package saccharine

import (
	"errors"
	"fmt"

	"git.maximhutz.com/max/lambda/pkg/iterator"
	"git.maximhutz.com/max/lambda/pkg/token"
)

type tokenIterator = iterator.Iterator[Token]

func passSoftBreaks(i *tokenIterator) {
	for {
		if _, err := token.ParseRawToken(i, TokenSoftBreak); err != nil {
			return
		}
	}
}

func parseToken(i *tokenIterator, expected TokenType, ignoreSoftBreaks bool) (*Token, error) {
	if ignoreSoftBreaks {
		passSoftBreaks(i)
	}

	return token.ParseRawToken(i, expected)
}

func parseString(i *tokenIterator) (string, error) {
	if tok, err := parseToken(i, TokenAtom, true); err != nil {
		return "", fmt.Errorf("no variable (col %d): %w", i.Index(), err)
	} else {
		return tok.Value, nil
	}
}

func parseBreak(i *tokenIterator) (*Token, error) {
	if tok, softErr := token.ParseRawToken(i, TokenSoftBreak); softErr == nil {
		return tok, nil
	} else if tok, hardErr := token.ParseRawToken(i, TokenHardBreak); hardErr == nil {
		return tok, nil
	} else {
		return nil, errors.Join(softErr, hardErr)
	}
}

func parseAbstraction(i *tokenIterator) (*Abstraction, error) {
	if _, err := parseToken(i, TokenSlash, true); err != nil {
		return nil, fmt.Errorf("no function slash (col %d): %w", i.MustGet().Column, err)
	} else if parameters, err := token.ParseList(i, parseString, 0); err != nil {
		return nil, err
	} else if _, err = parseToken(i, TokenDot, true); err != nil {
		return nil, fmt.Errorf("no function dot (col %d): %w", i.MustGet().Column, err)
	} else if body, err := parseExpression(i); err != nil {
		return nil, err
	} else {
		return &Abstraction{Parameters: parameters, Body: body}, nil
	}
}

func parseApplication(i *tokenIterator) (*Application, error) {
	if _, err := parseToken(i, TokenOpenParen, true); err != nil {
		return nil, fmt.Errorf("no openning brackets (col %d): %w", i.MustGet().Column, err)
	} else if expressions, err := token.ParseList(i, parseExpression, 1); err != nil {
		return nil, err
	} else if _, err := parseToken(i, TokenCloseParen, true); err != nil {
		return nil, fmt.Errorf("no closing brackets (col %d): %w", i.MustGet().Column, err)
	} else {
		return &Application{Abstraction: expressions[0], Arguments: expressions[1:]}, nil
	}
}

func parseAtom(i *tokenIterator) (*Atom, error) {
	if tok, err := parseToken(i, TokenAtom, true); err != nil {
		return nil, fmt.Errorf("no variable (col %d): %w", i.Index(), err)
	} else {
		return &Atom{Name: tok.Value}, nil
	}
}

func parseStatements(i *tokenIterator) ([]Statement, error) {
	statements := []Statement{}

	//nolint:errcheck
	token.ParseList(i, parseBreak, 0)

	for {
		if statement, err := parseStatement(i); err != nil {
			break
		} else if _, err := token.ParseList(i, parseBreak, 1); err != nil && !i.Done() {
			break
		} else {
			statements = append(statements, statement)
		}
	}

	return statements, nil
}

func parseClause(i *tokenIterator, braces bool) (*Clause, error) {
	if braces {
		if _, err := parseToken(i, TokenOpenBrace, true); err != nil {
			return nil, err
		}
	}

	var stmts []Statement
	var last *DeclareStatement
	var err error
	var ok bool

	if stmts, err = parseStatements(i); err != nil {
		return nil, err
	} else if len(stmts) == 0 {
		return nil, fmt.Errorf("no statements in clause")
	} else if last, ok = stmts[len(stmts)-1].(*DeclareStatement); !ok {
		return nil, fmt.Errorf("this clause contains no final return value (col %d)", i.MustGet().Column)
	}

	if braces {
		if _, err := parseToken(i, TokenCloseBrace, true); err != nil {
			return nil, err
		}
	}

	return &Clause{Statements: stmts[:len(stmts)-1], Returns: last.Value}, nil
}

func parseExpression(i *tokenIterator) (Expression, error) {
	passSoftBreaks(i)

	if i.Done() {
		return nil, fmt.Errorf("unexpected end of input")
	}

	switch peek := i.MustGet(); peek.Type {
	case TokenOpenParen:
		return parseApplication(i)
	case TokenSlash:
		return parseAbstraction(i)
	case TokenAtom:
		return parseAtom(i)
	case TokenOpenBrace:
		return parseClause(i, true)
	default:
		return nil, fmt.Errorf("expected expression, got '%v' (col %d)", peek.Value, peek.Column)
	}
}

func parseLet(i *tokenIterator) (*LetStatement, error) {
	if parameters, err := token.ParseList(i, parseString, 1); err != nil {
		return nil, err
	} else if _, err := parseToken(i, TokenAssign, true); err != nil {
		return nil, err
	} else if body, err := parseExpression(i); err != nil {
		return nil, err
	} else {
		return &LetStatement{Name: parameters[0], Parameters: parameters[1:], Body: body}, nil
	}
}

func parseDeclare(i *tokenIterator) (*DeclareStatement, error) {
	if value, err := parseExpression(i); err != nil {
		return nil, err
	} else {
		return &DeclareStatement{Value: value}, nil
	}
}

func parseStatement(i *tokenIterator) (Statement, error) {
	if let, letErr := iterator.Try(i, parseLet); letErr == nil {
		return let, nil
	} else if declare, declErr := iterator.Try(i, parseDeclare); declErr == nil {
		return declare, nil
	} else {
		return nil, errors.Join(letErr, declErr)
	}
}

// Given a list of tokens, attempt to parse it into an syntax tree.
func parse(tokens []Token) (Expression, error) {
	i := iterator.Of(tokens)

	exp, err := parseClause(i, false)
	if err != nil {
		return nil, err
	}

	if !i.Done() {
		return nil, fmt.Errorf("expected EOF, found more code (col %d)", i.MustGet().Column)
	}

	return exp, nil
}