package parser

import (
	"fmt"
	"matheval/ast"
	"matheval/token"
	"strconv"
)

// Parse converts a slice of tokens into an AST.
// Returns an error for malformed expressions.
func Parse(tokens []token.Token) (ast.Node, error) {
	p := &parser{tokens: tokens}
	node, err := p.expr()
	if err != nil {
		return nil, err
	}
	// Ensure all tokens have been consumed (only EOF remains).
	if p.current().Type != token.EOF {
		tok := p.current()
		return nil, fmt.Errorf("unexpected token %v at position %d", tok.Type, tok.Pos)
	}
	return node, nil
}

// parser holds the state for a single parse operation.
type parser struct {
	tokens []token.Token
	pos    int
}

// current returns the token at the current position.
func (p *parser) current() token.Token {
	if p.pos >= len(p.tokens) {
		return token.Token{Type: token.EOF}
	}
	return p.tokens[p.pos]
}

// advance moves to the next token and returns the previous one.
func (p *parser) advance() token.Token {
	tok := p.current()
	p.pos++
	return tok
}

// expect consumes a token of the given type or returns an error.
func (p *parser) expect(typ token.Type) (token.Token, error) {
	tok := p.current()
	if tok.Type != typ {
		return tok, fmt.Errorf("expected %v but got %v at position %d", typ, tok.Type, tok.Pos)
	}
	p.advance()
	return tok, nil
}

// expr → term (('+' | '-') term)*
func (p *parser) expr() (ast.Node, error) {
	left, err := p.term()
	if err != nil {
		return nil, err
	}
	for p.current().Type == token.Plus || p.current().Type == token.Minus {
		op := p.advance()
		right, err := p.term()
		if err != nil {
			return nil, err
		}
		left = &ast.BinaryExpr{
			Op:    op.Type,
			Left:  left,
			Right: right,
		}
	}
	return left, nil
}

// term → factor (('*' | '/') factor)*
func (p *parser) term() (ast.Node, error) {
	left, err := p.factor()
	if err != nil {
		return nil, err
	}
	for p.current().Type == token.Star || p.current().Type == token.Slash {
		op := p.advance()
		right, err := p.factor()
		if err != nil {
			return nil, err
		}
		left = &ast.BinaryExpr{
			Op:    op.Type,
			Left:  left,
			Right: right,
		}
	}
	return left, nil
}

// factor → NUMBER | '(' expr ')'
func (p *parser) factor() (ast.Node, error) {
	tok := p.current()
	switch tok.Type {
	case token.Number:
		p.advance()
		val, err := strconv.ParseFloat(tok.Literal, 64)
		if err != nil {
			return nil, fmt.Errorf("invalid number %q at position %d: %w", tok.Literal, tok.Pos, err)
		}
		return &ast.NumberLit{Value: val}, nil

	case token.LParen:
		p.advance() // consume '('
		node, err := p.expr()
		if err != nil {
			return nil, err
		}
		if _, err := p.expect(token.RParen); err != nil {
			return nil, fmt.Errorf("missing closing parenthesis at position %d", p.current().Pos)
		}
		return node, nil

	default:
		return nil, fmt.Errorf("unexpected token %v at position %d", tok.Type, tok.Pos)
	}
}