use crate::{
    expression::{Composite, HookKind},
    selection::{Selection, SelectionError, VectoriseError},
    selection_functions::{evaluate_selection_function, SFError},
    state::{EditorState, GroupedChangeError, StateChange, StateResult},
    text::Text,
};

use thiserror::Error;

#[derive(Error, Debug)]
pub enum EvaluationError {
    #[error("{0}")]
    SelectionFunctionError(VectoriseError<SFError>),
    #[error("{0}")]
    SelectionUnionError(SelectionError),
    #[error("Error in processing group {0}")]
    GroupError(GroupedChangeError),
    #[error("Invalid right-only application")]
    IROApplication,
    #[error("Not yet implemented {0}")]
    UnimplementedError(&'static str),
    #[error("Hook used in niladic context")]
    NiladicRightHook,
    #[error("Text argument provided to a selection function")]
    TextArgument,
}

// Helper functions

// Absolutely crazy that in 2026 Rust doesn't have a built-in Either type OR
// (even better) a way to do anonymous sum types! Of course it has anonymous
// product types, but big Cartesian doesn't want you to have +

#[derive(Clone)]
enum Either<L, R> {
    L(L),
    R(R),
}

fn ensure_selection(
    left: Option<Either<Selection, Text>>,
) -> Result<Option<Selection>, EvaluationError> {
    match left {
        Some(Either::L(left)) => Ok(Some(left)),
        None => Ok(None),
        _ => Err(EvaluationError::TextArgument),
    }
}

fn commit_if_needed(
    es: &mut EditorState,
    changes: Vec<StateChange>,
    commit: bool,
) -> Vec<StateChange> {
    if commit {
        es.commit_changes(changes);
        Vec::new()
    } else {
        changes
    }
}

fn _eval(
    es: &mut EditorState,
    comp: Composite,
    left: Option<Either<Selection, Text>>,
    right: Option<Selection>,
    commit: bool,
) -> Result<StateResult, EvaluationError> {
    if left.is_none() && right.is_some() {
        return Err(EvaluationError::IROApplication);
    };

    match comp {
        Composite::SelectionFunction {
            func,
            search_mod,
            result_transform,
        } => {
            let left: Option<Selection> = ensure_selection(left)?;
            let sel =
                evaluate_selection_function(es, func, search_mod, result_transform, left, right)
                    .map_err(EvaluationError::SelectionFunctionError)?;
            Ok((sel, Vec::new()))
        }
        Composite::TextFunction { func, swapped } => {
            Err(EvaluationError::UnimplementedError("text function"))
        }
        Composite::Hook { kind, f, g } => match kind {
            HookKind::Before => {
                let (f_result, mut changes) = _eval(es, *f, left.clone(), None, commit)?;
                changes = commit_if_needed(es, changes, commit);
                let right = if right.is_some() {
                    Ok(right)
                } else {
                    ensure_selection(left)
                }?;
                let (sel, g_changes) = _eval(es, *g, Either::L(f_result).into(), right, commit)?;
                let more_changes = commit_if_needed(es, g_changes, commit);
                changes.extend(more_changes);
                Ok((sel, changes))
            }
            HookKind::After => {
                if left.is_none() {
                    return Err(EvaluationError::NiladicRightHook);
                }
                let combi_left = match right {
                    Some(right) => Some(Either::L(right)),
                    None => left.clone(),
                };
                let (g_result, mut changes) = _eval(es, *g, combi_left, None, commit)?;
                changes = commit_if_needed(es, changes, commit);
                let (sel, f_changes) = _eval(es, *f, left.clone(), Some(g_result), commit)?;
                let more_changes = commit_if_needed(es, f_changes, commit);
                changes.extend(more_changes);
                Ok((sel, changes))
            }
        },
        Composite::Train2 { f, g } => {
            let (left, mut f_changes) = _eval(es, *f, left, right, commit)?;
            f_changes = commit_if_needed(es, f_changes, commit);
            let (sel, g_changes) = _eval(es, *g, Some(Either::L(left)), None, commit)?;
            let more_changes = commit_if_needed(es, g_changes, commit);
            f_changes.extend(more_changes);
            Ok((sel, f_changes))
        }
        Composite::Train3 { f, g, h } => {
            let (f_left, mut f_changes) = _eval(es, *f, left.clone(), right.clone(), commit)?;
            f_changes = commit_if_needed(es, f_changes, commit);
            let (g_right, h_changes) = _eval(es, *h, left, right, commit)?;
            let h_changes = commit_if_needed(es, h_changes, commit);
            let (sel, g_changes) = _eval(es, *g, Some(Either::L(f_left)), Some(g_right), commit)?;
            let g_changes = commit_if_needed(es, g_changes, commit);
            f_changes.extend(h_changes);
            f_changes.extend(g_changes);
            Ok((sel, f_changes))
        }
        Composite::Group { operations } => {
            let results: Vec<(Selection, Vec<StateChange>)> = operations
                .into_iter()
                .map(|op| _eval(es, op, left.clone(), right.clone(), commit))
                .collect::<Result<_, _>>()?;

            let (selections, arms) = results.into_iter().unzip();

            let states = if commit {
                es.commit_changes_grouped(arms).map(|_| Vec::new())
            } else {
                es.validate_grouped_changes(&arms)
                    .map(|_| arms.into_iter().flatten().collect())
            }
            .map_err(EvaluationError::GroupError)?;

            let sel = Selection::union(selections).map_err(EvaluationError::SelectionUnionError)?;
            Ok((sel, states))
        }
    }
}

pub fn evaluate(es: &mut EditorState, comp: Composite) -> Result<Selection, EvaluationError> {
    let (sel, changes) = _eval(es, comp, None, None, true)?;
    es.commit_changes(changes);
    Ok(sel)
}