package saccharine

import (
	"fmt"

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

type TokenIterator = iterator.Iterator[token.Token]

func parseToken(i *TokenIterator, expected token.Type) (*token.Token, error) {
	i2 := i.Copy()

	if tok, err := i2.Next(); err != nil {
		return nil, err
	} else if tok.Type != expected {
		return nil, fmt.Errorf("expected token %v, got %v'", token.Name(expected), tok.Value)
	} else {
		i.Sync(i2)
		return &tok, nil
	}
}

func parseExpression(i *TokenIterator) (ast.Expression, error) {
	var err error
	var exp ast.Expression
	peek := i.MustGet()

	switch peek.Type {
	case token.OpenParen:
		exp, err = parseApplication(i)
	case token.Slash:
		exp, err = parseAbstraction(i)
	case token.Atom:
		exp, err = parseAtom(i)
	default:
		return nil, fmt.Errorf("expected expression, got '%v' (col %d)", peek.Value, peek.Index)
	}

	return exp, err
}

func parseParameters(i *TokenIterator) ([]string, error) {
	i2 := i.Copy()
	variables := []string{}

	for {
		if tok, err := parseToken(i2, token.Atom); err != nil {
			break
		} else {
			variables = append(variables, tok.Value)
		}
	}

	i.Sync(i2)
	return variables, nil
}

func parseAbstraction(i *TokenIterator) (*ast.Abstraction, error) {
	i2 := i.Copy()

	if _, err := parseToken(i2, token.Slash); err != nil {
		return nil, trace.WrapError(fmt.Errorf("no function slash (col %d)", i2.MustGet().Index), err)
	} else if parameters, err := parseParameters(i2); err != nil {
		return nil, err
	} else if _, err = parseToken(i2, token.Dot); err != nil {
		return nil, trace.WrapError(fmt.Errorf("no function dot (col %d)", i2.MustGet().Index), err)
	} else if body, err := parseExpression(i2); err != nil {
		return nil, err
	} else {
		i.Sync(i2)
		return ast.NewAbstraction(parameters, body), nil
	}
}

func parseApplication(i *TokenIterator) (*ast.Application, error) {
	i2 := i.Copy()
	expressions := []ast.Expression{}

	if _, err := parseToken(i2, token.OpenParen); err != nil {
		return nil, trace.WrapError(fmt.Errorf("no openning brackets (col %d)", i2.MustGet().Index), err)
	}

	for {
		if exp, err := parseExpression(i2); err != nil {
			if len(expressions) == 0 {
				return nil, trace.WrapError(fmt.Errorf("application has no arguments"), err)
			}
			break
		} else {
			expressions = append(expressions, exp)
		}
	}

	if _, err := parseToken(i2, token.CloseParen); err != nil {
		return nil, trace.WrapError(fmt.Errorf("no closing brackets (col %d)", i2.MustGet().Index), err)
	}

	i.Sync(i2)
	return ast.NewApplication(expressions[0], expressions[1:]), nil
}

func parseAtom(i *TokenIterator) (*ast.Atom, error) {
	if tok, err := parseToken(i, token.Atom); err != nil {
		return nil, trace.WrapError(fmt.Errorf("no variable (col %d)", i.Index()), err)
	} else {
		return ast.NewAtom(tok.Value), nil
	}
}

func Parse(tokens []token.Token) (ast.Expression, error) {
	i := iterator.Of(tokens)

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

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

	return exp, nil
}