.candidates.iter() // Present in order in case of tie
		.map(|c| (c, state.candidates.get(c).unwrap()))
		.filter(|(_, cc)| &cc.votes > quota)
		.collect();
	let total_surpluses = has_surplus.iter()
		.fold(N::new(), |acc, (_, cc)| acc + &cc.votes - quota);
	
	if !has_surplus.is_empty() {
		// Determine if surplues can be deferred
		if opts.defer_surpluses {
			if super::can_defer_surpluses(state, opts, &total_surpluses) {
				state.logger.log_literal(format!("Distribution of surpluses totalling {:.dps$} votes will be deferred.", total_surpluses, dps=opts.pp_decimals));
				return Ok(false);
			}
		}
		
		match opts.surplus_order {
			SurplusOrder::BySize => {
				// Compare b with a to sort high-low
				has_surplus.sort_by(|a, b| b.1.votes.cmp(&a.1.votes));
			}
			SurplusOrder::ByOrder => {
				has_surplus.sort_by(|a, b| a.1.order_elected.cmp(&b.1.order_elected));
			}
		}
		
		// Distribute top candidate's surplus
		let elected_candidate;
		
		// Handle ties
		if has_surplus.len() > 1 && has_surplus[0].1.votes == has_surplus[1].1.votes {
			let max_votes = &has_surplus[0].1.votes;
			let has_surplus = has_surplus.into_iter().filter_map(|(c, cc)| if &cc.votes == max_votes { Some(c) } else { None }).collect();
			elected_candidate = super::choose_highest(state, opts, has_surplus)?;
		} else {
			elected_candidate = has_surplus[0].0;
		}
		
		distribute_surplus(state, &opts, elected_candidate);
		
		return Ok(true);
	}
	return Ok(false);
}

/// Return the denominator of the transfer value
fn calculate_surplus_denom<N: Number>(surplus: &N, result: &NextPreferencesResult<N>, transferable_votes: &N, weighted: bool, transferable_only: bool) -> Option<N>
where
	for<'r> &'r N: ops::Sub<&'r N, Output=N>
{
	if transferable_only {
		let total_units = if weighted { &result.total_votes } else { &result.total_ballots };
		let exhausted_units = if weighted { &result.exhausted.num_votes } else { &result.exhausted.num_ballots };
		let transferable_units = total_units - exhausted_units;
		
		if transferable_votes > surplus {
			return Some(transferable_units);
		} else {
			return None;
		}
	} else {
		if weighted {
			return Some(result.total_votes.clone());
		} else {
			return Some(result.total_ballots.clone());
		}
	}
}

/// Return the reweighted value of the vote after being transferred
fn reweight_vote<N: Number>(
	num_votes: &N,
	num_ballots: &N,
	surplus: &N,
	weighted: bool,
	surplus_fraction: &Option<N>,
	surplus_denom: &Option<N>,
	round_tvs: Option<usize>,
	rounding: Option<usize>) -> N
{
	let mut result;
	
	match surplus_denom {
		Some(v) => {
			if let Some(_) = round_tvs {
				// Rounding requested: use the rounded transfer value
				if weighted {
					result = num_votes.clone() * surplus_fraction.as_ref().unwrap();
				} else {
					result = num_ballots.clone() * surplus_fraction.as_ref().unwrap();
				}
			} else {
				// Avoid unnecessary rounding error by first multiplying by the surplus
				if weighted {
					result = num_votes.clone() * surplus / v;
				} else {
					result = num_ballots.clone() * surplus / v;
				}
			}
		}
		None => {
			result = num_votes.clone();
		}
	}
	
	// Round down if requested
	if let Some(dps) = rounding {
		result.floor_mut(dps);
	}
	
	return result;
}

/// Compute the number of votes to credit to a continuing candidate during a surplus transfer, based on [STVOptions::sum_surplus_transfers]
fn sum_surplus_transfers<N: Number>(entry: &NextPreferencesEntry<N>, surplus: &N, is_weighted: bool, surplus_fraction: &Option<N>, surplus_denom: &Option<N>, _state: &mut CountState<N>, opts: &STVOptions) -> N
where
	for<'r> &'r N: ops::Div<&'r N, Output=N>,
{
	match opts.sum_surplus_transfers {
		SumSurplusTransfersMode::SingleStep => {
			// Calculate transfer across all votes
			//state.logger.log_literal(format!("Transferring {:.0} ballot papers, totalling {:.dps$} votes.", entry.num_ballots, entry.num_votes, dps=opts.pp_decimals));
			return reweight_vote(&entry.num_votes, &entry.num_ballots, surplus, is_weighted, surplus_fraction, surplus_denom, opts.round_tvs, opts.round_votes);
		}
		SumSurplusTransfersMode::ByValue => {
			// Sum transfers by value
			let mut result = N::new();
			
			// Sort into parcels by value
			let mut votes: Vec<&Vote<N>> = entry.votes.iter().collect();
			votes.sort_unstable_by(|a, b| (&a.value / &a.ballot.orig_value).cmp(&(&b.value / &b.ballot.orig_value)));
			for (_value, parcel) in &votes.into_iter().group_by(|v| &v.value / &v.ballot.orig_value) {
				let mut num_votes = N::new();
				let mut num_ballots = N::new();
				for vote in parcel {
					num_votes += &vote.value;
					num_ballots += &vote.ballot.orig_value;
				}
				//state.logger.log_literal(format!("Transferring {:.0} ballot papers, totalling {:.dps$} votes, received at value {:.dps2$}.", num_ballots, num_votes, value, dps=opts.pp_decimals, dps2=max(opts.pp_decimals, 2)));
				result += reweight_vote(&num_votes, &num_ballots, surplus, is_weighted, surplus_fraction, surplus_denom, opts.round_tvs, opts.round_votes);
			}
			
			return result;
		}
		SumSurplusTransfersMode::PerBallot => {
			// Sum transfer per each individual ballot
			// TODO: This could be moved to distribute_surplus to avoid looping over the votes and calculating transfer values twice
			let mut result = N::new();
			for vote in entry.votes.iter() {
				result += reweight_vote(&vote.value, &vote.ballot.orig_value, surplus, is_weighted, surplus_fraction, surplus_denom, opts.round_tvs, opts.round_votes);
			}
			//state.logger.log_literal(format!("Transferring {:.0} ballot papers, totalling {:.dps$} votes.", entry.num_ballots, entry.num_votes, dps=opts.pp_decimals));
			return result;
		}
	}
}

/// Distribute the surplus of a given candidate according to the Gregory method, based on [STVOptions::surplus]
fn distribute_surplus<N: Number>(state: &mut CountState<N>, opts: &STVOptions, elected_candidate: &Candidate)
where
	for<'r> &'r N: ops::Sub<&'r N, Output=N>,
	for<'r> &'r N: ops::Div<&'r N, Output=N>,
	for<'r> &'r N: ops::Neg<Output=N>
{
	state.logger.log_literal(format!("Surplus of {} distributed.", elected_candidate.name));
	
	let count_card = state.candidates.get(elected_candidate).unwrap();
	let surplus = &count_card.votes - state.quota.as_ref().unwrap();
	
	let votes;
	match opts.surplus {
		SurplusMethod::WIG | SurplusMethod::UIG => {
			// Inclusive Gregory
			votes = state.candidates.get(elected_candidate).unwrap().parcels.concat();
		}
		SurplusMethod::EG => {
			// Exclusive Gregory
			// Should be safe to unwrap() - or else how did we get a quota!
			votes = state.candidates.get_mut(elected_candidate).unwrap().parcels.pop().unwrap();
		}
		_ => { panic!("Invalid --surplus for Gregory method"); }
	}
	
	// Count next preferences
	let result = super::next_preferences(state, votes);
	
	state.kind = Some("Surplus of");
	state.title = String::from(&elected_candidate.name);
	
	// Transfer candidate votes
	// TODO: Refactor??
	let is_weighted = match opts.surplus {
		SurplusMethod::WIG => { true }
		SurplusMethod::UIG | SurplusMethod::EG => { false }
		SurplusMethod::Meek => { todo!() }
	};
	
	let transferable_votes = &result.total_votes - &result.exhausted.num_votes;
	let surplus_denom = calculate_surplus_denom(&surplus, &result, &transferable_votes, is_weighted, opts.transferable_only);
	let mut surplus_fraction;
	match surplus_denom {
		Some(ref v) => {
			surplus_fraction = Some(surplus.clone() / v);
			
			// Round down if requested
			if let Some(dps) = opts.round_tvs {
				surplus_fraction.as_mut().unwrap().floor_mut(dps);
			}
			
			if opts.transferable_only {
				state.logger.log_literal(format!("Transferring {:.0} transferable ballots, totalling {:.dps$} transferable votes, with surplus fraction {:.dps2$}.", &result.total_ballots - &result.exhausted.num_ballots, transferable_votes, surplus_fraction.as_ref().unwrap(), dps=opts.pp_decimals, dps2=max(opts.pp_decimals, 2)));
			} else {
				state.logger.log_literal(format!("Transferring {:.0} ballots, totalling {:.dps$} votes, with surplus fraction {:.dps2$}.", result.total_ballots, result.total_votes, surplus_fraction.as_ref().unwrap(), dps=opts.pp_decimals, dps2=max(opts.pp_decimals, 2)));
			}
		}
		None => {
			surplus_fraction = None;
			
			if opts.transferable_only {
				state.logger.log_literal(format!("Transferring {:.0} transferable ballots, totalling {:.dps$} transferable votes, at values received.", &result.total_ballots - &result.exhausted.num_ballots, transferable_votes, dps=opts.pp_decimals));
			} else {
				state.logger.log_literal(format!("Transferring {:.0} ballots, totalling {:.dps$} votes, at values received.", result.total_ballots, result.total_votes, dps=opts.pp_decimals));
			}
		}
	}
	
	let mut checksum = N::new();
	
	for (candidate, entry) in result.candidates.into_iter() {
		// Credit transferred votes
		let candidate_transfers = sum_surplus_transfers(&entry, &surplus, is_weighted, &surplus_fraction, &surplus_denom, state, opts);
		let count_card = state.candidates.get_mut(candidate).unwrap();
		count_card.transfer(&candidate_transfers);
		checksum += candidate_transfers;
		
		let mut parcel = entry.votes as Parcel<N>;
		
		// Reweight votes
		for vote in parcel.iter_mut() {
			vote.value = reweight_vote(&vote.value, &vote.ballot.orig_value, &surplus, is_weighted, &surplus_fraction, &surplus_denom, opts.round_tvs, opts.round_weights);
		}
		
		count_card.parcels.push(parcel);
	}
	
	// Credit exhausted votes
	let mut exhausted_transfers;
	if opts.transferable_only {
		if transferable_votes > surplus {
			// No ballots exhaust
			exhausted_transfers = N::new();
		} else {
			exhausted_transfers = &surplus - &transferable_votes;
			
			if let Some(dps) = opts.round_votes {
				exhausted_transfers.floor_mut(dps);
			}
		}
	} else {
		exhausted_transfers = sum_surplus_transfers(&result.exhausted, &surplus, is_weighted, &surplus_fraction, &surplus_denom, state, opts);
	}
	
	state.exhausted.transfer(&exhausted_transfers);
	checksum += exhausted_transfers;
	
	// Transfer exhausted votes
	let parcel = result.exhausted.votes as Parcel<N>;
	state.exhausted.parcels.push(parcel);
	
	// Finalise candidate votes
	let count_card = state.candidates.get_mut(elected_candidate).unwrap();
	count_card.transfers = -&surplus;
	count_card.votes.assign(state.quota.as_ref().unwrap());
	checksum -= surplus;
	
	// Update loss by fraction
	state.loss_fraction.transfer(&-checksum);
}